Introduction
From the customer's point of view, it is mobile systems that are perhaps the most exciting telecommunications development since the invention of the telephone. The developments in optical fiber technology might sound very impressive and the statistics involved are mind-boggling, but the average subscriber does not usually appreciate the full extent of the benefits at a personal level. The developments in this field are taking place mainly behind the scenes, and are not really tangible service improvements to the subscriber. The pocket telephone, on the other hand, is a revelation that is center-stage and whose benefits can be instantly appreciated by anyone who purchases one of these devices. The cellular telephone industry has experienced explosive growth over recent years. It is an area of telecommunications that has benefited not only the developed world, but also many developing countries. From the service provider's point of view, cellular systems are very fast to install compared to extending new cables to customer premises. When a leading telecommunications company forecast in 1996 that by the year 2000 there would be 350 to 400 million mobile radio units worldwide, there was widespread disbelief. So far, all forecasts of this nature have turned out to be conservative.
Cellular mobile telephone systems are not easy to classify. They could be considered as part of the local loop because they extend out to the subscriber handset, or because of the long distances that can be bridged between a fixed
The analog systems that have been around for a number of years are giving way to digital technology. Narrowband TDMA is currently seeing widespread deployment, and CDMA started deployment in 1996. Significant equipment incompatibilities are encountered when going from one system to another or trying to incorporate two or three of the same network. Many of these problems stem from the difference in outputs. High-mobility cellular radios transmit at relatively high power in the region of 1 to 10 W, whereas the latest low-mobility portable units transmit at relatively low power levels of 1 to 10 mW. While this is fine for the customer, it makes coexistence of the two systems a network planner's nightmare High tier is a term often associated with high-mobility systems, whereas Iow tier is associated with low-mobility systems.
In summary, cellular telephony is the culmination of several technologies which have progressed in parallel over the past two decades. In fact. the progress has been so rapid that the standards bodies have had difficulty organizing meetings fast enough to determine standards that are consistent with the new technology. Go to Top
Historically, technology has lagged behind the design of mobile telephone systems, and it was only by 1983 that the first good-quality systems were put into operation. Since those early, low-capacity pioneering systems subscriber demand has mushroomed, despite the higher cost of calling from a mobile telephone. The different types of mobile radio systems, in terms of frequency spectrum usage. They differ primarily in modulatic technique and carrier spacing.
Analog FM. The first-generation cellular systems in operation were analog F radio systems that allocated a single carrier for each call. Each carrier frequency modulated by the caller. The carriers were typically spaced at 25-kl
Digital FDMA.
FDMA systems resemble analog FM, with the exception that the carrier is modulated by a digitally encoded speech signal. The bandwidth of each carrier is similar to the analog FM systems (typically 25 kHz).
Digital narrowband TDMA.
TDMA systems operate with several customers sharing one carrier. Each user is allocated a specific time slot for transmission and reception of short bursts or packets of information. The bandwidth of each carrier is typically 200 kHz, and the total bandwidth available is in the region of 10 to 30 MHz, so many FDMA carriers each contain several customers on a TDMA shared basis. This access combination allows a reasonably large channel capacity in the region of 500 to 1000 channels, before frequency reuse.
In the United States, for example, the 824- to 849-MHz frequency band is allocated for one-way transmission from the base station to the user, and the 869- to 894-MHz band is allocated for transmission from the user to the base station. To enable two competitive systems to operate simultaneously, only half of each of these bands is available to each operator. Each system therefore has 12.5 MHz available for transmission and 12.5 MHz for reception. Each of these 12.5-MHz bands is subdivided into several carriers in an FDMA manner. Each carrier is operated in a TDMA mode having time slots for voice or data channels. Digital wideband (spread spectrum). One form of digital wideband operation is CDMA.
In these systems there is a single carrier that is modulated by the speech signals of many users. Instead of allocating each user a different time slot, each is allocated a different modulation code. Mobile users in adjacent cells all use the same frequency band. Each user contributes some interfering energy to the receivers of fellow users, the magnitude of which depends on the processing gain. In addition to interference from users within a given cell, there is also interference from users in adjacent cells. The distance between adjacent cell users attenuates the interference considerably more than users within the same cell. Frequency reuse is therefore unnecessary. Consequently, each cell can use the full available bandwidth (12.5 MHz) for CDMA operation.
Analog Cellular Radio Go to Top
Analog cellular systems were used exclusively in the early days of mobile communications. Although they are being superseded by digital technology, a large number of systems are still in service and will probably remain in use for several years to come.
Analog FM.
Analog FM cellular radio systems are relatively old technology (1980 to 1985) in this fast-paced industry. These are the first-generation cellular radio systems. However, it is informative to discuss some of their features briefly, because they provide insight into how future systems are evolving. Analog cellular radio was initially designed for vehicle-mounted operation. By 1990, already more than 50 percent of mobile radios (stations) in most networks were hand-held portables, and the demand was growing.
As far back as 1979, Bell Labs designed and installed a trial cellular mobile system called the Advanced Mobile Phone Service (AMPS). This was really the birth of cellular radio in the United States, and is still the basis of the analog systems in operation today. The AMPS system uses the hexagonal cell structure, with a base station in each cell.
The cells are clustered into groups of seven cells (i.e., a seven-cell repeat pattern). AMPS covers large areas with large-sized cells, and high-traffic-density areas are covered by subdividing cells.
Sectorization is also used to enhance capacity. The overall control of the system is by a mobile telephone switching office (MTSO) in each metropolitan area. This digital switch connects into the regular telephone network and provides fault detection and diagnostics in addition to call processing. The mobile unit was originally installed in a car, truck, or bus. The frequencies for AMPS are 870 to 890 MHz from base to mobile, and 825 to 845 MHz from mobile to base. Each radio channel has a pair of one-way channels separated by 45 MHz. The spacing between adjacent channels is 30 kHz. The AMPS system uses FM with 12-kHz maximum deviation. FM has a convenient capture mechanism. If a receiver detects two different signals on the same frequency, it will lock onto the stronger signal and ignore the weaker, interfering signal.
Mobile units are microprocessor controlled. The MTSO periodically monitors the carrier signal quality coming from the active mobile. If, during a call, a mobile moves to the edge of a cell boundary and crosses the boundary, the signal quality from the adjacent cell gradually becomes better than the existing service provider, so handoffs initiated. The handoff command is a "blank and burst" message sent over the voice channel to the designated cell. A brief data burst is transmitted from the base providing service to instruct the mobile microprocessor to retune the radio to a new channel (carrier). The voice connection is momentarily blanked during the period of data transmission and base station switching. This interruption is so brief it is hardly noticeable, and most customers are unaware of its occurrence. Go to Top
All of the call setup is done by a separate channel. There are dedicated signaling channels that transmit information only in the form of binary data. These channels are monitored by all mobiles that do not have a call in progress. When a mobile is first switched to the on mode or is at the end of a call, it is in the idle state. It scans the frequencies used for call setup and monitors the one providing it with the strongest signal. Each cell has its own setup channel. The mobile periodically makes a scan to see if its change of position has made the
The AMPS system has nationwide roaming capability. This is possible by cooperation between the service providers in different parts of the country. The AMPS system has been very successful, but its main disadvantage is that its total system capacity is inferior to the more advanced digital cellular radio systems.
The AMPS cellular radio network structure.
Digital Cellular Radio Go to Top
Digital cellular radio systems can be divided into two categories, narrowband and wideband. Narrowband systems are often considered to be the second generation of cellular radio. Although the digital narrowband TDMA systems in North America and Europe have developed along similar lines, there remain many features that are different. Because of its global success, in this text the main focus of attention for digital narrowband TDMA cellular radio will be the European-designed system called GSM, to which the U.S. system called IS-54 will be compared.
The acronym GSM originally stood for the French name Groupe Speciale Mobile, the planning organization that did much of the groundwork for the TDMA cellular system.
GSM now stands for global system for mobile communications, A description of its features serves to highlight some of the intricacies of present-day cellular radio systems.
GSM operates in the primary spectrum range of 890–915 MHz (uplink) and 935–960 MHz (downlink), with subsequent adaptations to operate in 1800 MHz (Digital Cellular System or GSM 1800) and 1900 MHz (Personal Communications Services or GSM 1900). GSM 450 and GSM 800 (part of the IS-136(TDMA cellular standards) 850 Band) are planned to utilize the 450 MHz and 800 MHz spectra in the future.
GSM is the basis of a powerful family of platforms for the future - providing a direct link into next generation solutions including GPRS (General Packet Radio Services) EDGE (Enhanced Data for GSM Evolution) and 3G(3GSM).In the tables below will show frequency and the speed for the wireless transmission.
GSM terminals may incorporate one or more of the GSM frequency bands listed below to facilitate roaming on a global basis.
In the above bands mobile stations transmit in the lower frequency sub-band and base stations transmit in the higher frequency sub-band. |
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Theoretical vs. Actual Wireless Transmission Speeds |
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| Generation | Technology | Theoretical Top Speed | Avg. Delivered Speed |
| 1G | AMPS | 19.2 Kbps | Less than 9 Kbps |
| 1G | CDPD | 19.2 Kbps | 9.2 Kbps |
| 2G | TDMA, CDMA, iDEN, GSM | 19.2 Kbps | 9.6-19.2 Kbps |
| 2.5G | GPRS | 115 Kbps | 20-40 Kbps |
| 3G | 1xRTT | 153 Kbps | 60-80 Kbps |
| 3G | EDGE Phase II | 384 Kbps | 80-100 Kbps expected |
| 3G | 1xEV-DO | 2.4 Mbps | 200-300 Kbps |
| 3G | W-CDMA | 384 Kbps | 200-300 Kbps |
| 3G | 1xEV-DV | 4.8 Mbps | 200-300 Kbps |
The Wireless Generation is a function of speed and maturity of technology and is usually representative of a family of similar technologies, Theoretical Throughput is the best-case attainable speed over the network, and is typically 50 to 100% faster than real-world performance. 3G An industry term used to describe the next generation of public wireless voice + data networks. To qualify as 3G, a network must meet certain requirements for speed, availability, reliability and other criteria set forth by the International Telecommunications Union. There are many 3G network technologies being developed, generally they are packet-based "always on" networks.
GSM's unrivalled success can be attributed to many factors, including the
unparalleled co-operation and support between all those supplying, running and
exploiting the platform. It is based upon a true end-to-end solution, from
infrastructure and services to handsets and billing systems.
GSM is a standard that embraces all areas of technology, resulting in global,
seamless wireless services for all its customers. It's all part of the Wireless
Evolution.
The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the early 1970s. However, mobile cellular systems were not introduced for commercial use until the 1980s. During the early 1980s, analog cellular telephone systems experienced a very rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular systems still represent one of the fastest growing telecommunications systems.
But in the beginnings of cellular systems, each country developed its own system, which was an undesirable situation for the following reasons:
In 1989 the responsability for the GSM specifications passed from the CEPT to the European Telecommunications Standards Institute (ETSI). The aim of the GSM specifications is to describe the functionality and the interface for each component of the system, and to provide guidance on the design of the system. These specifications will then standardize the system in order to guarantee the proper interworking between the different elements of the GSM system. In 1990, the phase I of the GSM specifications were published but the commercial use of GSM did not start until mid-1991
The cellular structure
In a cellular system, the covering area of an operator is divided into cells. A cell corresponds to the covering area of one transmitter or a small collection of transmitters. The size of a cell is determined by the transmitter's power.
The concept of cellular systems is the use of low power transmitters in order to enable the efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the frequencies can not be reused for hundred of kilometers as they are limited to the covering area of the transmitter.
The frequency band allocated to a cellular mobile radio system is distributed over a group of cells and this distribution is repeated in all the covering area of an operator. The whole number of radio channels available can then be used in each group of cells that form the covering area of an operator. Frequencies used in a cell will be reused several cells away. The distance between the cells using the same frequency must be sufficient to avoid interference. The frequency reuse will increase considerably the capacity in number of users.
In order to work properly, a cellular system must verify the following two main conditions:
Cluster
The cells are grouped into clusters. The number of cells in a cluster must be determined so that the cluster can be repeated continuously within the covering area of an operator. The typical clusters contain 4, 7, 12 or 21 cells. The number of cells in each cluster is very important. The smaller the number of cells per cluster is, the bigger the number of channels per cell will be. The capacity of each cell will be therefore increased. However a balance must be found in order to avoid the interference that could occur between neighboring clusters. This interference is produced by the small size of the clusters (the size of the cluster is defined by the number of cells per cluster). The total number of channels per cell depends on the number of available channels and the type of cluster used.
Types of cells
The density of population in a country is so varied that different types of cells are used:
Microcells
These cells are used for densely populated areas. By splitting the existing areas into smaller cells, the number of channels available is increased as well as the capacity of the cells. The power level of the transmitters used in these cells is then decreased, reducing the possibility of interference between neighboring cells.
Selective cells
It is not always useful to define a cell with a full coverage
of 360 degrees. In some cases, cells with a particular shape and coverage are
needed. These cells are called selective cells. A typical example of selective
cells are the cells that may be located at the entrances of tunnels where a
coverage of 360 degrees is not needed. In this case, a selective cell with a
coverage of 120 degrees is used.
Umbrella cells
A freeway crossing very small cells produces an important number of handovers among the different small neighboring cells. In order to solve this problem, the concept of umbrella cells is introduced. An umbrella cell covers several microcells. The power level inside an umbrella cell is increased comparing to the power levels used in the microcells that form the umbrella cell. When the speed of the mobile is too high, the mobile is handed off to the umbrella cell. The mobile will then stay longer in the same cell (in this case the umbrella cell). This will reduce the number of handovers and the work of the network.
A too important number of handover demands and the propagation characteristics of a mobile can help to detect its high speed.
The transition from analog to digital technology Go to Top
In the 1980s most mobile cellular systems were based on analog systems. The GSM system can be considered as the first digital cellular system. The different reasons that explain this transition from analog to digital technology are presented in this section.
The capacity of the system
As it is explained in section 1, cellular systems have experienced a very important growth. Analog systems were not able to cope with this increasing demand. In order to overcome this problem, new frequency bands and new technologies were proposed. But the possibility of using new frequency bands was rejected by a big number of countries because of the restricted spectrum (even if later on, other frequency bands have been allocated for the development of mobile cellular radio). The new analog technologies proposed were able to overcome the problem to a certain degree but the costs were too important.
The digital radio was, therefore, the best option (but not the perfect one) to handle the capacity needs in a cost-efficiency way.
Compatibility with other systems such as ISDN
The decision of adopting a digital technology for GSM was made in the course of developing the standard. During the development of GSM, the telecommunications industry converted to digital methods. The ISDN network is an example of this evolution. In order to make GSM compatible with the services offered by ISDN, it was decide that the digital technology was the best option.
Additionally, a digital system allows,
easily than an analog one, the implementation of future improvements and the
change of its own characteristics.
Aspects of quality
The quality of the service can be considerably improved using a digital technology rather than an analog one. In fact, analog systems pass the physical disturbances in radio transmission (such as fades, multipath reception, spurious signals or interferences) to the receiver. These disturbances decrease the quality of the communication because they produce effects such as fadeouts, crosstalks, hisses, etc. On the other hand, digital systems avoid these effects transforming the signal into bits. This transformation combined with other techniques, such as digital coding, improve the quality of the transmission. The improvement of digital systems comparing to analog systems is more noticeable under difficult reception conditions than under good reception conditions.
The GSM network
Architecture of the GSM network
The GSM technical specifications define the different entities that form the GSM network by defining their functions and interface requirements.
The GSM network can be divided into four main parts:
Mobile Station
A Mobile Station consists of two main elements:
There are different types of terminals distinguished principally by their power and application:
The SIM is a smart card that identifies the terminal. By inserting the SIM card into the terminal, the user can have access to all the subscribed services. Without the SIM card, the terminal is not operational.
The SIM card is protected by a four-digit Personal Identification Number (PIN). In order to identify the subscriber to the system, the SIM card contains some parameters of the user such as its International Mobile Subscriber Identity (IMSI).
Another advantage of the SIM card is the mobility of the users. In fact, the only element that personalizes a terminal is the SIM card. Therefore, the user can have access to its subscribed services in any terminal using its SIM card.
The Base
Station Subsystem
The BSS connects the Mobile Station and the NSS. It is in charge of the transmission and reception. The BSS can be divided into two parts:
The BTS corresponds to the transceivers and antennas used in each cell of the network. A BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell. Each BTS has between one and sixteen transceivers depending on the density of users in the cell.
The Base Station Controller
The BSC controls a group of BTS and manages their radio resources. A BSC is principally in charge of handovers, frequency hopping, exchange functions and control of the radio frequency power levels of the BTSs.
The Network and Switching Subsystem
Its main role is to manage the communications between the mobile users and other users, such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases needed in order to store information about the subscribers and to manage their mobility. The different components of the NSS are described below.
The Mobile services Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of the network. It also provides connection to other networks.
The Gateway Mobile services Switching Center (GMSC)
A gateway is a node interconnecting two networks. The GMSC is the interface between the mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
Home Location Register (HLR)
The HLR is considered as a very important
database that stores information of the subscribers belonging to the covering
area of a MSC. It also stores the current location of these subscribers and the
services to which they have access. The location of the subscriber corresponds
to the SS7 address of the Visitor Location Register (VLR) associated to the
terminal.
Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR necessary in order to provide the subscribed services to visiting users. When a subscriber enters the covering area of a new MSC, the VLR associated to this MSC will request information about the new subscriber to its corresponding HLR. The VLR will then have enough information in order to assure the subscribed services without needing to ask the HLR each time a communication is established.
The VLR is always implemented together with a MSC; so the area under control of the MSC is also the area under control of the VLR.
The Authentication Center (AuC) Go to Top
The AuC register is used for security purposes. It provides the parameters needed for authentication and encryption functions. These parameters help to verify the user's identity.
The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register containing information about the mobile equipments. More particularly, it contains a list of all valid terminals. A terminal is identified by its International Mobile Equipment Identity (IMEI). The EIR allows then to forbid calls from stolen or unauthorized terminals (e.g, a terminal which does not respect the specifications concerning the output RF power).
The GSM Interworking Unit (GIWU)
The GIWU corresponds to an interface to various networks for data communications. During these communications, the transmission of speech and data can be alternated.
The Operation
and Support Subsystem (OSS)
The OSS is connected to the different components of the NSS and to the BSC, in order to control and monitor the GSM system. It is also in charge of controlling the traffic load of the BSS.
However, the increasing number of base stations, due to the development of cellular radio networks, has provoked that some of the maintenance tasks are transfered to the BTS. This transfer decreases considerably the costs of the maintenance of the system.
The geographical areas of the GSM network
The figure 2 presents the different areas that form a GSM network.
As it has already been explained a cell,
identified by its Cell Global Identity number (CGI), corresponds to the radio
coverage of a base transceiver station. A Location Area (LA), identified by its
Location Area Identity (LAI) number, is a group of cells served by a single MSC/VLR.
A group of location areas under the control of the same MSC/VLR defines the MSC/VLR
area. A Public Land Mobile Network (PLMN) is the area served by one network
operator.
The GSM functions
In this paragraph, the description of the GSM network is focused on the different functions to fulfill by the network and not on its physical components. In GSM, five main functions can be defined:
Transmission
The transmission function includes two sub-functions:
Radio
Resources management (RR)
The role of the RR function is to establish, maintain and release communication links between mobile stations and the MSC. The elements that are mainly concerned with the RR function are the mobile station and the base station. However, as the RR function is also in charge of maintaining a connection even if the user moves from one cell to another, the MSC, in charge of handovers, is also concerned with the RR functions.
The RR is also responsible for the management of the frequency spectrum and the reaction of the network to changing radio environment conditions. Some of the main RR procedures that assure its responsabilities are:
Handover
The user movements can produce the need to change the channel or cell, specially when the quality of the communication is decreasing. This procedure of changing the resources is called handover. Four different types of handovers can be distinguished:
The mobile station is the active participant in this procedure. In order to perform the handover, the mobile station controls continuously its own signal strength and the signal strength of the neighboring cells. The list of cells that must be monitored by the mobile station is given by the base station. The power measurements allow to decide which is the best cell in order to maintain the quality of the communication link. Two basic algorithms are used for the handover:
The MM function is in charge of all the aspects related with the mobility of the user, specially the location management and the authentication and security.
Location management
When a mobile station is powered on, it performs a location update procedure by indicating its IMSI to the network. The first location update procedure is called the IMSI attach procedure.
The mobile station also performs location updating, in order to indicate its current location, when it moves to a new Location Area or a different PLMN. This location updating message is sent to the new MSC/VLR, which gives the location information to the subscriber's HLR. If the mobile station is authorized in the new MSC/VLR, the subscriber's HLR cancells the registration of the mobile station with the old MSC/VLR.
A location updating is also performed periodically. If after the updating time period, the mobile station has not registered, it is then deregistered.
When a mobile station is powered off, it performs an IMSI detach procedure in order to tell the network that it is no longer connected.
Authentication and security
The authentication procedure involves the SIM card and the Authentication Center. A secret key, stored in the SIM card and the AuC, and a ciphering algorithm called A3 are used in order to verify the authenticity of the user. The mobile station and the AuC compute a SRES using the secret key, the algorithm A3 and a random number generated by the AuC. If the two computed SRES are the same, the subscriber is authenticated. The different services to which the subscriber has access are also checked.
Another security procedure is to check the equipment identity. If the IMEI number of the mobile is authorized in the EIR, the mobile station is allowed to connect the network.
In order to assure user confidentiality, the user is registered with a Temporary Mobile Subscriber Identity (TMSI) after its first location update procedure.
Enciphering is another option to guarantee a very strong security but this procedure is going to be described later on.
Communication
Management (CM)
The CM function is responsible for:
Call Control (CC) Go to Top
The CC is responsible for call establishing, maintaining and releasing as well as for selecting the type of service. One of the most important functions of the CC is the call routing. In order to reach a mobile subscriber, a user dials the Mobile Subscriber ISDN (MSISDN) number which includes:
Supplementary Services management
The mobile station and the HLR are the only components of the GSM network involved with this function. The different Supplementary Services (SS) to which the users have access are presented in section 6.3.
Short Message Services management
In order to support these services, a GSM network is in contact with a Short Message Service Center through the two following interfaces:
Operation, Administration and Maintenance (OAM)
The OAM function allows the operator to monitor and control the system as well as to modify the configuration of the elements of the system. Not only the OSS is part of the OAM, also the BSS and NSS participate in its functions as it is shown in the following examples:
The GSM radio interface
The radio interface is the interface between the mobile stations and the fixed infrastructure. It is one of the most important interfaces of the GSM system.
One of the main objectives of GSM is roaming. Therefore, in order to obtain a complete compatibility between mobile stations and networks of different manufacturers and operators, the radio interface must be completely defined.
The spectrum efficiency depends on the radio interface and the transmission, more particularly in aspects such as the capacity of the system and the techniques used in order to decrease the interference and to improve the frequency reuse scheme. The specification of the radio interface has then an important influence on the spectrum efficiency.
Frequency allocation
Two frequency bands, of 25 Mhz each one, have been allocated for the GSM system:
Multiple access scheme
The multiple access scheme defines how different simultaneous communications, between different mobile stations situated in different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM.
FDMA and TDMA
Using FDMA, a frequency is assigned to a user. So the larger the number of users in a FDMA system, the larger the number of available frequencies must be. The limited available radio spectrum and the fact that a user will not free its assigned frequency until he does not need it anymore, explain why the number of users in a FDMA system can be "quickly" limited.
On the other hand, TDMA allows several users to share the same channel. Each of the users, sharing the common channel, are assigned their own burst within a group of bursts called a frame. Usually TDMA is used with a FDMA structure.
In GSM, a 25 Mhz frequency band is divided, using a FDMA scheme, into 124 carrier frequencies spaced one from each other by a 200 khz frequency band. Normally a 25 Mhz frequency band can provide 125 carrier frequencies but the first carrier frequency is used as a guard band between GSM and other services working on lower frequencies. Each carrier frequency is then divided in time using a TDMA scheme. This scheme splits the radio channel, with a width of 200 khz, into 8 bursts. A burst is the unit of time in a TDMA system, and it lasts approximately 0.577 ms. A TDMA frame is formed with 8 bursts and lasts, consequently, 4.615 ms. Each of the eight bursts, that form a TDMA frame, are then assigned to a single user.
Channel
structure
A channel corresponds to the recurrence of one burst every frame. It is defined by its frequency and the position of its corresponding burst within a TDMA frame. In GSM there are two types of channels:
Full-rate traffic channels (TCH/F) are defined using a group of 26 TDMA frames called a 26-Multiframe. The 26-Multiframe lasts consequently 120 ms. In this 26-Multiframe structure, the traffic channels for the downlink and uplink are separated by 3 bursts. As a consequence, the mobiles will not need to transmit and receive at the same time which simplifies considerably the electronics of the system.
The frames that form the 26-Multiframe structure have different functions:
Control channels Go to Top
According to their functions, four different classes of control channels are defined:
Broadcast channels (BCH)
The BCH channels are used, by the base station, to provide the mobile station with the sufficient information it needs to synchronize with the network. Three different types of BCHs can be distinguished:
Common Control Channels (CCCH)
The CCCH channels help to establish the calls from the mobile station or the network. Three different types of CCCH can be defined:
Dedicated Control Channels (DCCH)
The DCCH channels are used for message exchange between several mobiles or a mobile and the network. Two different types of DCCH can be defined:
Associated Control Channels
The Fast Associated Control Channels (FACCH) replace all or part of a traffic channel when urgent signaling information must be transmitted. The FACCH channels carry the same information as the SDCCH channels.
Burst structure
As it has been stated before, the burst is the unit in time of a TDMA system. Four different types of bursts can be distinguished in GSM:
The tail bits (T) are a group of three bits
set to zero and placed at the beginning and the end of a burst. They are used to
cover the periods of ramping up and down of the mobile's power.
The coded data bits corresponds to two groups, of 57 bits each, containing signaling or user data.
The stealing flags (S) indicate, to the receiver, whether the information carried by a burst corresponds to traffic or signaling data.
The training sequence has a length of 26 bits. It is used to synchronize the receiver with the incoming information, avoiding then the negative effects produced by a multipath propagation.
The guard period (GP), with a length of 8.25 bits, is used to avoid a possible overlap of two mobiles during the ramping time.
Frequency hopping Go to Top
The propagation conditions and therefore the multipath fading depend on the radio frequency. In order to avoid important differences in the quality of the channels, the slow frequency hopping is introduced. The slow frequency hopping changes the frequency with every TDMA frame. A fast frequency hopping changes the frequency many times per frame but it is not used in GSM. The frequency hopping also reduces the effects of co-channel interference.
There are different types of frequency hopping algorithms. The algorithm selected is sent through the Broadcast Control Channels.
Even if frequency hopping can be very useful for the system, a base station does not have to support it necessarily On the other hand, a mobile station has to accept frequency hopping when a base station decides to use it.
From source information to radio waves
The figure 4 presents
the different operations that have to be performed in order to pass from the
speech source to radio waves and vice versa.
If the source of information is data and
not speech, the speech coding will not be performed.
Speech coding
The transmission of speech is, at the moment, the most important service of a mobile cellular system. The GSM speech codec, which will transform the analog signal (voice) into a digital representation, has to meet the following criterias:
Channel coding
Channel coding adds redundancy bits to the original information in order to detect and correct, if possible, errors occurred during the transmission.
Channel coding for the GSM data TCH channels Go to Top
The channel coding is performed using two codes: a block code and a convolutional code.
The block code corresponds to the block code defined in the GSM Recommendations 05.03. The block code receives an input block of 240 bits and adds four zero tail bits at the end of the input block. The output of the block code is consequently a block of 244 bits.
A convolutional code adds redundancy bits in order to protect the information. A convolutional encoder contains memory. This property differentiates a convolutional code from a block code. A convolutional code can be defined by three variables : n, k and K. The value n corresponds to the number of bits at the output of the encoder, k to the number of bits at the input of the block and K to the memory of the encoder. The ratio, R, of the code is defined as follows : R = k/n. Let's consider a convolutional code with the following values: k is equal to 1, n to 2 and K to 5. This convolutional code uses then a rate of R = 1/2 and a delay of K = 5, which means that it will add a redundant bit for each input bit. The convolutional code uses 5 consecutive bits in order to compute the redundancy bit. As the convolutional code is a 1/2 rate convolutional code, a block of 488 bits is generated. These 488 bits are punctured in order to produce a block of 456 bits. Thirty two bits, obtained as follows, are not transmitted :
C (11 + 15 j) for j = 0, 1, ..., 31
The block of 456 bits produced by the convolutional code is then passed to the interleaver.
Channel coding for the GSM speech channels
Before applying the channel coding, the 260 bits of a GSM speech frame are divided in three different classes according to their function and importance. The most important class is the class Ia containing 50 bits. Next in importance is the class Ib, which contains 132 bits. The least important is the class II, which contains the remaining 78 bits. The different classes are coded differently. First of all, the class Ia bits are block-coded. Three parity bits, used for error detection, are added to the 50 class Ia bits. The resultant 53 bits are added to the class Ib bits. Four zero bits are added to this block of 185 bits (50+3+132). A convolutional code, with r = 1/2 and K = 5, is then applied, obtaining an output block of 378 bits. The class II bits are added, without any protection, to the output block of the convolutional coder. An output block of 456 bits is finally obtained.
Channel coding for the GSM control channels
In GSM the signalling information is just contained in 184 bits. Forty parity bits, obtained using a fire code, and four zero bits are added to the 184 bits before applying the convolutional code (r = 1/2 and K = 5). The output of the convolutional code is then a block of 456 bits, which does not need to be punctured.
Interleaving
An interleaving rearranges a group of bits in a particular way. It is used in combination with FEC codes in order to improve the performance of the error correction mechanisms. The interleaving decreases the possibility of losing whole bursts during the transmission, by dispersing the errors. Being the errors less concentrated, it is then easier to correct them.
Interleaving for the GSM control channels
A burst in GSM transmits two blocks of 57 data bits each. Therefore the 456 bits corresponding to the output of the channel coder fit into four bursts (4*114 = 456). The 456 bits are divided into eight blocks of 57 bits. The first block of 57 bits contains the bit numbers (0, 8, 16, .....448), the second one the bit numbers (1, 9, 17, .....449), etc. The last block of 57 bits will then contain the bit numbers (7, 15, .....455). The first four blocks of 57 bits are placed in the even-numbered bits of four bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the same four bursts. Therefore the interleaving depth of the GSM interleaving for control channels is four and a new data block starts every four bursts. The interleaver for control channels is called a block rectangular interleaver.
Interleaving for the GSM speech channels
The block of 456 bits, obtained after the channel coding, is then divided in eight blocks of 57 bits in the same way as it is explained in the previous paragraph. But these eight blocks of 57 bits are distributed differently. The first four blocks of 57 bits are placed in the even-numbered bits of four consecutive bursts. The other four blocks of 57 bits are placed in the odd-numbered bits of the next four bursts. The interleaving depth of the GSM interleaving for speech channels is then eight. A new data block also starts every four bursts. The interleaver for speech channels is called a block diagonal interleaver.
Interleaving for the GSM data TCH channels
A particular interleaving scheme, with an interleaving depth equal to 22, is applied to the block of 456 bits obtained after the channel coding. The block is divided into 16 blocks of 24 bits each, 2 blocks of 18 bits each, 2 blocks of 12 bits each and 2 blocks of 6 bits each. It is spread over 22 bursts in the following way :
Burst assembling
The burst assembling procedure is in
charge of grouping the bits into bursts. Section 5.2.3 presents the different
bursts structures and describes in detail the structure of the normal burst.
Ciphering
Ciphering is used to protect signaling and
user data. First of all, a ciphering key is computed using the algorithm A8
stored on the SIM card, the subscriber key and a random number delivered by the
network (this random number is the same as the one used for the authentication
procedure). Secondly, a 114 bit sequence is produced using the ciphering key, an
algorithm called A5 and the burst numbers. This bit sequence is then XORed with
the two 57 bit blocks of data included in a normal burst.
In order to decipher correctly, the receiver has to use the same algorithm A5 for the deciphering procedure.
Modulation Go to Top
The modulation chosen for the GSM system is the Gaussian Minimum Shift Keying (GMSK).
The aim of this section is not to describe precisely the GMSK modulation as it is too long and it implies the presentation of too many mathematical concepts. Therefore, only brief aspects of the GMSK modulation are presented in this section.
The GMSK modulation has been chosen as a compromise between spectrum efficiency, complexity and low spurious radiations (that reduce the possibilities of adjacent channel interference). The GMSK modulation has a rate of 270 5/6 kbauds and a BT product equal to 0.3. Figure 5 presents the principle of a GMSK modulator.
Discontinuous transmission (DTX)
This is another aspect of GSM that could have been included as one of the requirements of the GSM speech codec. The function of the DTX is to suspend the radio transmission during the silence periods. This can become quite interesting if we take into consideration the fact that a person speaks less than 40 or 50 percent during a conversation. The DTX helps then to reduce interference between different cells and to increase the capacity of the system. It also extends the life of a mobile's battery. The DTX function is performed thanks to two main features:
Timing advance
The timing of the bursts transmissions is very important. Mobiles are at different distances from the base stations. Their delay depends, consequently, on their distance. The aim of the timing advance is that the signals coming from the different mobile stations arrive to the base station at the right time. The base station measures the timing delay of the mobile stations. If the bursts corresponding to a mobile station arrive too late and overlap with other bursts, the base station tells, this mobile, to advance the transmission of its bursts.
Power control
At the same time the base stations perform the timing measurements, they also perform measurements on the power level of the different mobile stations. These power levels are adjusted so that the power is nearly the same for each burst.
A base station also controls its power level. The mobile station measures the strength and the quality of the signal between itself and the base station. If the mobile station does not receive correctly the signal, the base station changes its power level.
Discontinuous reception
It is a method used to conserve the mobile station's power. The paging channel is divided into subchannels corresponding to single mobile stations. Each mobile station will then only 'listen' to its subchannel and will stay in the sleep mode during the other subchannels of the paging channel.
Multipath and equalisation
At the GSM frequency bands, radio waves reflect from buildings, cars, hills, etc. So not only the 'right' signal (the output signal of the emitter) is received by an antenna, but also many reflected signals, which corrupt the information, with different phases.
An equalizer is in charge of extracting the 'right' signal from the received signal. It estimates the channel impulse response of the GSM system and then constructs an inverse filter. The receiver knows which training sequence it must wait for. The equalizer will then , comparing the received training sequence with the training sequence it was expecting, compute the coefficients of the channel impulse response. In order to extract the 'right' signal, the received signal is passed through the inverse filter.
General
Packet Radio Service (GPRS) Go to Top
The General Packet Radio System (GPRS) is a new service that provides actual packet radio access for mobile Global System for Mobile Communications (GSM) and time-division multiple access (TDMA) users. The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. The increased functionality of GPRS will decrease the incremental cost to provide data services, an occurrence that will, in turn, increase the penetration of data services among consumer and business users. In addition, GPRS will allow improved quality of data services as measured in terms of reliability, response time, and features supported. The unique applications that will be developed with GPRS will appeal to a broad base of mobile subscribers and allow operators to differentiate their services. These new services will increase capacity requirements on the radio and base-station subsystem resources. One method GPRS uses to alleviate the capacity impacts is sharing the same radio resource among all mobile stations in a cell, providing effective use of the scarce resources. In addition, new core network elements will be deployed to support the high burstiness of data services more efficiently.
GPRS (General Packet Radio Service) is a step between GSM and 3G cellular networks. GPRS offers faster data transmission via a GSM network within a range 9.6Kbits to 115Kbits. This new technology makes it possible for users to make telephone calls and transmit data at the same time. (For example, if you have a mobile phone using GPRS, you will be able to simultaneously make calls and receive e-mail massages.) The main benefits of GPRS are that it reserves radio resources only when there is data to send and it reduces reliance on traditional circuit-switched network elements. The GPRS infrastructure and mobile phones support a data transmission speed of up to 13.4Kbits per channel.
In addition to providing new services for today's mobile user, GPRS is important as a migration step toward third-generation (3G) networks. GPRS will allow network operators to implement an IP-based core architecture for data applications, which will continue to be used and expanded upon for 3G services for integrated voice and data applications. In addition, GPRS will prove a testing and development area for new services and applications, which will also be used in the development of 3G services.
A complete understanding of the application availability and GPRS timeline requires understanding of terminal types and availability. The term "terminal equipment" is generally used to refer to the variety of mobile phones and mobile stations that can be used in a GPRS environment; the equipment is defined by terminal classes and types. Cisco Gateway GPRS Serving Node (GGSN) and data network components interoperate with GPRS terminals that follow the GPRS standards.
A GPRS terminal can be one of three classes: A, B, or C. A Class A terminal supports GPRS and other GSM services (such as SMS and voice) simultaneously. This support includes simultaneous attach, activation, monitor, and traffic. As such, a Class A terminal can make or receive calls on two services simultaneously. In the presence of circuit-switched services, GPRS virtual circuits will be held or placed on busy rather than being cleared.
A Class B terminal can monitor GSM and GPRS channels simultaneously, but can support only one of these services at a time. Therefore, a Class B terminal can support simultaneous attach, activation, and monitor, but not simultaneous traffic. As with Class A, the GPRS virtual circuits will not be closed down when circuit-switched traffic is present. Instead, they will be switched to busy or held mode. Thus, users can make or receive calls on either a packet or a switched call type sequentially, but not simultaneously.
A Class C terminal supports only non-simultaneous attach. The user must select which service to connect to. Therefore, a Class C terminal can make or receive calls from only the manually (or default) selected service. The service that is not selected is not reachable. Finally, the GPRS specifications state that support of SMS is optional for Class C terminals.
In addition to the three variables, each handset will have a unique form factor. Some of the form factors will be similar to current mobile wireless devices, while others will evolve to use the enhanced data capabilities of GPRS.
The earliest available type will be closely related to the current mobile phone. These will be available in the standard form factor with a numeric keypad and a relatively small display.
PC Cards are credit card-sized hardware devices that connect via a serial cable to the bottom of a mobile phone. Data cards for GPRS phones will enable laptops and other devices with PC Card slots to be connected to mobile GPRS-capable phones. Card phones provide functionality similar to that offered by PC Cards, without needing a separate phone. These devices may need an earpiece and microphone to support voice services.
Smart phones are mobile phones with built-in voice, non-voice, and Web-browsing services. Smart phones integrate mobile computing and mobile communications into a single terminal. They come in various form factors, which may include a keyboard or an icon drive screen. The Nokia 9000 series is a popular example of this form factor.
The increase in machine-to-machine communications has led to the adoption of application-specific devices. These "black-box" devices lack a display, keypad, and voice accessories of a standard phone. Communication is accomplished through a serial cable. Applications such as meter reading utilize such black-box devices.
Personal digital assistants (PDAs) such as the Palm Pilot series or Handspring Visor are data-centric devices that are adding mobile wireless access. These devices can either connect with a GPRS-capable mobile phone via a serial cable or have GPRS capability built in.
A final category of GPRS terminals is handheld communications. Again, these are primarily data-centric devices that are adding mobile wireless access. Access can be gained via a PC Card or via a serial cable to a GPRS-capable phone.
From a high level, GPRS can be thought of as an overlay network onto a second-generation GSM network. This data overlay network provides packet data transport at rates from 9.6 to 171 kbps. Additionally, multiple users can share the same air-interface resources.
GPRS attempts to reuse the existing GSM network elements as much as possible, but in order to effectively build a packet-based mobile cellular network, some new network elements, interfaces, and protocols that handle packet traffic are required. Therefore, GPRS requires modifications to numerous network elements.
New terminals (TEs) are required because existing GSM phones do not handle the enhanced air interface, nor do they have the ability to packetize traffic directly. A variety of terminals will exist, as described in a previous section, including a high-speed version of current phones to support high-speed data access, a new kind of PDA device with an embedded GSM phone, and PC Cards for laptop computers. All these TEs will be backward compatible with GSM for making voice calls using GSM.
Each BSC will require the installation of one or more PCUs and a software upgrade. The PCU provides a physical and logical data interface out of the base station system (BSS) for packet data traffic. The BTS may also require a software upgrade, but typically will not require hardware enhancements.
When either voice or data traffic is originated at the subscriber terminal, it is transported over the air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call. However, at the output of the BSC the traffic is separated; voice is sent to the mobile switching center (MSC) per standard GSM, and data is sent to a new device called the SGSN, via the PCU over a Frame Relay interface.
In the core network, the existing MSCs are based upon circuit-switched central-office technology, and they cannot handle packet traffic. Thus two new components, called GPRS Support Nodes, are added:
The SGSN can be viewed as a "packet-switched MSC;" it delivers packets to mobile stations (MSs) within its service area. SGSNs send queries to home location registers (HLRs) to obtain profile data of GPRS subscribers. SGSNs detect new GPRS MSs in a given service area, process registration of new mobile subscribers, and keep a record of their location inside a given area. Therefore, the SGSN performs mobility management functions such as mobile subscriber attach/detach and location management. The SGSN is connected to the base-station subsystem via a Frame Relay connection to the PCU in the BSC.
GGSNs are used as interfaces to external IP networks such as the public Internet, other mobile service providers' GPRS services, or enterprise intranets. GGSNs maintain routing information that is necessary to tunnel the protocol data units (PDUs) to the SGSNs that service particular MSs. Other functions include network and subscriber screening and address mapping. One (or more) GGSNs may be provided to support multiple SGSNs. More detailed technical descriptions of the SGSN and GGSN are provided in a later section.
Mobility management within GPRS builds on the mechanisms used in GSM networks; as a MS moves from one area to another, mobility management functions are used to track its location within each mobile network. The SGSNs communicate with each other and update the user location. The MS profiles are preserved in the visitor location registers (VLRs) that are accessible by the SGSNs via the local GSM MSC. A logical link is established and maintained between the MS and the SGSN in each mobile network. At the end of transmission or when a MS moves out of the area of a specific SGSN, the logical link is released and the resources associated with it can be reallocated
The operation of the GPRS is partly independent of the GSM network. However, some procedures share the network elements with current GSM functions to increase efficiency and to make optimum use of free GSM resources (such as unallocated time slots).
States of GPRS in a Mobile Station
An MS has three states in the GPRS system: idle, standby, and active . The three-state model represents the nature of packet radio relative to the GSM two-state model (idle or active).
Data is transmitted between a MS and the GPRS network only when the MS is in the active state. In the active state, the SGSN knows the cell location of the MS. However, in the standby state, the location of the MS is known only as to which routing area it is in. (The routing area can consist of one or more cells within a GSM location area.)
When the SGSN sends a packet to a MS that is in the standby state, the MS must be paged. Because the SGSN knows the routing area in which the MS is located, a packet paging message is sent to that routing area. After receiving the packet paging message, the MS gives its cell location to the SGSN to establish the active state.
Packet transmission to an active MS is initiated by packet paging to notify the MS of an incoming data packet. The data transmission proceeds immediately after packet paging through the channel indicated by the paging message. The purpose of the packet paging message is to simplify the process of receiving packets. The MS has to listen to only the packet paging messages, instead of all the data packets in the downlink channels, reducing battery use significantly.
When an MS has a packet to be transmitted, access to the uplink channel is needed. The uplink channel is shared by a number of MSs, and its use is allocated by a BSS. The MS requests use of the channel in a packet random access message. The transmission of the packet random access message follows Slotted Aloha procedures. The BSS allocates an unused channel to the MS and sends a packet access grant message in reply to the packet random access message. The description of the channel (one or multiple time slots) is included in the packet access grant message. The data is transmitted on the reserved channels.
The main reasons for the standby state are to reduce the load in the GPRS network caused by cell-based routing update messages and to conserve the MS battery. When a MS is in the standby state, there is no need to inform the SGSN of every cell change—only of every routing area change. The operator can define the size of the routing area and, in this way, adjust the number of routing update messages.
In the idle state, the MS does not have a logical GPRS context activated or any Packet-Switched Public Data Network (PSPDN) addresses allocated. In this state, the MS can receive only those multicast messages that can be received by any GPRS MS. Because the GPRS network infrastructure does not know the location of the MS, it is not possible to send messages to the MS from external data networks.
A cell-based routing update procedure is invoked when an active MS enters a new cell. In this case, the MS sends a short message containing information about its move (the message contains the identity of the MS and its new location) through GPRS channels to its current SGSN. This procedure is used only when the MS is in the active state.
When an MS in an active or a standby state moves from one routing area to another in the service area of one SGSN, it must again perform a routing update. The routing area information in the SGSN is updated and the success of the procedure is indicated in the response message.
The inter-SGSN routing update is the most complicated of the three routing updates. In this case, the MS changes from one SGSN area to another, and it must establish a new connection to a new SGSN. This means creating a new logical link context between the MS and the new SGSN, as well as informing the GGSN about the new location of the MS.
KEY USER FEATURES OF GPRS
The General Packet Radio Service (GPRS) is a new non-voice value added service that allows information to be sent and received across a mobile telephone network. It supplements today’s Circuit Switched Data and Short Message Service. GPRS is NOT related to GPS (the Global Positioning System), a similar acronym that is often used in mobile contexts. GPRS has several unique features which can be summarized as:
SPEED
Theoretical maximum speeds of up to 171.2 kilobits per second (kbps) are achievable with GPRS using all eight timeslots at the same time. This is about three times as fast as the data transmission speeds possible over today’s fixed telecommunications networks and ten times as fast as current Circuit Switched Data services on GSM networks. By allowing information to be transmitted more quickly, immediately and efficiently across the mobile network, GPRS may well be a relatively less costly mobile data service compared to SMS and Circuit Switched Data.
IMMEDIACY
GPRS facilitates instant connections whereby information can be sent or received immediately as the need arises, subject to radio coverage. No dial-up modem connection is necessary. This is why GPRS users are sometimes referred to be as being "always connected". Immediacy is one of the advantages of GPRS (and SMS) when compared to Circuit Switched Data. High immediacy is a very important feature for time critical applications such as remote credit card authorization where it would be unacceptable to keep the customer waiting for even thirty extra seconds.
NEW APPLICATIONS, BETTER APPLICATIONS
GPRS facilitates several new applications that have not previously been available over GSM networks due to the limitations in speed of Circuit Switched Data (9.6 kbps) and message length of the Short Message Service (160 characters). GPRS will fully enable the Internet applications you are used to on your desktop from web browsing to chat over the mobile network. Other new applications for GPRS, profiled later, include file transfer and home automation- the ability to remotely access and control in-house appliances and machines.
SERVICE ACCESS
To use GPRS, users specifically need:
· a mobile phone or terminal that supports GPRS
· a subscription to a mobile telephone network that supports GPRS
· use of GPRS must be enabled for that user. Automatic access to the GPRS may be allowed by some mobile network operators, others will require a specific opt-in
· knowledge of how to send and/ or receive GPRS information using their specific model of mobile phone, including software and hardware configuration (this
creates a customer service requirement)
· a destination to send or receive information through GPRS. Whereas with SMS this was often another mobile phone, in the case of GPRS, it is likely to be an
Internet address, since GPRS is designed to make the Internet fully available to mobile users for the first time. From day one, GPRS users can access any web
page or other Internet applications- providing an immediate critical mass of uses. Having looked at the key user features of GPRS, lets look at the key features from a network operator perspective. model of mobile phone, including software and hardware configuration .
KEY NETWORK FEATURES OF GPRS Go to Top
PACKET SWITCHING
GPRS involves overlaying a packet based air interface on the existing circuit switches GSM network. This gives the user an option to use a packet-based data service. To supplement a circuit switched network architecture with packet switching is quite a major upgrade. However, as we shall see later, the GPRS standard is delivered in a very elegant manner- with network operators needing only to add a couple of new infrastructure nodes and making a software upgrade to some existing network elements. With GPRS, the information is split into separate but related "packets" before being transmitted and reassembled at the receiving end. Packet switching is similar to a jigsaw puzzle- the image that the puzzle represents is divided into pieces at the manufacturing factory and put into a plastic bag. During transportation of the now boxed jigsaw from the factory to the end user, the pieces get jumbled up. When the recipient empties the bag with all the pieces, they are reassembled to form the original image. All the pieces are all related and fit together, but the way they are transported and assembled varies. The Internet itself is another example of a packet data network, the most famous of many such network types.
SPECTRUM EFFICIENCY
Packet switching means that GPRS radio resources are used only when users are actually sending or receiving data. Rather than dedicating a radio channel to a mobile data user for a fixed period of time, the available radio resource can be concurrently shared between several users. This efficient use of scarce radio resources means that large numbers of GPRS users can potentially share the same bandwidth and be served from a single cell. The actual number of users supported depends on the application being used and how much data is being transferred. Because of the spectrum efficiency of GPRS, there is less need to build in idle capacity that is only used in peak hours. GPRS therefore lets network operators maximize the use of their network resources in a dynamic and flexible way, along with user access to resources and revenues. GPRS should improve the peak time capacity of a GSM network since it simultaneously:
· allocates scarce radio resources more efficiently by supporting virtual connectivity
· migrates traffic that was previously sent using Circuit Switched Data to GPRS instead, and
· reduces SMS Center and signalling channel loading by migrating some traffic that previously was sent using SMS to GPRS instead using the GPRS/ SMS interconnect that is supported by the GPRS standards.
INTERNET AWARE
For the first time, GPRS fully enables Mobile Internet functionality by allowing interworking between the existing Internet and the new GPRS network. Any service that is used over the fixed Internet today- File Transfer Protocol (FTP), web browsing, chat, email, telnet- will be as available over the mobile network because of GPRS. In fact, many network operators are considering the opportunity to use GPRS to help become wireless Internet Service Providers in their own right. The World Wide Web is becoming the primary communications interface- people access the Internet for entertainment and information collection, the intranet for accessing company information and connecting with colleagues and the extranet for accessing customers and suppliers. These are all derivatives of the World Wide Web aimed at connecting different communities of interest. There is a trend away from storing information locally in specific software packages on PCs to remotely on the Internet. When you want to check your schedule or contacts, instead of using something like "Act!", you go onto the Internet site such as a portal. Hence, web browsing is a very important application for GPRS. Because it uses the same protocols, the GPRS network can be viewed as a sub-network of the Internet with GPRS capable mobile phones being viewed as mobile hosts. This means that each GPRS terminal can potentially have its own IP address and will be addressable as such.
SUPPORTS TDMA AND GSM
It should be noted right that the General Packet Radio Service is not only a service designed to be deployed on mobile networks that are based on the GSM digital mobile phone standard. The IS-136 Time Division Multiple Access (TDMA) standard, popular in North and South America, will also support GPRS. This follows an agreement to follow the same evolution path towards third generation mobile phone networks concluded in early 1999 by the industry associations that support these two network types.
LIMITATIONS OF GPRS
It should already be clear that GPRS is an important new enabling mobile data service which offers a major improvement in spectrum efficiency, capability and functionality compared with today’s non-voice mobile services. However, it is important to note that there are some limitations with GPRS, which can be summarized as:
LIMITED CELL CAPACITY FOR ALL USERS
GPRS does impact a network’s existing cell capacity. There are only limited radio resources that can be deployed for different uses- use for one purpose precludes simultaneous use for another. For example, voice and GPRS calls both use the same network resources. The extent of the impact depends upon the number of timeslots, if any, that are reserved for exclusive use of GPRS. However, GPRS does dynamically manage channel allocation and allow a reduction in peak time signalling channel loading by sending short messages over GPRS channels instead. RESULT: NEED FOR SMS as a complementary bearer that uses a different type of radio resource.
SPEEDS MUCH LOWER IN REALITY
Achieving the theoretical maximum GPRS data transmission speed of 172.2 kbps would require a single user taking over all eight timeslots without any error protection. Clearly, it is unlikely that a network operator will allow all timeslots to be used by a single GPRS user. Additionally, the initial GPRS terminals are expected be severely limited- supporting only one, two or three timeslots. The bandwidth available to a GPRS user will therefore be severely limited. As such, the theoretical maximum GPRS speeds should be checked against the reality of constraints in the networks and terminals. The reality is that mobile networks are always likely to have lower data transmission speeds than fixed networks. RESULT: Relatively high mobile data speeds may not be available to individual mobile users until Enhanced Data rates for GSM Evolution (EDGE) or Universal Mobile Telephone System (UMTS) are introduced.
SUBOPTIMAL MODULATION
GPRS is based on a modulation technique known as Gaussian minimum-shift keying (GMSK). EDGE is based on a new modulation scheme that allows a much higher bit rate across the air interface- this is called eight-phase-shift keying (8 PSK) modulation. Since 8 PSK will also be used for UMTS, network operators will need to incorporate it at some stage to make the transition to third generation mobile phone systems.
TRANSIT DELAYS
GPRS packets are sent in all different directions to reach the same destination. This opens up the potential for one or some of those packets to be lost or corrupted during the data transmission over the radio link. The GPRS standards recognize this inherent feature of wireless packet technologies and incorporate data integrity and retransmission strategies. However, the result is that potential transit delays can occur. Because of this, applications requiring broadcast quality video may well be implemented using High Speed Circuit Switched Data (HSCSD). HSCSD is simply a Circuit Switched Data call in which a single user can take over up to four separate channels at the same time. Because of its characteristic of end to end connection between sender and recipient, transmission delays are less likely.
NO STORE AND FORWARD
Whereas the Store and Forward Engine in the Short Message Service is the heart of the SMS Center and key feature of the SMS service, there is no storage mechanism incorporated into the GPRS standard, apart from the incorporation of interconnection links between SMS and GPRS.
APPLICATIONS FOR GPRS Go to Top
A wide range of corporate and consumer applications are enabled by nonvoice mobile services such as SMS and GPRS. This section will introduce those that are particularly suited to GPRS.
CHAT
Chat can be distinguished from general information services because the source of the information is a person with chat whereas it tends to be from an Internet site for information services. The "information intensity"- the amount of information transferred per message tends to be lower with chat, where people are more likely to state opinions than factual data. In the same way as Internet chat groups have proven a very popular application of the Internet, groups of likeminded people- so called communities of interest- have begun to use non-voice mobile services as a means to chat and communicate and discuss. Because of its synergy with the Internet, GPRS would allow mobile users to participate fully in existing Internet chat groups rather than needing to set up their own groups that are dedicated to mobile users. Since the number of participants is an important factor determining the value of participation in the newsgroup, the use of GPRS here would be advantageous. GPRS will not however support point to multipoint services in its first phase, hindering the distribution of a single message to a group of people. As such, given the installed base of SMS capable devices, we would expect SMS to remain the primary bearer for chat applications in the foreseeable future, although experimentation with using GPRS is likely to commence sooner rather than later.
TEXTUAL AND VISUAL INFORMATION
A wide range of content can be delivered to mobile phone users ranging from share prices, sports scores, weather, flight information, news headlines, prayer reminders, lottery results, jokes, horoscopes, traffic, location sensitive services and so on. This information need not necessarily be textual- it may be maps or graphs or other types of visual information. The length of a short message of 160 characters suffices for delivering information when it is quantitative- such as a share price or a sports score or temperature. When the information is of a qualitative nature however, such as a horoscope or news story, 160 characters is too short other than to tantalize or annoy the information recipient since they receive the headline or forecast but little else of substance. As such, GPRS will likely be used for qualitative information services when end users have GPRS capable devices, but SMS will continue to be used for delivering most quantitative information services. Interestingly, chat applications are a form of qualitative information that may remain delivered using SMS, in order to limit people to brevity and reduce the incidence of spurious and irrelevant posts to the mailing list that are a common occurrence on Internet chat groups.
STILL IMAGES
Still images such as photographs, pictures, postcards, greeting cards and presentations, static web pages can be sent and received over the mobile network as they are across fixed telephone networks. It will be possible with GPRS to post images from a digital camera connected to a GPRS radio device directly to an Internet site, allowing near real-time desktop publishing.
MOVING IMAGES
Over time, the nature and form of mobile communication is getting less textual and more visual. The wireless industry is moving from text messages to icons and picture messages to photographs and blueprints to video messages and movie previews being downloaded and on to full blown movie watching via data streaming on a mobile device. Sending moving images in a mobile environment has several vertical market applications including monitoring parking lots or building sites for intruders or thieves, and sending images of patients from an ambulance to a hospital. Videoconferencing applications, in which teams of distributed sales people can have a regular sales meeting without having to go to a particular physical location, is another application for moving images.
WEB BROWSING
Using Circuit Switched Data for web browsing has never been an enduring application for mobile users. Because of the slow speed of Circuit Switched Data, it takes a long time for data to arrive from the Internet server to the browser. Alternatively, users switch off the images and just access the text on the web, and end up with difficult to read text layouts on screens that are difficult to read from. As such, mobile Internet browsing is better suited to GPRS.
DOCUMENT SHARING/ COLLABORATIVE WORKING
Mobile data facilitates document sharing and remote collaborative working. This lets different people in different places work on the same document at the same time. Multimedia applications combining voice, text, pictures and images can even be envisaged. These kinds of applications could be useful in any problem solving exercise such as fire fighting, combat to plan the route of attack, medical treatment, advertising copy setting, architecture, journalism and so on. Even comments on which resort to book a holiday at could benefit from document sharing to save everyone having to visit the travel agent to make a decision. Anywhere somebody can benefit from having and being able to comment on a visual depiction of a situation or matter, such collaborative working can be useful. By providing sufficient bandwidth, GPRS facilitates multimedia applications such as document sharing.
AUDIO
Despite many improvements in the quality of voice calls on mobile networks such as Enhanced Full Rate (EFR), they are still not broadcast quality. There are scenarios where journalists or undercover police officers with portable professional broadcast quality microphones and amplifiers capture interviews with people or radio reports dictated by themselves and need to send this information back to their radio or police station. Leaving a mobile phone on, or dictating to a mobile phone, would simply not give sufficient voice quality to allow that transmission to be broadcast or analyzed for the purposes of background noise analysis or voice printing, where the speech autograph is taken and matched against those in police storage. Since even short voice clips occupy large file sizes, GPRS or other high speed mobile data services are needed.
JOB DISPATCH
Non-voice mobile services can be used to assign and communicate new jobs from office-based staff to mobile field staff. Customers typically telephone a call center whose staff take the call and categorize it. Those calls requiring a visit by field sales or service representative can then be escalated to those mobile workers. Job dispatch applications can optionally be combined with vehicle positioning applications- such that the nearest available suitable personnel can be deployed to serve a customer. GSM non-voice services can be used not only to send the job out, but also as a means for the service engineer or sales person can keep the office informed of progress towards meeting the customer’s requirement. The remote worker can send in a status message such as "Job 1234 complete, on my way to 1235". The 160 characters of a short message are sufficient for communicating most delivery addresses such as those needed for a sales, service or some other job dispatch application such as mobile pizza delivery and courier package delivery. However, 160 characters does require manipulation of the customer data such as the use of abbreviations such as "St" instead of "Street". Neither does 160 characters leave much space for giving the field representative any information about the problem that has been reported or the customer profile. The field representative is able to arrive at the customer premises but is not very well briefed beyond that. This is where GPRS will come in to allow more information to be sent and received more easily. With GPRS, a photograph of the customer and their premises could, for example, be sent to the field representative to assist in finding and identifying the customer. As such, we expect job dispatch applications will be an early adopter of GPRS-based communications.
CORPORATE EMAIL
With up to half of employees typically away from their desks at any one time, it is important for them to keep in touch with the office by extending the use of corporate email systems beyond an employee’s office PC. Corporate email systems run on Local Area computer Networks (LAN) and include Microsoft Mail, Outlook, Outlook Express, Microsoft Exchange, Lotus Notes and Lotus cc:Mail.
Since GPRS capable devices will be more widespread in corporations than amongst the general mobile phone user community, there are likely to be more corporate email applications using GPRS than Internet email ones whose target market is more general.
INTERNET EMAIL
Internet email services come in the form of a gateway service where the messages are not stored, or mailbox services in which messages are stored. In the case of gateway services, the wireless email platform simply translates the message from SMTP, the Internet email protocol, into SMS and sends to the SMS Center. In the case of mailbox email services, the emails are actually stored and the user gets a notification on their mobile phone and can then retrieve the full email by dialing in to collect it, forward it and so on. Upon receiving a new email, most Internet email users do not currently get notified of this fact on their mobile phone. When they are out of the office, they have to dial in speculatively and periodically to check their mailbox contents. However, by linking Internet email with an alert mechanism such as SMS or GPRS, users can be notified when a new email is received.
VEHICLE POSITIONING
This application integrates satellite positioning systems that tell people where they are with non-voice mobile services that let people tell others where they are. The Global Positioning System (GPS) is a free-to-use global network of 24 satellites run by the US Department of Defense. Anyone with a GPS receiver can receive their satellite position and thereby find out where they are. Vehicle positioning applications can be used to deliver several services including remote vehicle diagnostics, ad-hoc stolen vehicle tracking and new rental car fleet tariffs. The Short Message Service is ideal for sending Global Positioning System (GPS) position information such as longitude, latitude, bearing and altitude. GPS coordinates are typically about 60 characters in length. GPRS could alternatively be used.
REMOTE LAN ACCESS
When mobile workers are away from their desks, they clearly need to connect to the Local Area Network in their office. Remote LAN applications encompasses access to any applications that an employee would use when sitting at their desk, such as access to the intranet, their corporate email services such as Microsoft Exchange or Lotus Notes and to database applications running on Oracle or Sybase or whatever. The mobile terminal such as handheld or laptop computer has the same software programs as the desktop on it, or cut down client versions of the applications accessible through the corporate LAN. This application area is therefore likely to be a conglomeration of remote access to several different information types- email, intranet, databases. This information may all be accessible through web browsing tools, or require proprietary software applications on the mobile device. The ideal bearer for Remote LAN Access depends on the amount of data being transmitted, but the speed and latency of GPRS make it ideal.
FILE TRANSFER
As this generic term suggests, file transfer applications encompass any form of downloading sizeable data across the mobile network. This data could be a presentation document for a traveling salesperson, an appliance manual for a service engineer or a software application such as Adobe Acrobat Reader to read documents. The source of this information could be one of the Internet communication methods such as FTP (File Transfer Protocol), telnet, http or Java- or from a proprietary database or legacy platform. Irrespective of source and type of file being transferred, this kind of application tends to be bandwidth intensive. It therefore requires a high speed mobile data service such as GPRS, EDGE or UMTS to run satisfactorily across a mobile network.
HOME AUTOMATION
Home automation applications combine remote security with remote control. Basically, you can monitor your home from wherever you are- on the road, on holiday, or at the office. If your burglar alarm goes off, not only do you get alerted, but you get to go live and see who are perpetrators are and perhaps even lock them in. Not only can you see things at home, but you can do things too. You can program your video, switch your oven on so that the preheating is complete by the time you arrive home (traffic jams permitting) and so on. Your GPRS capable mobile phone really does become like the remote control devices we use today for our television, video, hi-fi and so on. As the Internet Protocol (IP) will soon be everywhere- not just in mobile phones because of GPRS but all manner of household appliances and in every machine- these devices can be addressed and instructed. A key enabler for home automation applications will be Bluetooth, which allows disparate devices to interwork.
OPTIMAL BEARER BY APPLICATION Go to Top
Currently, corporate applications that use the Short Message Service are few and far between. The reasons are the relatively older age of corporate mobile phone users and their lower price sensitivity, particularly since the employer usually pays mobile phones bills. Corporate users are less willing to learn how to and make the effort to send a short message- they tend to use voice as their primary communications method. Instead, the vast majority of SMS usage is accounted for by consumer applications. It is not uncommon to find 90% of the total SMS traffic accounted for by the consumer applications that have been described. Until GPRS terminals are consumer oriented, SMS will continue to be bearer for most consumer applications. However, since GPRS will be incorporated into high end mobile phones initially, it will be used more for corporate applications. Whatever the application, the Internet will become the primary communications interface. Previously, application developers wrote proprietary applications that worked with proprietary host terminals and often proprietary rugged terminal operating systems. For example, instead of corporate applications such as service engineering using platform and software specific interfaces, the mobile workers such as service engineers will access an intranet page using their GPRS capable terminal and fill in an electronic form. People increasingly use a web browser to access publicly available data on the Internet itself, the extranet for access to the data of business partners and other external collaborators and the intranet to access internal employee information. As such, all work will be carried out through the web interface. Often, by designing applications to minimize the effects of the limitations of existing mobile services- such as the length of a short message or the speed of a Circuit Switched Data call- existing non-voice mobile services can be successfully used for mobile working. However, many non-voice applications are graphics intensive and the new faster data services will allow BETTER VERSIONS of today’s existing non-voice applications. For example, instead of occasional information messages with SMS, information services via GPRS or UMTS will be more akin to the "push" Internet channels we see on Active PC Desktops today. Instead of the slow transmission of small video images, real-time broadcast quality images will be transmittable. Instead of using SMS to notify Internet users of new email, the whole email will be sent, and full-blown Internet access will be The same applications will be more immediate and convenient for users. The use of SMS has prepared customers for non-voice applications using GPRS and other non-voice services and most of the applications envisaged for GPRS already exist in some form today. It is therefore an important question to consider what the preferred bearer for each application will be- GPRS, Circuit Switched Data or SMS.
GPRS NETWORK NODES
Enabling GPRS on a GSM network requires the addition of two core modules, the Gateway GPRS Service Node (GGSN) and the Serving GPRS Service Node (SGSN). As
the word Gateway in its name suggests, the GGSN acts as a gateway between the GPRS network and Public Data Networks such as IP and X.25. GGSNs also connect to other GPRS networks to facilitate GPRS roaming. The Serving GPRS Support Node (SGSN) provides packet routing to and from the SGSN service area for all users in that service area. In addition to adding multiple GPRS nodes and a GPRS backbone, some other technical changes that need to be added to a GSM network to implement a GPRS service. These include the addition of Packet Control Units; often hosted in the Base Station Subsystems, mobility management to locate the GPRS Mobile Station, a new air interface for packet traffic, new security features such as ciphering and new GPRS specific signalling.
RELATED GPRS CHALLENGES
BILLING
GPRS is a different kind of service from those typically available on today’s mobile networks. GPRS is essentially a packet switching overlay on a circuit switching network. The GPRS specifications stipulate the minimum charging information that must be collected in the Stage 1 service description. These include destination and source addresses, usage of radio interface, usage of external Packet Data Networks, usage of the packet data protocol addresses, usage of general GPRS resources and location of the Mobile Station. Since GPRS networks break the information to be communicated down into packets, at a minimum, a GPRS network needs to be able to count packets to charging customers for the volume of packets they send and receive. Today’s billing systems have difficulties handling charging for today’s non-voice services. It is unlikely that circuit switched billing systems will be able to process a large number of new variables created by GPRS. GPRS call records are generated in the GPRS Service Nodes. The GGSN and SGSN may not be able to store charging information but this charging information needs to be processed. The incumbent billing systems are often not able to handle real time Call Detail Record flows. As such, an intermediary charging platform is a good idea to perform billing mediation by collecting the charging information from the GPRS nodes and preparing it for submission to the billing system. Packet counts are passed to a Charging Gateway that generates Call Detail Records that are sent to the billing system. However, the crucial challenge of being able to bill for GPRS and therefore earn a return on investment in GPRS is simplified by the fact that the major GPRS infrastructure vendors all support charging functions as part of their GPRS solutions. Additionally, a wide range of other existing non-GSM packet data networks such as X.25 and Cellular Digital Packet Data (CDPD) are in place along with associated billing systems. It may well be the case that the cost of measuring packets is greater than their value. The implication is that there will NOT be a per packet charge since there may be too many packets to warrant counting and charging for. For example, a single traffic monitoring application can generate tens of thousands of packets per day. Thus the charging gateway function is more a policing function than a charging function since network operators are likely to tariff certain amounts of GPRS traffic at a flat rate and then need to monitor whether these allocations are far exceeded. This is not to say that we will end up with the free Internet Service Provider model that has become established on the fixed Internet in which users pay no fixed monthly charge and network operators rely on advertising sales on mobile portal sites to make money. There is a premium for mobility and there is frankly a shortage of mobile bandwidth that limits the extent to which that bandwidth is viewed as a commodity. And given the additional customer care and billing complexity associated with mobile Internet and nonvoice services, network operators would be ill advised to reduce their prices in such a way as to devalue the perceived value of mobility.
TARIFFING
Decisions on charging for GPRS by packet or simply a flat monthly fee are contentious but need to be made. Charging different packets at different rates can make things complicated for the user, whilst flat rates favor heavy users more than occasional ones. We believe that the optimal GPRS pricing model will be based on two variables- time and packet. Network operators should levy a nominal per packet charge during peak times plus a flat rate, no per packet charge during non peak times. Time and packet related charging will encourage applications such as remote monitoring, meter reading and chat to use GPRS overnight when spare network capacity is available. Simultaneously, a nominal per packet charge during the day will help to allocate scarce radio resources and charge radio heavy applications such as file and image transfer more than applications with lower data intensity. It has the advantage that it will automatically adjust customer charging according to their application usage. As such the optimal charging model could well be a flat rate charge during off-peak times along with a per packet charge during peak times.
Digital Cellular System (DCS-1800)
The second generation of European cellular radio systems is the DCS-1800. This is an extension of the GSM system and, it has an improved capacity over GSM. This is achieved mainly by increasing the bandwidth from the GSM 25 MHz to DCS-1800 75 MHz, by moving up to the 1.710 to 1.880-GHz band. Bandwidth efficiency is doubled by using half-rate encoding at 11.4 kb/s (instead of 22.8 kb/s for GSM). This is therefore about a sixfold increase in the number of traffic channels per cluster before frequency reuse to 5984, compared to 992 for the original GSM.
The DCS-1800 has a low transmit power level of 125 mW average or 31 mW average, compared to 2.5 W down to 250 mW for GSM. The call range is clearly lower for DCS-1800, which means more cells are needed to cover a given area in the suburban-to-rural environment. The lower transmit power translates into less battery power consumption, and therefore longer talk and standby time between battery recharges. The higher operating frequency means higher free-space loss and higher shadowing loss due to signal refraction. That in turn means more signal fading in urban areas, and a higher cost of overcoming the free-space loss in rural area coverage.
North American Integrated Services-136 (IS-54) system Go to Top
The United States TDMA system was originally called IS-54 or digital AMPS, and has been superseded by the IS-136A upgraded standard. This system places its information on carriers spaced by 30 kHz (i.e., 30-kHz bandwidth).
This is the same as the AMPS system, and allows operating companies gradually to replace the analog channels with digital channels to ease base station traffic congestion. Each digital channel carries three user signals at the full-rate coding standard compared to eight in the GSM system. The half-rate doubles the number to six. The channel transmission bit rate for digitally modulating the carrier is 48.6 kb/s. Each frame has six time slots of 6.67-ms duration. Each time slot carries 324 bits of information, of which 260 bits are for the 13-kb/s full-rate traffic data. The other 64 bits are overhead; 28 of these are for synchronization, and they contain a specific bit sequence known by all receivers to establish frame alignment. Also, as with GSM, the known sequence acts as a training pattern to initialize an adaptive equalizer.
The IS-54 system has different synchronization sequences for each of the six time slots making up the frame, thereby allowing each receiver to synchronize to its own pre assigned time slots. An additional 12 bits in every time slot are for the SACCH (i.e., system control information). The digital verification color code (DVCC) is the equivalent of the supervisory audio tone used in the AMPS system. There are 256 different 8-bit color codes, which are protected by a (12,8,3) Hamming code. Each base station has its own pre assigned color code, so any incoming interfering signals from distant cells can be ignored.
Time slots for the mobile-to-base direction are constructed differently from the base-to-mobile direction. They essentially carry the same information but are arranged differently. Notice that the mobile-to-base direction has a 6-bit ramp time to enable its transmitter time to get up to full power, and a 6-bit guard band during which nothing is transmitted. These 12 extra bits in the base-to-mobile direction are reserved for future use. The modulation scheme for IS-54 is 7C/4 differential quaternary phase shift keying (DQPSK), otherwise known as differential 7t/4 4-PSK. This technique allows a bit rate of 48.6 kb/s with 30-kHz channel spacing, to give a bandwidth efficiency of 1.62 b/s/Hz. This value is 20 percent better than GSM. The major disadvantage with this type of linear modulation method is the power inefficiency, which translates into a heavier hand-held portable and, even more inconvenient, a shorter time between battery recharges. Go to Top
The IS-54 speech coder uses the technique called vector sum excited linear prediction (VSELP) coding. This is a special type of speech coder within a large class known as code-excited linear prediction (CELP) coders. The speech coding rate of 7.95 kb/s achieves a reconstructed speech quality similar to that of the analog AMPS system using frequency modulation. The 7.95-kb/s signal is then passed through a channel coder that loads the bit rate up to 13 kb/s. The new half-rate coding standard reduces the overall bit rate for each call to 6.5 kb/s, and should provide comparable quality to the 13-kb/s rate. This half-rate gives a channel capacity six times that of analog AMPS. So, the benefit of the D-AMPS (IS-54) compared to analog AMPS is the enhanced bandwidth efficiency, which translates into more channels for a given cost. However, the need for higher-quality speech in hostile propagation environments is leading to some higher, not lower, speech coding rates. This might not be backtracking so much as a step toward future variable coding bit rates automatically adjusted by received bit-error ratio, or some other quality measurement parameter.
IS-54 frame structure.
North American Integrated Services-95 system (CDMA) Go to Top
Introduction. The IS-95 standard is for CDMA cellular radio. Either frequency hopping (FH-CDMA) or direct sequence (DS-CDMA) provides a method of allowing multiple users to occupy the same channel (frequency band) with minimal interference. FH-CDMA is not used because the capacity would be too low for a bandwidth of 1.25 MHz. For DS-CDMA, the system is asynchronous, meaning that the pulses from each user do not have to bear any phase relationship to one another. Present research is progressing to maximize the number of users able to operate simultaneously with an acceptable level of interference.
DS-CDMA has a major drawback known as the near-far problem. This is defined as the differing amounts of interference contributed by different mobile stations depending on their distance from the base station. Clearly, interference from mobile stations closest to the base station is the dominant source of interference unless some transmit power control mechanism is used. Furthermore, this power control must be highly sophisticated, because the power level of multiple signals reaching the receiver must not vary by more than 1 to 2 dB if the maximum simultaneous user capability is to be achieved. Spread-spectrum systems require the phase of the incoming signal to be determined very precisely. Phase acquisition and tracking, which collectively are called the synchronization process in spread-spectrum technology, are of paramount importance; otherwise, dispreading of the desired signal is not possible.
Signals in a DS-CDMA system have different pseudorandom binary sequences that modulate a carrier and spread the spectrum of the carrier. This allows many CDMA signals to share the same frequency band. At the receiver, a correlator circuit is used to identify the signal with its specific binary sequence. The receiver dispreads that signal, but not ones whose binary sequences do not match. The unspread signals contribute to system noise and the maximum number of users is reached when the aggregate noise level becomes too high. The signal is filtered after the correlator so that the interfering signals are reduced by the ratio of the bandwidth before and after the correlator. This signal improvement is called the processing gain and is a major factor in determining the CDMA digital cellular system capacity. Other important parameters are the E E IN , efficiency of frequency reuse, number of cell sectors, and voice duty cycle. Go to Top
Comparison of Cellular Radio and Cordless
Specifications
Cellular Structures and Planning
Cells. In a mobile telecommunications system the mobile handset beams into a nearby radio receiver called the base station. As the mobile user moves further away from the base station the received signal level decreases until communication is too noisy and eventually, if extra measures are not taken, the call would be dropped. This area around the base is called the cell, and multiple base stations form what is now known as a cellular radio structure. In a mobile telecommunications system the mobile handset beams into a nearby radio receiver called the base station. As the mobile user moves further away from the base station the received signal level decreases until communication is too noisy and eventually, if extra measures are not taken, the call would be dropped. This area around the base is called the cell, and multiple base stations form what is now known as a cellular radio structure. Go to Top
Cellular systems use a hexagonal honeycomb structure of cells, and the base station at the center of each cell connects into the public telephone network. The hexagonal cell pattern arises from the best method of covering a given area, remembering that radio coverage is ideally radial in nature. The area of overlap is calculated for a completely surrounded cell (i.e., by six cells for the hexagon, four for the square, and three for the triangle). The hexagon has a small overlap compared to the triangle. To cover an area of three hexagonal cells, or 7.8r2, would require six triangular or four square cells. The obvious conclusion from this simple analysis is that the regular hexagon is the most advantageous and therefore widely used structure, with the triangle suitable only in difficult propagation areas that require deep overlapping of radio zones. Initially, the limited available power transmitted by the mobile radio telephone set determined the cell size. As the user moves ((roams) between cells during a journey, the communication with the base station of the departing cell ceases and communication with the base station of the entering cell commences. This process is known as handoff North America and handover in Europe. Each adjacent base station transmits at a different frequency from its neighbor.
Connection of the mobile telephone to the PSTN
Cordless telephones were first designed to allow tetherless use in and around the home. They have now become a useful low-cost option for mobility around the workplace or in some downtown areas. They are primarily an access technology, in contrast to cellular radios (which form a full network).
Digital European cordless telecommunications Go to Top
The digital European cordless telecommunications (DECT) Standard was approved by ETSI in 1992. It was intended to allow connection to PABXs to give mobility within the vicinity of a PABX, or to be used as a single cell with the base at a home or small company premises. The DECT Standard also enables users to make calls at participating locations such as airports, shopping malls, sports arenas, etc. The cost of cordless systems is lower than cellular and the data rates are higher. It must be appreciated that mobility is limited with cordless technology because the transmit power is low compared to cellular systems and is designed only for short distances. With a 10-mW transmit coverage power(250 mW peak), the line-of-sight distance is about 3 km, but more realistically, in a city environment the operating distance is only about 100 to 200 m. While a cordless system might have many cells that give the appearance of a cellular system, some of the early cordless systems did not permit a user to maintain a call while moving from one cell to another. In other words, handoff is not a feature offered by all cordless system operators. Handoff is offered by DECT operators, and this seems to be a trend that others are following.
DECT systems operate a TDMA platform using ten carriers in the 1881.792-to 1897.344-MHz band. Each carrier supports 12 channels, totaling 120 over the available 20 MHz.High demand is met by multiple small-cell coverage depending on traffic density, which varies with urban, suburban, or rural coverage areas.
DECT differs from the uplink-downlink frequency division duplex (FDD) scheme by operating time division duplex (TDD) transmission, so both handset and base transmit on the same frequency at different times. Each of the 120 channels has 24 time slots; 12 for transmitting and 12 for receiving. The DECT protocol is described in ETSI documents ETS300175-1 to 5. The 10-ms frame contains 24 time slots in which the first 12 are for base station to handset (downlink) channels, and the second 12 are for handset to base (uplink) channels. The duration of one time slot is 416.7 us and 424 bits are transmitted in 368.1 u.s, leaving a guard band of 48.6 u.s between each time slot. The voice or data bits constitute only 320 bits per time slot, so the other 104 bits are for overhead. Because the speech coding in the DECT system is 32 kb/s ADPCM per slot (B-field), the speech quality in the absence of interference is toll quality. The B-field has an unprotected mode for voice and a protected mode for reliable transmission of internal control signaling and limited user data at 25.6 kb/s. There is also a 6.4 kb/s per slot A-field for control and signaling. The gross bit rate is 1.152 Mb/s. 16 frames are packed into a 160-ms multiframe.
Personal Communications Services (or Networks) Go to Top
It must be stressed that PCS or PCN is a general concept and not a specific communications system. There are several definitions of PCS. Perhaps the most appealing is the following one: The availability of all communication services anytime, anywhere, to everybody, by a single identity number and a pocketable This simple statement is loaded with technological hitches. With the potential customer base in the major U.S. urban areas alone estimated to be over 100 million, the stakes are high. The cost of PCS systems is not small, especially because they are evolving out of cellular radio systems. For example, because of the microcellular nature of PCSs, they will need more than five times the number of base stations of present cellular networks. Designers did not have the luxury of starting their designs from the beginning, and too much money has already been invested in cellular mobile radio to discard it.
The importance of hand-held telephones as a strong and vital market force in the telecommunications industry cannot be overemphasized. Hand-held portable radios first gained widespread recognition with the introduction of cordless extension phones, which allowed a customer to place or receive calls anywhere within the home or garden.
Because the portable phone transmitter power was only about 1 mW, the operating range was not more than a 100 m or so. The convenience of this tetherless phone facility soon prompted the idea of universal coverage for portable phones, and today's cellular radio systems go a long way toward achieving that goal.
The future promises to be very exciting in this area of telecommunications. Although there are still many questions to be answered. For example, "How you make radio contact into obscure places, such as elevators made complex from steel?" The solution might be a communication fiber or wires down the center of the steel cable operating the elevator, or to have elevators made from ni metallic material. There are further obstacles to overcome in providing service supersonic airplanes. In these circumstances, the Doppler effect adds another dimension of complication to the rather intricate problem of setting up cells to cover the vast areas of the Atlantic and Pacific Oceans. By the above definite of PCS, it will be some years beyond 2000 before the objective is fully realized Go to Top
PCS is primarily concerned with increased levels of mobility and high data rate communications compared to the first and second generation of cellular, radio systems. This can be in the form of either terminal mobility or person mobility. Terminal mobility means a terminal can connect to any point in the cellular network. A unique identifier is assigned to the terminal by the manufacturer which enables validation and location information for delivery of calls. Personal mobility allows a user not only terminal mobility but also access by any terminal. This extra level of mobility requires a means of identifying the user and authenticating or validating the user's access. A unique number is a desirable option, perhaps via a portable smart card or speed recognition. A PCS system needs to keep track of terminal locations, and the terminal number will be different from the user's universal personal telecom (UPT) number. A user can then log on to any user terminal and once authentication is completed there is full access to the telephone network Clearly, the main problems for the service provider are routing calls to and from the user.
Cordless, cellular, and satellite mobile systems all have their strength weaknesses. The dual or triple mode terminals are an interim or possibly term solution for providing PCS. There is a trend toward the converge cellular and cordless technologies. However, there remain the incompatibility aspects of transmit power (low for cordless, high for cellular) and therefore cell size, and of source coding (high bit rate for cordless, low for cellular) which greatly affects system capacity and QoS. This is being resolved by the dual handset concept.
The system environments, and the mobility covers all forms of transport including pedestrians, motor vehicles, boats, trains, and airplanes. The UMTS scheme, which should start to take shape in the first decade of the new century, should provide a multiband, multiservice, multifunction PCS with communication rates from 144 kb/s to 2 Mb/s with global roaming. It will support ATM for B-ISDN at rates up to 155 Mb/s utilizing the millimeter bands in the range 38 to 60 GHz for wireless LAN connect capability.
Point to multipoint system
Comparison of CDMA and TDMA Go to Top
Despite all of the capacity improvement predictions for CDMA over TDMA, practical delivery of theoretical improvement factors is proving elusive, and the progress of this technological "cat-and-mouse" chase is intriguing. System capacity has many parameters in its equation that are not easy to model or clearly define. There is, however, a clear relationship between capacity and quality of service that affects both CDMA and TDMA. As more capacity-improving signal processing is used to reduce the speech encoded bit rate of each user, the quality of service to each user drops. As the delay associated with the speech encoding and error correction (channel coding) increases, the quality drops and echo cancellation eventually becomes essential. The dc power used for extensive signal processing can offset the battery-saving benefits of lower RF transmit power. Talk time and standby time are of major concern to the customer. Unless there is a breakthrough in battery technology in the near future, there could well be a backtracking in the seemingly never-ending speech coding quest for high bandwidth efficiency.
In other words, lower-capacity systems that give good speech quality might be worth paying for. Also of concern for QoS is voice activation. The 2- to 2.5fold capacity improvement comes at a price. In acoustically noisy environments
The complexity of CDMA, which leads to delay times as high as 200 ms, could be a major drawback in the future when QoS becomes a more serious issue. However, CDMA has some technically very pleasing attributes. When fully functional, it might well outperform TDMA or other systems. An unbiased comparison must have preestablished ground rules, and the following factors will be used here. First, any meaningful comparison between CDMA and TDMA must be based on a unit bandwidth; 25 MHz is used here. The QoS must also be taken into account.
UMTS
Conclusion.
The values derived here show a capacity in favor of CDMA, but not by the large factor reported in comparisons in some other texts. However, it must be remembered that these calculations are simplified, and only give approximate results. Important factors such as environmental conditions are not considered. For example, CDMA performs better than TDMA in a hostile multipath fading situation. Clearly, a direct comparison between TDMA and CDMA is not simple, and most comparisons, including this one, are inexact. This is merely a snapshot of the present time, which could change radically with future improvements in technology, such as steerable smart antennas and statistical multiplexers. Go to Top
While CDMA was in its development phase, TDMA GSM systems are being rapidly deployed. , GSM have more than 100 million customers in over 100 countries, which is a strong position in the global market. Depending on the source of information, the CDMA capacity per cell is quoted as several times that of TDMA. The exact figure is difficult to ascertain. The questions that really matter are, How does any capacity advantage translate into a resulting cost-benefit to users? and Is the quality of service any better? The answers to these questions will be known only when CDMA achieves widespread deployment.
