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History of GSM

During the early 1980s, analog cellular telephone systems wereexperiencing rapid growth in Europe, particularly in Scandinavia and theUnited Kingdom, but also in France and Germany. Each country

developed its own system, which was incompatible with everyone else'sin equipment and operation. This was an undesirable situation, becausenot only was the mobile equipment limited to operation within nationalboundaries, which in a unified Europe were increasingly unimportant,but there was also a very limited market for each type of equipment, soeconomies of scale and the subsequent savings could not be realized.

The Europeans realized this early on, and in 1982 the Conference ofEuropean Posts and Telegraphs (CEPT) formed a study group called theGroupe Spcial Mobile (GSM) to study and develop a pan-Europeanpublic land mobile system. The proposed system had to meet certain

criteria:

Good subjective speech quality Low terminal and service cost Support for international roaming Ability to support handheld terminals Support for range of new services and facilities Spectral efficiency ISDN compatibility

In 1989, GSM responsibility was transferred to the European

Telecommunication Standards Institute (ETSI), and phase I of the GSMspecifications were published in 1990. Commercial service was startedin mid-1991, and by 1993 there were 36 GSM networks in 22 countries.Although standardized in Europe, GSM is not only a European standard.Over 200 GSM networks (including DCS1800 and PCS1900) areoperational in 110 countries around the world. In the beginning of 1994,there were 1.3 million subscribers worldwide [18], which had grown tomore than 55 million by October 1997. With North America making adelayed entry into the GSM field with a derivative of GSM calledPCS1900, GSM systems exist on every continent, and the acronym GSMnow aptly stands for Global System for Mobile communications.

The developers of GSM chose an unproven (at the time) digital system,as opposed to the then-standard analog cellular systems like AMPS inthe United States and TACS in the United Kingdom. They had faith thatadvancements in compression algorithms and digital signal processorswould allow the fulfillment of the original criteria and the continualimprovement of the system in terms of quality and cost. The over 8000pages of GSM recommendations try to allow flexibility and competitive


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innovation among suppliers, but provide enough standardization toguarantee proper inter-working between the components of the system.This is done by providing functional and interface descriptions for eachof the functional entities defined in the system.

Services provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility interms of the services offered and the control signalling used. However,radio transmission limitations, in terms of bandwidth and cost, do notallow the standard ISDN B-channel bit rate of 64 kbps to be practicallyachieved.

Using the ITU-T definitions, telecommunication services can be dividedinto bearer services, teleservices, and supplementary services. The mostbasic teleservice supported by GSM is telephony. As with all other

communications, speech is digitally encoded and transmitted throughthe GSM network as a digital stream. There is also an emergencyservice, where the nearest emergency-service provider is notified bydialing three digits (similar to 911).

A variety of data services is offered. GSM users can send and receivedata, at rates up to 9600 bps, to users on POTS (Plain Old TelephoneService), ISDN, Packet Switched Public Data Networks, and CircuitSwitched Public Data Networks using a variety of access methods andprotocols, such as X.25 or X.32. Since GSM is a digital network, amodem is not required between the user and GSM network, although an

audio modem is required inside the GSM network to interwork withPOTS.

Other data services include Group 3 facsimile, as described in ITU-Trecommendation T.30, which is supported by use of an appropriate faxadaptor. A unique feature of GSM, not found in older analog systems, isthe Short Message Service (SMS). SMS is a bidirectional service for shortalphanumeric (up to 160 bytes) messages. Messages are transported ina store-and-forward fashion. For point-to-point SMS, a message can besent to another subscriber to the service, and an acknowledgement ofreceipt is provided to the sender. SMS can also be used in a cell-

broadcast mode, for sending messages such as traffic updates or newsupdates. Messages can also be stored in the SIM card for later retrieval[2].

Supplementary services are provided on top of teleservices or bearerservices. In the current (Phase I) specifications, they include severalforms of call forward (such as call forwarding when the mobilesubscriber is unreachable by the network), and call barring of outgoing


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or incoming calls, for example when roaming in another country. Manyadditional supplementary services will be provided in the Phase 2specifications, such as caller identification, call waiting, multi-partyconversations.

Architecture of the GSM network

A GSM network is composed of several functional entities, whosefunctions and interfaces are specified. Figure 1 shows the layout of ageneric GSM network. The GSM network can be divided into three broadparts. The Mobile Station is carried by the subscriber. The Base StationSubsystem controls the radio link with the Mobile Station. The NetworkSubsystem, the main part of which is the Mobile services SwitchingCenter (MSC), performs the switching of calls between the mobile users,and between mobile and fixed network users. The MSC also handles themobility management operations. Not shown is the Operations and

Maintenance Center, which oversees the proper operation and setup ofthe network. The Mobile Station and the Base Station Subsystemcommunicate across the Um interface, also known as the air interface orradio link. The Base Station Subsystem communicates with the Mobileservices Switching Center across the A interface.

Figure 1. General architecture of a GSM network

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal)and a smart card called the Subscriber Identity Module (SIM). The SIMprovides personal mobility, so that the user can have access to


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subscribed services irrespective of a specific terminal. By inserting theSIM card into another GSM terminal, the user is able to receive calls atthat terminal, make calls from that terminal, and receive othersubscribed services.

The mobile equipment is uniquely identified by the International MobileEquipment Identity (IMEI). The SIM card contains the International MobileSubscriber Identity (IMSI) used to identify the subscriber to the system,a secret key for authentication, and other information. The IMEI and theIMSI are independent, thereby allowing personal mobility. The SIM cardmay be protected against unauthorized use by a password or personalidentity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base

Transceiver Station (BTS) and the Base Station Controller (BSC). Thesecommunicate across the standardized Abis interface, allowing (as in therest of the system) operation between components made by differentsuppliers.

The Base Transceiver Station houses the radio tranceivers that define acell and handles the radio-link protocols with the Mobile Station. In alarge urban area, there will potentially be a large number of BTSsdeployed, thus the requirements for a BTS are ruggedness, reliability,portability, and minimum cost.

The Base Station Controller manages the radio resources for one ormore BTSs. It handles radio-channel setup, frequency hopping, andhandovers, as described below. The BSC is the connection between themobile station and the Mobile service Switching Center (MSC).

Network Subsystem

The central component of the Network Subsystem is the Mobile servicesSwitching Center (MSC). It acts like a normal switching node of the PSTNor ISDN, and additionally provides all the functionality needed to handlea mobile subscriber, such as registration, authentication, location

updating, handovers, and call routing to a roaming subscriber. Theseservices are provided in conjuction with several functional entities,which together form the Network Subsystem. The MSC provides theconnection to the fixed networks (such as the PSTN or ISDN). Signallingbetween functional entities in the Network Subsystem uses SignallingSystem Number 7 (SS7), used for trunk signalling in ISDN and widelyused in current public networks.


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The Home Location Register (HLR) and Visitor Location Register (VLR),together with the MSC, provide the call-routing and roaming capabilitiesof GSM. The HLR contains all the administrative information of eachsubscriber registered in the corresponding GSM network, along with thecurrent location of the mobile. The location of the mobile is typically in

the form of the signalling address of the VLR associated with the mobilestation. The actual routing procedure will be described later. There islogically one HLR per GSM network, although it may be implemented asa distributed database.

The Visitor Location Register (VLR) contains selected administrativeinformation from the HLR, necessary for call control and provision of thesubscribed services, for each mobile currently located in thegeographical area controlled by the VLR. Although each functional entitycan be implemented as an independent unit, all manufacturers ofswitching equipment to date implement the VLR together with the MSC,

so that the geographical area controlled by the MSC corresponds to thatcontrolled by the VLR, thus simplifying the signalling required. Note thatthe MSC contains no information about particular mobile stations --- thisinformation is stored in the location registers.

The other two registers are used for authentication and securitypurposes. The Equipment Identity Register (EIR) is a database thatcontains a list of all valid mobile equipment on the network, where eachmobile station is identified by its International Mobile Equipment Identity(IMEI). An IMEI is marked as invalid if it has been reported stolen or isnot type approved. The Authentication Center (AuC) is a protected

database that stores a copy of the secret key stored in each subscriber'sSIM card, which is used for authentication and encryption over the radiochannel.

Radio link aspects

The International Telecommunication Union (ITU), which manages theinternational allocation of radio spectrum (among many other functions),allocated the bands 890-915 MHz for the uplink (mobile station to basestation) and 935-960 MHz for the downlink (base station to mobilestation) for mobile networks in Europe. Since this range was already

being used in the early 1980s by the analog systems of the day, theCEPT had the foresight to reserve the top 10 MHz of each band for theGSM network that was still being developed. Eventually, GSM will beallocated the entire 2x25 MHz bandwidth.

Multiple access and channel structure


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Since radio spectrum is a limited resource shared by all users, a methodmust be devised to divide up the bandwidth among as many users aspossible. The method chosen by GSM is a combination of Time- andFrequency-Division Multiple Access (TDMA/FDMA). The FDMA partinvolves the division by frequency of the (maximum) 25 MHz bandwidth

into 124 carrier frequencies spaced 200 kHz apart. One or more carrierfrequencies are assigned to each base station. Each of these carrierfrequencies is then divided in time, using a TDMA scheme. Thefundamental unit of time in this TDMA scheme is called a burst periodand it lasts 15/26 ms (or approx. 0.577 ms). Eight burst periods aregrouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), whichforms the basic unit for the definition of logical channels. One physicalchannel is one burst period per TDMA frame.

Channels are defined by the number and position of their correspondingburst periods. All these definitions are cyclic, and the entire pattern

repeats approximately every 3 hours. Channels can be divided intodedicated channels, which are allocated to a mobile station, andcommon channels, which are used by mobile stations in idle mode.

Traffic channels

A traffic channel (TCH) is used to carry speech and data traffic. Trafficchannels are defined using a 26-frame multiframe, or group of 26 TDMAframes. The length of a 26-frame multiframe is 120 ms, which is how thelength of a burst period is defined (120 ms divided by 26 frames dividedby 8 burst periods per frame). Out of the 26 frames, 24 are used for

traffic, 1 is used for the Slow Associated Control Channel (SACCH) and 1is currently unused (see Figure 2). TCHs for the uplink and downlink areseparated in time by 3 burst periods, so that the mobile station does nothave to transmit and receive simultaneously, thus simplifying theelectronics.

In addition to these full-rate TCHs, there are also half-rate TCHs defined,although they are not yet implemented. Half-rate TCHs will effectivelydouble the capacity of a system once half-rate speech coders arespecified (i.e., speech coding at around 7 kbps, instead of 13 kbps).Eighth-rate TCHs are also specified, and are used for signalling. In the

recommendations, they are called Stand-alone Dedicated ControlChannels (SDCCH).


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Figure 2. Organization of bursts, TDMA frames, and multiframesfor speech and data

Control channels

Common channels can be accessed both by idle mode and dedicatedmode mobiles. The common channels are used by idle mode mobiles toexchange the signalling information required to change to dedicatedmode. Mobiles already in dedicated mode monitor the surrounding basestations for handover and other information. The common channels are

defined within a 51-frame multiframe, so that dedicated mobiles usingthe 26-frame multiframe TCH structure can still monitor controlchannels. The common channels include:Broadcast Control Channel (BCCH)

Continually broadcasts, on the downlink, information includingbase station identity, frequency allocations, and frequency-hopping sequences.

Frequency Correction Channel (FCCH) and Synchronisation Channel(SCH)

Used to synchronise the mobile to the time slot structure of a cellby defining the boundaries of burst periods, and the time slot

numbering. Every cell in a GSM network broadcasts exactly oneFCCH and one SCH, which are by definition on time slot number 0(within a TDMA frame).

Random Access Channel (RACH)Slotted Aloha channel used by the mobile to request access to thenetwork.

Paging Channel (PCH)Used to alert the mobile station of an incoming call.


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Access Grant Channel (AGCH)Used to allocate an SDCCH to a mobile for signalling (in order toobtain a dedicated channel), following a request on the RACH.

Burst structure

There are four different types of bursts used for transmission in GSM.The normal burst is used to carry data and most signalling. It has a totallength of 156.25 bits, made up of two 57 bit information bits, a 26 bittraining sequence used for equalization, 1 stealing bit for eachinformation block (used for FACCH), 3 tail bits at each end, and an 8.25bit guard sequence, as shown in Figure 2. The 156.25 bits aretransmitted in 0.577 ms, giving a gross bit rate of 270.833 kbps.

The F burst, used on the FCCH, and the S burst, used on the SCH, havethe same length as a normal burst, but a different internal structure,

which differentiates them from normal bursts (thus allowingsynchronization). The access burst is shorter than the normal burst, andis used only on the RACH.

Speech coding

GSM is a digital system, so speech which is inherently analog, has to bedigitized. The method employed by ISDN, and by current telephonesystems for multiplexing voice lines over high speed trunks and opticalfiber lines, is Pulse Coded Modulation (PCM). The output stream fromPCM is 64 kbps, too high a rate to be feasible over a radio link. The 64

kbps signal, although simple to implement, contains much redundancy.The GSM group studied several speech coding algorithms on the basis ofsubjective speech quality and complexity (which is related to cost,processing delay, and power consumption once implemented) beforearriving at the choice of a Regular Pulse Excited -- Linear PredictiveCoder (RPE--LPC) with a Long Term Predictor loop. Basically, informationfrom previous samples, which does not change very quickly, is used topredict the current sample. The coefficients of the linear combination ofthe previous samples, plus an encoded form of the residual, thedifference between the predicted and actual sample, represent thesignal. Speech is divided into 20 millisecond samples, each of which is

encoded as 260 bits, giving a total bit rate of 13 kbps. This is the so-called Full-Rate speech coding. Recently, an Enhanced Full-Rate (EFR)speech coding algorithm has been implemented by some NorthAmerican GSM1900 operators. This is said to provide improved speechquality using the existing 13 kbps bit rate.


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Channel coding and modulation

Because of natural and man-made electromagnetic interference, theencoded speech or data signal transmitted over the radio interface mustbe protected from errors. GSM uses convolutional encoding and block

interleaving to achieve this protection. The exact algorithms used differfor speech and for different data rates. The method used for speechblocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 msspeech sample. From subjective testing, it was found that some bits ofthis block were more important for perceived speech quality thanothers. The bits are thus divided into three classes:

Class Ia 50 bits - most sensitive to bit errors Class Ib 132 bits - moderately sensitive to bit errors

Class II 78 bits - least sensitive to bit errors

Class Ia bits have a 3 bit Cyclic Redundancy Code added for errordetection. If an error is detected, the frame is judged too damaged to becomprehensible and it is discarded. It is replaced by a slightlyattenuated version of the previous correctly received frame. These 53bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a totalof 189 bits), are input into a 1/2 rate convolutional encoder of constraintlength 4. Each input bit is encoded as two output bits, based on acombination of the previous 4 input bits. The convolutional encoder thusoutputs 378 bits, to which are added the 78 remaining Class II bits,

which are unprotected. Thus every 20 ms speech sample is encoded as456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radiointerface, each sample is interleaved. The 456 bits output by theconvolutional encoder are divided into 8 blocks of 57 bits, and theseblocks are transmitted in eight consecutive time-slot bursts. Since eachtime-slot burst can carry two 57 bit blocks, each burst carries trafficfrom two different speech samples.

Recall that each time-slot burst is transmitted at a gross bit rate of

270.833 kbps. This digital signal is modulated onto the analog carrierfrequency using Gaussian-filtered Minimum Shift Keying (GMSK). GMSKwas selected over other modulation schemes as a compromise betweenspectral efficiency, complexity of the transmitter, and limited spuriousemissions. The complexity of the transmitter is related to powerconsumption, which should be minimized for the mobile station. Thespurious radio emissions, outside of the allotted bandwidth, must bestrictly controlled so as to limit adjacent channel interference, and allow


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for the co-existence of GSM and the older analog systems (at least forthe time being).

Multipath equalization

At the 900 MHz range, radio waves bounce off everything - buildings,hills, cars, airplanes, etc. Thus many reflected signals, each with adifferent phase, can reach an antenna. Equalization is used to extractthe desired signal from the unwanted reflections. It works by finding outhow a known transmitted signal is modified by multipath fading, andconstructing an inverse filter to extract the rest of the desired signal.This known signal is the 26-bit training sequence transmitted in themiddle of every time-slot burst. The actual implementation of theequalizer is not specified in the GSM specifications.

Frequency hopping

The mobile station already has to be frequency agile, meaning it canmove between a transmit, receive, and monitor time slot within oneTDMA frame, which normally are on different frequencies. GSM makesuse of this inherent frequency agility to implement slow frequencyhopping, where the mobile and BTS transmit each TDMA frame on adifferent carrier frequency. The frequency hopping algorithm is

broadcast on the Broadcast Control Channel. Since multipath fading isdependent on carrier frequency, slow frequency hopping helps alleviatethe problem. In addition, co-channel interference is in effectrandomized.

Discontinuous transmission

Minimizing co-channel interference is a goal in any cellular system, sinceit allows better service for a given cell size, or the use of smaller cells,thus increasing the overall capacity of the system. Discontinuoustransmission (DTX) is a method that takes advantage of the fact that a

person speaks less that 40 percent of the time in normal conversation,by turning the transmitter off during silence periods. An added benefit ofDTX is that power is conserved at the mobile unit.

The most important component of DTX is, of course, Voice ActivityDetection. It must distinguish between voice and noise inputs, a taskthat is not as trivial as it appears, considering background noise. If avoice signal is misinterpreted as noise, the transmitter is turned off and


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a very annoying effect called clipping is heard at the receiving end. If, onthe other hand, noise is misinterpreted as a voice signal too often, theefficiency of DTX is dramatically decreased. Another factor to consider isthat when the transmitter is turned off, there is total silence heard at thereceiving end, due to the digital nature of GSM. To assure the receiver

that the connection is not dead, comfort noise is created at the receivingend by trying to match the characteristics of the transmitting end'sbackground noise.

Discontinuous reception

Another method used to conserve power at the mobile station isdiscontinuous reception. The paging channel, used by the base stationto signal an incoming call, is structured into sub-channels. Each mobile

station needs to listen only to its own sub-channel. In the time betweensuccessive paging sub-channels, the mobile can go into sleep mode,when almost no power is used.

Power control

There are five classes of mobile stations defined, according to their peaktransmitter power, rated at 20, 8, 5, 2, and 0.8 watts. To minimize co-channel interference and to conserve power, both the mobiles and theBase Transceiver Stations operate at the lowest power level that willmaintain an acceptable signal quality. Power levels can be stepped up or

down in steps of 2 dB from the peak power for the class down to aminimum of 13 dBm (20 milliwatts).

The mobile station measures the signal strength or signal quality (basedon the Bit Error Ratio), and passes the information to the Base StationController, which ultimately decides if and when the power level shouldbe changed. Power control should be handled carefully, since there isthe possibility of instability. This arises from having mobiles in co-channel cells alternatingly increase their power in response to increasedco-channel interference caused by the other mobile increasing its power.This in unlikely to occur in practice but it is (or was as of 1991) under

study.

Network aspects

Ensuring the transmission of voice or data of a given quality over theradio link is only part of the function of a cellular mobile network. A GSMmobile can seamlessly roam nationally and internationally, whichrequires that registration, authentication, call routing and location


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updating functions exist and are standardized in GSM networks. Inaddition, the fact that the geographical area covered by the network isdivided into cells necessitates the implementation of a handovermechanism. These functions are performed by the Network Subsystem,mainly using the Mobile Application Part (MAP) built on top of the

Signalling System No. 7 protocol.

Figure 3. Signalling protocol structure in GSM

The signalling protocol in GSM is structured into three general layers,depending on the interface, as shown in Figure 3. Layer 1 is the physicallayer, which uses the channel structures discussed above over the airinterface. Layer 2 is the data link layer. Across the Um interface, thedata link layer is a modified version of the LAPD protocol used in ISDN,

called LAPDm. Across the A interface, the Message Transfer Part layer 2of Signalling System Number 7 is used. Layer 3 of the GSM signallingprotocol is itself divided into 3 sublayers.

Radio Resources ManagementControls the setup, maintenance, and termination of radio andfixed channels, including handovers.

Mobility ManagementManages the location updating and registration procedures, aswell as security and authentication.

Connection ManagementHandles general call control, similar to CCITT RecommendationQ.931, and manages Supplementary Services and the ShortMessage Service.

Signalling between the different entities in the fixed part of the network,such as between the HLR and VLR, is accomplished throught the MobileApplication Part (MAP). MAP is built on top of the Transaction CapabilitiesApplication Part (TCAP, the top layer of Signalling System Number 7. The


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specification of the MAP is quite complex, and at over 500 pages, it isone of the longest documents in the GSM recommendations.

Radio resources management

The radio resources management (RR) layer oversees the establishmentof a link, both radio and fixed, between the mobile station and the MSC.The main functional components involved are the mobile station, andthe Base Station Subsystem, as well as the MSC. The RR layer isconcerned with the management of an RR-session, which is the timethat a mobile is in dedicated mode, as well as the configuration of radiochannels including the allocation of dedicated channels.

An RR-session is always initiated by a mobile station through the accessprocedure, either for an outgoing call, or in response to a pagingmessage. The details of the access and paging procedures, such as

when a dedicated channel is actually assigned to the mobile, and thepaging sub-channel structure, are handled in the RR layer. In addition, ithandles the management of radio features such as power control,discontinuous transmission and reception, and timing advance.

Handover

In a cellular network, the radio and fixed links required are notpermanently allocated for the duration of a call. Handover, or handoff asit is called in North America, is the switching of an on-going call to adifferent channel or cell. The execution and measurements required for

handover form one of basic functions of the RR layer.

There are four different types of handover in the GSM system, whichinvolve transferring a call between:

Channels (time slots) in the same cell Cells (Base Transceiver Stations) under the control of the same

Base Station Controller (BSC), Cells under the control of different BSCs, but belonging to the

same Mobile services Switching Center (MSC), and Cells under the control of different MSCs.

The first two types of handover, called internal handovers, involve onlyone Base Station Controller (BSC). To save signalling bandwidth, theyare managed by the BSC without involving the Mobile services SwitchingCenter (MSC), except to notify it at the completion of the handover. Thelast two types of handover, called external handovers, are handled bythe MSCs involved. An important aspect of GSM is that the original MSC,the anchor MSC, remains responsible for most call-related functions,


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with the exception of subsequent inter-BSC handovers under the controlof the new MSC, called the relay MSC.

Handovers can be initiated by either the mobile or the MSC (as a meansof traffic load balancing). During its idle time slots, the mobile scans the

Broadcast Control Channel of up to 16 neighboring cells, and forms a listof the six best candidates for possible handover, based on the receivedsignal strength. This information is passed to the BSC and MSC, at leastonce per second, and is used by the handover algorithm.

The algorithm for when a handover decision should be taken is notspecified in the GSM recommendations. There are two basic algorithmsused, both closely tied in with power control. This is because the BSCusually does not know whether the poor signal quality is due tomultipath fading or to the mobile having moved to another cell. This isespecially true in small urban cells.

The 'minimum acceptable performance' algorithm gives precedence topower control over handover, so that when the signal degrades beyonda certain point, the power level of the mobile is increased. If furtherpower increases do not improve the signal, then a handover isconsidered. This is the simpler and more common method, but it creates'smeared' cell boundaries when a mobile transmitting at peak powergoes some distance beyond its original cell boundaries into another cell.

The 'power budget' method uses handover to try to maintain or improvea certain level of signal quality at the same or lower power level. It thus

gives precedence to handover over power control. It avoids the'smeared' cell boundary problem and reduces co-channel interference,but it is quite complicated.

Mobility management

The Mobility Management layer (MM) is built on top of the RR layer, andhandles the functions that arise from the mobility of the subscriber, aswell as the authentication and security aspects. Location management isconcerned with the procedures that enable the system to know thecurrent location of a powered-on mobile station so that incoming call

routing can be completed.

Location updating

A powered-on mobile is informed of an incoming call by a pagingmessage sent over the PAGCH channel of a cell. One extreme would beto page every cell in the network for each call, which is obviously awaste of radio bandwidth. The other extreme would be for the mobile to


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notify the system, via location updating messages, of its current locationat the individual cell level. This would require paging messages to besent to exactly one cell, but would be very wasteful due to the largenumber of location updating messages. A compromise solution used inGSM is to group cells into location areas. Updating messages are

required when moving between location areas, and mobile stations arepaged in the cells of their current location area.

The location updating procedures, and subsequent call routing, use theMSC and two location registers: the Home Location Register (HLR) andthe Visitor Location Register (VLR). When a mobile station is switched onin a new location area, or it moves to a new location area or differentoperator's PLMN, it must register with the network to indicate its currentlocation. In the normal case, a location update message is sent to thenew MSC/VLR, which records the location area information, and thensends the location information to the subscriber's HLR. The information

sent to the HLR is normally the SS7 address of the new VLR, although itmay be a routing number. The reason a routing number is not normallyassigned, even though it would reduce signalling, is that there is only alimited number of routing numbers available in the new MSC/VLR andthey are allocated on demand for incoming calls. If the subscriber isentitled to service, the HLR sends a subset of the subscriber information,needed for call control, to the new MSC/VLR, and sends a message tothe old MSC/VLR to cancel the old registration.

For reliability reasons, GSM also has a periodic location updatingprocedure. If an HLR or MSC/VLR fails, to have each mobile register

simultaneously to bring the database up to date would causeoverloading. Therefore, the database is updated as location updatingevents occur. The enabling of periodic updating, and the time periodbetween periodic updates, is controlled by the operator, and is a trade-off between signalling traffic and speed of recovery. If a mobile does notregister after the updating time period, it is deregistered.

A procedure related to location updating is the IMSI attach and detach. Adetach lets the network know that the mobile station is unreachable,and avoids having to needlessly allocate channels and send pagingmessages. An attach is similar to a location update, and informs the

system that the mobile is reachable again. The activation of IMSIattach/detach is up to the operator on an individual cell basis.

Authentication and security

Since the radio medium can be accessed by anyone, authentication ofusers to prove that they are who they claim to be, is a very importantelement of a mobile network. Authentication involves two functional


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entities, the SIM card in the mobile, and the Authentication Center(AuC). Each subscriber is given a secret key, one copy of which is storedin the SIM card and the other in the AuC. During authentication, the AuCgenerates a random number that it sends to the mobile. Both the mobileand the AuC then use the random number, in conjuction with the

subscriber's secret key and a ciphering algorithm called A3, to generatea signed response (SRES) that is sent back to the AuC. If the numbersent by the mobile is the same as the one calculated by the AuC, thesubscriber is authenticated.

The same initial random number and subscriber key are also used tocompute the ciphering key using an algorithm called A8. This cipheringkey, together with the TDMA frame number, use the A5 algorithm tocreate a 114 bit sequence that is XORed with the 114 bits of a burst (thetwo 57 bit blocks). Enciphering is an option for the fairly paranoid, sincethe signal is already coded, interleaved, and transmitted in a TDMA

manner, thus providing protection from all but the most persistent anddedicated eavesdroppers.

Another level of security is performed on the mobile equipment itself, asopposed to the mobile subscriber. As mentioned earlier, each GSMterminal is identified by a unique International Mobile EquipmentIdentity (IMEI) number. A list of IMEIs in the network is stored in theEquipment Identity Register (EIR). The status returned in response to anIMEI query to the EIR is one of the following:

White-listed

The terminal is allowed to connect to the network.Grey-listed

The terminal is under observation from the network for possibleproblems.

Black-listedThe terminal has either been reported stolen, or is not typeapproved (the correct type of terminal for a GSM network). Theterminal is not allowed to connect to the network.

Communication management

The Communication Management layer (CM) is responsible for CallControl (CC), supplementary service management, and short messageservice management. Each of these may be considered as a separatesublayer within the CM layer. Call control attempts to follow the ISDNprocedures specified in Q.931, although routing to a roaming mobilesubscriber is obviously unique to GSM. Other functions of the CCsublayer include call establishment, selection of the type of service(including alternating between services during a call), and call release.


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Call routing

Unlike routing in the fixed network, where a terminal is semi-permanently wired to a central office, a GSM user can roam nationallyand even internationally. The directory number dialed to reach a mobile

subscriber is called the Mobile Subscriber ISDN (MSISDN), which isdefined by the E.164 numbering plan. This number includes a countrycode and a National Destination Code which identifies the subscriber'soperator. The first few digits of the remaining subscriber number mayidentify the subscriber's HLR within the home PLM

An incoming mobile terminating call is directed to the Gateway MSC(GMSC) function. The GMSC is basically a switch which is able tointerrogate the subscriber's HLR to obtain routing information, and thuscontains a table linking MSISDNs to their corresponding HLR. Asimplification is to have a GSMC handle one specific PLMN. It should be

noted that the GMSC function is distinct from the MSC function, but isusually implemented in an MSC.

The routing information that is returned to the GMSC is the MobileStation Roaming Number (MSRN), which is also defined by the E.164numbering plan. MSRNs are related to the geographical numbering plan,and not assigned to subscribers, nor are they visible to subscribers.

The most general routing procedure begins with the GMSC querying thecalled subscriber's HLR for an MSRN. The HLR typically stores only theSS7 address of the subscriber's current VLR, and does not have the

MSRN (see the location updating section). The HLR must therefore querythe subscriber's current VLR, which will temporarily allocate an MSRNfrom its pool for the call. This MSRN is returned to the HLR and back tothe GMSC, which can then route the call to the new MSC. At the newMSC, the IMSI corresponding to the MSRN is looked up, and the mobile ispaged in its current location area (see Figure 4).


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Figure 4. Call routing for a mobile terminating call

NETWORK PARAMETER ADJUSTMENT

The GSM system can be divided into three parts in physical structure ofthe network: the network subsystem (NSS), the base station subsystem(BSS), and the MS (MS). In the signaling structure, the GSM system consists of MAP interface, A-interface (interface between MSC and BSC),Abis interface (interface between BSC and BTS) and Um interface(interface between BTS and MS). All these entities and interfaces haveplenty of configuration parameters and performance parameters. Some

of them have already been determined during the equipmentdevelopment stage. While most network parameters are determined bythe network operators according to the actual requirements and actualrunning. The settings and adjustments of those parameters haveconsiderable impacts on the operation of the whole GSM network.Therefore, network optimization is the process of settings andadjustments of various parameters.The GSM network has parameters related to radio devices and radiointerfaces that can impact the network performance. The radioparameters in the GSM network refer to those related to radio devicesand radio resources. These parameters have vital impact on the

coverage, distribution of signaling flow and network performance. Thus,adjustment of radio parameters is an important part of the optimization.The GSM radio parameters can be divided into two types as according tothe service targets of the radio parameters, i.e., engineering parametersand resource parameters. The engineering parameters are related toengineering design, installation and commissioning such as antennaheight, antenna direction, antenna gain, antenna down-tilt and cableattenuation. These parameters must be determined during network


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design. These parameters can hardly be changed during networkoperation. The resources parameters refer to those related to theconfiguration and usage of radio resources. They are normallytransmitted on Um interface to keep the consistency between the basestation and MS. Most resources parameters can be dynamically adjusted

through certain man machine interfaces (MMI) during network operation.The radio parameters involved in this document are mainly radioresources parameters (unless otherwise specify, the parametersdescribed here are radio resources parameters).When an operator is to construct a mobile communications network, hemust first make engineering design according to geographicenvironments, service forecast, radio channel features, and etc. Thedesign should include network structure design, base station locationselection and frequency planning. During the network operation, theoperator may need to adjust the network configurations and parametersso as to improve network performance. This is an important part of the

whole network optimization process.Radio parameter optimization is a process to improve thecommunication quality, the network performance, and the equipmentutilization rate by adjusting the partial or global radio parameters asaccording to the actual radio channel features, traffic features, andsignaling flow bearing. The basic principle of radio parameteradjustment is to make full use of existing radio resources, balance theglobal traffic and signaling flow through load-sharing so as to achieve abetter network performance.

Types of Radio Parameter Adjustment

There are two types of radio parameter adjustment. The first type ofRadio Parameter Adjustment is to solve static problems as according tothe actual average traffic and signaling flow. The other type of RadioParameter Adjustment is to solve problems in traffic overloading andchannel congestion.For the first type of adjustment, the operator has to test the actualrunning of the network periodically. On the basis of the test resultsadjust the global or partial network parameters and configurations.While for the second type of adjustment, the operator needs to adjustsome radio parameters as according to the real-time driver test and

the traffic statistics data. This document describes the meaning of theparameters and analyses the impact of parameter adjustment on thewhole network performance.

Prerequisites for Radio Parameter Adjustment

The network operator must know the meaning, adjustment method andthe result of the adjustments of each radio network parameter. The


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network operator should be very experienced in the radio networkparameters. This is a necessary condition to adjust radio networkparameters effectively. Meanwhile, the adjustment of radio parametersdepends on many testing data during network operation. Generally,these data can be obtained in two ways. Firstly, the statistical data can

be obtained from the Operation Maintenance Center (OMC) such as theload of CCCH, RACH channels. Secondly, other data such as thecoverage, and the MS speech quality, should be obtained from actualmeasurements and tests. Therefore, frequent and long-termmeasurements of various network features are necessary for effectiveradio parameter adjustment.

Points for Attention in Radio Parameter Adjustment

In the GSM system, many radio network parameters are set on the basisof cells and local areas (LA). As inter-region parameters are often

strongly interrelated, during adjustment of those parameters,consideration must be given to the impact on other areas, especially onneighboring cells. Otherwise, parameter adjustment will bring aboutnegative consequences.Besides, when a problem occurs in a region, make sure whether it iscaused by equipment faults (including connection failure). Only when itis confirmed that network problems are caused by service causes thenradio parameter adjustment is performed. The radio parameteradjustments recommended in this document assume that no deviceproblems exist.

Developed till now, capacity of the cellular system is limited in a certaindegree by the frequency bandwidth. The frequency can meet thecapacity requirements in a certain area only when it is multiplexed. Butthe frequency multiplexing, especially the close frequency multiplexingmode, will certainly make us face the problem of how to reduce theinterference of the same and neighboring frequencies. In the same area,if the frequency multiplexing distance is longer, interference of the sameand neighboring frequencies will be less and the capacity will be smaller.If the frequency multiplexing distance is closer, the capacity will belarger. But this will cause the increasing of the interference from thesame and neighboring frequencies. How to get the capacity and voice

quality balanced is a problem that must be solved in frequency planning.In other words, a good frequency plan can increase the network capacitywhile maintaining good voice quality.


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Requirements for Frequency Division and Carrier to InterferenceRatio

The cellular system is generally divided into GSM900M and DCS1800Msystems. The carrier frequency interval is 200KHz. The up and downfrequencies are divided as follows:

Network type Frequency bandwidth up/downMHz

GSM900 890915/935960DCS1800 17101785/18051880

GSM900

There are 124 frequency points totally, the sequence numbersARFCNare 1124. Each end is left with a protection bandwidth of200KHz.. Relation of the frequency and the sequence number nis as

followsThe base-station receivesf1n890.2n10.2 MHzThe base-station sends f2nf1n45 MHz

DCS1800

There are 374 frequency points totally. The sequencenumbersARFCNare from 512885. Relation of the frequency andthe sequence numbernis as follows:The base-station receives: f1n1710.2n5120.2 MHzThe base-station sendsf2nf1n95 MHz

Requirements for Carrier to Interference Ratio

GSM is an interference-restricted system. According to the requirementsfor signal demodulation in space interface, GSM stipulates the protectionratio of the same and neighboring frequencies should meet thefollowing requirementsCarrier to interference ratio of the same frequency C/I9dB With3dB allowance left in the project, i.e., C/I12dBRejection ratio of the neighboring frequencies C/A 9dBWith 3dBallowance left in the project, i.e., C/A6dB


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Frequency Multiplexing Technologies

4*3 Multiplexing Mode

Frequency points in the available frequency bandwidth are groupedaccording to different frequency multiplexing modes prior to frequencyplanning. The most common frequency multiplexing mode in the cellularsystem is 4*3 frequency multiplexing mode, as shown in the followingFigure:

Figure 1 4*3 Frequency multiplexing mode

As can be seen from the above Figure, in 4*3 mode,4 indicate 4stations 3indicates each station has 3 cellsaltogether 12 cells arefrequency families. Different cells in the same family have different

frequencies. But cells of other families use a group of frequencies fromthe 12 frequency families repeatedly.

Close Multiplexing Technology

The development of network construction will certainly bring a rapidincreasing of network capacity. In turn, it will raise an even higherdemand on network construction. With the dividing of cells and the use


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of micro-cellular and distributed antenna system, how to plan thefrequency properly becomes a subject of challenge. And various closefrequency multiplexing technologies have emerged. At present, themost popular ones are MRP (Multiple Re-use Pattern) of ERISSONcompany, IUO (Intelligent Overlay and underlay) of NOKIA company and

FRACTIONAL REUSE13of North Telecom. etc.Different frequency multiplexing technologies needs the support of thecorresponding software and hardware technologies. At the same time,the handover, functional control, channel allocation algorithm, DTX andfrequency hopping are perfected and developed. It also puts forwardhigher demands for the selection of antenna. In other words, how toreduce the interference is the key factor to guarantee the systemservice quality since the frequency multiplexing is very close. Infrequency planning, this is shown as the proper selection of base-stationposition and cell direction, proper setting of antenna height anddeclination angle to reduce the cross-area coverage. In system

functions, it is shown as the use of DTX, frequency hopping techniqueand the use of BGT handover, functional control of up and down lines,allocation algorithm of optimization channels etc. In parameter setting,it includes proper setting of static power of the base station, varioushand-over thresholds, proper setting of frequency hopping parametersto reduce the possibility of the collision of the same and neighboringfrequencies in a maximum degree. In addition, it raises higher demandsfor stray index and the adjacent channel selection index of the basestation equipment. This paper will not describe these in detail. See therelated documents for the specific contents. Refer to Appendix 1 for thefrequency hopping.

Nearly all-different close multiplexing technologies use the hierarchicalconcept. Plan BCCH and TCH in different layers. A different layer uses adifferent frequency-multiplexing mode. Considering that BCCH mustalways send system information for the test and contact of the mobilephone so as to make the mobile phone decode correctly. At the sametime, 12 frequency points must be allocated to BCCH at least to enhancethe correctness of mobile phone test report. In the actual allocation,considering the irregularity of base station allocation and variation ofazimuth angle of the cellmore than 14 frequency points are generallyallocated to BCCH. Its frequency planning is generally done with 4*3 orlooser frequency multiplexing mode. Frequency planning of TCH uses

3*3, 2*3 and 1*3 modes etc. It should be pointed out that the purpose ofdifferent carrier layer using different multiplexing degree is to avoid theinterference as far as possible. It is shown specifically as follows:

In case of the unevenness of network station types, sincenot all cells will use TRX in the last layer or the last severallayers, so TRX in the last layer or the last several layers canrealize closer multiplexing degree even if no frequencyhopping.


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As each carrier layer tries to use different multiplexingmode, thus making frequency points of any two cells in thenetwork not completely the same. That is to say, cells withcomplete the same frequency doesnt exist.

When multiple frequency multiplexing is implemented,

although the interference is increased, TRX is added in thecell at the same time, making more frequencies taken partin frequency hopping, thus make the gain increased.

If frequency points with less interference and greaterinterference exist in the same cell, make them combinedtogether when the frequency hopping technique is used.The interfered frequency points can still be used normallyaccording to the characteristics of Viterbi decoder. For eachburst, although the interference is variable, the voicequality depends on the average value of the interferencefor a special connection.

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