The GSM Radio Interface

AIR INTERFACE

UPLINK 890 - 915 MHz

DOWNLINK 935 - 960 MHz

MOBILE

BASE TRANSCEIVER STATION

The GSM Network ArchitectureTime division multiple access-TDMA124 radio carriers, inter carrier spacing 200khz.890 to 915mhz mobile to base - UPLINK935 to 960mhz base to mobile - DOWNLINK8 channels/carrier

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

Access MechanismFDMA, TDMA, CDMA

Frequency multiplexSeparation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signalsDisadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

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Time multiplexA channel gets the whole spectrum for a certain amount of timeAdvantages:only one carrier in the medium at any timethroughput high even for many usersDisadvantages:precise synchronization necessaryk2k3k4k5k6k1

Time and Frequency MultiplexCombination of both methodsA channel gets a certain frequency band for a certain amount of timeftck2k3k4k5k6k1

Time and Frequency MultiplexExample: GSM Advantages:Better protection against tappingProtection against frequency selective interferenceHigher data rates compared to code multiplexBut: precise coordination requiredftck2k3k4k5k6k1

GSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

Code MultiplexEach channel has a unique codeAll channels use the same spectrum at the same timeAdvantages:Bandwidth efficientNo coordination and synchronization necessaryGood protection against interference and tappingDisadvantages:Lower user data ratesMore complex signal regenerationImplemented using spread spectrum technologyk2k3k4k5k6k1ftc

Various Access Method

Cells

Capacity & Spectrum Utilization SolutionThe need:Optimum spectrum usageMore capacityHigh quality of serviceLow cost

Representation of CellsIdeal cellsFictitious cells

Cell size and capacityCell size determines number of cells available to cover geographic area and (with frequency reuse) the total capacity available to all usersCapacity within cell limited by available bandwidth and operational requirementsEach network operator has to size cells to handle expected traffic demand

Cell structureImplements space division multiplex: base station covers a certain transmission area (cell)Mobile stations communicate only via the base stationAdvantages of cell structures:higher capacity, higher number of usersless transmission power neededmore robust, decentralizedbase station deals with interference, transmission area etc. locallyProblems:fixed network needed for the base stationshandover (changing from one cell to another) necessaryinterference with other cellsCell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

Capacity of a Cellular SystemFrequency Re-Use DistanceThe K factor or the cluster sizeCellular coverage or Signal to interference ratioSectoring

Increasing cellular system capacityCell sectoringDirectional antennas subdivide cell into 3 or 6 sectorsMight also increase cell capacity by factor of 3 or 6

Increasing cellular system capacityCell splittingDecrease transmission power in base and mobileResults in more and smaller cellsReuse frequencies in non-contiguous cell groupsExample: cell radius leads 4 fold capacity increase

Tri-Sector antenna for a cell

Cell Distribution in a NetworkHighwayTownSuburbRural

Concept of TDMA Frames and ChannelsGSM combines FDM and TDM: bandwidth is subdivided into channels of 200khz, shared by up to eight stations, assigning slots for transmission on demand.

GSM uses paired radio channels01240124890MHz915MHz935MHz960MHzUPLINKDOWNLINK

GSM delays uplink TDMA framesUplink TDMA Frame F1 + 45MHzDownlink TDMA F1MHzThe start of the uplink TDMA is delayed of three time slotsTDMA frame (4.615 ms)Fixed transmit Delay of three time-slots

GSM - TDMA/FDMA935-960 MHz124 channels (200 kHz)downlink890-915 MHz124 channels (200 kHz)uplinkfrequencytimeGSM TDMA frameGSM time-slot (normal burst)guardspaceguardspace

LOGICAL CHANNELSTRAFFICSIGNALLINGFULL RATEBm 22.8 Kb/SHALF RATELm 11.4 Kb/SBROADCASTCOMMON CONTROL DEDICATED CONTROLFCCHSCHBCCHPCHRACHAGCHSDCCHSACCHFACCHFCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNELDOWN LINK ONLYUPLINK ONLYBOTH UP & DOWNLINKS

Broadcast Channel - BCHBroadcast control channel (BCCH) is a base to mobile channel which provides general information about the network, the cell in which the mobile is currently located and the adjacent cellsFrequency correction channel (FCCH) is a base to mobile channel which provides information for carrier synchronizationSynchronization channel (SCH) is a base to mobile channel which carries information for frame synchronization and identification of the base station transceiver

Common Control Channel - CCHPaging channel (PCH) is a base to mobile channel used to alert a mobile to a call originating from the networkRandom access channel (RACH) is a mobile to base channel used to request for dedicated resourcesAccess grant channel (AGCH) is a base to mobile which is used to assign dedicated resources (SDCCH or TCH)

Dedicated Control Channel - DCCHStand-alone dedicated control channel (SDCCH) is a bi-directional channel allocated to a specific mobile for exchange of location update information and call set up information

Dedicated Control Channel - DCCHSlow associated control channel (SACCH) is a bi-directional channel used for exchanging control information between base and a mobile during the progress of a call set up procedure. The SACCH is associated with a particular traffic channel or stand alone dedicated control channelFast associated control channel (FACCH) is a bi-directional channel which is used for exchange of time critical information between mobile and base station during the progress of a call. The FACCH transmits control information by stealing capacity from the associated TCH

DEFINITION OF TIME SLOT - 156.25 BITS 15/26ms = 0.577ms

LOGICAL CHANNELSTRAFFICSIGNALLINGFULL RATEBm 22.8 Kb/SHALF RATELm 11.4 Kb/SBROADCASTCOMMON CONTROL DEDICATED CONTROLFCCHSCHBCCHPCHRACHAGCHSDCCHSACCHFACCHFCCH -- FREQUENCY CORRECTION CHANNELSCH -- SYNCHRONISATION CHANNELBCCH -- BROADCAST CONTROL CHANNELPCH -- PAGING CHANNELRACH -- RANDOM ACCESS CHANNELAGCH -- ACCESS GRANTED CHANNELSDCCH -- STAND ALONE DEDICATED CONTROL CHANNELSACCH -- SLOW ASSOCIATED CONTROL CHANNELFACCH -- FAST ASSOCIATED CONTROL CHANNELDOWN LINK ONLYUPLINK ONLYBOTH UP & DOWNLINKS

Location update from the mobileMobile looks for BCCH after switching onRACH send channel request AGCH receive SDCCHSDCCH authenticateSDCCH switch to cipher modeSDCCH request for location updatingSDCCH authenticate responseSDCCH cipher mode acknowledgeSDCCH allocate TMSISDCCH acknowledge new TMSISDCCH switch idle update mode

Call establishment from a mobileMobile looks for BCCH after switching onRACH send channel request AGCH receive SDCCHSDCCH do the authentication and TMSI allocationSDCCH require traffic channel assignmentSDCCH send call establishment requestSDCCH send the setup message and desired numberFACCH switch to traffic channel and send ack (steal bits)FACCH receive alert signal ringing soundFACCH acknowledge connect message and use TCHTCH conversation continuesFACCH receive connect message

Call establishment to a mobileMobile looks for BCCH after switching onReceive signaling channel SDCCH on AGCHReceive alert signal and generate ringing on FACCHReceive authentication request on SDCCHGenerate Channel Request on RACHAnswer paging message on SDCCHAuthenticate on SDCCHReceive setup message on SDCCHFACCH acknowledge connect message and switch to TCHReceive connect message on FACCHReceive traffic channel assignment on SDCCHMobile receives paging message on PCHFACCH switch to traffic channel and send ack (steal bits)

Thank You

270.833 kb/s per carrierGMSK with a time bandwidth product BT =0.3Slow frequency hoping 217/hops/second.Synchronization compensation for up to 233micro seconds absolute delayBlock and convolutional channel coding copuled with interleaving to combat channel perturbations- overall channel rate of 22.8 kb/sFull rate channel 13 kb/s voice coder rate using regular pulse excitation/linear predictive coding RPE/LPC, half rate channel 6.5 kb/s usingVector coder rate using vector sum excited linear predictivie coding VSELPOverall full rate channel bit rate of 22.8 kb/s.Each cell can have from 1 to 16 pairs of carriers.The system capacity depends on :The total number of radio channelsThe size of the cellThe frequency re-use factor or distance

The minimum distance which allows the same frequencies to be re-used will depend on many factors,The number of co-channel cells in the vicinity of the center cellThe geography of the terrain,The antenna heightThe transmitted power within each cellThe start of the uplink TDMA frame is delayed with respect to downlink by a fixed period of three timeslots. Why ? Staggering TDMA frames allows the same timeslot number to be used in both the down and uplink while avoiding the requirement for mobile to transmit and receive simultaneously. Between T and R the MS is in the IDLE mode, makes measurement of signal strength of neighboring cells.Because of natural and man-made electromagnetic interference, the encoded speech or data signal transmitted over the radio interface must be protected from errors. GSM uses convolutional encoding andblock interleaving to achieve this protection. The exact algorithms used differ for speech and for different data rates. The method used for speech blocks will be described below.

Recall that the speech codec produces a 260 bit block for every 20 ms speech sample. From subjective testing, it was found that some bits of this 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 error detection. If an error is detected, the frame is judged too damaged to be comprehensible and it is discarded. It is replaced by a slightlyattenuated version of the previous correctly received frame. These 53 bits, together with the 132 Class Ib bits and a 4 bit tail sequence (a total of 189 bits), are input into a 1/2 rate convolutional encoder ofconstraint length 4. Each input bit is encoded as two output bits, based on a combination of the previous 4 input bits. The convolutional encoder thus outputs 378 bits, to which are added the 78 remainingClass II bits, which are unprotected. Thus every 20 ms speech sample is encoded as 456 bits, giving a bit rate of 22.8 kbps.

To further protect against the burst errors common to the radio interface, each sample is interleaved. The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocksare transmitted in eight consecutive time-slot bursts. Since each time-slot burst can carry two 57 bit blocks, each burst carries traffic from 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 carrier frequency using Gaussian-filtered Minimum Shift Keying (GMSK).GMSK was selected over other modulation schemes as a compromise between spectral efficiency, complexity of the transmitter, and limited spurious emissions. The complexity of the transmitter is related topower consumption, which should be minimized for the mobile station.Normal burst 148 bits + 8.25 guard bitsFrequency correction burst 148 bits + 8.25 guard bitsSynchronizing burst 148 bits + 8.25 guard bitsAccess burst 88 bits +68.25 guard bits used to access a cell for the first time in case of a call set up or handoverThe data structure within a normal burst consists of 148 bits transmitted at a rate of 270.833 kb/s. Each burst in GSM system modulates one of the carriers assigned to a particular cell using GMSK.When a mobile station is first switched on it is necessary to read the BCCH in order to determine its orientation within the network.The mobile must first synchronize in frequency and then in time. The FCCH, SCH and BCCH are all transmitted on the same carrier frequency which has a higher power density than any of the other channels in a cell because steps are taken to ensure that it is transmitted information at all times. The mobile scans around the available frequencies, picks the strongest and then selects the FCCH. Fc+67.7kHz