gsm frame structure

5/9/2018 GSM Frame Structure - slidepdf.com

http://slidepdf.com/reader/full/gsm-frame-structure-559ca0c576398 1/25

Um

Um

Abis Interface

A InterfaceF Interface

D Interface

C Interface B Interface

E Interface

F Interface

The GMSC represents the gateway to other networks like public switched telephone network(PSTN), Integrated services digital network ISDN etc.

Dimensioning and Optimization Strategies of GSM Control Channels

By: Bhishma Bhardwaj Anuj Kumar Sanjay Thakur Head, RF Planning RF Planner RF Planner

ConvergeLabs ConvergeLabs ConvergeLabs

Abstract:“ The paper deals with a detailed analytical overview of GSM framstructure and their dimensioning. The channel structure and frames in GSM hav

been discussed. The concept of bursts used in GSM has been elaborated. Effect oRayleigh fading and frequency hopping has been dealt with. Optimization oconfiguration of channel structure has been discussed as applicable to particulatypes of service areas. Impact of various timers & counters on networperformance, Computation of paging loads and location area planning undevarious traffic mobility scenarios and optimization of the same are alsdiscussed .”

1. Introduction :The mobile station (MS) of a GSM public land mobile network (PLMNcommunicates with the serving & adjacent base stations (BSS) subsystem via thradio interface Um, the Base Trans Receivers Stations (BTS) communicate witthe Base Station Controller (BSC) through the Abis Interface while the BSCcommunicates with the Network Switching Sub – System (NSS) through the interface ( Figure 1 presents the basic architecture of the GSM Network)

The Home Location Register (HLR)

assists the mobility management by storing partof MS’s location information and routing incomingcalls to the visitor location register VLR in charge of thearea where the paged MS roams. The authentification centeAuC is implemented as a part of HLR and helps in authentification of the MS through itsinternational mobile subscriber identity (IMSI). Stolen, fraudulent or faulty mobile stationare identified with the help of equipment identity register (EIR). TheBSC is principally in charge of handovers initiation, frequencyhopping, channel allocation, link quality, power budget control, signaling and broadcastraffic control etc. The MSC’s functions include paging, MS location updating, handovecontrol etc. The GMSC is often implemented in the same machines as the MSC. The VLR

is always implemented together with a MSC; so the area under the control of MSC is alsthe area under control of the VLR.

2. Channel Structure :

MS

BTSMS

MS

BTS

TS

TS

TS

MSC

HLR VLR

G-MSC

EIR

V

Um

Figure 1 : Basic GSM Architecture



A channel corresponds to the recurrence of one burst every frame. It is defined bits frequency and the position of its corresponding burst within a TDMA frame. IGSM there are two types of logical channels:

The traffic Channels used to transport encoded speech and datinformation. Full rate traffic channels TCH/F are defined using a group of 2TDMA frames called a 26 multiframe. The 26 multiframe lasts 120ms anthe traffic channels for the downlink and uplink are separated by threbursts. As a consequence the mobiles will not need to transmit and receiv

at the same time which simplifies considerably the electronics of the systemand preventing high level transmitted power leakage back to the sensitivreceiver. Half rate traffic (TCH/H) double the capacity of the system aralso grouped in a 26 mutiframe. The net bit rate, block length, blocrecurrence for full rate and half rate traffic channels are 13Kbps, 260 bits20ms and 5.6 Kbps, 112 bits, 20 ms. For full rate speech the block idivided into two classes according to the importance of the bits (182 bits foclass I and 78 bits for class II). For half rate speech, the block is divided inttwo classes as 95 bits for Class I and 17 bits for class II. The TCH/consists of one time slot in each TDMA frame i.e one slot every 4.615ms .

The control Channels used for network management messages and som

channel maintenance tasks. These can be subdivided into BCH ( BroadcasChannel ), CCCH ( Common Control Channel), SDCCH ( Stand alondedicated Control Channel), ACCH ( Associated Control Channel)

An associated control channel is for down link and uplink and always associated iconjunction with, either a TCH or an SDCCH. Two types of ACCH for circuit switcheconnections are defined: continued stream (Slow ACCH) and burst stealing mode (fasACCH). The FACCH carry the same information as the SDCCH channels. The SACCHcan be of four types - SACCH/TF (associated with TCH/F), SACCH/TH (associated witTCH/H), SACCH/C4 (associated with SDCCH/4), SACCH/C8 (associated witSDCCH/8). The FACCH is used for signaling over TCH itself to indicate ca

establishment progress, to command handover etc. during transmission of fast associatesignalling on a traffic channel before a call actually commences and for handovecommands. The SACCH is used for measurement report. The broadcast channels ardown link channel and of three types: Broadcast Control Channel (BCCH) which gives thmobile station the parameters needed in order to identify & access the networkFrequency Correction Channel (FCH), which supplies the mobile station with thfrequency reference of the system in order to synchronize it with the network anSynchronization Channel (SCH) which gives the mobile station the training sequencneeded in order to demodulate the information transmitted by the base station.The common control channel helps to establish the calls from the mobile station or thnetwork and are used to allocate an SDCCH to the mobile station. The SDCCH allocate

is used for signaling between the mobile station and the network and is used to allocate Traffic Channel (if required) to the mobile station, for location updates, authentification othe MS etc. After Traffic Channel is allocated to the MS, the SDCCH channels arreleased. Three types of CCCH can be defined – The paging Channel (Downlink only)PCH, which is used to alert the MS of an incoming call; The Random Access Channe(RACH), Uplink only, which is used by the MS to request access to the network i.e. foallotment of an SDCCH. The Access Grant Channel (AGCH), down link only, which iused by the base station to inform the MS about which channel i.e SDCCH it should useThis channel is the answer of a base station to a RACH from the mobile station. Th



SDCCH can share a physical channel with a BCH or CCCH but not with a TCH. Thus wsee from the above that the only channels that are for both downlink and uplink are thassociated control channels (FACCH & SACCH). All other control channels are either fodownlink only (BCCH, FCH, SCH, PCH, AGCH) or for uplink only (RACH). The controchannels FCH and SCH are always sent on Time Slot 0 of the BCCH carrier which for thireason does not follow frequency hopping. The control Channel BCCH, RACH, PCH anAGCH must be assigned to the BCCH carrier only on any even numbered time slot. TheSDCCH can be assigned to any carrier and only time slot. This means that except fo

SDCCH, FACCH and SACCH, all other control channels have to be on the BCCH carriefrequency only. The net bit rate, block length and block recurrence time of the controchannels is summarized below in Table 1.

Table 1: Control Channel Block StructureControl Channel Net Bit Rate

KbpsBlock Length

(bits)Block Recurrence

(ms)Remarks

SACCH (with TCH) 115/300 168 + 16 480 (after every four 26multiframe)

16 bits are resaved for coninformation on layer 1, 168are used for higher layers,SACCH carries about 2messages per second

SACCH (with SDCCH) 299/765 168 + 16 6120/13 ( after every two 51

multiframe)

16 bits are resaved for con

information on layer 1, 168are used for higher layers,SACCH carries about 2messages per second

SDCCH 598/765 184 3060/13=235.38(after every 51 Multiframe)

BCCH 598/765 184 3060/13(after every 51 Multiframe)

AGCH n*598/765 184 3060/13(after every 51 Multiframe)

The total number of blocksper recurrence period isadjustable on a cell by cel& depends on parametersbroadcast on the BCCH

PCH P*598/765 184 3060/13

(after every 51 Multiframe)

The total number of blocks

per recurrence period isadjustable on a cell by cel& depends on parametersbroadcast on the BCCH

RACH V*26/765 8 3060/13(after every 51 Multiframe)

The total number of blocksper recurrence period isadjustable on a cell by cel& depends on parametersbroadcast on the BCCH

FACCH/F 9.2 184 20

FACCH/H 4.6 184 40

Note: One 51 frame multiframe lasts 15/ 26 * 8* 51 i.e. 3060/13 msOne 26 frame multiframe lasts 15/26*8*26 i.e. 120ms

3. Time Division Multiple Access and Time Slot Structure:

Eight basic physical channels per carrier i.e. eight time slot are used to make up TDMA frame. The carrier separation is 200KHz. A physical channel is therefordefined as a sequence of TDMA frames, a time slot number and a frequenchopping sequence. The principle of frequency hopping is that each TDMA frame itransmitted over a different frequency except the BCCH (beacon ) frequency.The logical channels are mapped on to a physical channel i.e. on to a particulatime slot of the TDMA frame which repeats after every 4.615ms. The TCH ar



mapped in a 26 frame multiframe and the control channels in a 51 frammultiframe. The basic radio resource is thus a time slot lasting 15/26 m(.5769ms) and transmitting information at a modulation rate of 1625/6 Kbits/sewhich is the input to the GMSK modulator. This means that one time slot, includinguard time is 156.25 bits duration (15/20 * 1625/6). The bandwidth B of thGaussian filter in the GMSK modulator is 81.3Khz. Hence the BT product comeout to 81.3KHz Tbit= 81.3 KHz6/1625*1/1000= 0.3A time slot may be pictured in a time/frequency diagram as a small rectangl

15/26ms long and 200KHz wide.

3.1 From Multiframe to Hyperframe:One multiframe consists of either 26 TDMA frames (each TDMA framconsisting of eight time slots) used to carry Traffic Channels, SACCH anFACCH ( if required) ,or ,51 TDMA frames which is used to carry controchannels. Thus we have two types of multiframes:

A 26 multiframe with a duration of 120 msec=(15/26)*8*26 in which TCH/bursts are sent for 24 frames, SACCH bursts – on one frame with one slovacant.

A 51 multiframe with a duration of 235.38ms = (15/26)*8*51msA TDMA frame with eight time slots is of duration (15/26)*8= 4.615ms.A Super frame lasts for 6.12 seconds and contains either 51 numbers of 2multiframe or 26 numbers of 51 multiframe. Hence the duration of Superframis the same for Traffic Channels and Control Channels. One hyperframcontains 2K superframe and lasts 3hrs 28mins 53.76 seconds. The framnumber FN thus can have 26*51*2048 values from 0 to 2715647. This FN itransmitted by base station as a part of Synchronisation burst. Figure 1 givethe schematic arrangement of TDMA frames, multiframes, superframes anhyperframes. The 26 multiframe lasts for 120ms which was chosen as multiple of 20ms in order to obtain some synchronization with fixed networksISDN, in particular. This leads to the value of TDMA frame as 120/26 and thaof one TS as 120/(26*8)= 15/26ms.

Figure 2:Frames , MultiFrames ,SuperFrames ,HyperFrames

3.2 Bursts :The physical content of a Time Slot , TS is called a burst. There are five typeof bursts each having 15/26ms and having 156.25 bits. A schematrepresentation of burst in power over time presentation is given in Figure 2.

Figure 3: Burst Used in GSM The effective transmission power is constant over the entire transmissioperiod. It must be noted that the power ramp and down envelope at the leadinand trailing edges of the transmission bursts is attenuated by 70dB during a 28and 18 micro sec. interval respectively. The actual data transmission takeplace only during the period of 147 bits which is 542.8ms long. The remainintime in the time slot is used for power ramp up and down. Each burst has tabits added at both ends to reset the memory of the Viterbi Channel Equalize(VE) which is responsible for removing, both the channel induced anintentional controlled inter symbol interference. Each burst ends with a guarperiod to prevent burst overlapping due to propagation delay fluctuations an



for multiple path echoes. The tail bits are not set to 1 as the transition from ‘1’ tothe first ‘0’ bit of the burst and from the last ‘0’ bit of the burst to ‘1’ fall exactly inthe ramping portion of the burst amplitude profile. In the absence of transitiothe modulated signal is shifted towards higher frequencies and the interferenccreated by ramping outside the frequency slot would be greater then with a btransition .Every normal burst contains 114 bits of useful encoded data sent itwo packets of 57 bits each. The 26 bit training sequence is placed in betweethe two packets of 57 bits each. This means that the receiver has to memoriz

the first packet 57 bits before being able to demodulate it. There are eighdifferent training periods & for neighboring base stations one of the eighdifferent training patterns is used associated with the so called BS colour codewhich assist in identifying the BS’s. The 26 bit training segment is constructeby a 16 bit Viterbi channel equalizer training pattern surrounded by fivquasiperiodically repeated bits on both sides .Quality of the received signaRXQUAL is a key parameter for evaluating network performance. RXQUAL ithe Bit Error rate BER derived from the 26 bit midamble from the TDMA bursRXQUAL levels characterize speech quality and dropped calls , where indicates the highest quality and 7 the worst . The stealing flag indicatewhether a 57 bit packet actually contains user data ( set to 0) or FCC

information (set to 1). The autocorrelation function of the eight traininsequences calculated between the central 16 bits and the whole 26 bsequence has a central correlation peak surrounded by 5 zero’s on each side.

Figure 4 : Normal Burst (156.25 Bits) T : 3 TAIL BITS ; F: I STEALING BIT

G: 8.25 GUARD BITS

3.2.2 Synchronization Burst :

Figure 5 : Synchronization Burst (156.25 Bits) T : 3 TAIL BITS ; G: 8.25 GUARD BITS

The training sequence of 64 bits is identical for all BTS. The BS sendsynchronization burst on timeslot 0 of the BCCH carrier. The MS sets up ittime base counters after receiving a synch burst by detecting QN (Quarter BNumber = 0- 624) counting the quarter bit intervals in burst, BN (Bit Number= 0156), TN (Time Slot Number= 0-7) and FN (TDMA frame number= 026.51.2048). The value of QN is determined from the 64 bit training sequencethe value of TN is set to 0 .QN increments every 12/13 micro seconds; BN ithe integer part of QN/4; TN increments when QN changes from count 624 t

0 ;FN increments whenever TN changes from count 7 to 0. The 78 encryptebits are decoded to arrive at the 25-SCH control bits. These 25 control bitcontain the PLMN color code and BS color code (BSIC) and the TDMA framnumber. FN is determined by the relation FN=51((T3-T2) mod(26+T3+51*26*T1, where T3=(10*T3’)+1; T1,T2,T3’ being contained in the 25-SCHbitsThe synch burst is the first burst that the mobile station needs to demodulate ithe downlink direction.

T PAYLOAD F TRAINING SEQUENCE F PAYLOAD 57 BITS 26 BITS 57 BITS

T SCH DATA EXTENDED TRAINING SEQUENCE SCH DATA T G

39 BITS 64 BITS 39 BITS

T 1 SCH SEQUENCE RACH DATA T2 GUARD BAND 68.25 BITS

41 BITS 36 BITS



Figure 6 : Access Burst (156.25 Bits) T1: 8 TAIL BITS ; T2: 3 TAIL BITS

The access burst is used only for the initial access by the MS to the BTS whichapplies in two cases:1. For a connection setup from idle state wherein a CHAN-REQ message is

sent using access burst.2. For handover wherein it sends HND-ACC message. The access burst ha

longer guard period of 68.25 bits to ensure that the access burst fits in threceiver window of a BTS. We must note that the MS has alreadsynchronized with the network. The BTS determines the actual propagatiodelay when the access burst arrives at the BTS and calculates the distancof an MS from the BTS and provides the offset time as a 6 bit numbe(Timing Advance) to the MS which in turn advances its time base over thrange 0-63 bits to transmit its signal earlier to enable the normal burst to fin the receiver window of the BTS. The 36 bit contain among otheparameters the encoded 6 bit BSIC (BS Identifier Code) and contains eithe

a CHAN- REQ or an HND-ACC message. The access burst always startwith the bit sequence 00111010 followed by 41 bit synchronizatiosequence allows the BTS to recognize the access burst. The access bursarrives at the base station with a time error of twice the propagation delacompared to the reception window. The access burst is the first burst that base station needs to demodulate in the uplink direction. This allows maximum cell distance of 35kms. The exact shift between downlink & uplinas seen by the mobile station is 3 Burst Period minus TA.

3.2.4 Frequency Correction Burst:

Figure 7: Frequency Correction Burst (156.25 Bits) T: 3 TAIL BIT;G:8.25 GUARD BIT

AlI 148 bits (142+6) are coded with 0. The output of GMSK modulator is a fixefrequency signal exactly 67.7 KHz above the BCCH carrier frequency. Thus thMS on receiving this fixed frequency signal fine tunes to the BCCH frequency anwaits for the synch burst to arrive after one TDMA frame i.e. 4.615ms.

3.2.5 Dummy Burst

Figure 8: Dummy Burst (156.25 Bits) T: 3 TAIL BITS; G:8.25 GUARD BITS

To enable the BCCH frequency to be transmitted with a constant power levedummy bursts are inserted into otherwise empty time slots on the BCCfrequency. The dummy burst are coded with a predefined pseudo random b

T ALL ZERO 142 BITS T G

T PREDEFINED BIT SEQUENCE 142 BITS T G



1 ≤ n ≤ 124

512 ≤ n ≤ 885

sequence to prevent accidental confusion with frequency correction bursts. A kedifference between BCH and TCH ARFCN is that a BCH ARFCN has continuoutransmission at a constant power level on all time slots , whereas, a TCH ARFCNhas bursted transmission with power levels that can be different in different timslots .

4. Frequencies Available:The following frequency bands are specified in GSM :

a. Primary Band: 890- 915 (MHz) mobile transmit, base receive935 – 960 MHz: base transmit, mobile receiveAllowed Frequencies = 124 with 200khz spacing

b. Extended GSM 900 Band: 880- 915 MHz mobile transmit, base receive(including standard GSM 900 band) 925-960 MHz: base transmit, mobile receive

Allowed Frequencies = 194 with 200khz spacingc. DCS 1800 Band: 1710-1785 MHz: mobile transmit, base receive

1805-1880 MHz: base transmit, mobile receiveAllowed Frequencies = 374 with 200khz spacing

d. PCS 1900 Band: 1850-1910 MHz: mobile transmit, base receive

1930- 1990 MHz: base transmit, mobile receiveAllowed Frequencies = 299 with 200khz spacing

For GSM 900 different categories of mobile there are four power classes with thmaximum power class having 8W peak output power and the minimum havin0.8W peak output power. For DCS 1800 there are three power classes of 4W peaoutput power, 1W peak output power, and the minimum having 0.25W peak outpupower. For PCS 1900 there are three power classes of 2W, 1W and 0.25W peaoutput power. Easy formulas to describe the actual frequency of an ARFCN ar(n=ARFCN):

Primary Band: Fuplink (n) = (890 + 0.2n) MHzFdownlink (n) = Fuplink (n) + 45 MHz

Extended GSM: Fuplink (n) = (80 + 0.2n) MHz 0 ≤ n ≤ 124

Fuplink (n) = 890 MHz + 0.2 (n-1024) 975 ≤ n ≤ 1023

Fdownlink (n) = Fuplink (n) + 45 MHz

DCS- 1800: Fuplink (n) = 1710 MHz + 0.2 (n- 511) Fdownlink (n) = Fuplink (n) + 95 MHz

The radio interface of GSM uses slow frequency hopping. The transmission frequencremains the same during the transmission of a TDMA burst having eight time slots. Imost cases, the emitting and receiving antennas are not within direct line of sight and threceived signal is a sum of a number of copies of one signal with different phases due tmultipath propagation and reflection. The sum of a lot of phase shifted signals with random distribution of phases has an envelope following the Rayleigh distribution. Thfading is frequency dependent. With frequency hopping all the bursts containing the partof one code word are transmitted on different frequencies and are hence not damaged ithe same way by Rayleigh fading. When the mobile station moves at high speed, th



difference between its position during the reception of two successive bursts of the samchannel (i.e. 4.615ms) is sufficient to decorrelate Rayleigh fading. In this case, slowfrequency hopping does no harm but it does not help much either. However, when thMS is stationary or moves at slow speeds, SFH allows the transmission to reach the leveof performance of high speeds (around 6.5 dB gain). The second advantage of frequenchopping is “Interferer Diversity” where due to different hopping sequences of neighborininterfacing cells using the same frequencies, the quality improves as the receiveinterfering signal follows a different hopping pattern than that of the cell where the MS i

receiving the signal. For a set of n given frequencies, GSM allows 64*n different hoppinsequences to be built. They are described by two parameters, the MAIO (MobilAllocation Index Offset) which may take as many values as the number of frequencies ithe set and the HSN (Hopping Sequence Number) which may take 64 different valuesTwo channels bearing the same HSN but different MAIO never use the same frequencon the same burst. On the opposite two channels using the same frequency list and thesame TN, but bearing different HSN, interface I/n th of the bursts. The sequences arpseudo random except for the special case of HSN=0,where the frequencies are useone after the other in order. Usually channels in one cell bear the same HSN and differenMAIO’s. In distant cells using the same frequency set, different HSN should be used tgain from interferer diversity. It is best to avoid HSN=0 which leads to poor interfere

diversity, even with non-identical frequency sets. The BCCH Carrier frequency (BeacoFrequency) is not hopped i.e. the channels BCCH, SCH, FCH, RACH, AGCH, PCH mususe a fixed frequency to ease initial synchronization acquisition and reduce systemcomplexity. In most applications, a cell is equipped with exactly as many TRXs aallocated frequencies. In cells of smaller capacity the operator may choose to let thchannels other than those on the beacon frequency to hop only on as many frequencieas there are TRXs , or, on as many frequencies as available . A mobile station transmit(or receives) on a fixed frequency during one time slot (≃577µm) and then must hobefore the time slot on the next TDMA frame after 4.615ms.

5. Cycles

5.1 TCH/F and its SACCH:A TCH/F is always allocated together with its associated slow rate channel (SACCH

For the TCH/F, a cycle contains 6 times 4 bursts in the 26-multiframe of 120 msCoding follows cycles based on the grouping of four successive bursts.However, for the SACCH, the full cycle lasts four 26-multiframes i.e. 8*26*4 bursperiods i.e. 480ms. In order to spread the arrival of SACCH messages at the basstation, the cycle of two SACCH using successive time slots are separated by 9bursts periods (i.e. 12*8 plus the difference of one time slot). This results in an eveload at the base station. It is important to note that slots of one channel bear the sametime slot number in both uplink and downlink directions, even, though they arseparated by three burst period in time domain.

T T T T T T T T T T T T S T T T T T T T T T T T T

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2

Figure 9: TCH/F FRAME (120 ms=26 Frames) T:TRAFFIC ; S: SACCH;I: IDLE



5.2 TCH/H A TCH/H in time domain is described as one slot every 16 burst periods( two TDMA

Frames) in average.

t1 t2 t1 t2 t1 t2 t1 t2 t1 t2 t1 t2 s1

t1 t2 t1 t2 t1 t2 t1 t2 t1 t2 t1 t2 s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2

Figure 10: TCH/H FRAME t1,t2: two half rate TCHs ; s1,s2: their SACCH/Hs

5.3 SDCCH SDCCH are of two types: SDCCH/8 and SDCCH/4SDCCH /8 are grouped by 8 along with its associated SACCH/C8 to form theequivalent of a TCH/F & its SACCH/F. SDCCH/4 are grouped by 4 along withits associated SACCH/C4 and combined with common channels to form anequivalent of TCH/F and its SACCH/F. All SDCCH follow a cycle of 102*8(two51-multiframe) burst periods i.e. a group of four slots separated by 4.615ms (8bursts periods) every 51-multiframe. These are combined with 4 slots for SACCH separated by 4.615ms (8 bursts periods) every two 51-multiframe. Thus

during the cycle of SDCCH two blocks of 4 slots are used for SDCCH/8 and oneblock of 4 slots for its SACCH/C8. There are maximum 16 different schedulingfor mobile stations in connection with a SDCCH/8. The SDCCH/ 4 can becombined with common control channels and sent on TS0. Only one SDCCH/4combination can be defined for each cell.

D0 D1 D2 D3 D4 D5 D6 D7 S1 S2 S3 S4 I I I

Figure 11 (51 MULTIFRAME=235.38 ms) D0 – D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS

S1-S4 : FOUR SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCHI : IDLE FRAMES

5.4 Common Control Channels:The cycles of traffic channels (26frames) and control channels (51 frames) do nohave a common divider. This allows the mobile station in dedicated mode to listen tsynchronization channel, SCH, and frequency correction channel, FCCH, osurrounding base stations. A BCCH Allocation (BA) Table or list is a set of ARFCNbroadcast to the mobile in the idle and dedicated modes for monitoring as potentianeighbor cells . In the idle mode , this list is broadcast on the BCCH in a SystemInformation type 2 message , in the dedicated mode on the SACCH in SystemInformation Type 5 message . This dedicated table can contain the same list o

ARFCNs as the idle mode table or a different list .

5.4.1 FCCH and SCH: (Down link)One SCH slot follows each FCCH slot 4.615ms later. Each of these two channeluse 5 slots in each 51-multiframe of TS0 of the beacon frequency. The mobilstation recognizes the time slot as TS0 whenever it receives FCCH and SCH.

f s bcch

ccch

f s ccch

ccch f s ccch ccch f s ccch ccch f s ccch

ccch



f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;i : IDLE ( 1time slot )

Figure 12(51 MultiFrames=235.38ms)

5.4.2 BCCH, PCH, AGCH: (Down link)A BCCH together with PCH + AGCH uses 40 slots per 51-multiframe on the sam

TN of the beacon frequency. These 40 slots are built into 10 groups of 4, the fouslots of first group are used by BCCH and the remaining nine by PCH + AGCHThe other combination is that BCCH with PCH + AGCH uses 16 slots per 51multiframe all on the same TN of the beacon frequency. BCCH then uses the firsblock of four slots and PCH +AGCH the remaining three. In both cases the BCCHinformation can be sent only once every 51- multiframe i.e. only once ever235.38ms.

5.4.3 RACH (Uplink )Two combinations exist: RACH/F and RACH/H.The RACH/F uses one slot every TDMA frame of 4.615ms and its organization isimilar to TCH/F with its SACCH/F in the uplink direction.The RACH/H uses only 27 slots in the 51-multiframe. A RACH/H fits in the burst lefree uplink by 4 numbers of SDCCH/8.

RACH /F ON ALL 51 FRAMES RACH (ACCESS BURST)

RACH / F: RACH IS SENT ON THE UPLINK FOR ALL THE 51 MULTIFRAMES

D3 R R SA2 SA3 RACH FROM 14 TO 36 FRAMES D0 D1 R R

RACH/H :RACH IS SENT ON THE UPLINK FOR 27 FRAMES D0-D3 : SDCCH/4 FOUR TIME SLOTS

SA : SACCH /C4 FOUR TIME SLOTSI : IDLE FOUR TIME SLOTS ; R : RACH 1 T

Figure 13(51 MultiFrame=235.38ms)

5.4.4 Common Channel Combinations:Every cell broadcasts one single FCCH and one single SCH on TS0 of the Beacofrequency. The common channels are always arranged in three combinations tmake a 51-multiframe.

a) Medium Capacity Cells:In the downlink direction:FCCH (5 frames), SCH (5 frames), BCCH (4 frames) , PCH + AGCH (36frames) all on TS0. This allows seven time slots for TCH and SDCCH ineach TDMA frame .In the Uplink direction:

RACH/F on TS0b) Small Capacity Cells:

In the downlink direction:FCCH (5 frames), SCH (5 frames), BCCH (4 frames) , PCH + AGCH (12frames), SDCCH/4 (16 frames), SACCH/C4 (4 frames). This allows seventime slots for TCH in each TDMA frame .In the Uplink direction:RACH/H (27 frames), SDCCH/4 (16 frames), SACCH/C4 (4 frames)

c) Large Capacity Cells:

1



For large capacity cells combination (a) is used along with up to threextension sets on even time slots only. An extension set contains the samchannels as combination. (a) except FCCH and SCH ( which are only oTS0). BCCH appears on the extension set to enable the mobile to listen tbursts on one TS only since BCCH contains information about RACH of thaparticular Time Slot.

5.4.5 CBCH A Cell Broadcast Channel CBCH follows a cycle of 8*51*8 burst periods i.e8 numbers of 51-multiframe. In each multiframe the CBCH can be seen as part of SDCCH/4. There are two combinations possible:(a) If the common channel configuration is that of case (b) in para 5.4.4 thethe CBCH can use the same time slot 0 and frequency as the commochannels . It then replaces one of the four SDCCH/4s(b) The CBCH can use TS0 (but not on the beacon frequency), 1,2, or 3; Ithis case the MS in idle mode has to listen regularly to bursts of differentime slot numbers. When CBCH is used, the first block of PCH + AGCH ithe 51- multiframe cannot be used for paging.

It is allowed to stop the termination of the CBCH incase of congestion anthen these resources can be used by SDCCH during such periods. ThCBCH reduces the number of available SDCCH’s.

f s bcch

ccch

f s ccch ccch f s D0 D1 f s CBCH D3 f s SA0 SA1

f s bcch

ccch

f s ccch ccch f s D0 D1 f s CBCH D3 f s SA2 SA3

TSO : DOWN LINK (two 51 frames shown to show cycle of SACCH/C4 )f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;

i : IDLE ( 1time slot ) ; D0-D3 : SDCCH/4 ; SA0-SA3 :SACCH/C4 ASSOCIATED WITH SDCCH/4 (requires two 5multiframes )

Figure 14: CBCH used in place of D2

6. Channel Organisation in a cell:In order to optimize implementation costs in a base station we must chooschannels so that they form groups where at most one burst is emitted at anone time, and to fill the time slots within these groups as much as possibleEvery TRX is able to cope with 8 channels, each channel corresponding ta given Time Slot number. Table 2 gives the possible combinations ochannels on a particular time slot.

Table 2

Channels Unused SlotsTCH/F with SACCH/F 1 out of 26

2 numbers of TCH/F with SACCH /H none

8 numbers of SDCCH/8 3 out of 51

FCCH + SCH+BCCH+PCH+AGCHIn down Link

1 out of 51

RACH/F in uplink None

BCCH +PCH+AGCHIn downlink

11 out of 51

BCCH +PCH+ AGCH+ SDCCH/4In downlink

3 out of 51

1



RACH/H+ SDCCH/4In uplink

none

A TRX may combine eight such groups with restrictions on time slots asdiscussed earlier.

A. A small capacity cell with a single TRX can typically be organized asTS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH+SDCCH/4+SACCH/C4

Uplink : RACH/H+ SDCCH/4TS1 to 7: Downlink : TCH/F + SACCH/F

Uplink: TCH/F + SACCH/F

f s bcch

ccch

f s ccch ccch f s D0 D1 f s D2 D3 f s SA0 SA1

f s bcch

ccch

f s ccch ccch f s D0 D1 f s D2 D3 f s SA2 SA3

TSO : DOWN LINK (two 51 frames shown to show cycle of SACCH/C4 )

f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;i : IDLE ( 1time slot ) ; D0-D3 : SDCCH/4 ; SA0-SA3 :SACCH/C4 ASSOCIATED WITH SDCCH/4 (requires two 5

multiframes )



TS O UPLINK :RACH + SDCCH/4 D0-D3 : SDCCH/4 FOUR TIME SLOTS

( two 51 multiframe shown to show complete cycle of SACCH/C4 SA : SACCH /C4 FOUR TIME SLOTSI : IDLE FOUR TIME SLOTS ; R : RACH 1 T


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2 TS 1 TO TS 7 ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE

( one 26 multiframe shown )

Figure 15 :Channel organization for a small capacity cell

B. A medium capacity cell with 4 TRX’s may typically be organized asOne group on TS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH

Uplink : RACH/FTwo groups of SDCCHon two time slots

Remaining 29 Time Slots: Downlink: TCH/F+ SACCH/F

Uplink : TCH/F + SACCH/F

f s bcch

ccch

f s ccch


ccch

TS 0 DOWNLINK ( one 51 multiframe shown ) of Beacon Frequencyf : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;i : IDLE ( 1time slot )


TS O UPLINK ( one 51 multiframe shown ) of Beacon frequency

D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I

TWO SUCH GROUPS OF 51 MULTIFRAME ON ANY TS OTHER THAN TS0 OF Beacon Frequency:DOWNLINKD0 – D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTSS0-S3 : FOUR SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8I : IDLE FRAMES EACH OF ONE SLOT

S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S0

TWO SUCH GROUPS OF 51 MULTIFRAME ON ANY TS OTHER THAN TS0 OF Beacon Frequency:UPINK

1

Downlink: SDCCH/8 + SACCH/8Uplink : SDCCH/8 + SACCH/8



Downlink: BCCH+PCH+AGCH

Uplink: RACH/F

D0 – D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTSS0-S3 : FOUR SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8I : IDLE FRAMES


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2Remaining 29 Time Slots ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE

( one 26 multiframe shown for single time slot )

Figure 16: Channel Capacity for a medium capacity cell

C. A large capacity cell with 12TRXs may include :( A BS may typically have maximum 16 TRXs)One group on TS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH

Uplink : RACH/F

One group on TS2, one groupOn TS4 & one group on TS6

Five groups of SDCCH: Downlink : SDCCH/8 +SACCH/8

One five time slots Uplink : SDCCH/8 + SACCH/8

Remaining 87 time slots: Downlink : TCH/F +SACCH/8 Uplink : TCH/F + SACCH/F

f s bcch

ccch

f s ccch


ccch

TS 0 DOWNLINK ( one 51 multiframe shown ) of Beacon Frequencyf : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;i : IDLE ( 1time slot )


TS O UPLINK ( one 51 multiframe shown ) of Beacon frequency

i i bcch

ccch

i i ccch

ccch i i ccch ccch i i ccch ccch i i ccch

ccch

ONE GROUP EACH ON TS 2,TS4 & TS6 DOWNLINK ( one 51 multiframe shown ) of Beacon Frequencyf : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; BCCH ; ccch (4 time slots ) : PCH + AGCH ;i : IDLE ( 1time slot )


TS 2,4,6 UPLINK ( one 51 multiframe shown ) of Beacon frequency

D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I

FIVE SUCH GROUPS OF 51 MULTIFRAME ON ANY TS OTHER THAN TS0,2,4,6 OF Beacon Frequency:DOWNLINK &(one 51 multiframe shown ) UPLINKD0 – D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTSS0-S3 : FOUR SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8I : IDLE FRAMES


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 2Remaining 87 Time Slots ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE

( one 26 multiframe shown for single time slot )

Figure 17 : Channel capacity for a very large capacity cell

While configuring a cell, a network operator has to consider the peculiaritieof a service area and the frequency situation, to optimize the configurationAn important factor is the average and maximum loads that are expected foBTS and how the load is shared between signaling and pay load. For cellhaving several carriers and with a large amount of expected traffic o

1



Common Control Channel eg. Paging, channel requests, channeassignments, the combination B discussed above is most likely to be usedThe signaling needs for mobiles like those for call setup, location updateetc. are then taken care by the SDCCH’s.For cells having one or two carriers the combination A is most likely to bused with SDCCH’s combined with Common Control Channels on time slo0. Here the paging capacity of the cell is lower as only three paging blockare sent as compared to nine in combination B. we must note the position o

the SDCCH’s in the uplink and downlink direction. If the base statiocommands the MS to authenticate itself the response can be sent only 1frames later (i.e after 15*4.615ms). Thus the command response cycle ireduced to one multiframe. If the base station manages a huge amount otransreceivers it is probable that the number of Common Control Channelprovided by combination B is not enough to handle the work and in succases combination C is preferred wherein additional Common ControChannels are allotted. The CBCH if used is always mapped on to thsecond subslot of SDCCH i.e. on TS0 of combination A, & on SDCCH timslots of combination B & C.

7. Dimensioning of Logical Channels:SDCCH load is affected by the following events:

Mobility management Procedures like location updates, Periodic Registration, IMSattach, IMSI detach

Call Setup, Short Message Service point to point, Supplementary Services.An optimum Configuration of SDCCH depends on Cell statistics like SDCCH load,SDCCH congestion, TCH load and TCH congestion .The values of holding time of SDCCH is determined by several timers whosemaximum values and functions are defined briefly as under:Table 3

PROCESS TIMER MAXIMUM VALUE REMARKSLocation Updating Timer T 3210 in ms Maximum value is 10 sec, stops

when LOC- UPD- ACC message isreceived by the ms i.e. locationupdate is acknowledged by thenetwork.

Starts when SDCCH is allottedexpiry it starts timer T 3211 atwhose expiry location update isrestarted. Maximum 4 attemptsbe made.

Mobile OriginatingCall

Timer T3230 in ms Maximum value 15 sec, It stopswhen CM-SERV-ACC or CM-SERV-REJ or AUTH- REJ is received i.eauthentification is successful,

allotment of Traffic Channel is doneafter which SDCCH is released

Starts when SDCCH is allotted expiry provides release indicati

Authentification Timer 3240 in ms Maximum value 10 sec starts whenthe ms receives an AUTH-REJmessage

At expiry it releases the SDCCH

Timer T 3260 in thenetwork

Maximum value 12 sec, starts whenAUTH-REQ is sent and stops whenAUTH-RSP/AUTH-REJ is receivedby the network

On expiry releases SDCCH

Identification Timer T3270 in thenetwork

Maximum value 12 sec, starts whenIDENT-REQ is sent and stops whenIDENT-RSP is received


1



Timer T3250 in thenetwork

Maximum value 12 sec, starts whenTMSI-REAL-CMD is sent and stopswhen TMSI-REAL-COM is received


a) The Common Control Channel, CCCH ( consisting of PCH+AGCH) in the downlincan work in stealing mode which means replacing paging blocks with Access GranBlocks if required. If dedicated blocks are used, each multiframe contains twopaging blocks (for combined i.e. combination ‘A’) or eight paging blocks (for noncombined i.e. combination ‘B’)

b) The number of TRX’s limits the possible number of SDCCH/8s in a cell. It is notpossible to have more SDCCH/8s in a cell than the number of TRX’s. However, itis possible to add an SDCCH/4 even if the number of SDCCH/8s equals thenumber of TRX’s in the cell. SDCCH/4 is generally not used incase of high pagingload in the location area.

c) A connection for speech or data requires an SDCCH for call setup signaling and aTCH for the remaining of the call. As a general rule we can say that blocking rate(GOS) for SDCCH/4 & SDCCH/8 should be less than 0.5 & 0.25 respectively timesthe blocking rate for TCH which means that for a 2% GOS of TCH the GOS of SDCCH/8 should be less than 0.5%. If the number of SDCCH are increased theSDCCH GOS improves but the capacity of TCH reduces . So the SDCCHdimensioning is a compromise between TCH capacity and SDCCH Grade of Service . SDCCH use the physical channels more effectively than TCH.

d) When all SDCCHs are occupied additional call setup signaling can be performedon TCH whenever more TCHs are available. This means that the traffic load onTCH increases since a TCH instead of SDCCH is allotted on IMM-ASS-CMDmessage. In this technique we can reduce the number of time slots reserved for SDCCHs.

SDCCH Configuration when no TCH is used for signaling with TCH-GOS as 2%and 1% can be selected as below for combination A, B and C discussed under para 6 earlier. Other combinations are also discussed. The Figure in parenthesisare those for 1% GOS of TCH .

Table 4

1



The SDCCH/TCH ratio given in the above table indicates the SDCCH configurationrequired for a given Grade of Service of the TCH .It is important to note that for TCH GOS 1% and 2% the ratio of SDCCH/TCH remainsabout the same i.e. the same table applies to different values of GOS for TCH. Thismeans that the same table can be applied for different grades of service of the TCH andthat the SDCCH configuration does not depend on the GOS of TCH and instead only onthe relationship between TCH-GOS and SDCCH-GOS The same is shown in Figure 18

No. of TRX

SDCCH type Number of SDCCH

SubChannelswithoutCBCH

Number of

SDCCHSub

Channelwith

CBCH

Capacity SDCCH inErlangs

Number of TCH

TCHCapacityErlangs

SDCCH/TCHratio

Without With CBCH WithoutCBCH

With CBCH

1 SDCCH/4( Combination A)

4 3 0.8694(0.7012)

0.4555(0.3490)

7 2.935(2.501)

29.62%(28.03%)

15.51%(13.95%)

SDCCH/8(When pagingsignaling load ishigher andSDCCH/8 areconfigured onother than TS0)

8 7 2.730(2.4037)

2.158(1.8778)

6 2.276(1.909)

119.47%(125.91%)

94.81%(98.36%)

2 SDCCH/4(On TS0)

4 3 0.8694(0.7012)

0.4555(0.349)

15 9.0096(8.108)

9.64%(8.64%)

5.055%(4.31%)

SDCCH/4+SDCCH/8

(SDCCH/4 on TS0,SDCCH/8 on anyother TS)

12 11 5.2789(4.7807)

4.6104(4.1533)

14 8.2003(7.3517)

64.37%(65.02%)

56.22%(56.49%)

SDCCH/8(On any TS other than TS0)

8 7 2.7299(2.4037)

2.1575(1.8778)

14 8.2003(7.3517)

33.29%(32.69%)

26.31%(25.54%)

3 SDCCH/4 4 3 0.8694(0.7012)

0.4555(0.3490)

23 15.761(14.47)

5.51%(4.84%)

2.89%(2.41%)

SDCCH/8 8 7 2.730(2.4037)

2.158(1.8778)

22 14.896(13.651)

18.32%(17.61%)

14.48%(13.76%)

4 SDCCH/4 4 3 0.8694

(0.7012)

0.4555

(0.3490)

31 22.827

(21.191)

3.81%

(3.31%)

1.99%

(1.65%)

SDCCH/8 8 7 2.730(2.4037)

2.158(1.8778)

30 21.932(20.337)

12.44%(11.82%)

9.83%(9.24%)

2* SDCCH/8(combination B)

16 15 8.0095(7.4475)

7.3755(6.7606)

29 21.039(19.489)

38.49%(38.22%)

35.06%(34.69%)

SDCCH/4 +SDCCH/8

12 11 5.2789(4.7807)

4.6104(4.1533)

30 21.932(20.337)

24.07%(23.51%)

21.02%(20.42%)

12 5*SDCCH/8(combination C)

40 39 27.382(26.003)

26.534(25.181)

87 75.415(71.881)

36.31%(36.18%)

35.18%(35.03%)

1



The above table also tells us that the reduction in SDCCH/TCH ratio due to use of CBCHis maximum for smaller cells i.e. for smaller cells whenever CBCH is used the SDCCHresources are more severely constrained .

Figure 18

The above table can be used for choice of SDCCH configuration B:Number of TRX’s= 4, Cell Broadcast not used

Estimated SDCCH load= 5 mE/ subscriber Estimated TCH load= 20 mE/subscriber SDCCH/TCH ratio= 5/20 = 25%From the above table we can select the configuration which gives SDCCH/TCH ratio of aleast 25%. This leads us to the combination: 2*SDCCH/8. However if the paging load isless the combination SDCCH/4 + SDCCH/8 can also be used as it is ≃25%.The SDCCH/TCH ratio depends on parameter setting , subscriber behavior, size of location area and service provided in the network. SDCCH traffic in mErlang per subscriber for each type of procedure ( location update, IMSI attach/detach, Periodicregistration, Call set up etc )can be calculated by multiplying, for each type of procedure,the number of performances per busy hour by holding time of the channel (in sec) and

dividing the result by 3.6 . A margin for traffic peaks of 15% can be added to theestimated SDCCH load . Contributions from each procedure added together give the totaSDCCH load per subscriber .

SDCCH configuration when TCH is allotted for signaling when all SDCCHs are occupiedwith TCH GOS 2% and 1% can be selected from Table 5 below. It has been assumedthat the limit capacity is reached when 0.5 Erlang of signaling traffic is served by the TCH

1



Table 5

Thus we see that using the Immediate Assignment Command of TCH when all SDCCH’are busy leads to higher SDCCH/TCH ratios. The situation is depicted graphically iFigure 19 below.

Figure 19

(e) Whenever location updates are increased, the demand for SDCCH resourcesincreases. Dimensioning of the location area depends on the paging load. A paginmessage must be sent to all cells belonging to the LA where the MS is registeredThe BTS broadcasts all incoming paging messages. Too large LA may lead to paging load in the BTS that is too high resulting in congestion and lost pages. Th

No. of TRX

SDCCH type Number of SDCCH Sub

ChannelswithoutCBCH

Number of

SDCCHSub

Channelwith

CBCH

Capacity SDCCH Number of TCH

TCHCapacity

E

SDCCH/TCHratio

Without With CBCH(Er)

WithoutCBCH

With CBCH

1 SDCCH/4Combination A 4 3 2.8 2.0 7 2.93 115.22% 82.3%

SDCCH/8 8 7 5.8 5.0 6 2.27 327.6% 282.48%

2 SDCCH/4 4 3 2.8 2.0 15 9.009 32.9% 23.5%

SDCCH/8 8 7 5.8 5.0 14 8.2003 75.32% 64.93%

3SDCCH/4 4 3 2.8 2.0 23 15.761 18.34% 13.1%

SDCCH/8 8 7 5.8 5.0 22 14.896 40.28% 34.73%

SDCCH/4 +SDCCH/8

12 11 9.1 8.3 22 14.896 63.21% 57.65%

4 SDCH/4 4 3 2.8 2.0 31 22.827 12.54% 8.95%

SDCCH/8 8 7 5.8 5.0 30 21.932 27.06% 23.32%

SDCCH/4+SDCCH/8 12 11 9.1 8.3 30 21.932 42.45% 38.72%

2*SDCCH/8Combination B

16 15 12.4 11.5 29 21.039 60.37% 55.99%

5 SDCCH/8 8 7 5.8 5.0 38 29.166 20.23% 17.44%

SDCCH/4+SDCCH/8

12 11 9.1 8.3 38 29.166 31.74% 28.95%

2*SDCCH/8 16 15 12.4 11.5 37 28.254 44.67% 41.43%

1



upper boundary of a LA is set by the paging load and the lower boundary by thlocation updating load. Smaller LAs means larger number of border cells in thnetwork and hence larger updating load. The LA border cells should not be in higmobility areas such as highways etc and instead should be in low subscriber densitareas to reduce the load on SDCCH due to location updates and number ohandovers.

(f) Each paging block can fit up to four page requests i.e., either 2 IMSI pagin

requests, or, 4TMSI paging requests or 1IMSI+2TMSI paging requests. If thnumber of paging groups (to which an MS belongs) is large the paging timincreases as the time before which the right paging block arrives is longer. If thnumber of paging groups in a cell is small than call set up time reduces but the MSpower consumption increases at its paging group arrives more frequently. To savbattery a MS does not monitor all the paging channels in a multiframe , it onlmonitors the paging channel belonging to its paging group depending on thsetting of the cell parameter BS_PA_MFRMS which informs the MS after howmany multiframes ( ranging from 1 to 9) the same paging group is repeated . Thimeans that a mobile paging block can occur at intervals ranging from 470 ms to2.1 seconds .

(g) The paging messages are controlled by timer T3113 which starts when the paginmessage is sent by the network. On expiry, the network may repeat paginmessage and start T3113 gain. The number of attempts is a network dependenchoice. Time T3113 stops when PAG-RSP message is received by the network. there are too many paging messages increases the queuing time at the BTSsomething that leads to an increase of the average time for a paging response.

(h) Paging load is also affected by the strategy followed in paging- whether the seconpage, after no response to the paging message in the cell where the ms iregistered, is a local page in the same cells or in all the cells under the same MSC

area as the former reduces the paging load but the latter has a better chance osuccessful paging. Paging load is also affected by whether TMSI or IMSI is usefor paging. Use of TMSI reduces paging load but at the same time use of IMSI haa better chance of successful second paging message. If the paging message iglobal (when LA is not known in the VLR) its is recommended that IMSI must bused.

(i) If IMSI/ attach/detach and periodic location update are successfully and regularcarried out, paging load is reduced as the network more or less knows the locatioof the MS. Timer T3212 controls the periodicity of regular location update. shorter time period reduces the paging load but increases the location updatin

load i.e. load on SDCCH. The value of timer T3212 can vary from 1 deci hour i.e. minutes to 255 deci hour i.e. 25.5 hours. The initial recommended setting can bfor periodic location update every third hour.

(j) The MS’s Down Link Signaling Counter (DSC) is initialized to the integer that nearest to the value of 90/BS_PA_MFRMS when the mobile camps on to a celThis counter decrements by 1 when a mobile is not able to decode a paginmessage and increments by 1 when a mobile successfully decodes a messageOnce the DSC reaches a value of 0, a radio link failure is declared and the mobil

1



does a cell reselection. BS_PA_MFRMS can have value in the range of 1 to multiframes, so the DSC will range between 45 and 10 . Thus for BS_PA_MFRMS=1 it needs 45 bad consecutive messages ( 90 multiframes) tdeclare a radio failure and for BS_PA_MFRMS =9 it needs 10 such messages (9multiframes)

(k) Paging Capacity of BTS: The paging capacity depends on all the above factorviz the dimensioning of control channels, size of LA, type of paging request used

paging strategy, setting of timer T3113, periodicity of periodic location update i.eefficiency of the location updates which reduce paging load.

The paging block capacity of a BTS can be defined as:

For combined case when SDCCH/4 is combined with common controchannels resulting in reduced paging blocks availability:[(3-( number of paging blocks per mulitframe reserved for AGCH))/ 0.2354] Paging Blocks/Second

For the non combined case when SDCCH/8 is used on a separate time slo

resulting in increased paging blocks availability.[(9-(number of paging blocks per multiframe reserved for AGCH))0.2354] Paging Blocks/ Second

If no blocks are reserved for AGCH the paging capacity becomes for thcombined case as 3/0.2354 paging blocks /second and for the nocombined case as 9/0.2354 paging blocks /second. In this case, the AccesGrant will work in stealing mode which means that paging blocks arreplaced with Access Grant blocks if required .To calculate the pagincapacity of a BTS, two cases are considered :Case I - it is assumed that all second pages use IMSI to identify the MSand , that typically 25% of the pages of an MS result in a second page.Case II - it is assumed that all second pages use TMSI to identify the MS

and , that typically 25% of the pages of an MS result in a second pageCase III - It is assumed that no second pages are sent .

There are no global pages in a properly dimensional VLR.

A. Case I : Thus for each mobile terminated call 1.25 paging commandare issued which contain 1 TMSI and 1/4 th IMSI.The number of paging attempt per paging block is: 4/ (1+2*25%)= 2.66 Paging Attempt/ Paging Block (Paging attempt = 1 TMSI + ¼ IMSI)(since one fourth IMSI equals one half TMSI)Thus the maximum paging capacity in the BTS for case (i) above isSDCCH Combined Case : 2.66*3/(0.2354)= 33.89 paging attempts/secondThe number of paging commands the BTS can handle hence comes out to1.25*33.89=42.36 paging commands/ second when no blocks are reservefor AGCH.It is reasonable to assume that the maximum allowed paging load is 50% othe maximum paging capacity in the BTS to ensure that no pages are losdue to paging queue in the BTS being full, and that the BTS is able tretransmit all the paging requests.This leads to maximum paging attempt/ second capacity in the BTS as

2



16.94 paging attempts/ sec and the number of paging commands thereforcomes out to 1.25*16.94=21.17 paging commands/second.

SDCCH Non Combined Case : 2.66*9/(0.2354)= 101.69 paginattempts/secondThe number of paging commands the BTS can handle hence comes out to1.25*101.69=127.11 paging commands/ second when no blocks arreserved for ACH.

Assuming that the maximum allowed paging load is 50% of the maximumpaging capacity in the BTS leads to maximum paging attempt/ seconcapacity in the BTS as 50.84 paging attempts/ sec and the number opaging commands therefore comes out to 1.25*50.84=63.55 pagincommands/second.

B. Case II : Thus for each mobile terminated call 1.25 paging commandare issued which contain 1 TMSI and 1/2 th TMSI.The number of paging attempt per paging block is: 4/ (1+25%)= 3.2 , PaginAttempt/ Paging Block (Paging attempt = 1 TMSI + ¼ IMSI)Thus the maximum paging capacity in the BTS for case (ii) above is

SDCCH Combined Case : 3.2*3/(0.2354)= 40.78 paging attempts/secondThe number of paging commands the BTS can handle hence comes out to1.25*40.78=50.97 paging commands/ second when no blocks are reservefor AGCHThis leads to maximum paging attempt/ second capacity in the BTS a20.39 paging attempts/ sec and the number of paging commands thereforcomes out to 1.25*20.39=25.48 paging commands/second.SDCCH Non Combined Case : 3.2*9/(0.2354)= 122.34 paginattempts/secondThe number of paging commands the BTS can handle hence comes out to1.25*122.34=152.92 paging commands/ second.

This leads to maximum paging attempt/ second capacity in the BTS a61.17 paging attempts/ sec and the number of paging commands thereforcomes out to 1.25*61.17=76.46 paging commands/second

Case III : Thus for each mobile terminated call one paging commands arissued which contain 1 TMSI .The number of paging attempt per paging block is: 4, Paging AttempPaging Block (Paging attempt = 1 TMSI )Thus the maximum paging capacity in the BTS for case (ii) above is SDCCH Combined Case 4*3/(0.2354)= 50.97 paging attempts/secondThe number of paging commands the BTS can handle hence comes out to

50.97 paging commands/ second.This leads to maximum paging attempt/ second capacity in the BTS a50.97 paging attempts/ sec and the number of paging commands thereforcomes out to 50.97 paging commands/second.SDCCH Non Combined Case 4*9/(0.2354)=152.93paging attempts/seconThe number of paging commands the BTS can handle hence comes out to152.93 paging commands/ second when no blocks are reserved for AGCH

2



This leads to maximum paging attempt/ second capacity in the BTS a152.93 paging attempts/ sec and the number of paging commands thereforcomes out to 152.93 paging commands/second.The most important rule is that the maximum paging capacity of a BTshould not be exceeded. Similar calculations have been carried out for thcases when one block is reserved for AGCH . A summary of results ishown as under in Table 6.

Table 6 :Type of SDCCH

usedNumber

of pagingblocks

reservedfor AGCH

Pagingblocks/Second

Paging Capacity

Maximum TheoreticalPaging Capacity

Maximum Paging Capacity

PagingAttempt/Second

PagingCommandsper second

Pagingattempts

per second

PagingCommands pe

second

SDCCH/4

a. Case I O 12.7 33.89 42.36 16.94 21.17b. Case II O 12.7 40.78 50.97 20.39 25.48

c. Case III O 12.7 50.97 50.97 50.97 50.97

SDCCH/4

a. Case I 1 8.5 22.59 28.24 11.29 14.11

b. Case II 1 8.5 27.18 33.98 13.59 16.98

c. Case III 1 8.5 33.98 33.98 33.98 33.98

SDCCH/8

a. Case I 0 38.2 101.69 127.11 50.84 63.55

b. Case II 0 38.2 122.34 152.92 61.17 76.49

c. Case III 0 38.2 152.93 152.93 152.93 152.93SDCCH/8

a. Case I 1 33.9 90.39 112.98 45.19 56.48

b. Case II 1 33.9 108.74 135.92 54.37 67.99

c. Case III 1 33.9 135.93 135.93 135.93 135.93

We can see from the above that the paging capacity for Case III is thhighest i.e. when no second page is sent . After that comes the pagincapacity when TMSI is used for second page i.e. Case II .The third in term

of capacity is the case when IMSI is used for second page . If strategy oCase III is adopted then, the risk of unsuccessful paging increases and ithe first case the paging capacity reduces although the pages are morlikely to be successful . Hence the recommended strategy is that of CasII i.e. a second page is sent using the TMSI in areas where paging load ilarge . In this case if MS has the wrong TMSI in the VLR, the page may bunsuccessful . In areas where paging load is smaller then strategy of caseis suitable . Also the above table tells us that using AGCH in a stealin

2



mode increases the paging capacity by about 50% for SDCCH/4 and b12.5% for SDCCH/8.

8. Impact of Paging Load on Dimensioning of Location Areas:Base on the earlier calculations of maximum paging capacity of a BTS , wcan arrive at the maximum size of a LA in terms the TRXs a LA can serv.The maximum paging capacity and hence maximum paging load dependon the type of SDCCH combination used and the number of blocks reserve

for AGCH .For a combination using Case I i.e. SDCCH/4 the maximum loain mErlang comes out to 24.44 mE .Now for a BTS with one TRX the traffic load at 2% GOS as per Erlang table comes out to 2.935 Er. Assuming an average call duration of 15seconds the average number of call in an hour comes out to 70.44 Assuming that 50% of these calls are mobile terminating calls the pagintraffic comes out to35.22*1.25*0.577/3600 mE ( assuming 25% second pages are sent)=0.007 mE which is much less than the maximum paging load . Similacalculations have been done for 2% GOS for various combinations of TRXand SDCCH and the results are tabulated in Table 7 below for Case

where IMSI is used for second page

Table 6:

TRX Type of SDCCHUsed

No of BlocksReservedfor AGCH

MaximumPagingCommandsper second

MaximumpagingLoad ,mEr

TrafficLoadas per ErlangTable

PagingLoad asErlangcalculatiomEr

1 SDCCH/4 0 42.36 24.44 2.935 0.007

SDCCH/4 1 28.24 16.29 2.935 0.007

SDCCH/4 2 14.12 8.14 2.935 0.007

SDCCH/8 0 127.11 73.34 2.276 0.005

SDCCH/8 1 112.98 65.18 2.276 0.005

SDCCH/8 2 98.86 57.04 2.276 0.005

2 SDCCH/4 0 42.36 24.44 9.01 0.021

SDCCH/4 1 28.24 16.29 9.01 0.021

SDCCH/4 2 14.12 8.14 9.01 0.021

SDCCH/8 0 127.11 73.34 8.2 0.019

SDCCH/8 1 112.98 65.18 8.2 0.019

SDCCH/8 2 98.86 57.04 8.2 0.019

3 SDCCH/4 0 42.36 24.44 15.76 0.037

SDCCH/4 1 28.24 16.29 15.76 0.037

SDCCH/4 2 14.12 8.14 15.76 0.037

SDCCH/8 0 127.11 73.34 14.9 0.035

SDCCH/8 1 112.98 65.18 14.9 0.035

SDCCH/8 2 98.86 57.04 14.9 0.035

4 SDCCH/4 0 42.36 24.44 22.83 0.053

2



SDCCH/4 1 28.24 16.29 22.83 0.035

SDCCH/4 2 14.12 8.14 22.83 0.035

SDCCH/8 0 127.11 73.34 21.93 0.052

SDCCH/8 1 112.98 65.18 21.93 0.052

SDCCH/8 2 98.86 57.04 21.93 0.052

The above table tells us that for 2% GOS of TCH channels , 25% secondpages using IMSI for second pages , the Paging capacity is much greatethan the paging load . Even if we configure our systems for retransmissioof all pages then also the paging capacity exceeds the paging load. Also thabove indicates that even if two blocks are reserved for AGCH the pagincapacity is adequate . Hence it is the requirement of SDCCH resources thadecides the location area dimensioning and the limit due to SDCCH/ TCHratio requirements will be reached earlier .

8. Conclusions :

In this paper an analytical analysis of GSM frame structure in general an

Control Channels ,in particular, has been done .Bursts Structures and theusage has been elaborated. Effect of frequency hopping has beediscussed especially for slowly moving users under Rayleigh Fading Control Channel Configurations under various traffic scenarios has beedetailed. Effect of various timers and counter on network performance habeen described .The dimensioning of SDCCH channels under various GOhas been done . We have seen that the SDCCH configuration requirecomes out to be independent of the TCH-GOS. Calculations for SDCCHconfiguration for estimated traffic and SDCCH loads has been given Dynamic Allotment of TCH for signaling in case all SDCCHs are busy habeen discussed and the trunking gain as a result thereof has bee

tabulated. Various factors that affect SDCCH load and Paging Load havbeen dealt in detail. Paging Strategies and their comparisons have beedescribed . Calculations for Paging capacity of BTS for various SDCCconfigurations has been done under static and dynamic allotment conditionof AGCH .We see that for Location Area dimensioning the requirement oSDCCH resources is the deciding factor as paging capacity exceeds threquired paging load.

Ok10

2



2

gsm frame structure

Documents