GSM Frame Structure-Sanjay

GSM Frame Structure : Their Dimensioning & Analytical Overview

Dimensioning and Optimization Strategies of GSM Frame Structure & Analytical Overview

By: Bhishma Bhardwaj

Sanjay Thakur

Anuj Kumar

Head, RF Planning

RF Planner

RF Planner

ConvergeLabs

ConvergeLabs ConvergeLabsAbstract:The paper deals with a detailed analytical overview of GSM frame structure and their dimensioning. The channel structure and frames in GSM have been discussed. The concept of bursts used in GSM has been elaborated. Effect of Rayleigh fading and frequency hopping has been dealt with. Optimization of configuration of channel structure has been discussed as applicable to particular types of service areas. Impact of various timers & counters on network performance, Computation of paging loads and location area planning under various traffic mobility scenarios and optimization of the same are also discussed.

1. Introduction:

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

Um

The Home Location Register (HLR) assists the mobility management by storing part of MSs location information and routing incoming calls to the visitor location register VLR in charge of the area where the paged MS roams. The authentication center AuC is implemented as a part of HLR and helps in authentication of the MS through its international mobile subscriber identity (IMSI). Stolen, fraudulent or faulty mobile stations are identified with the help of equipment identity register (EIR). The BSC is principally in charge of handovers initiation, frequency hopping, channel allocation, link quality, power budget control, signaling and broadcast traffic control etc. The MSCs functions include paging, MS location updating, handover control 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 also the area under control of the VLR.

2. Channel Structure:

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

The traffic Channels used to transport encoded speech and data information. Full rate traffic channels TCH/F are defined using a group of 26 TDMA frames called a 26 multiframe. The 26 multiframe lasts 120ms and the traffic channels for the downlink and uplink are separated by three bursts. As a consequence the mobiles will not need to transmit and receive at the same time which simplifies considerably the electronics of the system and preventing high level transmitted power leakage back to the sensitive receiver. Half rate traffic (TCH/H) double the capacity of the system are also grouped in a 26 mutiframe. The net bit rate, block length, block recurrence for full rate and half rate traffic channels are 13Kbps, 260 bits, 20ms and 5.6 Kbps, 112 bits, 20 ms. For full rate speech the block is divided into two classes according to the importance of the bits (182 bits for class I and 78 bits for class II). For half rate speech, the block is divided into two classes as 95 bits for Class I and 17 bits for class II. The TCH/F 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 some channel maintenance tasks. These can be subdivided into BCH ( Broadcast Channel ), CCCH ( Common Control Channel), SDCCH ( Stand alone dedicated Control Channel), ACCH ( Associated Control Channel)

An associated control channel is for down link and uplink and always associated in conjunction with, either a TCH or an SDCCH. Two types of ACCH for circuit switched connections are defined: continued stream (Slow ACCH) and burst stealing mode (fast ACCH). The FACCH carry the same information as the SDCCH channels. The SACCH can be of four types - SACCH/TF (associated with TCH/F), SACCH/TH (associated with TCH/H), SACCH/C4 (associated with SDCCH/4), SACCH/C8 (associated with SDCCH/8). The FACCH is used for signaling over TCH itself to indicate call establishment progress, to command handover etc. during transmission of fast associated signaling on a traffic channel before a call actually commences and for handover commands. The SACCH is used for measurement report. The broadcast channels are down link channel and of three types: Broadcast Control Channel (BCCH) which gives the mobile station the parameters needed in order to identify & access the network, Frequency Correction Channel (FCH), which supplies the mobile station with the frequency reference of the system in order to synchronize it with the network and Synchronization Channel (SCH) which gives the mobile station the training sequence needed 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 the network and are used to allocate an SDCCH to the mobile station. The SDCCH allocated is used for signaling between the mobile station and the network and is used to allocate a Traffic Channel (if required) to the mobile station, for location updates, authentication of the MS etc. After Traffic Channel is allocated to the MS, the SDCCH channels are released. 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 Channel (RACH), Uplink only, which is used by the MS to request access to the network i.e. for allotment of an SDCCH. The Access Grant Channel (AGCH), down link only, which is used by the base station to inform the MS about which channel i.e SDCCH it should use. This channel is the answer of a base station to a RACH from the mobile station. The SDCCH can share a physical channel with a BCH or CCCH but not with a TCH. Thus we see from the above that the only channels that are for both downlink and uplink are the associated control channels (FACCH & SACCH). All other control channels are either for downlink only (BCCH, FCH, SCH, PCH, AGCH) or for uplink only (RACH). The control channels FCH and SCH are always sent on Time Slot 0 of the BCCH carrier which for this reason does not follow frequency hopping. The control Channel BCCH, RACH, PCH and AGCH must be assigned to the BCCH carrier only on any even numbered time slot. The SDCCH can be assigned to any carrier and only time slot. This means that except for SDCCH, FACCH and SACCH, all other control channels have to be on the BCCH carrier frequency only. The net bit rate, block length and block recurrence time of the control channels is summarized below in Table 1.

Table 1: Control Channel Block Structure

Control ChannelNet Bit Rate

KbpsBlock Length

(bits)Block Recurrence

(ms)Remarks

SACCH (with TCH)115/300168 + 16480 (after every four 26 multiframe)16 bits are resaved for control information on layer 1, 168 bits are used for higher layers, SACCH carries about 2 messages per second

SACCH (with SDCCH)299/765168 + 166120/13 ( after every two 51 multiframe)16 bits are resaved for control information on layer 1, 168 bits are used for higher layers, SACCH carries about 2 messages per second

SDCCH598/7651843060/13=235.38

(after every 51 Multiframe)

BCCH598/7651843060/13

(after every 51 Multiframe)

AGCHn*598/7651843060/13

(after every 51 Multiframe)The total number of blocks and per recurrence period is adjustable on a cell by cell basis & depends on parameters broadcast on the BCCH

PCHP*598/7651843060/13


RACHV*26/76583060/13


FACCH/F9.218420

FACCH/H4.618440

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

One 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 a TDMA frame. The carrier separation is 200KHz. A physical channel is therefore defined as a sequence of TDMA frames, a time slot number and a frequency hopping sequence. The principle of frequency hopping is that each TDMA frame is transmitted over a different frequency except the BCCH (beacon ) frequency.

The logical channels are mapped on to a physical channel i.e. on to a particular time slot of the TDMA frame which repeats after every 4.615ms. The TCH are mapped in a 26 frame multiframe and the control channels in a 51 frame multiframe. The basic radio resource is thus a time slot lasting 15/26 ms (.5769ms) and transmitting information at a modulation rate of 1625/6 Kbits/sec which is the input to the GMSK modulator. This means that one time slot, including guard time is 156.25 bits duration (15/20 * 1625/6). The bandwidth B of the Gaussian filter in the GMSK modulator is 81.3Khz. Hence the BT product comes out to 81.3KHz Tbit= 81.3 KHz6/1625*1/1000= 0.3

A time slot may be pictured in a time/frequency diagram as a small rectangle 15/26ms long and 200KHz wide.

3.1 From Multiframe to Hyperframe:

One multiframe consists of either 26 TDMA frames (each TDMA frame consisting of eight time slots) used to carry Traffic Channels, SACCH and FACCH ( if required) ,or ,51 TDMA frames which is used to carry control channels. Thus we have two types of multiframes:

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

A 51 multiframe with a duration of 235.38ms = (15/26)*8*51ms

A 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 26 multiframe or 26 numbers of 51 multiframe. Hence the duration of Superframe is the same for Traffic Channels and Control Channels. One hyperframe contains 2K superframe and lasts 3hrs 28mins 53.76 seconds. The frame number FN thus can have 26*51*2048 values from 0 to 2715647. This FN is transmitted by base station as a part of Synchronisation burst. Figure 1 gives the schematic arrangement of TDMA frames, multiframes, superframes and hyperframes. The 26 multiframe lasts for 120ms which was chosen as a multiple of 20ms in order to obtain some synchronization with fixed networks, ISDN, in particular. This leads to the value of TDMA frame as 120/26 and that of one TS as 120/(26*8)= 15/26ms.

3.2 Bursts:

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

Figure 2

The effective transmission power is constant over the entire transmission period. It must be noted that the power ramp and down envelope at the leading and trailing edges of the transmission bursts is attenuated by 70dB during a 28- and 18 micro sec. interval respectively. The actual data transmission takes place only during the period of 147 bits which is 542.8ms long. The remaining time in the time slot is used for power ramp up and down. Each burst has tail bits added at both ends to reset the memory of the Viterbi Channel Equalizer (VE) which is responsible for removing, both the channel induced and intentional controlled inter symbol interference. Each burst ends with a guard period to prevent burst overlapping due to propagation delay fluctuations and for multiple path echoes. The tail bits are not set to 1 as the transition from 1 to the first 0 bit of the burst and from the last 0 bit of the burst to 1 fall exactly in the ramping portion of the burst amplitude profile. In the absence of transition the modulated signal is shifted towards higher frequencies and the interference created by ramping outside the frequency slot would be greater then with a bit transition .Every normal burst contains 114 bits of useful encoded data sent in two packets of 57 bits each. The 26 bit training sequence is placed in between the two packets of 57 bits each. This means that the receiver has to memorize the first packet 57 bits before being able to demodulate it. There are eight different training periods & for neighboring base stations one of the eight different training patterns is used associated with the so called BS colour codes which assist in identifying the BSs. The 26 bit training segment is constructed by a 16 bit Viterbi channel equalizer training pattern surrounded by five quasiperiodically repeated bits on both sides .Quality of the received signal RXQUAL is a key parameter for evaluating network performance. RXQUAL is the Bit Error rate BER derived from the 26 bit midamble from the TDMA burst. RXQUAL levels characterize speech quality and dropped calls , where 0 indicates the highest quality and 7 the worst . The stealing flag indicates whether a 57 bit packet actually contains user data ( set to 0) or FCCH information (set to 1). The autocorrelation function of the eight training sequences calculated between the central 16 bits and the whole 26 bit sequence has a central correlation peak surrounded by 5 zeros on each side.

3.2.2 Synchronization Burst:

Figure 4

The training sequence of 64 bits is identical for all BTS. The BS sends synchronization burst on timeslot 0 of the BCCH carrier. The MS sets up its time base counters after receiving a synch burst by detecting QN (Quarter Bit Number = 0- 624) counting the quarter bit intervals in burst, BN (Bit Number= 0-156), TN (Time Slot Number= 0-7) and FN (TDMA frame number= 0- 26.51.2048). The value of QN is determined from the 64 bit training sequence, the value of TN is set to 0 .QN increments every 12/13 micro seconds; BN is the integer part of QN/4; TN increments when QN changes from count 624 to 0 ;FN increments whenever TN changes from count 7 to 0. The 78 encrypted bits are decoded to arrive at the 25-SCH control bits. These 25 control bits contain the PLMN color code and BS color code (BSIC) and the TDMA frame number. 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-SCH bits

The synch burst is the first burst that the mobile station needs to demodulate in the downlink direction.

Figure 5

The access burst is used only for the initial access by the MS to the BTS which applies 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 has longer guard period of 68.25 bits to ensure that the access burst fits in the receiver window of a BTS. We must note that the MS has already synchronized with the network. The BTS determines the actual propagation delay when the access burst arrives at the BTS and calculates the distance of an MS from the BTS and provides the offset time as a 6 bit number (Timing Advance) to the MS which in turn advances its time base over the range 0-63 bits to transmit its signal earlier to enable the normal burst to fit in the receiver window of the BTS. The 36 bit contain among other parameters the encoded 6 bit BSIC (BS Identifier Code) and contains either a CHAN- REQ or an HND-ACC message. The access burst always starts with the bit sequence 00111010 followed by 41 bit synchronization sequence allows the BTS to recognize the access burst. The access burst arrives at the base station with a time error of twice the propagation delay compared to the reception window. The access burst is the first burst that a base station needs to demodulate in the uplink direction. This allows a maximum cell distance of 35kms. The exact shift between downlink & uplink as seen by the mobile station is 3 Burst Period minus TA.

3.2.4Frequency Correction Burst:

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

3.2.5 Dummy Burst

Figure 7

To enable the BCCH frequency to be transmitted with a constant power level, dummy bursts are inserted into otherwise empty time slots on the BCCH frequency. The dummy burst are coded with a predefined pseudo random bit sequence to prevent accidental confusion with frequency correction bursts. A key difference between BCH and TCH ARFCN is that a BCH ARFCN has continuous transmission at a constant power level on all time slots , whereas, a TCH ARFCN has bursted transmission with power levels that can be different in different time slots .

4. Frequencies Available:

The following frequency bands are specified in GSM :

a. Primary Band: 890- 915 (MHz) mobile transmit, base receive

935 960 MHz: base transmit, mobile receive

Allowed 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


c. DCS 1800 Band: 1710-1785 MHz: mobile transmit, base receive

1805-1880 MHz: base transmit, mobile receive


d. PCS 1900 Band: 1850-1910 MHz: mobile transmit, base receive 1930- 1990 MHz: base transmit, mobile receive


For GSM 900 different categories of mobile there are four power classes with the maximum power class having 8W peak output power and the minimum having 0.8W peak output power. For DCS 1800 there are three power classes of 4W peak output power, 1W peak output power, and the minimum having 0.25W peak output power. For PCS 1900 there are three power classes of 2W, 1W and 0.25W peak output power. Easy formulas to describe the actual frequency of an ARFCN are (n=ARFCN):

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

Fdownlink (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 frequency remains the same during the transmission of a TDMA burst having eight time slots. In most cases, the emitting and receiving antennas are not within direct line of sight and the received signal is a sum of a number of copies of one signal with different phases due to multipath propagation and reflection. The sum of a lot of phase shifted signals with a random distribution of phases has an envelope following the Rayleigh distribution. The fading is frequency dependent. With frequency hopping all the bursts containing the parts of one code word are transmitted on different frequencies and are hence not damaged in the same way by Rayleigh fading. When the mobile station moves at high speed, the difference between its position during the reception of two successive bursts of the same channel (i.e. 4.615ms) is sufficient to decorrelate Rayleigh fading. In this case, slow frequency hopping does no harm but it does not help much either. However, when the MS is stationary or moves at slow speeds, SFH allows the transmission to reach the level of performance of high speeds (around 6.5 dB gain). The second advantage of frequency hopping is Interferer Diversity where due to different hopping sequences of neighboring interfacing cells using the same frequencies, the quality improves as the received interfering signal follows a different hopping pattern than that of the cell where the MS is receiving the signal. For a set of n given frequencies, GSM allows 64*n different hopping sequences to be built. They are described by two parameters, the MAIO (Mobile Allocation Index Offset) which may take as many values as the number of frequencies in the set and the HSN (Hopping Sequence Number) which may take 64 different values. Two channels bearing the same HSN but different MAIO never use the same frequency on the same burst. On the opposite two channels using the same frequency list and the same TN, but bearing different HSN, interface I/n th of the bursts. The sequences are pseudo random except for the special case of HSN=0,where the frequencies are used one after the other in order. Usually channels in one cell bear the same HSN and different MAIOs. In distant cells using the same frequency set, different HSN should be used to gain from interferer diversity. It is best to avoid HSN=0 which leads to poor interferer diversity, even with non-identical frequency sets. The BCCH Carrier frequency (Beacon Frequency) is not hopped i.e. the channels BCCH, SCH, FCH, RACH, AGCH, PCH must use a fixed frequency to ease initial synchronization acquisition and reduce system complexity. In most applications, a cell is equipped with exactly as many TRXs as allocated frequencies. In cells of smaller capacity the operator may choose to let the channels other than those on the beacon frequency to hop only on as many frequencies as there are TRXs , or, on as many frequencies as available . A mobile station transmits (or receives) on a fixed frequency during one time slot (577m) and then must hop before the time slot on the next TDMA frame after 4.615ms.

5. Cycles

5.1 TCH/F and its SACCH (Figure 8):

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 ms. Coding 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 burst periods i.e. 480ms. In order to spread the arrival of SACCH messages at the base station, the cycle of two SACCH using successive time slots are separated by 97 bursts periods (i.e. 12*8 plus the difference of one time slot). This results in an even load at the base station. It is important to note that slots of one channel bear the same time slot number in both uplink and downlink directions, even, though they are separated by three burst period in time domain.

5.2 TCH/H (Figure 9)

A TCH/H in domain is describes as one slot every 16 burst periods in average.

5.3 SDCCH (Figure 10)

SDCCH are of two types: SDCCH/8 and SDCCH/4

SDCCH /8 are grouped by 8 along with its associated SACCH/C8 to form the

equivalent of a TCH/F & its SACCH/F. SDCCH/4 are grouped by 4 along with

its associated SACCH/C4 and combined with common channels to form an

equivalent of TCH/F and its SACCH/F. All SDCCH follow a cycle of 102*8(two

51-multiframe) burst periods i.e. a group of four slots separated by 4.615ms (8

bursts 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 one

block of 4 slots for its SACCH/C8. There are maximum 16 different scheduling

for mobile stations in connection with a SDCCH/8. The SDCCH/ 4 can be

combined with common control channels and sent on TS0. Only one SDCCH/4

combination can be defined for each cell.

5.4 Common Control Channels:

The cycles of traffic channels (26frames) and control channels (51 frames) do not have a common divider. This allows the mobile station in dedicated mode to listen to synchronization channel, SCH, and frequency correction channel, FCCH, of surrounding base stations. A BCCH Allocation (BA) Table or list is a set of ARFCNs broadcast to the mobile in the idle and dedicated modes for monitoring as potential neighbor cells . In the idle mode , this list is broadcast on the BCCH in a System Information type 2 message , in the dedicated mode on the SACCH in System Information Type 5 message . This dedicated table can contain the same list of ARFCNs as the idle mode table or a different list .

5.4.1 FCCH and SCH: (Down link) Figure 11

One SCH slot follows each FCCH slot 4.615ms later. Each of these two channels use 5 slots in each 51-multiframe of TS0 of the beacon frequency. The mobile station recognizes the time slot as TS0 whenever it receives FCCH and SCH.

5.4.2 BCCH, PCH, AGCH: (Down link) Figure 12

A BCCH together with PCH + AGCH uses 40 slots per 51-multiframe on the same TN of the beacon frequency. These 40 slots are built into 10 groups of 4, the four slots of first group are used by BCCH and the remaining nine by PCH + AGCH. The other combination is that BCCH with PCH + AGCH uses 16 slots per 51-multiframe all on the same TN of the beacon frequency. BCCH then uses the first block of four slots and PCH +AGCH the remaining three. In both cases the BCCH information can be sent only once every 51- multiframe i.e. only once every 235.38ms.

5.4.3 RACH (Uplink ) Figure 13

Two combinations exist: RACH/F and RACH/H.

The RACH/F uses one slot every TDMA frame of 4.615ms and its organization is similar 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 left free uplink by 4 numbers of SDCCH/8.

5.4.4 Common Channel Combinations:

Every cell broadcasts one single FCCH and one single SCH on TS0 of the Beacon frequency. The common channels are always arranged in three combinations to make a 51-multiframe.

a) Medium Capacity Cells:

In the downlink direction:

FCCH (5 frames), SCH (5 frames), BCCH (4 frames) , PCH + AGCH (36 frames) all on TS0. This allows seven time slots for TCH and SDCCH in each TDMA frame .

In the Uplink direction:

RACH/F on TS0

b) Small Capacity Cells:

In the downlink direction:

FCCH (5 frames), SCH (5 frames), BCCH (4 frames) , PCH + AGCH (12 frames), SDCCH/4 (16 frames), SACCH/C4 (4 frames). This allows seven time 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:

For large capacity cells combination (a) is used along with up to three extension sets on even time slots only. An extension set contains the same channels as combination. (a) except FCCH and SCH ( which are only on TS0). BCCH appears on the extension set to enable the mobile to listen to bursts on one TS only since BCCH contains information about RACH of that particular Time Slot.

5.4.5 CBCH (Figure 14)

A Cell Broadcast Channel CBCH follows a cycle of 8*51*8 burst periods i.e. 8 numbers of 51-multiframe. In each multiframe the CBCH can be seen as a 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 then the CBCH can use the same time slot 0 and frequency as the common channels . 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; In this case the MS in idle mode has to listen regularly to bursts of different time slot numbers. When CBCH is used, the first block of PCH + AGCH in the 51- multiframe cannot be used for paging.

It is allowed to stop the termination of the CBCH incase of congestion and then these resources can be used by SDCCH during such periods. The CBCH reduces the number of available SDCCHs.

6. Channel Organisation in a cell:

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

Table 2

ChannelsUnused Slots

TCH/F with SACCH/F1 out of 26

2 numbers of TCH/F with SACCH /Hnone

8 numbers of SDCCH/83 out of 51

FCCH + SCH+BCCH+PCH+AGCH

In down Link1 out of 51

RACH/F in uplinkNone

BCCH +PCH+AGCH

In downlink11 out of 51

BCCH +PCH+ AGCH+ SDCCH/4

In downlink3 out of 51

RACH/H+ SDCCH/4

In uplinknone

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

A. A small capacity cell with a single TRX can typically be organized as (Figure 15)

TS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH+SDCCH/4+SACCH/C4

Uplink : RACH/4+ SDCCH/4

TSO to 7: Downlink : TCH/F + SACCH/F Uplink: TCH/F + SACCH/F

Figure 15

B. A medium capacity cell with 4 TRXs may typically be organized as (Figure 16)

One group on TS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH

Uplink : RACH/F

Two groups of SDCCH

on two time slots

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

Uplink : TCH/F + SACCH/F

Figure 16

C. A large capacity cell with 12TRXs may include :Figure 17

( 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 group

On 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

Figure 17: While configuring a cell, a network operator has to consider the peculiarities of a service area and the frequency situation, to optimize the configuration. An important factor is the average and maximum loads that are expected for BTS and how the load is shared between signaling and pay load. For cells having several carriers and with a large amount of expected traffic on Common Control Channel eg. Paging, channel requests, channel assignments, the combination B discuseed above is most likely to be used. The signaling needs for mobiles like those for call setup, location updates etc. are then taken care by the SDCCHs.

For cells having one or two carriers the combination A is most likely to be used with SDCCHs combined with Common Control Channels on time slot 0. Here the paging capacity of the cell is lower as only three paging blocks are sent as compared to nine in combination B. we must note the position of the SDCCHs in the uplink and downlink direction. If the base station commands the MS to authenticate itself the response can be sent only 15 frames later (i.e after 15*4.615ms). Thus the command response cycle is reduced to one multiframe. If the base station manages a huge amount of transreceivers it is probable that the number of Common Control Channels provided by combination B is not enough to handle the work and in such cases combination C is preferred wherein additional Common Control Channels are allotted. The CBCH if used is always mapped on to the second subslot of SDCCH i.e. on TS0 of combination A, & on SDCCH time slots 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, IMSI attach, IMSI detach

Call Setup, Short Message Service point to point, Supplementary Services.

An optimum Configuration of SDCCH depends on Cell statistics .

The values of holding time of SDCCH is determined by several timers whose maximum values and functions are defined briefly as under:

Table 3

PROCESSTIMERMAXIMUM VALUEREMARKS

Location UpdatingTimer T 3210 in msMaximum value is 10 sec, stops when LOC- UPD- ACC message is received by the ms i.e. location update is acknowledged by the network.Starts when SDCCH is allotted. At expiry it starts timer T 3211 at whose expiry location update is restarted. Maximum 4 attempts can be made.

Mobile Originating CallTimer T3230 in msMaximum value 15 sec, It stops when CM-SERV-ACC or CM-SERV-REJ or AUTH- REJ is received i.e authentication is successful, allotment of Traffic Channel is done after which SDCCH is released Starts when SDCCH is allotted at expiry provides release indication

AuthenticationTimer 3240 in msMaximum value 10 sec starts when the ms receives an AUTH-REJ messageAt expiry it releases the SDCCH

Timer T 3260 in the networkMaximum value 12 sec, starts when AUTH-REQ is sent and stops when AUTH-RSP/AUTH-REJ is received by the networkOn expiry releases SDCCH

IdentificationTimer T3270 in the networkMaximum value 12 sec, starts when IDENT-REQ is sent and stops when IDENT-RSP is received On expiry releases SDCCH

Timer T3250 in the network Maximum value 12 sec, starts when TMSI-REAL-CMD is sent and stops when TMSI-REAL-COM is received On expiry releases SDCCH

a) The Common Control Channel, CCCH ( consisting of PCH+AGCH) in the downlink can work in stealing mode which means replacing paging blocks with Access Grant Blocks if required. If dedicated blocks are used, each multiframe contains two paging blocks (for combined i.e. combination A) or eight paging blocks (for non combined i.e. combination B)

b) The number of TRXs limits the possible number of SDCCH/8s in a cell. It is not possible to have more SDCCH/8s in a cell than the number of TRXs. However, it is possible to add an SDCCH/4 even if the number of SDCCH/8s equals the number of TRXs in the cell. SDCCH/4 is generally not used incase of high paging load in the location area.

c) A connection for speech or data requires an SDCCH for call setup signaling and a TCH 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 times the blocking rate for TCH which means that for a 2% GOS of TCH the GOS of SDCCH/8 should be less than 0.5%.

d) When all SDCCHs are occupied additional call setup signaling can be performed on TCH whenever more TCHs are available. This means that the traffic load on TCH increases since a TCH instead of SDCCH is allotted on IMM-ASS-CMD message. 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 parenthesis are those for 1% GOS.

Table 4

No. of TRXSDCCH typeNumber of SDCCH Sub Channels without CBCHNumber of SDCCH Sub Channel with CBCHCapacity SDCCHNumber of TCHTCH Capacity ESDCCH/TCH

ratio

Without CBCH (Er)With CBCH

(Er)Without CBCHWith CBCH

1SDCCH/4

( Combination A)

SDCCH/8

(When paging signaling load is higher and SDCCH/8 are configured on other than TS0)4

8

3

70.8694

GOS=1%

(0.7012)

2.730

GOS=.5%

(2.4037)0.4555

GOS=1%

(0.3490)

2.158

GOS=.5%

(1.8778)7

GOS=2%

6

GOS=2%2.935

(2.501)

2.276

(1.909)29.62%

(28.03%)

119.47%

(125.91%)15.51%

(13.95%)

94.81%

(98.36%)

2SDCCH/4

(On TS0)

SDCCH/4+ SDCCH/8

(SDCCH/4 on TS0, SDCCH/8 on any other TS)

SDCCH/8

(On any TS other than TS0)4

12

83

11

70.8694

(0.7012)

5.2789

(4.7807)

2.7299

(2.4037)0.4555

(0.349)

4.6104

(4.1533)

2.1575

(1.8778)15

14

149.0096

(8.108)

8.2003

(7.3517)

8.2003

(7.3517)9.64%

(8.64%)

64.37%

(65.02%)

33.29%

(32.69%)

5.055%

(4.31%)

56.22%

(56.49%)

26.31%

(25.54%)

42* SDCCH/8

(combination B)

SDCCH/4 + SDCCH/816

1215

118.0095

(7.4475)

5.2789

(4.7807)7.3755

(6.7606)

4.6104

(4.1533)

29

3021.039

(19.489)

21.932

(20.337)38.49%

(38.22%)

24.07%

(23.51%)35.06%

(34.69%)

21.02%

(20.42%)

125*SDCCH/8

(combination C)

403927.382

(26.003)26.534

(25.181)8775.415

(71.881)36.31%

(36.18%)35.18%

(35.03%)

It is important to note that for TCH GOS 1% and 2% the ratio of SDCCH/TCH remains about the same i.e. the same table applies to different values of GOS for TCH.

The above table can be used for choice of SDCCH configuration B:

Number of TRXs= 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 at least 25%. This leads us to the combination: 2*SDCCH/8. However if the paging load is less 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 configuration when TCH is allotted for signaling when all SDCCHs are occupied with TCH GOS 2% and 1% can be selected from Table 5 below. It has been assumed that the limit capacity is reached.

Table 5

No. of TRXSDCCH typeNumber of SDCCH Sub Channels without CBCHNumber of SDCCH Sub Channel with CBCHCapacity SDCCHNumber of TCHTCH Capacity ESDCCH/TCH

ratio

Without CBCH (Er)With CBCH

(Er)Without CBCHWith CBCH

1SDCCH/4

Combination A

SDCCH/8

2SDCCH/4

SDCCH/4+SDCCH/8

SDCCH/8

42*SDCCH/8

(Combination B)

SDCCH/4+SDCCH/8

125*SDCCH/8

Combination C

Thus we see that using the Immediate Assignment Command of TCH when all SDCCHs are busy leads to higher SDCCH/TCH ratios. The situation is depicted graphically in Figure 18 below.

Figure 18

(e) Whenever location updates are increased, the demand for SDCCH resources

increases. Dimensioning of the location area depends on the paging load. A paging message must be sent to all cells belonging to the LA where the MS is registered. The BTS broadcasts all incoming paging messages. Too large LA may lead to a paging load in the BTS that is too high resulting in congestion and lost pages. The upper boundary of a LA is set by the paging load and the lower boundary by the location updating load. Smaller LAs means larger number of border cells in the network and hence larger updating load. The LA border cells should not be in high mobility areas such as highways etc and instead should be in low subscriber density areas to reduce the load on SDCCH due to location updates and number of handovers.

(f) Each paging block can fit up to four page requests i.e., either 2 IMSI paging requests, or, 4TMSI paging requests or 1IMSI+2TMSI paging requests. If the number of paging groups (to which an MS belongs) is large the paging time increases as the time before which the right paging block arrives is longer. If the number of paging groups in a cell is small than call set up time reduces but the MS power consumption increases at its paging group arrives more frequently. To save battery a MS does not monitor all the paging channels in a multiframe , it only monitors the paging channel belonging to its paging group depending on the setting of the cell parameter BS_PA_MFRMS which informs the MS after how many multiframes ( ranging from 1 to 9) the same paging group is repeated . This means that a mobile paging block can occur at intervals ranging from 470 ms to 2.1 seconds .

(g) The paging messages are controlled by timer T3113 which starts when the paging message is sent by the network. On expiry, the network may repeat paging message and start T3113 gain. The number of attempts is a network dependent choice. Time T3113 stops when PAG-RSP message is received by the network. If there are too many paging messages increases the queuing time at the BTS, something 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 second page, after no response to the paging message in the cell where the ms is registered, 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 of successful paging. Paging load is also affected by whether TMSI or IMSI is used for paging. Use of TMSI reduces paging load but at the same time use of IMSI has a better chance of successful second paging message. If the paging message is global (when LA is not known in the VLR) its is recommended that IMSI must be used.

(i) If IMSI/ attach/detach and periodic location update are successfully and regularly carried out, paging load is reduced as the network more or less knows the location of the MS. Timer T3212 controls the periodicity of regular location update. A shorter time period reduces the paging load but increases the location updating load i.e. load on SDCCH. The value of timer T3212 can vary from 1 deci hour i.e. 6 minutes to 255 deci hour i.e. 25.5 hours. The initial recommended setting can be for periodic location update every third hour.

(j) The MSs Down Link Signaling Counter (DSC) is initialized to the integer that is nearest to the value of 90/BS_PA_MFRMS when the mobile camps on to a cell. This counter decrements by 1 when a mobile is not able to decode a paging message and increments by 1 when a mobile successfully decodes a message. Once the DSC reaches a value of 0, a radio link failure is declared and the mobile does a cell reselection. BS_PA_MFRMS can have value in the range of 1 to 9 multiframes, so the DSC will range between 45 and 10 . Thus for a BS_PA_MFRMS=1 it needs 45 bad consecutive messages ( 90 multiframes) to declare a radio failure and for BS_PA_MFRMS =9 it needs 10 such messages (90 multiframes)

(k) Paging Capacity of BTS: The paging capacity depends on all the above factors viz 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.e. efficiency 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 control channels 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 slot resulting in increased paging blocks availability.

[(9-(number of paging blocks per multiframe reserved for AGCH))0.2354] Paging Blocks/ SecondIf no blocks are reserved or AGCH the paging capacity becomes combined case= 3/0.2354 paging blocks /second. To calculate the paging capacity of a BTS, it is assumed that all second pages use IMSI to identify the MS, and , that typically 25% of the pages of an MS result in a second page. There are no global pages in a properly dimensional VLR. Thus for each mobile terminated call 1.25 paging commands are issued which contain 1 TMSI and 1/4th IMSI.

The number of paging attempt per paging block is: 4/ (1+2*25%)= 2.7 Paging Attempt/ Paging Block (Paging attempt = 1 TMSI + 1/4IMSI)

Thus the maximum paging capacity in the BTS for case (i) above is 2.7*3/(0.2354)= 34 paging attempts/second

The number of paging commands the BTS can handle hence comes out to= 1.25*34=42.5 paging commands/ second.

It is reasonable to assume that the maximum allowed paging load is 50% of the maximum paging capacity in the BTS to ensure that no pages are lost due to paging queue in the BTS being full, and that the BTS is able to retransmit all the paging requests.

This leads to maximum paging attempt/ second capacity in the BTS as 17 paging attempts/ sec and the number of paging commands therefore comes out to 1.25*17=21.2 paging commands/second. The most important rule is that the maximum paging capacity of a BTS should not be exceeded. Similar calculations can be carried out for non-combined SDCCH and Common Control Channels. A summary of results is shown as under in Table 6.

Table 6

Type of SDCCH usedNumber of paging blocks reserved for AGCHPaging blocks/SecondPaging Capacity

Maximum Theoretical Paging CapacityMaximum Paging Capacity

Paging Attempt/ SecondPaging Commands per secondPaging attempts per secondPaging Commands per second

SDCCH/4

(combined case)

0

112.753442.51721.2

14.4

SDCCH/8

(Non combined case)0

163.8

57.5

MS

MS

MS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

MSC

HLR

EIR

G-MSC

VLR

VLR

Um

Um

Abis Interface

A Interface

F 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.

Figure 1

1 n 124

512 n 885

Downlink: SDCCH/8 + SACCH/8

Uplink : SDCCH/8 + SACCH/8

Downlink: BCCH+PCH+AGCH

Uplink: RACH/F

PAGE 3

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