gsm frame structure-finalwith graphs
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Abis Interface
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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 Sanjay Thakur Anuj Kumar Head, RF Planning RF Planner RF Planner ConvergeLabs ConvergeLabs ConvergeLabs Abstract:�The paper deals with a detailed analytical overview of GSM frame structure, GSM Channels 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 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 [1] and the interfaces between the network entities)
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The Home Location Register (HLR) stores part of MS�s location information and assists the mobility management by routing incoming calls to the visitor location register VLR in charge of the area where the paged MS currently roams. The authentication center AuC is implemented as a part of HLR and helps in authentication of the MS through its
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Figure 1 : Basic GSM Architecture
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international mobile subscriber identity (IMSI). Stolen, fraudulent or faulty MSs are identified with the help of equipment identity register (EIR). All radio resources are controlled by BSC and it is principally in charge of all the processes related to handovers initiation, frequency hopping, channel allocation, link quality, power budget control, signaling and broadcast traffic control etc. The MSC�s functions include paging, MS location updating, handover control etc.[2] 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 recurrence of one burst every frame (one burst on a TDMA frame). It is defined by its frequency and burst position within a TDMA frame. In GSM there are two types of logical channels:
��The traffic Channels are 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 frame multi-frame. The 26 frame multi-frame 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 ( single synthesizer chip in MS) 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 frame multi-frame. The net bit rate, block length, block recurrence [3] 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 are used for network management messages(call set up, control signaling etc.) 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 bi-directional (downlink and uplink) and is always associated 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) [1]. The FACCH is used for signaling over TCH itself to indicate call establishment progress, to command handover etc. and transmission of fast associated signaling on a traffic channel while a call is in progress leads to loss of user data and hence the term �stealing mode� .The SACCH is used for measurement report. The broadcast channels are the down link channels and are of three types: Broadcast Control Channel (BCCH) which provides the MS (MS) the parameters needed to identify and access the network, Frequency Correction Channel (FCH), which supplies the MS with the frequency reference of the system in order to synchronize it with the network and Synchronization Channel (SCH) gives the MS the frame synchronization to demodulate the information transmitted by the base station in system information frames.
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The common control channels help to establish the calls from the MS or the network and are used to allocate an SDCCH to the MS. The SDCCH allocated is used for signaling between the MS and the network for location updates, authentication of the MS etc. and is used for subsequent TCH allocation (for data and speech calls). After Traffic Channel is allocated to the MS, the SDCCH channels are released. Three types of CCCH can be defined � The Paging Channel ( PCH -Downlink only) is used to alert the MS of an incoming call; The Random Access Channel (RACH -Uplink only) is used by the MS to request access to the network i.e. for allotment of an SDCCH. The Access Grant Channel (AGCH � downlink only), is used by the base station to inform the MS about which channel i.e. SDCCH should be used. This channel is the answer of a base station to a RACH from the MS. 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 bi-directional are the associated control channels (FACCH & SACCH). All other control channels are either for downlink (BCCH, FCH, SCH, PCH, AGCH) or for uplink (RACH) only. 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 any 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 [3] below in Table 1. Table 1: Control Channel Block Structure
Control Channel Net Bit Rate Kbps
Block Length (bits)
Block Recurrence (ms)
Remarks
SACCH (with TCH) 115/300 168 + 16 480 (after every four 26 multi-frame)
16 bits are reserved for control information on layer 1, 168 bits are used for higher layers, SACCH carries about 2 messages per second
SACCH (with SDCCH) 299/765 168 + 16 6120/13 ( after every two 51 multi-frame)
16 bits are reserved for control information on layer 1, 168 bits are used for higher layers, SACCH carries about 2 messages per second
SDCCH 598/765 184 3060/13=235.38 (after every 51 Multi-frame)
BCCH 598/765 184 3060/13 (after every 51 Multi-frame)
AGCH m*598/765 184 3060/13 (after every 51 Multi-frame)
The total number of blocks, m, per recurrence period is adjustable on a cell by cell basis & depends on parameters broadcast on the BCCH like BS_CC_CHANS, BS_BCCH_SDCCH_COMB etc
PCH P*598/765 184 3060/13 (after every 51 Multi-frame)
The total number of blocks ,p, per recurrence period is adjustable on a cell by cell basis & depends on parameters broadcast on the BCCH
RACH n*26/765 8 3060/13 (after every 51 Multi-frame)
The total number of blocks,n, per recurrence period is adjustable on a cell by cell basis & depends on parameters broadcast on the BCCH
FACCH/F 9.2 184 20 FACCH/H 4.6 184 40
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Note: One 51 frame multi-frame lasts 15/ 26 * 8* 51 i.e. 3060/13 ms One 26 frame multi-frame 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 timeslots 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 on a TRX ,which in turn may use a frequency hopping sequence. The principle of frequency hopping is that each TDMA frame is transmitted over a different frequency except the the frames on the BCCH 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 multi-frame and the control channels in a 51 frame multi-frame. 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 (15/26 * 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*T(bit)= 81.3 *6/1625= 0.3 Hz per second A time slot may be pictured in a time/frequency diagram as a small rectangle 15/26ms long and 200KHz wide.[4]
3.1 From Multi-frame to Hyper-frame: A TDMA frame with eight time slots is of duration (15/26)*8= 4.615ms. One multi-frame consists of either 26 TDMA frames (each TDMA frame consisting of eight time slots) used to carry TCH, SACCH and FACCH ( if required) (TCH-Multi-frame ), or 51 TDMA frames which is used to carry control channels, Control-Multi-frame. Thus we have two types of multi-frames: ��A 26 frame TCH-Multi-frame 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 frame Control-Multi-frame with a duration of 235.38ms = (15/26)*8*51ms A Super frame lasts for 6.12 seconds and contains either 51 numbers of TCH-Multi-frame or 26 numbers of Control-Multi-frame. Hence the duration of Super-frame is the same for Traffic Channels and Control Channels. One Hyper-frame contains 2K super-frame 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 Synchronization burst which we discuss later. Figure 2 gives the schematic arrangement of TDMA frames, multi-frames, super-frames and Hyper-frames. The 26 multi-frame 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.
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0 1 2 3 2046 2047
Normal Burst Figure 2:Frames, Multi-frames, Super-frames, Hyper-frames
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 duration and having 156.25 bits. A schematic representation of burst in power over time presentation is given in Figure 3.
0 1 2 49 50
0 1 25
0 1 25 0 1 2 49 50
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TB 3
58 Encrypted Bits with stealing bit
26 bits Training Seq.
58 Encrypted bits with stealing bit
TB 3
GP 8.25
1 Hyper Frame=2048 Superframes=2,715,648 TDMA 3 hours 28 min.53.76 sec
1 Super Frame=1326 TDMA Frames(6.12 s)
1 TCH Multi Frame=26 TDMA (120 ms)1 Control Multi Frame= 51 TDMA Frames(235 ms)
1 TDMA Frame=8 timeslots(4.165 ms)
1 Time Slot=(156.25) bit durations=0.577ms)
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Figure 3: Burst Used in GSM 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 [3] which is 542.8micros 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.
3.2.1 Normal Burst : 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 patterns & for neighboring base stations one of the eight
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147 bit = 542.8 micro sec. (TCH) 312.2 micro sec. (RACH)
156.25 bit = 577micro sec.
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different training patterns is used associated with the so called BS color codes which assist in identifying the BS�s. The 26 bit training segment is constructed by a 16 bit Viterbi channel equalizer training pattern surrounded by five quasi-periodically 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 FACCH 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 zero�s on each side.[4] 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 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- 2715647). 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 [2] The synch burst is the first burst that the MS needs to demodulate in the downlink direction.
3.2.3 Access Burst : 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 which applies in two cases:
T PAYLOAD F TRAINING SEQUENCE F PAYLOAD T G 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
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1. For a connection setup from idle state where a CHAN-REQ message is sent using access burst.
2. For handover when MS 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 [1] 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 MS is 3 Burst Period minus TA. [4]
3.2.4 Frequency Correction Burst: Figure 7: Frequency Correction Burst (156.25 Bits) T: 3 TAIL BIT;G:8.25 GUARD BITS
AlI 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 [1]. 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 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 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 .
T ALL ZERO 142 BITS T G
T PREDEFINED BIT SEQUENCE 142 BITS T G
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1 � n � 124
512 � n � 885
4. Frequencies Available: The following frequency bands are specified in GSM [3]:
a. Primary Band: 890- 915 (MHz) mobile transmit, base receive 935 � 960 MHz: base transmit, mobile receive Allowed Frequencies in one direction = 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 in one direction= 174 with 200khz spacing c. DCS 1800 Band: 1710-1785 MHz: mobile transmit, base receive
1805-1880 MHz: base transmit, mobile receive Allowed Frequencies in one direction = 374 with 200khz spacing
d. PCS 1900 Band: 1850-1910 MHz: mobile transmit, base receive 1930- 1990 MHz: base transmit, mobile receive
Allowed Frequencies in one direction = 299 with 200khz spacing 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):[6]
��Primary Band: Fuplink (n) = (890 + 0.2n) MHz Fdownlink (n) = Fuplink (n) + 45 MHz
��Extended GSM: Fuplink (n) = (890 + 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 MS 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 not help much . 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
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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.[4] 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 MAIO�s. 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 MS transmits (or receives) on a fixed frequency during one time slot (�577µm) 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:
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-multi-frame of 120 ms. Coding follows cycles based on the grouping of four successive bursts. However, for the SACCH, the full cycle lasts four 26-multi-frames 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.[4] T
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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.
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Figure 10: TCH/H FRAME t1,t2: two half rate TCHs ; s1,s2: their SACCH/Hs
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5.3 SDCCH 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-multi-frame) burst periods i.e. eight groups of four slots separated by 4.615ms (8 bursts periods) every 51-multi-frame for SDCCH/8. These are combined with four groups of 4 slots for SACCH/8 separated by 4.615ms (8 bursts periods) every 51-multi-frame. Thus during the cycle of SDCCH/8 we need two 51 multiframes. There are maximum 16 different scheduling for MSs in connection with a SDCCH/8. Similarly SDCCH/4 follows two 51 multiframes cycle [5]. 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. D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S4 S5 S6 S7 I I I
(Two 51 multi frames shown for complete cycle of SACCH ) Figure 11 (51 MULTI-FRAME=235.38 ms) D0 � D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS S0-S7 : EIGHT SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8 I : IDLE FRAMES
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 MS 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) One SCH slot follows each FCCH slot 4.615ms later. Each of these two channels use 5 slots in each 51-multi-frame of TS0 of the beacon frequency. The MS 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 i f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; ccch (4 time slots ) - PCH + AGCH ; i : IDLE ( 1time slot )
Figure 12(51 Multi-frames=235.38ms) 5.4.2 BCCH, PCH, AGCH: (Down link)
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A BCCH together with PCH + AGCH uses 40 slots per 51-multi-frame 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 when SDCCH is not combined with CCCH. The other combination is that BCCH with PCH + AGCH uses 16 slots per 51-multi-frame all on the same TN of the beacon frequency when SDCCH is combined with CCCH . 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- multi-frame i.e. only once every 235.38ms.
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 is similar to TCH/F with its SACCH/F in the uplink direction.[2] The RACH/H uses only 27 slots in the 51-multi-frame. A RACH/H fits in the burst left free uplink by 4 numbers of SDCCH/4.
RACH /F ON ALL 51 FRAMES RACH (ACCESS BURST)
RACH / F: RACH IS SENT ON THE UPLINK FOR ALL THE 51 MULTI-FRAMES
D3 R R SA2 SA3 RACH FROM 14 TO 36 FRAMES D0 D1 R R D2
RACH/H :RACH IS SENT ON THE UPLINK FOR 27 FRAMES D0-D3 : SDCCH/4 FOUR TIME SLOTS SA : SACCH /C4 FOUR TIME SLOTS R : RACH 1 TS
Figure 13(51 Multi-frame=235.38ms) 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-multi-frame.
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 (8 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 (8 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.
13
5.4.5 CBCH A Cell Broadcast Channel CBCH follows a cycle of 8*51*8 burst periods i.e. 8 numbers of 51-multi-frame. In each multi-frame 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- multi-frame cannot be used for paging.[4] 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 SDCCH�s.
f s bcch ccch f s ccch ccch f s D0 D1 f s CBCH D3 f s SA0 SA1 i f s bcch ccch f s ccch ccch f s D0 D1 f s CBCH D3 f s SA2 SA3 i 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 ) ; ccch (4 time slots )- PCH + AGCH ;
i : IDLE ( 1time slot ) ; D0-D3 : SDCCH/4 ; SA0-SA3 :SACCH/C4 ASSOCIATED WITH SDCCH/4 (requires two 51 multi-frames )
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 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. [4] Table 2
Channels Unused Slots TCH/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+AGCH In down Link
1 out of 51
RACH/F in uplink None BCCH +PCH+AGCH In downlink
11 out of 51
BCCH +PCH+ AGCH+ SDCCH/4 In downlink
3 out of 51
RACH/H+ SDCCH/4 In uplink
none
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
14
TS0: Downlink: FCCH+SCH+BCCH+PCH+AGCH+SDCCH/4+SACCH/C4
Uplink : RACH/H+ SDCCH/4
TS1 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 i F s bcch ccch f s ccch Ccch f s D0 D1 f s D2 D3 f s SA2 SA3 i 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 ) ; ccch (4 time slots ) - PCH + AGCH ;
i : IDLE ( 1time slot ) ; D0-D3 : SDCCH/4 ; SA0-SA3 :SACCH/C4 ASSOCIATED WITH SDCCH/4 (requires two 51 multi-frames )
D3 R R SA2 SA3 RACH FROM 14 TO 36 FRAMES D0 D1 R R D2 D3 R R SA0 SA1 RACH FROM 14 TO 36 FRAMES D0 D1 R R D2
TS O UPLINK :RACH + SDCCH/4 D0-D3 : SDCCH/4 FOUR TIME SLOTS ( two 51 multi-frame shown to show complete cycle of SACCH/C4 SA : SACCH /C4 FOUR TIME SLOTS
R : RACH 1 TS
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
I
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 26 TS 1 TO TS 7 ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE
( one 26 multi-frame shown )
Figure 15 :Channel organization for a small capacity cell
B. A medium capacity cell with 4 TRX�s may typically be organized as 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
f S bcch ccch f s ccch ccch f s ccch ccch f s ccch ccch f S ccch ccch i TS 0 DOWNLINK ( one 51 multi-frame shown ) of Beacon Frequency f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; ccch (4 time slots ) - PCH + AGCH ; i : IDLE ( 1time slot ) RACH /F ON ALL 51 FRAMES RACH (ACCESS BURST)
TS O UPLINK ( one 51 multi-frame shown ) of Beacon frequency
D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S4 S5 S6 S7
( Two 51 multiframes of a time slot shown for complete cycle of SACCH ) TWO SUCH GROUPS OF 51 MULTI-FRAME ON ANY TWO TS OTHER THAN TS0 OF Beacon Frequency: DOWNLINK D0 � D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS S0-S7 : EIGHT SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8
I : IDLE FRAMES EACH OF ONE SLOT
S5 S6 S7 I I I D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S4
( Two 51 Multiframes of a TS shown for complete cycle of SACCH ) TWO SUCH GROUPS OF 51 MULTI-FRAME ON ANY TWO TS OTHER THAN TS0 OF Beacon Frequency :UPINK D0 � D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS S0-S7 : EIGHT SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8
I : IDLE FRAMES 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
I
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 26 Remaining 29 Time Slots ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE
Downlink: SDCCH/8 + SACCH/8 Uplink : SDCCH/8 + SACCH/8
15
Downlink: BCCH+PCH+AGCH Uplink: RACH/F
( one 26 multi-frame 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 group On TS4 & one group on TS6
Five groups of SDCCH: Downlink : SDCCH/8 +SACCH/8
On 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 f s ccch ccch f s ccch ccch f s ccch ccch i
TS 0 DOWNLINK ( one 51 multi-frame shown ) of Beacon Frequency f : FCCH (1 time slot) ; s : SCCH ( 1 time slot) ; bcch ( 4 time slots ) ; ccch (4 time slots ) : PCH + AGCH ; i : IDLE ( 1time slot ) RACH /F ON ALL 51 FRAMES RACH (ACCESS BURST)
TS O UPLINK ( one 51 multi-frame shown ) of Beacon frequency
i i bcch ccch i i ccch ccch i i ccch ccch i i ccch ccch i i ccch ccch i ONE GROUP EACH ON TS 2,TS4 & TS6 DOWNLINK ( one 51 multi-frame shown ) of Beacon Frequency 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 ) RACH /F ON ALL 51 FRAMES RACH (ACCESS BURST)
TS 2,4,6 UPLINK ( one 51 multi-frame shown ) of Beacon frequency
D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S4 S5 S6 S7
FIVE SUCH GROUPS OF 51 MULTI-FRAME ON ANY FIVE TS OTHER THAN TS0,2,4,6 OF Beacon Frequency: DOWNLINK (TWO 51 multi-frame of a TS shown for complete cycle of SACCH shown ) D0 � D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS S0-S7 : EIGHT SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8
I : IDLE FRAMES
S5 S6 S7 I I I D0 D1 D2 D3 D4 D5 D6 D7 S0 S1 S2 S3 I I I D0 D1 D2 D3 D4 D5 D6 D7 S4
FIVE SUCH GROUPS OF 51 MULTI-FRAME ON ANY FIVE TS OTHER THAN TS0,2,4,6 OF Beacon Frequency: UP LINK (TWO 51 multi-frame of a TS shown for complete cycle of SACCH shown ) D0 � D7 : EIGHT SDCCH/8 CHANNELS EACH OF FOUR SLOTS S0-S7 : EIGHT SACCH/C8 CHANNELS EACH OF FOUR SLOTS ,ASSOCIATED WITH SDCCH/8
I : IDLE FRAMES
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
I
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 26 Remaining 87 Time Slots ( DOWNLINK & UPLINK ) T:TRAFFIC ; S: SACCH;I: IDLE
( one 26 multi-frame 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 peculiarities of a service area and the frequency situation, to optimize the configuration.
16
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 discussed 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 SDCCH�s. [7] For cells having one or two carriers the combination A is most likely to be used with SDCCH�s 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 SDCCH�s 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 multi-frame. 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 like SDCCH load, SDCCH congestion, TCH load and TCH congestion . The values of holding time of SDCCH is determined by several timers whose maximum values and functions are defined briefly as under: [3] Table 3
PROCESS TIMER MAXIMUM VALUE REMARKS Location Updating Timer T 3210 in ms Maximum value is 20 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 (MAX. VALUE 15 s ) at whose expiry location update is restarted. Maximum 4 attempts can be made.
Mobile Originating Call
Timer T3230 in ms Maximum value 15 sec, It stops when CM-SERV-ACC or CM-SERV-REJ or AUTH- REJ is received i.e authentication is successful/unsuccessful ; allotment of Traffic Channel is done if authentification is successful, after which SDCCH is released
Starts when SDCCH is allotted at expiry provides release indication
Authentication Timer 3240 in ms Maximum value 10 sec starts when the ms receives an AUTH-REJ message
At expiry it releases the SDCCH
Timer T 3260 in the Maximum value 12 sec, starts when On expiry releases SDCCH
17
network AUTH-REQ is sent and stops when AUTH-RSP/AUTH-REJ is received by the network
Identification Timer T3270 in the network
Maximum 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
The time out value of each MS timer is broadcasted in a SYSTEM INFORMATION message .
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 multi-frame contains two paging blocks (for combined case i.e. combination �A�) or eight paging blocks (for non combined case i.e. combination �B�)
b) The number of TRX�s 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 TRX�s. However, it is possible to add an SDCCH/4 even if the number of SDCCH/8s equals the number of TRX�s 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%. If the number of SDCCH are increased the SDCCH GOS obviously improves but the capacity of TCH reduces . So the SDCCH dimensioning 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 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 of TCH . Table 4
18
The SDCCH/TCH ratio given in the above table indicates the SDCCH configuration required 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 remains about the same i.e. the same table applies to different values of GOS for TCH. This means that the SDCCH configuration does not depend on the GOS of TCH and instead only on the relationship between TCH-GOS and SDCCH-GOS The same is shown in Figure 18
No. of TRX
SDCCH type Number of SDCCH
Sub Channels without CBCH
Number of
SDCCH Sub
Channel with
CBCH
Capacity SDCCH in Erlangs
Number of TCH
TCH Capacity Erlangs
SDCCH/TCH ratio
Without CBCH
With CBCH
Without CBCH
With CBCH
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%)
1
SDCCH/8 (When paging signaling load is higher and SDCCH/8 are configured on other 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%)
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 any other TS)
12
11
5.2789 (4.7807)
4.6104 (4.1533)
14
8.2003 (7.3517)
64.37% (65.02%)
56.22% (56.49%)
2
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%)
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%)
3
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%)
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%)
4
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%)
19
The above table also tells us that the reduction in SDCCH/TCH ratio due to use of CBCH is maximum for smaller cells i.e. for smaller cells whenever CBCH is used the SDCCH resources are more severely constrained .
Figure 18:- Ratio of SDCCH / TCH Loads for Static allotment of SDCCH 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
% of SDCCH / TCH using SDCCH 4 for 1 & 2 % grade of Service
0.00%
5.00%
10.00%
15.00%
20.00%
25.00%
30.00%
35.00%
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
No. of TRX
% o
f SD
CC
H /
TC
H L
oa
d
Without CBCH 1 % GOS With CBCH 1 % GOS
Without CBCH 2 % GOS With CBCH 2 % GOS
% of SDCCH / TCH using SDCCH 8 for 1 & 2 % grade of Service
0.00%
50.00%
100.00%
150.00%
0 1 2 3 4 5
No. of TRX
% o
f S
DC
CH
/ T
CH
L
oa
d
Without CBCH 1 % GOS With CBCH 1 % GOS
Without CBCH 2 % GOS With CBCH 2 % GOS
20
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 traffic in mErlang per subscriber for each type of procedure ( location update, IMSI attach/detach, Periodic registration, 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 the estimated SDCCH load . Contributions from each procedure added together give the total SDCCH load per subscriber . 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 when 0.5 Erlang of signaling traffic is served by the TCH. Table 5
No. of TRX
SDCCH type Number of SDCCH Sub
Channels without CBCH
Number of
SDCCH Sub
Channel with
CBCH
Capacity SDCCH Number of TCH
TCH Capacity
E
SDCCH/TCH ratio
Without CBCH (Er)
With CBCH (Er)
Without CBCH
With CBCH
SDCCH/4 Combination A
4 3 2.8 2.0 7 2.93 115.22% 82.3% 1
SDCCH/8
8 7 5.8 5.0 6 2.27 327.6% 282.48%
SDCCH/4
4 3 2.8 2.0 15 9.009 32.9% 23.5% 2
SDCCH/8 8 7 5.8 5.0 14 8.2003 75.32% 64.93%
SDCCH/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%
3
SDCCH/4 + SDCCH/8
12 11 9.1 8.3 22 14.896 63.21% 57.65%
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%
4
2*SDCCH/8 Combination B
16 15 12.4 11.5 29 21.039 60.37% 55.99%
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%
5
2*SDCCH/8
16 15 12.4 11.5 37 28.254 44.67% 41.43%
21
Thus we see that using the Immediate Assignment Command of TCH when all SDCCH�s are busy leads to higher SDCCH/TCH ratios. The situation is depicted graphically in Figure 19 below.
SDCCH 4 / TCH Ratio using Static & Dynamic Allotment for 2 % GOS
0.00%
20.00%
40.00%
60.00%
80.00%
100.00%
120.00%
140.00%
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5
No. of TRX
SD
CC
H /
TC
H L
oad
(%
)
SDCCH / TCH Ratio (Static Allotment) without CBCH SDCCH / TCH Ratio (Dynamic Allotment) without CBCH
SDCCH / TCH Ratio (Static Allotment) with CBCH "SDCCH / TCH Ratio (Dynamic Allotment) with CBCH"
SDCCH 8 / TCH Ratio using Static & Dynamic Allotment for 2 % GOS
0.00%
50.00%
100.00%
150.00%
200.00%
250.00%
300.00%
350.00%
0 1 2 3 4 5
No. of TRX
SD
CC
H / T
CH
Load (
%)
SDCCH / TCH Ratio (Static Allotment) without CBCHSDCCH / TCH Ratio (Static Allotment) with CBCHSDCCH / TCH Ratio (Dynamic Allotment) without CBCHSDCCH / TCH Ratio (Dynamic Allotment) with CBCH
Figure 19
22
(e) Whenever location updates are increased, the demand for SDCCH resources
increases. Dimensioning of the location area also 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 multi-frame , 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 multi-frames ( 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 [3] 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 cells 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.[1] The initial recommended setting can be for periodic location update every two hours.
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(j) The MS�s Down Link Signaling Counter (DSC) is initialized [10] 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 multi-frames, so the DSC will range between 90 and 10 . Thus for a BS_PA_MFRMS=1 it needs 90 bad consecutive messages ( 90 multi-frames) to declare a radio failure and for BS_PA_MFRMS =9 it needs 10 such messages (90 multi-frames) Thus in either case a radio link failure is declared after 90 multi-frames (21s).
(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 multi-frame reserved for AGCH))0.2354] Paging Blocks/ Second If no blocks are reserved for AGCH the paging capacity becomes for the combined case as 3/0.2354 paging blocks /second and for the non combined case as 9/0.2354 paging blocks /second. In this case, the Access Grant will work in stealing mode which means that paging blocks are replaced with Access Grant blocks if required .To calculate the paging capacity of a BTS, three cases are considered : Case I - 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. 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 page Case III - It is assumed that no second pages are sent . It is assumed that there are no global pages i.e. the VLR is properly dimensional . A. Case I : 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%)= 2.66 , 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 is SDCCH Combined Case : 2.66*3/(0.2354)= 33.89 paging attempts/second
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The number of paging commands the BTS can handle hence comes out to= 1.25*33.89=42.36 paging commands/ second when no blocks are reserved for AGCH. We may 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.[5] This leads to maximum paging attempt/ second capacity in the BTS as 16.94 paging attempts/ sec and the number of paging commands therefore comes out to 1.25*16.94=21.17 paging commands/second. SDCCH Non Combined Case : 2.66*9/(0.2354)= 101.69 paging attempts/second The number of paging commands the BTS can handle hence comes out to= 1.25*101.69=127.11 paging commands/ second when no blocks are reserved for ACH. Assuming that the maximum allowed paging load is 50% of the maximum paging capacity in the BTS leads to maximum paging attempt/ second capacity in the BTS as 50.84 paging attempts/ sec and the number of paging commands therefore comes out to 1.25*50.84=63.55 paging commands/second. B. Case II : Thus for each mobile terminated call 1.25 paging commands are issued which contain 1 TMSI and 1/2th TMSI. The number of paging attempt per paging block is: 4/ (1+25%)= 3.2 , Paging Attempt/ 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/second The number of paging commands the BTS can handle hence comes out to= 1.25*40.78=50.97 paging commands/ second when no blocks are reserved for AGCH This leads to maximum paging attempt/ second capacity in the BTS as 20.39 paging attempts/ sec and the number of paging commands therefore comes out to 1.25*20.39=25.48 paging commands/second. SDCCH Non Combined Case : 3.2*9/(0.2354)= 122.34 paging attempts/second The number of paging commands the BTS can handle hence comes out to= 1.25*122.34=152.92 paging commands/ second. This leads to maximum paging attempt/ second capacity in the BTS as 61.17 paging attempts/ sec and the number of paging commands therefore comes out to 1.25*61.17=76.46 paging commands/second Case III : Thus for each mobile terminated call one paging commands are issued which contain 1 TMSI . The number of paging attempt per paging block is: 4, Paging Attempt/ Paging 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/second The number of paging commands the BTS can handle hence comes out to= 50.97 paging commands/ second.
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This leads to maximum paging attempt/ second capacity in the BTS as 50.97 paging attempts/ sec and the number of paging commands therefore comes out to 50.97 paging commands/second. SDCCH Non Combined Case 4*9/(0.2354)=152.93paging attempts/second The number of paging commands the BTS can handle hence comes out to= 152.93 paging commands/ second when no blocks are reserved for AGCH This leads to maximum paging attempt/ second capacity in the BTS as 152.93 paging attempts/ sec and the number of paging commands therefore comes out to 152.93 paging commands/second. The most important rule is that the maximum paging capacity of a BTS should not be exceeded. Similar calculations have been carried out for the cases when one block is reserved for AGCH . A summary of results is shown as under in Table 6. Table 6 :
Type of SDCCH used
Number of paging
blocks reserved for AGCH
Paging blocks/ Second
Paging Capacity
Maximum Theoretical Paging Capacity
Maximum Paging Capacity
Paging Attempt/ Second
Paging Commands per second
Paging attempts
per second
Paging Commands per
second assuming 50%
max load SDCCH/4
a. Case I O 12.7 33.89 42.36 16.94 21.17 b. 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 38.2 152.93 152.93 152.93 152.93
SDCCH/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 the highest i.e. when no second page is sent . After that comes the paging capacity when TMSI is used for second page i.e. Case II .The third in terms of capacity is the case when IMSI is used for second page . If strategy of Case III is adopted then, the risk of unsuccessful paging increases and in
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the first case the paging capacity reduces although the pages are more likely to be successful . Hence the recommended strategy is that of Case II i.e. a second page is sent using the TMSI in areas where paging load is large . In this case if MS has the wrong TMSI in the VLR, the page may be unsuccessful . In areas where paging load is smaller then strategy of case I is suitable . Also the above table tells us that using AGCH in a stealing mode increases the paging capacity by about 50% for SDCCH/4 and by 12.5% for SDCCH/8.
8. Impact of Paging Load on Dimensioning of Location Areas: Based on the above calculations of maximum paging capacity of a BTS , we can arrive at the maximum size of a LA in terms the TRXs a LA can serve .The maximum paging capacity and hence maximum paging load depends on the type of SDCCH combination used and the number of blocks reserved for AGCH .For a combination using Case I i.e. SDCCH/4 the maximum load in mErlang comes out to 12.21 mE assuming that all paging requests are transmitted twice. Now for a BTS with one TRX the traffic load at 2% GOS as per Erlang B table comes out to 2.935 Er. Assuming an average call duration of 72 seconds the average number of call in an hour comes out to 146.75. Assuming that 50% of these calls are mobile terminating calls the paging traffic comes out to 73.375*2*0.577/3600 mE ( assuming 100% second pages are sent) =0.023 mE which is much less than the maximum paging capacity . Similar calculations have been done for 2% GOS for various combinations of TRX and SDCCH and the results are tabulated in Table 7 below for the Case where IMSI is used for second page and all page messages are transmitted twice . Table 6: TRX Type of
SDCCH Used
No of Blocks Reserved for AGCH
Maximum Paging Commands per second
Maximum paging Load , mEr
Traffic Load as per Erlang Table
Paging Load as per Erlang calculations, mEr
1 SDCCH/4 0 42.36 12.21 2.935 0.023 SDCCH/4 1 28.24 8.14 2.935 0.023 SDCCH/4 2 14.12 4.07 2.935 0.023 SDCCH/8 0 127.11 36.66 2.276 0.018 SDCCH/8 1 112.98 32.59 2.276 0.018 SDCCH/8 2 98.86 28.52 2.276 0.018 2 SDCCH/4 0 42.36 12.21 9.01 0.073 SDCCH/4 1 28.24 8.14 9.01 0.073 SDCCH/4 2 14.12 4.07 9.01 0.073 SDCCH/8 0 127.11 36.66 8.2 0.066 SDCCH/8 1 112.98 32.59 8.2 0.066 SDCCH/8 2 98.86 28.52 8.2 0.066 3 SDCCH/4 0 42.36 12.21 15.76 0.126
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SDCCH/4 1 28.24 8.14 15.76 0.126 SDCCH/4 2 14.12 4.07 15.76 0.126 SDCCH/8 0 127.11 36.66 14.9 0.119 SDCCH/8 1 112.98 32.59 14.9 0.119 SDCCH/8 2 98.86 28.52 14.9 0.119 4 SDCCH/4 0 42.36 12.21 22.83 0.182 SDCCH/4 1 28.24 8.14 22.83 0.182 SDCCH/4 2 14.12 4.07 22.83 0.182 SDCCH/8 0 127.11 36.66 21.93 0.176 SDCCH/8 1 112.98 32.59 21.93 0.176 SDCCH/8 2 98.86 28.52 21.93 0.176 The above table tells us that for 2% GOS of TCH channels , 100% second pages using IMSI for second pages , the Paging capacity is much greater than the paging load . Even if we configure our systems for retransmission of all pages then also the paging capacity exceeds the paging load. Also the above indicates that even if two blocks are reserved for AGCH the paging capacity is adequate . Hence it is the requirement of SDCCH resources that decides the location area dimensioning and the limit due to SDCCH/ TCH ratio requirements will be reached earlier .The factors that may lead to increased paging load from that calculated above may be : increased global pages when the VLR does not know the LA of the MS, and hardware constraints on the BTS �BSC interface i.e. the capacity of paging queue , unsuccessful pages due to wrong IMSI/TMSI, lower settings of timer T3113 requiring more frequent paging etc. All of the these are however a part of proper functioning of the VLR and hardware planning . The paging capacity per se is adequate even for the lower call holding times taken in the above calculations . 8. Conclusions : In this paper an analytical analysis of GSM frame structure in general and Control Channels ,in particular, has been done .Bursts Structures and their usage has been elaborated. Effect of frequency hopping has been discussed especially for slowly moving users under Rayleigh Fading . Control Channel Configurations under various traffic scenarios has been detailed. Effect of various timers and counter on network performance has been described .The dimensioning of SDCCH channels under various GOS has been done . We have seen that the SDCCH configuration required comes out to be independent of the TCH-GOS. Calculations for SDCCH configuration for estimated traffic and SDCCH loads has been given . Dynamic Allotment of TCH for signaling in case all SDCCHs are busy has been discussed and the trunking gain as a result thereof has been tabulated. Various factors that affect SDCCH load and Paging Load have been dealt in detail. Paging Strategies and their comparisons have been described . Calculations for Paging capacity of BTS for various SDCCH configurations has been done under static and dynamic allotment conditions of AGCH .We see that for Location Area dimensioning the requirement of
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SDCCH resources is the deciding factor as paging capacity exceeds the required paging load. Bibliography [1] �GSM Networks, Protocols, Terminology and Implementation� by Gunnar Heine, Artech House Mobile Communications Series . [2] �The Pan European Mobile Radio System � by Hanzo L. and Steele, European Trans. On Telecomm [3] �Global System of Mobile Communication (GSM) Specification Series 05.01-5.10 � , European Telecomunications Standardization Institute ,ETSI Secretariat, Sophia Antipolis Cedex, France [4] �The GSM System for Mobile Communications� by Mouly and Pautet, Published by Cell & Sys.ISBN 2-9507190-0-7 [5]�Principles & Applications of GSM� by Vijay K. Garg and Joseph E. Wilkes, Prentice Hall Communications Engineering and Emerging Technology Series, ISBN 0-13-949124-4 [6]�An Introduction to GSM� by Redl, Weber and Oliphant, Artech House, ISBN 0-89006-785-6 [7]�Mobile Radio Communications� by Raymond Steele, Pentech Press Publishers and IEEE Press ISBN 0-7803-1102-7 [8]�Overview of Global System for Mobile Communications � by John Scourias , Web Document http:ccnga.uwaterloo.ca-jscouriaGSM index.html [9] �GSM System Engineering� by Mehrotra A. , Artech House, Boston.