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					The GSM Radio Interface
Introduction
The radio interface is the interface between the mobile stations and the fixed infrastructure. It is probably the most important interface of the GSM system, as it is the key element to enable mobility and wireless access. One of the main objectives of GSM is roaming1. Therefore, in order to obtain a complete compatibility between mobile stations and networks of different manufacturers and operators, the radio interface must be completely defined. The spectrum efficiency depends on the radio interface and the transmission, more particularly in aspects such as the capacity of the system and the techniques used in order to decrease the interference and to improve the frequency reuse scheme. The specification of the radio interface has then an important influence on the spectrum efficiency.

Multiplexing and Multiple Access
Multiple Access Scheme Since the radio spectrum is a limited resource shared by all users, a method must to be devised to divide up the bandwidth among as many users as possible. The multiple access scheme defines how different simultaneous communications, between different mobile stations situated in different cells, share the GSM radio spectrum. A mix of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA), combined with frequency hopping, has been adopted as the multiple access scheme for GSM. The Figure 1 shows how the available 25MHz spectrum are divided into radio channels by the FDMA/TDMA multiple access scheme. The TDMA multiple access scheme on a carrier frequency together with the GMSK modulation used by GSM, a 200KHz carrier spacing is required to provide the necessary bit rate per carrier frequency. The 200kHz carrier spacing yields 124 carriers from the 25MHz spectrum allocation. Because some of the energy in a GMSK modulated signal lies outside the nominal 200KHz band, GSM recommends that carriers 1 and 124 will not normally be used (guard band of 200 KHz) in order to protect services using adjacent spectrum bands. These 124 possible carriers are defined for the uplink (Fu) and downlink (Fd) as follows: Fu(n) = 890.2 MHz + 0.2(n-1) MHz Fd(n) = 925.2 MHz + 0.2(n-1) MHz (1≤ n ≤ 124) (1≤ n ≤ 124)

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Roam: (In wireless communications) to move around freely and still be in contact with a wireless communications transmitter.  Dr. Dirk H Pesch, CIT, 2000-2002 1

Frequency ch 1 ch 2 ch 3 . . . . . ch 124

Slot 0

Slot 1

Slot 2

…

Slot 7

Time 1 Frame = 8 timeslots Frame duration = 4.615 ms timeslot duration = 0.577 ms Figure 1. FDMA/TDMA based radio channel concept TDMA Frame Structure Each of the frequency carriers is divided into frames of 8 timeslots of approximately 577µs (15/26 µs) duration with 156.25 bits per timeslot. The duration of a TDMA frame is 4.615ms (577µs x 8 = 4.615 ms). The bits per timeslot and frame duration yield a gross bit rate of about 271kbps per TDMA frame. TDMA frames are grouped into two types of multiframes: 26-frame multiframe (4.615ms x 26 = 120 ms) comprising of 26 TDMA frames. This multiframe is used to carry traffic channels and their associated control channels. 51-frame multiframe (4.615ms x 51 ≅ 235.4 ms) comprising 51 TDMA frames. This multiframe is exclusively used for control channels. The multiframe structure is further multiplexed into a single superframe of duration of 6.12sec. This means a superframe consists of 51 multiframes of 26 frames. 26 multiframes of 51 frames. The last multiplexing level of the frame hierarchy, consisting of 2048 superframes (2715648 TDMA frames), is a hyperframe. This long time period is needed to support the GSM data encryption mechanisms. The frame hierarchy is shown in Figure 2.

 Dr. Dirk H Pesch, CIT, 2000-2002

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Figure 2. GSM frame hierarchy

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CHANNEL TYPES Physical channel: channel defined by specifying both, a carrier frequency and a TDMA timeslot number. It is important to note that the frame structure used on each physical channel is independent of those on the other channels, most notably those with the same carrier frequency assignment but different timeslot designations. This means that the physical channels using the same carrier frequency (8 of them, differentiated by their timeslot numbers) do not have to use the same multiframe type. Logic channel: logical channels are multiplexed into the physical channels. Logical channels are, so to speak, laid over the grid of physical channels. Each logic channel performs a specific task. Consequently the data of a logical channel is transmitted in the corresponding timeslots of the physical channel. During this process, logical channels can occupy a part of the physical channel or even the entire channel. There are two different types of logical channel within the GSM system: Traffic channels (TCHs). Control channels (CCHs). Traffic channels. Traffic channels carry user information such as encoded speech or user data. Traffic channels are defined by using a 26-frame multiframe. Two general forms are defined: i. Full rate traffic channels (TCH/F), at a gross bit rate of 22.8 kbps (456bits / 20ms) ii. Half rate traffic channels (TCH/H), at a gross bit rate of 11.4 kbps. Uplink and downlink are separated by three slots (bursts) in the 26-multiframe structure. This simplifies the duplexing function in mobile terminals design, as mobiles will not need to transmit and receive at the same time. The 26-frame multiframe structure, as shown in Figure 3, multiplexes two types of logical channels, a TCH and a Slow Associated Control CHannel (SACCH). However, if required, a Fast Associated Control CHannel (FACCH) can steal TCH in order to transmit control information at a higher bit rate. This is usually the case during the handover process. In total 24 TCH/F are transmitted and one SACCH. The logical channels are multiplexed as shown in Figure 3. The last frame is unused when full rate traffic channels are used to transmit user information. This idle frame allows the mobile station to perform other functions, such as measuring the signal strength of neighbour cells for the handover process. However, when user data is transmitted in half rate traffic channels, the idle frame carries the SACCH of the second TCH/H.

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26-frame multiframe T1 T2 T3 T4 T5 T1 T1 A T1 0 1 2 T2 T2 T2 I 1 2 3

Tn: time frame number n for traffic data. A: slow associated control channel. I: idle frame. Figure 3. Traffic channel multiframe. Control channels Control channels carry system signalling and synchronisation data for control procedures such as location registration, mobile station synchronisation, paging, random access etc. between base station and mobile station. Three categories of control channel are defined: Broadcast Common Dedicated Control channels are multiplexed into the 51-frame multiframe as shown in DOWNLINK (51 time frames) F S B B B B C C F S C C C C C C F S C C C C C C C C I

F: frequency correction channel S: synchronisation channel B: broadcast control channel C: common control channel I: idle frame Figure 4. Control channel multiframe Broadcast Channel (BCH) As the name implies, these channels are transmitted by the BS to many MSs in order to allow MSs to synchronise to the network, acquire system status information, etc. Broadcast channels are only transmitted on the downlink. The following broadcast channels are defined: Frequency Correction Channel (FCCH) - provides MSs with the frequency reference of the system to allow synchronisation with the network and frequency drift correction. Synchronisation Channel (SCH) - provides frame synchronisation for MSs and identification of the BS. The SCH transmits the training sequence that is needed for link quality estimation and equalisation.

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Broadcast Control Channel (BCCH) – provides system parameters needed to identify and access the cellular network. Common Control Channel (CCCH)

The CCCH consist of a combination of common control channel types and is used between MS and BS before a dedicated control channel has been allocated. There are three downlink only channels, for MS paging, access grant and cell broadcast and one uplink only channel, for random access attempts. − Paging Channel (PCH) - This channel exists only on the downlink and is activated for the selective addressing of a called mobile terminal during a connect request from the network (incoming call). - Random Access Channel (RACH) - This access channel only occurs on the uplink and allows MSs, using the S-ALOHA access protocol, to access the network and request channel capacity from the BS to establish a connection. - Access Grant Channel (AGCH) - This channel is transmitted only on the downlink by a BS in response to a RACH from the MS. In accordance with the call setup mechanism selected by the network operator, the MS is allocated an SDCCH or a TCH through the AGCH. - Cell Broadcast Channel (CBCH) – This channel is only transmitted on the downlink and carries broadcast messages containing information such as traffic information, weather news, etc. The channel is received by all or only a limited number of mobile stations. Dedicated Control CHannel (DCCH) DCCH are used for control signalling message exchange between a mobile and the network. Dedicated control channels can be transmitted in either the 26-frame or the 51frame multiframe. Three basic types exist - Stand-alone Dedicated Control CHannel (SDCCH) - This channel is always used when a traffic channel has not been assigned and is allocated to a MS only as long as control information is being transmitted. Control information transmitted on the SDCCH includes registration, authentication, location area updating and data for call setup. - Slow Associated Control CHannel (SACCH) - This channel is always associated with a TCH or a SDCCH and maps onto the same physical channel. The SACCH carries general information between MS and BS, e.g. measurement reports sent by the MS giving details of the current and neighbour radio signal strength. - Fast Associated Control CHannel (FACCH) – This channel carries the same signalling data as the SDCCH. A FACCH will only be assigned when a SDCCH has not been assigned and will obtain access to the physical resource by ‘stealing’ frames from the traffic channel with which it is associated. Stealing TCH increases the signalling data rate available on the radio link in order to rapidly transmit control

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signalling messages that do not tolerate long delays such as handover signalling for example. Burst Types The modulating bit rate for a GSM carrier is 270.8 Kbits/s (1625/ 6 Kbits/s) which means that the timeslot of approximately 577µs corresponds to 156.25 bits. During the time slot period a frequency carrier is modulated by a stream of data bits organised into blocks, which are called a burst. A burst is made up of a useful part and a guard period. In the guard period nothing is transmitted and its purpose is to allow for a variation in the arrival time of the radio signal. The guard period avoids that adjacently (in time) received burst have their useful parts overlapping. If the useful parts of two received bursts overlap then their data will be corrupted. In addition to the guard period an adaptive time alignment process is used by the MS for all bursts except the access burst. This is used to vary the time of start of transmission of bursts to avoid overlapping with adjacent bursts at the receiver, e.g. BS. Five types of bursts are defined by GSM, four full bursts (i.e. with long useful parts) and one short burst. Normal burst – This burst consists of 8.25 bits guard period and 116 data bits (see Figure 5). The data bits may be encrypted, if encryption has been initiated by the network. The coded data bits correspond to two groups of 57 bits each, containing signalling or user data. The stealing flags (S) indicate to the receiver whether the information carried by the burst is traffic or signalling data. The training sequence has a length of 26 bits and is used for channel quality estimation and equalisation. The remaining bits are used as start and stop tail bits. These tail bits are a group of three bits set to zero and placed at the beginning and the end of the burst. The tail bits are used to cover the periods of ramping up and down of the mobile transmitter’s power amplifier.
Start(3) Tail bits Encrypted data (58) 57 bits data + 1 Stealing flag training (26) encrypted data (58) 57 bits data + 1 Stealing flag stop (3) Tail bits Guard period (8.25)

Figure 5. Normal burst Frequency correction burst – This burst has the same guard period and start and stop bits as the normal burst, but instead of a sequence of encrypted data bits and training sequence, the remaining bits (142) form a fixed series of zeros. This burst is used for frequency synchronisation of the mobile (see Figure 6).
Start(3) fixed bits (142) Stop (3) guard period (8.25)

Figure 6. Frequency correction burst

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Synchronisation burst - This burst is used for time synchronisation of the mobile station. It contains the same start and stop bits as the frequency correction burst but uses an ‘extended training sequence’ of 64 bits and a consequently reduced set of data bits (see Figure 7).
start(3) encrypted data (39) Extended training (64) Encrypted data (39) stop (3) guard period (8.25)

Figure 7. Synchronisation burst Dummy burst – This burst has the same structure as the normal burst but it carries no data. It is used in place of a normal burst when no other data need be sent. Access burst – This burst is used by the mobile for initial access to the system. The burst is shorter than the others, because it does not require the MS to be fully synchronous with the BS. It is characterised by an extended guard period of 68.25 bits to cater for burst transmission without knowledge the timing advance. The maximum spatial distance accommodated by the 68.25 bits is ca. 75.6km, which is well in excess of the maximum cell radius anticipated for GSM radio cells.
extended start(8) synch. seq. (41) encrypted data (36) Stop (3) extended guard period (68.25)

Figure 8. Access burst Typical Channel Combinations Logical channels are not simply distributed over the two types of multiframes, but follow a particular pattern. The following list is a set of approved channel combinations (CC) represented by their multiframes: CC1: CC2: CC3: CC4: CC5: CC6: CC7: TCH/F + FACCH/F + SACCH/TF TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1) TCH/H(0) + FACCH/H(0) + SACCH/TH(0) + TCH/H(1) FCCH + SCH + BCCH + CCCH FCCH + SCH + BCCH + CCCH + SDCCH/4(0,1,2,3) + SACCH/C4(0,1,2,3) BCCH + CCCH SDCCH/8 + SACCH/8

NOTE: The numbers in brackets after the logical channels give the numbers of the logical subchannels. This means that in the case of SDCCH/4 there are four SDCCHs on a physical channel, each of which is used by one MS. A physical channel contains exactly one of these combinations. The shared use of a physical channel by several logical channel types does not mean that all these channels can be used at the same time. Although the mapping of combinations of logical channels onto a physical channel may mean that several logical channels can appear on one physical channel, they occur sequentially at intervals of at least one TDMA frame length. GSM 05.02 specifies the multiplexing on the GSM radio interface and the following tables indicate example

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multiplexing sequences of logical channels onto the physical channels in the respective multiframes.
T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 A0 T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 T0 T1 A1 T0: TCH/H(0) T1: TCH/H(1) A0: SACCH/TH(0) A1: SACCH/TH(1)

26-frame multiframe

Figure 9. Channel combination 2: TCH/H(0,1) + FACCH/H(0,1) + SACCH/TH(0,1)
Downlink F S B B B B C C C C F S C C C C C C C C F S D0 D0 D0 D0

D1 D1 D1 D1 F F S

S D2 D2 D2 D2 D3 D3 D3 D3 F C C C F S C C C

S A0 A0 A0 A0 A1 A1 A1 A1 C C C C C F S D0 D0 D0 D0

B B B B C

D1 D1 D1 D1 F Uplink

S D2 D2 D2 D2 D3 D3 D3 D3 F

S A2 A2 A2 A2 A3 A3 A3 A3 -

D3 D3 D3 D3 R R A2 A2 A2 A2 A3 A3 A3 A3 R R R R R R R R R R R R R R R

R R R R

R R R R D0 D0 D0 D0 D1 D1 D1 D1 R R D2 D2 D2 D2 R R R R

D3 D3 D3 D3 R R A0 A0 A0 A0 A1 A1 A1 A1 R R R R R R R R R R R R R R R F: FCCH F: FCCH F: FCCH F: FCCH

R R R R D0 D0 D0 D0 D1 D1 D1 D1 R R D2 D2 D2 D2 A0: SACCH/C4(0) A1: SACCH/C4(1) A2: SACCH/C4(2) A3: SACCH/C4(3) R: RACH

D0: SDCCH/4(0) D1: SDCCH/4(1) D2: SDCCH/4(2) D3: SDCCH/4(3)

2 x 51-frame multiframe

Figure 10. Channel combination 5: FCCH + SCH + BCCH + CCCH + SDCCH/4 (0, 1, 2, 3) + SACCH/C4(0, 1, 2, 3) Channel Combinations Depending on Anticipated Cell Load The BS provides a cell with a set of logical channels occupying several physical channels. On the bases of anticipated traffic load of a cell, the network operator establishes a channel configuration that must adhere to the rules mentioned in the previous section. Each individual transceiver (TRX) can offer eight channel combinations in each time slot. The timeslot is identified through a timeslot number (TN). Three common combinations are present: Low capacity cell with one TRX

- TN 0:

FCCH + SCH + BCCH + CCCH + SDCCH/4(0,1,2,3) +

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TN1 to TN7:

SACCH/C4(0,1,2,3) TCH/F + FACCH/F + SACCH/TF

Medium capacity cell with four TRX

- Once (on TN 0): - Twice (on TN2 and TN4):
29 times: Once on TN0: Once on TN2: Once on TN4: Once on TN6: 5 times: 87 times:

FCCH + SCH + BCCH + CCCH SDCCH/8 + SACCH/8 TCH/F + FACCH/F + SACCH/TF

High capacity cell with 12 TRXs FCCH + SCH + BCCH + CCCH BCCH + CCCH BCCH + CCCH BCCH + CCCH SDCCH/8 + SACCH/8 TCH/F + FACCH/F + SACCH/TF

1. Notice that a BCCH always appears in TN 0 together with the logical channels SCH and FCCH. 2. Additional combinations CC6 are added when traffic is expected to be heavy.

Channel Coding and Interleaving
All mobile radio systems, unlike fixed transmission links, are characterised by a widely changing received signal strength as the mobile moves. This is due to the nature of radio propagation comprising of path loss, shadow fading and multipath fading. To protect the transmission against errors, forward error correction (channel coding and decoding) is employed in the GSM system. Obviously, there is a penalty to be paid for channel coding in increased bit rate requirements and GSM has been designed to achieve an ‘optimum’ compromise between overhead and protection. GSM employs both block coding and convolutional coding for FEC. GSM’s implementation of the two coding techniques is briefly summarised in the following. Block Coding Block coding is not extensively used in GSM, as the main channel coding technique is convolutional coding. However, parity codes are used on the control channels to add additional error protection before convolutional encoding. The main block code in use is a Fire code, which adds 40 bits of redundancy to a layer 2 data block on FACCH, SACCH, SDCCH, BCCH, AGCH, and PCH. The advantage of the Fire code is that it offers good error detection and/or correction properties when errors occur in bursts. The generator polynomial of the Fire code used in GSM is P( X ) = ( X 23 + 1)( X 17 + X 3 + 1) . This Fire code allows groups of errors of up to 11 bits to be detected and corrected. The class I bits of a 20ms speech frame are also protected by a 3 bit parity code before submission to the convolutional coder.

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Convolutional Coding Convolutional coding is the main FEC technique used in GSM as it has good performance, that is the error capability is high considering the extra redundancy that the code adds. However, compared with block codes, convolutional codes do have one disadvantage in that they are more difficult to decode. The decoding of a single bit depends on the values of all previous k⋅K decoded bits, and hence, heuristic procedures such as the Viterbi algorithm are being used. The convolutional codes used in GSM have a rate of 1/2, 1/3 and 1/6 as well as 244/456. The latter rate is achieved by puncturing 32 code bits in order to reduce the bits per block from 488 to 456. All convolutional codes used in GSM have a constraint length of K = 5. For a detailed specification of the convolutional generator polynomials refer to the GSM specification GSM 05.03, available for download free of charge from www.etsi.org. Interleaving As mentioned in the first part of this course, bit errors in mobile radio systems are largely due to signal fading. The fades occur at a much slower rate than the transmission bit rate of the GSM radio interface and hence errors tend to occur in bursts. Radio reception is, therefore, characterised by periods of high error rates with longer gaps of very low error rates. In order for a convolutional code to provide efficient error protection, the errors have to be evenly distributed in time. Scrambling and interleaving are ways in which this is achieved in GSM. The aim of the interleaving algorithm is to avoid the risk of loosing consecutive data bits. Figure 12 depicts the interleaving concept on a full rate TCH.
0 8 M 440 448 1 9 M 441 449 2 10 M 442 450 3 11 M 443 451 4 12 M 444 452 5 13 M 445 453 6 14 M 446 454 7 15 M 2 10 … 442 450 447 455 3 11 … 443 451 4 12 … 444 452 5 13 … 445 453 6 14 … 446 454 7 15 … 447 455 Burst N+2 (even bit) Burst N+3 (even bit) Burst N+4 (odd bit) Burst N+5 (odd bit) Burst N+6 (odd bit) Burst N+7 (odd bit) 0 8 … 440 448 1 9 … 441 449 Burst N (even bit) Burst N+1 (even bit)

Figure 11. 57 bit block assembly for interleaving The channel encoding process yields blocks of 456 bits of user data or control signalling information every 20 ms. In order to spread the bits in the 456 bit block so that bursts of errors are avoided, 8 sub-blocks are created as shown in Figure 11. A sub-block is defined as either the odd- or the even-numbered bits of the coded data within one burst. Each sub-block of 57 bits is carried by a different burst and in a different TDMA frame as shown in Figure 12. That means each burst contains the contribution of two successive speech bocks A and B. Single bit flags are added to

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indicate when either the odd or even bits have been stolen by a FACCH for signalling purposes. Coding for full and half rate data channels is defined in a slightly different way. The main difference is that an interleaving depth of 19 is used in most cases as shown Table 1. This interleaving depth results in long transmission delays, which can be tolerated for data traffic but not for real-time speech traffic.

Figure 12. Interleaving speech frames onto TDMA frames

Table 1. Coding methods for logial channels Channel Type TCH/FS Class I Class II TCH/F9.6 TCH/F4.8 TCH/H4.8 TCH/F2.4 TCH/H2.4 FACCHs SDCCHs, SACCHs BCCH, AGCH, PCH RACH SCH Bit/Block Data+Parity+Tail 182 + 3 + 4 78 + 0 + 0 4 * 60 + 0 + 4 60 + 0 + 16 4 * 60 + 0 + 4 72 + 0 + 4 72 + 0 + 4 184 + 40 + 4 184 + 40 + 4 184 + 40 + 4 8+6+4 25 + 10 + 4 Convolutional Coding Rate 1/2 244/456 1/3 244/456 1/6 1/3 1/2 1/2 1/2 1/2 1/2 Bit/ Block 456 (378) (78) 456 228 456 456 228 456 456 456 36 78 Interleaving depth 8

19 19 19 8 19 8 4 4 1 1

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Modulation
Modulation is the process where the baseband data bits are converted into an analogue signal at the carrier frequency. The analogue signal is designed to match both the transmission requirements of the medium and any imposed by the system design and operation. GSM uses a digital modulation techniques called Gaussian Minimum Shift Keying (GMSK). The advantages of digital modulation are as follows Greater noise immunity. Robustness to channel impairments. Easier multiplexing of various forms of information and greater security. They accommodate digital error-control codes, which detect and correct transmission errors and support complex signal conditioning and processing techniques to improve the performance of overall communication link. The following are the main requirements of the GSM modulation scheme Frequency translation into the correct band. Relatively narrow bandwidth to allow good spectrum efficiency. Constant envelop to allow the use of simple and efficient power amplifiers (e.g. class C). Low out- of -band radiation so that adjacent channel interference is low. Minimum Shift Keying (MSK), which is binary digital frequency modulation, achieves the first three of the above requirements. Unfortunately it fails to meet the fourth requirement, having excessive out-of-band radiation. From the point of view of mobile radio use, the out-of-band radiation power in the adjacent channel should be generally suppressed to 60 dB - 80dB below that in the desired channel. However, by pre-filtering the modulating signal it is possible to reduce out-of-band radiation whilst retaining the constant envelope property. It has been found that a Gaussian premodulation filter (see Figure 13) produces a signal, Gaussian MSK (GMSK), with the desired properties. Baseband Gaussian pulse shaping makes the phase trajectory of the MSK signal smoother and hence stabilises the instantaneous frequency variation over time. This has the effect of considerably reducing the side lobe levels in the transmitted spectrum. The bit stream of Figure 14 is separated in even stream and odd stream. This is shown in the next figure (FIG.12). Once we have the bit streams separated, we filter them (FIG.13). Note the effect of Gaussian filtering has smoothened the sharp phase transitions. Finally, the power spectrum density as a function of normalised frequency, with BT as a parameter, is shown in FIG.15. Note that as BT decreases, the spectral efficiency decreases but only at the expense of increased the central lobe. A normalised bandwidth of BT is 0.3 is used in GSM. This BT value is a compromise between BER (Bit Error Rate) and out-of-band interference.

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Figure 13. Impulse response of Gaussian filter

Figure 14. Random bit stream

Figure 15. Random bit stream divided into even and odd streams  Dr. Dirk H Pesch, CIT, 2000-2002 14

Figure 16. Bit streams (even and odd) before and after filtering

Figure 17. Modulated streams

Figure 18. Power spectral densities for different values of BT  Dr. Dirk H Pesch, CIT, 2000-2002 15

Timing Advance Mechanism
For technical reasons, it is necessary that the MS and BS do not transmit simultaneously. Therefore, the MS is transmitting three timeslots after receiving data from the BS. The time between sending and receiving data is used by the MS to perform various tasks such as processing data, measuring signal quality of the receivable neigbour cells, etc. As shown in Figure 19 the MS actually does not send exactly three timeslots after receiving data from the BS. Depending on the distance between MS and BS, a considerable propagation delay needs to be taken into account. That propagation delay, known as timing advance (TA), requires the MS to transmit its data a little earlier as determined by the “three timeslot delay rule”. Without timing advance, bursts transmitted by two different MSs in slots adjacent in time would overlap and interfere with each other. 3 TSs

Receivi Sending

TS 0 TS 1 TS 2 TS 3 TS 4 TS 5 TS 5 TS 6 TS 7 TA
The actual point in time of the transmission is shifted by the Timing Advance.

TS 1 TS 2

Figure 19. Receiving and sending from the perspective of the MS

Control Signalling on the GSM Radio Interface
Layer 2 LAPDm Signalling The only GSM specific signalling protocols of OSI layers 1 and 2 can be found on the radio interface. Layer 2 signalling employs a modified version of the ISDN layer two signalling protocol, LAPD, that is called LAPDm (m for modified). The protocol is related two other layer 2 protocols such as HDLC and LAPB. In the development of the LAPDm protocol, the LAPD protocol was taken and all dispensable parts were removed to save resources. The TEI and FCS parts of the LAPD frame are not in use as other GSM processes such as FEC take care of these functions. LAPDm uses three frame formats − A-format – This frame can be sent on any DCCH and does not carry any higher layer data but is used for filling. − B-format – This frame carries the actual signalling data on the radio interface and is transmitted in every DCCH and ACCH. The maximum length of Layer 3 information is restricted based on the logical channel and is defined by parameter N201.  Dr. Dirk H Pesch, CIT, 2000-2002 16

− Bbis-format – This frame format is used for transmission on BCCH, PCH, and AGCH. This frame does not have any address field as this is not required on a broadcast channel.

Figure 20. LAPDm frame format  Dr. Dirk H Pesch, CIT, 2000-2002 17

A-format and B-format frames are send in both uplink and downlink, whereas Bbis frames are only sent on the downlink. The maximum frame length is 23bytes = 184bits, which is the length of the Layer 2 data block into channel coding (see above). Figure 20 shows the LAPDm frame format and coding of fields for the several message types used by the protocol. The LAPDm address field has as its main element the SAPI through which the layer 3 message was received. On the radio interface two values of SAPI are used, SAPI = 0 for messages from the Radio Resource Management (RR), Mobility Management (MM), and Call Control (CC), and SAPI = 3 for message from the SMS and Supplementary Services (SS) messages. The Control field is used in the same fashion as in HDLC or LAPB and contains the sequence and retransmission counters N(S) and N(R), respectively. The frame length field contains the length of the layer 3 message within the information field of the LAPDm frame. If the message is less than the length specified in parameter N201 of the radio interface, fill-in octets are used to make up for the space. If the layer three message to be transmitted is longer than N201, segmentation occurs. Whether segmentation has occurred or not is indicated in the M-bit of the length field. Layer 3 Signalling Layer 3 signalling on the GSM radio interface contains control message exchanges between a number of protocol control processes. These processes are Call Control (CC), Mobility Management (MM), Radio Resource management (RR), as well as Short Message Service (SMS) management and Supplementary Services (SS) management. SS contains functions such as call waiting, call forwarding, group call, called party identity, etc. One can debate at length whether the GSM radio interface layer 3 is equivalent to OSI network layer or OSI application layer. We leave it to the reader to get involved into the debate if so desired and concentrate here on a brief description of the structure of layer 3 messages. A layer 3 message consists of three fields, the first field is the Type ID, the second field is the Message Type, and the third field is a Data field. The Type ID consists of a 4 bit Protocol Discriminator (PD), which identifies the control process that originated this message and a Transaction Identifier (TI), which is used to identify concurrent transactions between two layer 3 processes. The Message Type field consists of an 8 bit code that identifies the type of message sent. The Data field is of variable length and contains Information Elements (IE), which convey the data to the receiver. Each message data consists of mandatory and optional IEs.

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Figure 21. Layer 3 message format on the GSM radio interface

Table 2. Selected Radio Resource management messages Message ID (Hex) Name CHANnel REQuest Direction MS→ BTS Description CHAN_REQ is used to request a channel when in idle state. The message contains the reason for the request and the preferred channel type. This message is sent in an access burst HND_ACC is sent on a new traffic channel to perform a handover. This message is also sent in access bursts. This message contains system information that is sent in a BCCH. The CHAN_REL message is used in order to release the radio resource on when a connection is disconnected. PAG_REQ is sent to alert a MS of an incoming call. PAG_RSP is the first message sent by the MS on the SDCCH as a reply to a PAG_REQ message. After an unsuccessful handover the MS sends a HND_FAI message over the still existing link to the old BTS.

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HaNDover ACCess SYStem INFOrmation 2bis CHANnel RELease PAGing REQuest PAGing ReSPonse HaNDover FAIlure

MS→ BTS

02

BTS→ MS

0D

BTS→ MS

21 27

BTS→ MS MS→ BTS

28

MS→ BTS

 Dr. Dirk H Pesch, CIT, 2000-2002

19

Message ID (Hex) 29

Name ASSignment COMplete

Direction MS→ BTS

Description The MS confirms that it has successfully changed to a new traffic channel that was previously assigned by an ASS_CMD message. Channel assignment for handover in which the BTS changes is always performed with HND_CMD. Assignment of a traffic channel in the case of an intra-cell handover or during call setup. The BTS informs the MS with CIPH_MOD_CMD that all data in both, uplink and downlink, are to be encrypted.

2B

HaNDover CoMmanD ASSignment CoMmanD CIPHering MODe CoMmanD

BTS→ MS

2E

BTS→ MS

35

BTS→ MS

 Dr. Dirk H Pesch, CIT, 2000-2002

20


				
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