Signalling During a Call - Global System Mobile, GSM, 2G

Global System Mobile, GSM, 2G

7.7 Signalling During a Call

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 ...

...

26-frame of broadcast carrier

Logical T1 channel

D - Stand-alone Dedicated Control Channel (SDCCH)

F S D D D D D D F S D

51 TDMA frame, BCCH + CCCH + SDCCH, downlink

Figure 7.26 Signalling on dedicated downlink control channel during the call setup.

used for transmission of the calling and called-party numbers, for authentication, for sending encryption keys and so on. Finally, a traﬃc channel is allocated to the terminal.

For setting up a callfrom a terminal, the terminal sends a call request via the RACH.

The network sends back details of the allocated SDCCH on the AGCH. Further signalling takes place as described previously, the procedure is shown in Figure 7.25 and mapping of signalling frame to a broadcast carrier is shown in Figure 7.26.

94 Introduction to Mobile Network Engineering

T T T A T T T T T T A T T T

T T T A T T T T T T A T T

T T T A T T T T T T A T

T T T A T T T T T T A

T T T A T T T T T T

T T T A T T T T T

T T T A T T T T

T T T A T T T

T TT

TT TT

T TT

TT TT

T TT

TT TT

T TT

TT TT

T TT

TT TT

T TT

TT TT

TT T

TT T T

TT TT T

T TT

TT T T A T T T

AA AA

F S B C F S C C F S C

51 frames = 235 s

C F S C C F S C C

TCH1326 frames = 6.12 s

Figure 7.27 Sliding frames [4].

during each TDMA frame available for the MS to measure the level of a carrier from an adjacent cell. To ensure reliable readings averaging must be carried out over the fast fading. Several measurements are made of each carrier before the mean values are sent over the SACCH to the base. The terminal also needs to associate the BSIC for the measured neighbour cell. It means that terminal has to read the BSIC (Base-Station Identity Codec) transmitted on the neighbour SCH. The terminal measures and reads a neighbour carrier during the last, idle TDMA frame in the multiframe.

A complication here is that the base stations may not be mutually time synchronized.

On the other hand, the MS cannot read the neighbour BSIC if not synchronized to neighbour SCH. This means that the terminal has to listen to the BCCH on the broadcast carrier from an adjacent cell for an entire TDMA frame to capture a 0 time slot. This alone is not enough since the SCH uses only 1 in 10 of the 0 time slots. For this reason, two diﬀerent multiframes are used in GSM. The inward and outward traﬃc channels use multiframe A (26 basic TDMA frames), whereas the broadcast channels use multiframe B (51 frames). This means that the idle TDMA frame that is used for listening will slide over the TDMA frames in multiframe B, thus ensuring that, after a number of A multiframes, the terminal will have reached the correct 0 time slot in the broadcast channel of an adjacent cell, see Figure 7.27 [4].

7.7.2 Handover

Handover is a key feature of a cellular network supporting mobility of mobile user when it moves from the coverage area of one cell to that of another. The network switches an air-interface communication link from one cell to another by allocating radio and

other traﬃc resources in a new cell and possibly BSC and MSC depending on the type of handover. The reasons for handover decisions could be based either on poor signal quality or traﬃc congestion in serving cell (or selected cell during call setup).

7.7.2.1 Intra-Cell and Inter-Cell Handover

In case of an intra-cell handover, the MS does not leave the cell at all but is allocated to another carrier in the same cell. With an inter-cell handover, the MS is moved from one cell to another.

7.7.2.2 Intra- and Inter-BSC Handover

An inter-cell handover can be an intra or an inter-BSC handover. In the ﬁrst case, the target cell and the source cell are controlled by the same BSC. In the latter, source and target cells are located in diﬀerent BSC areas. The inter-BSC handover has to be handled by the MSC, but nevertheless the decision about it is made by the BSC controlling the source cell.

7.7.2.3 Intra- and Inter-MSC Handover

In case of an inter-BSC handover, the target cell might be located in a diﬀerent MSC area to the source cell. For such an inter-MSC handover, the current MSC/VLR must contact the target MSC/VLR and then transfer the call to it. In case of an intra-MSC handover, source and target cells belong to the same MSC area.

7.7.2.4 Intra- and Inter-PLMN Handover

Finally, the MS might be moved even from one PLMN to another one. This is called inter-PLMN handover or roaming.

7.7.2.5 Handover Triggering

The decision for a handover is made by the BSC on the basis of the uplink and downlink measurements taken by the MS and BS, respectively. Since the MS partly provides the necessary data or handover decision, the GSM handover was named the Mobile Assisted Handover (MAHO). When the decision about the MAHO is made, the BSC looks for a suitable target cell and moves the ongoing call to a new cell or transceiver (intra-cell handover).

The measured data on local radio conditions at the MS are transmitted to the BS via the SACCH. The following information is used by the MSC when deciding the best cell for handover:

• Carrier level and connection quality measured in Frame-Error Rate probability (both BS and MS) averaged over 12 s.

• Signal levels of neighbour carriers in the MS receiver (MS).

• Distance from BS calculated from the timing advance (BS).

• Interference level in the base receiver in idle time slots (BS).

During the call, the BSC continuously monitors these measurements taken by the MS and BS. Handover becomes mandatory when the signal level in the serving cell drops below a certain threshold. The next step in the handover process is to select a suitable target neighbour cell with a good downlink signal level. Given the fact that the radio signal

96 Introduction to Mobile Network Engineering

Received signal strength from serving BS (BS #1) Received signal strengthfrom neighbour BS (BS#2)

Handover (HO) margin

MS movement with time

MS served by BS#1 MS served by BS#2

Figure 7.28 Handover decision.

ﬂuctuates due to shadowing that is an important factor at the borders of the cell, some Handover Margin should be introduced in decision factors, as shown in Figure 7.28.

This is the case of power budget handover. The handover can also be triggered via timing advance data when a mobile moves too far from the base and the base is not able further adjust timing advance.

7.7.3 Power Control

Power control in the base and terminal transmitters reduces the average interference level to other cells and optimizes power consumption of mobile terminal. Power control decisions to increase or reduce transmit power are based on the carrier level and transmission quality at both the terminal and base. In addition to power control, GSM uses Discontinuous Transmission and Reception (DRX). During the natural gaps in incoming speech there is no need to use entire signal processing chain and transmit at the same power level as in presence of speech. The system use voice activity detector in both BS and the MS and generate a background noise at lowest but comfortable level during the pauses in speech. The ‘comfortable’ noise is transmitted via SACCH instead of traﬃc channel.

The DRX is also used to replace speech frames with too-low transmission quality.

The process is controlled by a channel decoder. When an isolated speech frame is dis- carded, the previous frame is repeated. If conditions persist, several repetitions of the speech frame will cause signiﬁcant degradation of quality. In that case, after each repe- tition the output level is progressively reduced down to 0 and comfort noise is inserted instead.

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