Air interface

The GSM Air Interface (Urn) provides the physical link between the mobile and the network. Some of the key characteristics of the air interface are given in Table 47.3. As already described, GSM is a digital system employing time division multiple access (TDMA) techniques and operating in the 900MHz band. The CEPT have made available two frequency bands to be used throughout Europe by the GSM system, namely;

1. 890MHz to 915MHz forthe mobile to base station (uplink)direction.

2. 935MHz to 960MHz for the base station to mobile (downlink)direction.

These 25MHz bands are divided into 124 pairs of carriers spaced by 200kHz. In addition, consideration is now being given to specif­ying additional carriers in a pair of extension bands 872MHz to 888MHz and 917MHz to 933MHz. Each of the carriers is divided up into eight TDMA timeslots of length 0.577ms such that the frame length is 4.615ms. The recurrence of each timeslot makes up one physical channel, such that each carrier can support eight physical channels, in both the uplink and downlink directions.

The timeslot allocation in either direction is staggered so that the mobile station does not need to transmit and receive at the same time. Data is transmitted in bursts within the timeslots and a number of different types ofburst can be carried as shown in Figure 47.10. The normal burst has a data structure as shown. It consists of148 bits of which 114 are available for data transmission, 26 are used for a training sequence which allows the receiver to estimate the radio propagation characteristics and to set up a dispersion equaliser, 6 bits as tail bits, and two stealing flags. These physical channels therefore provide a data throughput of 114 bits every 4.615ms or 24.7kbit/s.

The bursts modulate one of the RF carriers using Gaussian Mini­mum Shift Keying (GMSK) modulation with a BT index of 0.3. The allocation ofthe carrier can be such that frequency hopping is achieved, i.e consecutive bursts of a physical channel will be carried by differing RF carriers. This "hopping" is performed every TDMA frame, or every 4.615ms and provides extra protection against channel fading and co-channel interference.

A number oflogical channels can be carried by the physical channels described above. These are summarised in Table 47.4.

There are two categories of traffic channels; speech, whether full rate using 22.8kbit/s or half rate using 11.4kbit/s, and data, provid­ing a variety of data rates. There are four basic categories of control channels, known as the broadcast control channel (BCCH), the common control channel (CCCH), the standalone dedicated control channel (SDCCH) and the associated control channel (ACCH).

These are further divided into channels with specific purposes and for a detailed description of these channels the reader is referred to the GSM Recommendations published by ETS1.

Each of these logical channels is mapped onto the physical chan­nels, using the appropriate burst type as shown in Figure 47.10.

TDMA frames are built up into 26 or 51 frame multiframes, such that individual timeslots can use either of the multiframe types, and then into superframes and hyperframes as shown in Figure 47.10. The TCH and the associated ACCH uses the 26 frame structure, whilst the BCCH and CCCH use the 51 frame structure. The SDCCH may occupy one physical channel, providing 8 SDCCH, or may share a physical channel with the BCCH/CCCH. Typical ar­rangements for allocating the 8 physical channels could be:

1. 7 channels TCH and SACCH + 1 channelBCCH/CCCH/SDCCH

2. 6 channels TCH and SACCH + 1 channel BCCH/CCCH + 1channel SDCCH.

Each cell must have at least one physical channel assigned to the BCCH/CCCH, where there are 2 or more carriers per cell, the non-BCCH carriers may have all 8 channels allocated to TCH.

Table 47-3 GSM air interface parameters

Figure 47.10 GSM timeframes, timeslots and bursts (Extract from GSM Recommendation 05.01)

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