GSM Radio Aspects

In GSM the uplink (mobile-to-base) frequency band is 890-915 MHz and the corresponding downlink (base-to-mobile)
band is 935-960 MHz , resulting in a 45 MHZ spacing for duplex operation. The GSM uses time division multiple access (TDMA) and frequency division multiple access (FDMA) , whereby the available 25 MHz spectrum is partitions into 124 carriers ( carrier spacing = 200KHz ) , and each carrier in turn is divided into 8 time slots ( radio channels ). Each user transmits periodically in every eighth time slot in an uplink radio carrier & receives a corresponding time slot on the downlink carrier. Thus several conversations can takes place simultaneously at the same pair of tansmit/receive radio frequencies. The radio parameters for GSM are summerised in the following table :

Table - Radio parameters and characteristics for GSM

In the GSM system a digitized speech is passed at 64 Kb/s through a speech coder ( transcoder ) , which compresses the
64 Kb/s PCM(pulse code modulated ) speech to a 13 Kb/s data rate.

The transcoder models the vocal tract of user and generate a set of filter parameters that are used to represent a segment of speech ( 20 ms long ) , and only the filter parameters and impulse input to the filter are transmitted on the radio interface.The speech coding improves the spectral efficiency of the radio interface thereby increasing a traffic capacity of the system (more users over a limited bandwidth ). The linear predictive , low bit rate (LBR) transcoder is based on Residual Pulse Excitation Long Term prediction(RPE-LTE) techniques. The GSM transcoder also permits the detection of silent periods in the speech sample , during which transmit power at the mobile station can be turned off to save power and extend battery life.

The transcoded speech is error protected by passing it through a channel encoder , which utilises both a parity code and a convolution code .The channel encoding increases the bit rate of transmitted speech from 13 kb/s (260 bits/20 ms of speech) to 22.8 kb/s (456 bits/20ms of speech) for GSM full rate coder (22.8 kb/s, 8 slots /frame ) . A half rate coder for GSM (11.4 kb/s,16 slots/frame ) has also been specified . The 456 bits of convolutionally encoded data (representing 20ms speech sample ) is then interleaved to combat the effect of burst error on the radio path. An interleaving depth of 8 is used in GSM for full rate speech , which requires that the channel encoded bits produced over two adjecent 20ms intervals
(2 X 256 bits) be split into 8 blocks (114 bits each) and transmitted over 8 frames . The interleaved data are then modulated by means of Gaussian Minimum Shift Keying (GMSK) and passed through a duplexer ,which provides filterring to isolate transmit and receive signals.

The digital signal proccessing in GSM radio leads to high capacity and better speech quality , it introduces additional delays. eg. , the delays introduced by speech coding (20ms),interleaving (37ms) ,analog-to-digital (A/D) conversion( 8ms) and procesing for transmission and switching ( 15ms) add up to 80ms.

Logical Channels

Logical Channel structure in GSM

There are 2 distinct catagories in logical channel structure of GSM

1. Traffic channel (TCH)

2. Control channel (CCH)

• Each traffic channel type is always associated with control channel. These channels used for communication of information ( speech or data ).
• The broadcast control channel (BCCH) is a unidirectional channel (base-to-mobile) that is used to broadcast information regarding the mobile's servicing cell as well as other cells.It may include FCCH and SCH to provide accurate tuning to BS and frame synchronisation resp.
• CCCH may be used either for uplink (mobile-to-base) or downlink (base-to-mobile). PCH ( for paging a mobile) and AGCH ( to assign dedicated resources to mobile ) channels operate in downlink direction.When in the idle mode , MS always listens to the paging channel of incoming calls.
• The DCCH are used for call setup or for measurement and handoff and are assigned to single mobile connection .The SACCH is either assigned to a TCH or to a SDCCH .The SACCH is deployed to transfer link quality and signal strength information in uplink direction and power control frame adjustment(timing) information in downlink direction . FACCH is assigned only to a TCH and relies on frame stealing to transfer handoff information during an active call. The SDCCH provides data transfer in both uplink and downlink direction for call setup or short messages and released on comletion of data transfer.

Frequency Hopping and discontinuous transmission

• To reduce the effect of multipath fading when a mobile station is stationary or slowly moving , GSM provides frequency hopping capability on the radio interface . Frequncy hopping is achieved in such a way that , according to the calculated sequence ( based on simple algorithm ) on both sides of interface , the MS sends and receives each time slot (burst) on a different frequency . Frequency hopping is applied only on the traffic and non-broadcast channels. It also provides security against unauthorised evesdropping on a call ,in progress.
• GSM utilises voice activated transmissions and discontinuous transmission (DTx) to maximise spectrum efficiency.The technique is based on detecting voice activity and switching on the trasnmitter only during the period when there is a active speech to transmit & switching off the transmitter during silent periods reduces interference in the air, thereby allowing the use of smaller frequency reuse clusters and reduces battery power consumption int the MS.Upto 50% gain can be acheived through DTx.
• An adaptive threshold voice activity detector (VAD) algorithm is used in GSM radio equipment to detect silent period sand empty frames are filled with comfort noise.