GSM Wireless Communications (2)

GSM Wireless Communications (2)

GSM

Multiple access technology:

TDM­A is the used access technology for Global System for Mobile communications (GSM). 

TDMA is the access method used by the Electronics Industry Alliance and the Telecommunications Industry Association for Interim Standard 54 (IS-54) and Interim Standard 136 (IS-136). Using TDMA, a narrow band that is 30 kHz wide and 6.7 milliseconds long is split time-wise into three time slots.

Narrow band means "channels" in the traditional sense. Each conversation gets the radio for one-third of the time. This is possible because voice data that has been converted to digital information is compressed so that it takes up significantly less transmission space. Therefore, TDMA has three times the capacity of an analog system using the same number of channels. TDMA systems operate in either the 800-MHz (IS-54) or 1900-MHz (IS-136) frequency bands.

GSM Transmission Process:

§  Analog to Digital Conversion

Analog to digital conversion takes place in 3 steps:

          1. Sampling

          2. Quantization

          3. Coding

§  Segmentation

§  Speech Coding

         

               Instead of using 13 bits per sample as in A/D conversion, GSM

             speech coding uses 260   bits to encode one segment.

             This calculates as 260 bits / 20 ms = 13 kb/s. This provides a

              speech quality which is acceptable for mobile telephony and

              comparable with wire line PSTN phones.

§  Channel Coding

      

speech coding does not consider the problems which may be             encountered on the radio transmission path. The next stages in the transmission process, channel coding and interleaving, help to overcome these problems.

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