What is Bit Rate in Gsm?

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What is the BIT-RATE at which the GSM phone transmits data? 

  • 270 kbps shared by 8 users.

  • When you receive a mobile call, the base station transmits at 270 kbps by combining the speech data of 8 telephone calls and signaling data.

  • Actually there are 8 time-slots defined, where each time-slot carries the speech data for one speech call (13 kbps data) plus some over-head bits.

What is the BIT-RATE of Speech in  GSM mobile Phone? 

  • 13 kbps. [full rate]
  • 6.5 kbps [half rate]
  • Compare the above figures with that of 64 kbps used in a fixed phone case.

  • In cellphone, we do not have sufficient bandwidth capacity of the radio freq carrier and therefore we had done some tech of reducing the speech rate of speech used(by degrading the quality of speech to some extent).

Numbers we may remember

  • The digital bit rate of speech in the MSC=64kbps.

  • The digital bit rate used in mobile phone=13 kbps.

  • The digital bit rate used in mobile phone=6.5kbps.

  • Bit rate of signal carried by one GSM carrier=270.8kbps.

  • Bandwidth of one RF carrier required to carry 270.8kbps=200 kHz.

  • One RF carrier carries speech from BTS for 8 mobile together.

SPEECH PROCESSING

  • SPEECH IS DIVIDED INTO 20MS SAMPLES, EACH OF WHICH IS ENCODED AS 260 BITS GIVING A TOTAL BIT RATE OF 13 kbps.

STEPS:

                A to D conversion

                PCM sample blocks

                LPC=linear prediction coding

                LTP=long term prediction

                RPE=regular phase excited      

Speech Encoding

 

  • We shall start with a raw voice signal fed into the microphone, travel through the various stages involving vocoding, channel coding etc till it reaches the final burst format on the Air Interface.

 

Channel coding

 

  • Channel Coding is done to protect the logical channels from transmission errors introduced by the radio path.

  • The coding schemes depend on the type of the logical channels, hence the coding can differ from speech, control and data .

Convolution coder CC

  • The Convolution coder is a series of shift registers implemented using logic gates, where for every one input bit we get 2 output bits. Hence it is called ½ coder.

  • Here k=5 is the constraint length, it means thereare 5 shift register and each bit has memory depthof 4 , meaning it can influence the output of up to four next successive bits. This is useful during reception as bits can be derived even if a few consecutive bits are lost due to errors or corruption.

Data Channel Coding

  • The data bits are received in blocks of 240 bits. These are directly convolution coded after adding 4 tail bits.

  • The output of the CC is now 488 bits, which actually increases the bitrate to 24.4 Kbps.

  • To keep the bitrate constant on the air interface we need to puncture the output of the CC. Hence, we have a final bitrate of 22.8 Kbps again .

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