In the last few posts i wrote about the Speech Transmission in GSM, later i was reading more on it then i found this brief from a book. Flow Diagram below shows the transmitting and receiving chain of a GSM receiver. Several successive operations have to be performed to convert a speech signal into a radio signal and back.
The following operations take place on the transmitting side:
- Source coding: Converts the analogue speech signal into a digital equivalent.
- Channel coding: Adds extra bits to the data flow. This way redundancy is introduced into the data flow, increasing its rate by adding information calculated from the source data, in order to allow detection or even correction of bit errors that might be introduced during transmission. This is described in more detail below.
- Interleaving: Consists of mixing up the bits of the coded data blocks. The goal is to have adjacent bits in the modulated signal spread out over several data blocks. The error probability of successive bits in the modulated stream is typically highly correlated, and the channel coding performance is better when errors are decorrelated. Therefore, interleaving improves the coding performance by decorrelating errors and their position in the coded blocks.
- Ciphering: Modifies the contents of these blocks through a secret code known only by the mobile station and the base station.
- Burst formatting: Adds synchronisation and equalisation information to the ciphered data. Part of this is the addition of a training sequence.
- Modulation: Transforms the binary signal into an analogue signal at the right frequency. Thereby the signal can be transmitted as radio waves.
The receiver side performs the reverse operations as follows:
- Demodulation: Transforms the radio signal received at the antenna into a binary signal. Today most demodulators also deliver an estimated probability of correctness for each bit. This extra information is referred to as soft decision or soft information.
- Deciphering: Modifies the bits by reversing the ciphering code.
- Deinterleaving: Puts the bits of the different bursts back in order to rebuild the original code words.
- Channel decoding: Tries to reconstruct the source information from the output of the demodulator, using the added coding bits to detect or correct possible errors, caused between the coding and the decoding.
- Source decoding: Converts the digitally decoded source information into an analogue signal to produce the speech.
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Satya Sravan
