Exercise 3.2Z: Components of the GSM System
The diagram shows the entire transmission path of the GSM,
- On the left side the transmitter,
- on the right side the receiver.
The representation only refers to voice transmission. In GSM data transmission, only the uppermost block (voice encoder or decoder) is replaced by another channel encoder/decoder (concatenation of two channel codes).
This task deals with some basic properties of
- voice encoder and voice decoder,
- convolution encoder and convolution decoder,
- interleaver and de–interleaver and
- encryption and decryption.
- The task belongs to the chapter Similarities between GSM and UMTS.
- Reference is also made to the chapter Characteristics of GSM.
(1) The answers 1 and 4 are correct:
- The goal of voice encoding (or speech encoding) is to compress data and thus reduce redundancy.
- This is a typical task of source coding.
(2) The answers 2 and 3 are correct:
- Convolutional coding is a form of channel coding that enables the receiver (decoder) to detect and possibly correct errors.
- The channel encoder adds (meaningful) redundancy to this, while the voice encoder removes irrelevant redundancy.
- In many cases, both components are implemented together or at least closely coordinated.
- This is called "Common Source– and Channel Coding".
(3) Only the answer 2 is correct:
- The convolution decoder has big problems if the transmission errors do not occur statistically independent, but bundled.
- Task of interleaver and de–interleaver is to break up such bundle errors and distribute them over a longer period of time.
- The redundancy is not changed by this procedure.
- In the AWGN channel, bit errors occur statistically independently, so that interleaver and de–interleaver can be dispensed with.
(4) The answers 1 and 4 are correct:
- Encryption and decryption – the counterpart on the receiving side – serve only to protect user data against unauthorized access.
- They are not used for error correction and do not add redundancy.
- A distinction is made between symmetric and asymmetric encryption. GSM mainly uses the first variant.