Exercise 3.2Z: Components of the GSM System

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GSM system components

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.

Notes:

• The task belongs to the chapter  Similarities between GSM and UMTS.
• Reference is also made to the chapter  Characteristics of GSM.

Questionnaire

1

Which statements apply to voice encoding or decoding?

	They are components of source coding.
	They are components of channel coding.
	The voice encoder adds redundancy.
	The voice encoder removes redundancy.

2

Which statements apply to the convolutional encoder/decoder?

	These are components of the source coding.
	These are components of the channel coding.
	Redundancy is added by the convolutional encoder.
	Redundancy is removed by the convolutional encoder.

3

What statements apply to the interleaver and de-interleaver?

	The interleaver adds redundancy.
	The de–interleaver is used to distribute bundling errors.
	The interleaver is most important in the AWGN channel.

4

What statements apply to encryption and decryption?

	Both serve data security in the sense of data protection.
	These components are used for error correction.
	The encryption adds redundancy.
	GSM usually uses symmetric encryption.

Solution

Solution

(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.