Unit 3 : Digital Cellular Mobile

Standards
Lecture 4: Signal Processing in GSM

S. B. Bhosle
Government Polytechnic, Yavatmal
GSM Signal Processing
 Speech Coding

Based on Residually Excited Linear Predictive Coder

The coder provides 260 bits for each 20 ms blocks of speech, which yields a bit
rate of 13kbps

This speech coder was selected after extensive subjective evaluation of various
candidate coders available in the late 1980s

takes advantage of the fact that in a normal conversation, each person speaks on
average for less than 40% of the time

Incorporation of a voice activity detector (VAD) in the speech coder

GSM systems operate in a discontinuous transmission mode (DTX) which provides
a longer subscriber battery life and reduces instantaneous radio interference since
the GSM transmitter is not active during silent periods.
 Speech Coding

A comfort noise subsystem (CNS) at the receiving end introduces a background
acoustic noise to compensate for the annoying switched muting which occurs due
to DTX.
TCH/FS, SACCH, and FACCH Channel
Coding

The output bits of the speech coder
are ordered into groups for error
protection, based upon their
significance in contributing to speech
quality.

Out of the total 260 bits in a frame,
the most important 50 bits, called
type Ia bits, have 3 parity check
(CRC) bits added to them.

This facilitates the detection of non-
correctable errors at the receiver
TCH/FS, SACCH, and FACCH Channel
Coding

The next 132 bits along with the first 53
(50 type Ia bits + 3 parity bits) are
reordered and appended by 4 trailing
zero bits, thus providing a data block of
189 bits.

This block is then encoded for error
protection using a rate 112 convolutional
encoder with constraint length K = 5

The least important 78 bits do not have
any error protection and are
concatenated to the existing sequence to
form a block of 456 bits in a 20 ins frame
TCH/FS, SACCH, and FACCH Channel
Coding

The error protection coding scheme
increases the gross data rate of the
GSM speech signal, with channel
coding, to 22.8 kbps
 Channel Coding for Data Channels

The coding provided for GSM full rate data channels (TCH/F9.6) is based on
handling 60 bits of user data at 5ms intervals, in accordance with the modified
CCITT V.110 modem standard.

240 bits of user data are applied with 4 tailing bits to a half-rate punctured
convolutional coder with constraint length K = 5.

The resulting 488 coded bits are reduced to 456 encoded data bits through
puncturing (32 bits are not transmitted), and the data is separated into four 114 bit
data bursts that are applied in an interleaved fashion to consecutive time slots.
Channel Coding for Control Channels

GSM control channel messages are 184 bits long

encoded using a shortened binary cyclic fire code followed by a half-rate
convolutional coder.

The fire code uses the generator polynomial
G 5 ( x )= x 40 + x 26 + x 23 + x 17 + x 3 + 1

It produces 184 message bits, followed by 40 parity bits

Four tail bits are added to clear the convolutional coder yielding a 228 bit data
block

This block is applied to a half-rate K = 5 convolutional code

The resulting 456 encoded bits are interleaved onto eight consecutive frames in
the same manner as TCH speech data.
 Interleaving

The total of 456 encoded bits within each 20 ma speech frame or control
message frame are broken into eight 57 bit sub-blocks to minimize the effect of
sudden fades on the received data

These eight subblocks which make up a single speech frame are spread over
eight consecutive TCH time slots.

If a burst is lost due to interference or fading, channel coding ensures that
enough bits will still be received correctly to allow the error correction to work

Each TCH time slot carries two 57 bit blocks of data from two different 20 ma
(456 bit) speech (or control) segments
 Ciphering

Modifies the contents of interleaved blocks through encryption technique

Known only to MS and BTS

Encryption algorithm is changed from call to call

Two types of encryption algorithms, called A3 and A5, are used to prevent
unauthorized network access and privacy for radio transmission respectively

A3 algorithm is used to authenticate each mobile by verifying users passcode
within SIM with the cryptographic key at MSC

A5 algorithm provides scrambling of 114 coded data bits sent in each TS
 Burst Formatting

Adds binary data to the ciphered blocks, in order to help synchronization and
equalization of the received signal.
 Modulation

The modulation scheme used by GSM is 0.3 GMSK where 0.3 describes the 3 dB
bandwidth of the Gaussian pulse shaping filter with

GMSK is a special type of digital FM modulation

Binary ones and zeros are represented in GSM by shifting the RF carrier by ±67.708
kHz

The channel data rate of GSM is 270.833333 kbps, which is exactly four tines the RF
frequency shift

This minimizes the bandwidth occupied by the modulation spectrum and hence
improves channel capacity.

The MSK modulated signal is passed through a Gaussian filter to smooth the rapid
frequency transitions which would otherwise spread energy into adjacent channels.
 Frequency Hopping

Under normal conditions, each data burst belonging to a particular physical
channel is transmitted using the same carrier frequency

If users in a particular cell have severe multipath problems, the cell may be
defined as a hopping cell by the network operator

In this case slow frequency hopping may be implemented to combat the
multipath or interference effects in that cell.

Frequency hopping is carried out on a frame-by-frame basis, thus hopping
occurs at a maximum rate of 217.6 hops per second

As many as 64 different channels may be used before a hopping sequence is
repeated.

Frequency hopping is completely specified by the service provider.
 Equalization

performed at the receiver with the help of the training sequences transmitted in
the midamble of every time slot.

The type of equalizer for GSM is not specified and is left up to the manufacturer
 Demodulation

The portion of the transmitted forward channel signal which is of interest to a
particular user is determined by the assigned TS and ARFCN

The appropriate TS is demodulated with the aid of synchronization data
provided by the burst formatting

After demodulation, the binary information is deciphered, de-interleaved, channel
decoded, and speech decoded.