U. Glaeser

Point B
FIGURE 42.49 Fundamental building blocks in a communication system.
42 kbps
Point A
Point B
FIGURE 42.50 Relationship between speech signal and the transmitted signal.
Second generation (2G) cellular standards required the introduction of optimized instructions for
speech processing and for communication algorithms used in the channel coding and modulation/demodulation.
The fundamental components of a wireless system are shown on Fig. 42.49.
Speech Coding
The source coder/decoder in 2G cellular standards (GSM, IS-136, IS-95 CDMA, Japanese PDC) is mainly
a speech coder/decoder. The main function of a speech coder is to remove the redundancy and compress
the speech signal and hence, reduce the bandwidth requirements for storage or transmission over the
air. The required reduction in bit rate is illustrated in Fig. 42.50 for the Japanese PDC standard.
At point A, a “toll quality” digital speech signal requires the sampling of the analog speech waveform
at 8 kHz. Each sample requires 8 bits of storage (µ-law compressed) thus resulting in a bit rate of
64 kbits/s or 2560 bits for one 40 ms TDMA frame. This speech signal needs to be compressed to increase
the capacity of the channel. One TDMA frame, which has a basic time period of 40 ms, is shared by six
users. The bit rate at point B is 42 kbits/s. Thus, one user slot gets only 7 kbits/s. The 2560 bits have to
be reduced to 138 bits, to which 86 bits are added for forward error correction (FEC), resulting in a total
of 5.6 kbits/s.
The higher the compression ratio and the higher the quality of the speech coder, the more calculations,
usually expressed in MIPS, are required. This is illustrated in Fig. 42.51. The first generation GSM digital
cellular standard employs the Regular Pulse Excitation-Long Term Prediction (RPE-LTP) algorithm and
requires a few thousand MIPS to implement it on a current generation DSP processor. For instance, it
requires 2000 MIPS on the lode processor [21]. The Japanese half-rate coder Pitch Synchronous Innovation-
Code Excited Linear Prediction (PSI-CELP) requires at least ten times more MIPS.
Viterbi Decoding
The function of the channel codec is to add controlled redundancy to the bit stream on the encoder side
and to decode, detect, and correct transmission errors on the receiver side. Thus, channel encoding and
decoding is a form of error control coding. The most common decoding method for convolutional codes
© 2002 by CRC Press LLC
Receiver
Synthesizer
Demodulator
Transmitter Modulator
User 6
8 bits
(µ-Law)
Channel CODEC
125 µs (8 kHz)
-> 2,560 bits/40 ms -> 64 kbps(8 bits* 8 kHz)
Speech: 138 bits/40 ms -> 3.45 kbps
FEC: 86 bits/40 ms -> 2.15 kbps
1 slot (~6.7 ms, 7 kbps)
1 TDMA Frame (40 ms)
Total: 5.6 kbps
Point A
Speech Speaker
CODEC Microphone
User 1 User 2 User 3 User 4 User 5 User 6
User 1