User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–22
Original 11/97
The CDMA Signal
1.25 MHz BW1.25 MHz BW
9.6 kbps 19.2 kbps 1228.8 kbps
CDMA
Transmitter
CDMA
Receiver
1228.8 kbps
Baseband
Data
Encoding &
Interleaving
Walsh Code
Spreading
Walsh Code
Correlator
Decode & De–
interleaving
19.2 kbps 9.6 kbps
Baseband
Data
Background Noise External Interference Other cell interference Other User Noise
10 kHz BW10 kHz BW
Interference Sources
CDMA05.DRW
Figure 21. CDMA Waveforms
To explain CDMA, some terms will have to be used that most persons are not
familiar with, but have patience they will be given a full explanation later in this Study
Guide. Forward link (BS to MS) CDMA starts with a narrowband signal that is
digitized speech. In this example the
full rate speech data rate of 9600 bps is
shown.
Speech data rates from the VOCODER can vary from 1200 BPS to 9600 BPS when
using “Rate Set One” and 14.4, 7.2, 3.6, and 1.8 kbps when using “Rate Set Two”.
A specialized digital code called a Walsh Code provides “user” channelization for the
forward link (B.S to M.S.) and is used to encode the reverse link (B.S. to M.S.) user
data. The short code PN sequence
SPREADS the baseband for both forward and
reverse links. The short code also provides channelization for BASE STATIONS on
the forward link by using a masking circuit. Masking will be explained later.
Processing Gain
One of the unique aspects of IS–95 standard CDMA is 21 dB of processing gain.
Processing gain is computed by using the formula 10 log(spread data rate) divided
by (Symbol rate). [10 log (1,228,800 / 19.2kBPS) = 21 dB]. If you calculate the
processing gain using the numbers in the last sentence the answer is 18 dB. The
extra 3 dB is comes from the same data being transmitted by the Q channel. If rate
set 2 is used the processing gain is 19.31 dB. When “your” CDMA signal is
transmitted all other CDMA signals along with background noise and any spurious
signals are considered interference.