User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–40
Original 11/97
CDMA Reverse Link
Walsh
Code 0
Walsh
Code 1
Walsh
Code 2
Walsh
Code 61
Walsh
Code 62
Walsh
Code 63
307.2
kbps
Long Code
1.2288 Mbps
1.2288 Mbps
6–bit
words
@ 4.8 k/s
xx
x
Q Short Code
I Short Code
1/2 Chip
Delay
1/2
I
Q
Data Burst
Randomizer
28.8
kbps
28.8
kbps
Interleaver
Convolutional
Encoder
20 msec
blocks
Vocoded
Speech
Data
9.6
kbps
1/3
rate
Figure 35. CDMA Reverse Link
The CDMA reverse link (Mobile to Base Station) is quite a bit different from the
forward link. The mobile does not have a pilot channel. This is because each
phone would have to have its own unique pilot channel and there are only 64 Walsh
codes, also the pilot channel power requirements would be severe for the mobile.
Because of the lack of a pilot channel and OQPSK (Offset Quadraphase Shift
Keyed) modulation the base station will have a tougher time demodulating the
mobiles signal. To give the reverse link better performance, a one–third rate
Convolutional Encoder is used and six data bits at a time is used to point at one of
the 64 Walsh codes. The last sentence will be explained shortly. The 307.2 kbps
data is XOR’ed with a long code that is unique for each CDMA cellular phone
creating a 1.2288 Mbps data stream. Finally, just like a base station the 1.2288
Mbps data stream is split and XOR’ed with an I & Q short code. The mobile cell
phone has one more process, the Q Channel is delayed by one–half clock period.
Data Burst Randomizer
When the vocoder lowers its data rate, the mobile starts turning off its transmitter.
At the lowest data rate the mobile transmitter is only on one–eighth of the time. The
average output power will drop 3 dB each time the data rate is cut in half. Average
output power drops because the mobile’s transmit time is cut in half, the peak output
power does not change. Now if all the mobiles transmitted at the same time there
would not be any reduction of interference. The Data Burst Randomizer
randomizes the mobiles transmit time to keep them from transmitting at the same
time. Randomizing instructions come from the Frame Rate determination algorithm
and the Long Code state in the previous frame.