User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–24
Original 11/97
The CDMA Forward Link
20 MSEC
BLOCKS
Vocoded
Speech
data
9.6
kbps
1/2
Rate
Convolutional
Encoder
19.2
kbps
Interleaver
XOR
19.2
kbps
Long Code
Decimator
1 of 64 bits
1.2288
Mbps
Long Code
Generator
CDMA Forward Link
XOR
Walsh Cover
1.2288
Mbps
I Short Code
Q Short Code
Lo Pass
Filter
Lo Pass
Filter
1.2288
Mbps
1.2288
Mbps
To I/Q
Modulator
I Channel
Q Channel
Walsh Code
Generator
CDMA06,DRW
1 in 24
Decimator
800 Hz
Power
Control
Bit
MUX
Figure 22. CDMA Forward Link
When discussing the CDMA Forward Link, voice data will be shown at 9600 BPS
(full rate). Keep in mind that the Vocoded Speech rate can be 9600, 4800, 2400 or
1200 BPS when using Rate Set One. The Vocoded Speech rate is developed after
the CODEC in both the Base Station and the Mobile Phone.
Speech data is passed through a Convolutional Encoder that doubles the data rate.
This data is then Interleaved. Interleaving does not change the data rate but will
introduce some data time delay. The Long Code Generator running at 1.2288
Mbps develops the 242 bits long PN (Pseudo–random noise) code. The long code
Decimator uses one out of every 64 bits of the PN long code and exclusive OR’s this
decimated bit stream with the output of the Interleaver. At this point the data stream
is still running at 19.2 kbps. The 64 bit Walsh Code Generator output running at
1.2288 Mbps is exclusive OR’ed with the pervious exclusive OR gate’s output. The
baseband is now running at a data rate of 1.2288 Mbps, 64 times 19.2 kbps. The
Walsh encoded data stream is then split into I and Q channels, and then each
channel is spread with a short code. Then finally, signals are sent through a low
pass filter to the I/Q modulators.