User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–28
Original 11/97
Offset T0 T1 T2 T3 T4 T5 T6 Base
Stations
01 1 0 0 1 0 1 1 1
10 0 0 1 0 1 1 1 2
11 1 0 1 1 1 0 0 3
100 0 1 0 1 1 1 0 4
101 1 1 0 0 1 0 1 5
110 0 1 1 1 0 0 1 6
1111 1 1 1 0 0 1 0 7
Figure 27. Mask offset example
The above example shows how different offsets will create different codes. Note
that none of the codes has been altered. Each one just starts at a different time.
Remember the CDMA system uses the same 15 bit linear feedback shift register to
generate the PN short code for both forward and reverse links. If figure 28 were
expanded to a 15–bit shift register the time shifted short codes for the 512 base
station channels would be shown.
Long Code Scrambling
In the forward link the long code is used to scramble voice data and provide some
measure of security. However the complete long code is not used, refer to Figure
28. A Long Code Decimator allows only one in every 64 bits of the Long Code to
be exclusively OR’ed with the Encoded Voice Data. This scrambling does not
increase the data rate because two 19.2 kbps data streams are being exclusive
OR’ed with each other.
Walsh Code User Channelization
The CDMA forward link figure will be repeated here to show where we are in the
CDMA forward link (base station to mobile) generation.
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
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 28. CDMA Forward Link
The 20 msec VOCODED speech data blocks have had an error correction routine
added in the Convolutional Encoder that increased the data rate to 19.2 ksps (kilo
symbols per second).