User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–29
Original 11/97
The Interleaver changes the data order so only bits instead of whole words would be
lost because of data errors. The Long Code Generator generates a code that is
242 bits long. This code runs at 1.2288 Mbps and takes about 41.5 days before it
repeats. The PN (Pseudo–random) code is decimated by a factor of 64 that means
only one out of 64 bits is XOR’ed with the output of the Interleaver. The data rate at
this point is still 19.2 ksps because two 19.2 ksps data streams have been XOR’ed.
The 64 Walsh codes are used in the forward link as a means to uniquely identify
each user. The Walsh code generator runs at 1.2288 Mbps while the encoded
voice data runs at 19.2 kbps the ratio is 64 or 21 dB of processing gain. This
means that each data bit is XOR’ed with 64 Walsh code bits, one complete 64 bit
Walsh code. The voice data determines the polarity of the Walsh code. This
makes it easier for the CDMA mobile to find and decode its assigned Walsh code.
All base station’s use the same Walsh code 64 set. What gives each base station
its own unique identity will be explained in “Short Code Spreading”
The forward link is now running at its final rate of 1.2288 Mbps.
Walsh Codes
Walsh Codes in the CDMA forward link are used to “make” the CDMA forward
channels. Remember in analog phones a different frequency channel is used to
separate one cell phone user from another. TDMA cell phones use different time
slots to allow 3 phones to share one frequency channel. CDMA uses different
frequency channels like analog and TDMA cell phones. However, to separate
CDMA users on the same base station, different codes are used on the forward link
(Base Station to Mobile). IS–95 Standard CDMA uses Walsh code set 64. This
Walsh set has 64 unique codes each having 64 bits. Figure 29 shows how a Walsh
code set is built up.
W =
W W
W W
2n
4
1
W = 0
2
W =
0 0
0 1
W =
0 0 0 0
0 1 0 1
0 0 1 1
0 1 1 0
CDMA11.DRW
Figure 29. Walsh code example
Walsh code sets are generated by using the formula W2n = W W
W W .
In Walsh code set 2 it can be seen that the lower right digit is the logical not of the
other three digits. In Walsh code 4 the set 2 code is repeated three times with the
logical not being used in the lower right corner. The expansion number is always a
power of 2 and also notice that for each set the first code is always all zeros.