User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–41
Original 11/97
Reverse Link Error Protection
To improve the reverse link performance a one–third rate convolutional encoder is
used. This encoder has one 9600 bps input and three 9600 bps outputs which
when combined result in a 28.8 kbps data stream. Each data bit is encoded with 3
error correction bits to improve the error correction rate. The forward link uses
one–half rate encoding.
64–ary Modulation
Remember that Walsh codes are orthogonal with each other, which means that
several can be broadcast on one frequency without interfering with each other. The
mobile does not XOR voice data with a Walsh code. Every six–bits of voice data is
used to select one of the 64 Walsh codes. 26 = 64, when six bits of voice data “1 0
1 1 0 1” for example, are converted to a base 10 number it equals 45. So instead of
XOR’ing “1 0 1 1 0 1” with one Walsh code, Walsh code 45 represents the six data
bits. Again the reason for using Walsh codes is because they are orthogonal with
each other, they do not interfere with each other. The six–bit words have a rate of
4800 words per second that means that 4800 64–bit Walsh codes are selected each
second. This works out to a data rate of 307.2 kbps.
Reverse Channel Long Code Spreading
The long code shift register is 42 bits long, runs at a rate of 1.2288 Mbps, and
repeats it’s self approximately once every 41.5 days. Mobile cellular phones use
one of the 4.3 billion long codes for their reverse link channel, each mobile has its
own long code. The long codes are uncorrelated, which means they are all
different, but they are not orthogonal with each other. Not being orthogonal is a
draw–back but the base station knows when the mobiles long code started plus or
minus doppler and range uncertainty and this helps with correlation. High speed
searcher circuits in the base station allow a quick search over a wide range to lock
on a particular user’s signal. The long code at 1.2288 Mbps is XOR’ed with the
307.2 kbps data stream to create a 1.2288 Mbps data rate.
Reverse Channel Short Code Spreading
CDMA mobile phones use the same PN short code sequence as the base station’s
use, however the PN code’s purpose is different. The mobile’s use OQPSK (Offset
Quadraphase Shift Keyed). OQPSK is accomplished by adding a half period clock
delay to the mobile’s Q channel. OQPSK prevents the signal from going to zero
magnitude and greatly reduces the dynamic range of the modulated signal. Less
costly amplifiers can be used on CDMA mobiles because of the reduced linear
dynamic range obtained with OQPSK modulation. The mobile’s short code is not
delayed with a mask like the base stations short code is.