User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–23
Original 11/97
When the wanted CDMA signal, “yours”, is received the correlation receiver
recovers “your” signal and rejects the rest. Looking at figure 21, the upper right
most part of the drawing shows what happens to the unwanted signals. The
unwanted signals are not de–spread so that each interfering signal only contributes
a little to the noise floor while “your” wanted signal is de–spread and will have an
acceptable signal–to–noise ratio. This is where the processing gain comes into
play. The processing gain is 21 dB and it takes a signal–to–noise ratio of about 7
dB for acceptable voice quality. This leaves 14 dB of processing gain to extract
“your” signal from the noise.
Here are some of the differences between CDMA and analog FM (AMPS).
Multiple users are on one frequency at the same time. RF engineers have spent a
lot of time and effort trying to keep signals on one channel so that adjacent channel
signals would not cause interference. CDMA technology places a great many
conversations (signals) on the same frequency.
In CDMA a channel is defined by various digital codes in addition to having different
frequencies. Analog FM channels are defined by different frequencies only.
An analog FM (AMPS) cell site has a hard limit on the number of users it can
accommodate, only one call per frequency channel. CDMA has a soft capacity
limit. If cells surrounding a heavily loaded cell are lightly loaded then the heavily
loaded cell site can accommodate additional users. CDMA has a soft limit because
less “other cell” interference causes the total interference to be less. More calls can
also be accommodated at the expense of lower voice quality (S/N), this because
each additional user adds only a small amount of interference to the total.