User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–17
Original 11/97
Code Division Multiple Access (CDMA)
Amplitude
Time
Frequency
Channelization – FDMA
RX Ch1 RX Ch...n TX Ch 1 TX Ch...n
Time
Amplitude
Channelization – CDMA
CDMA01.DRW
Reverse LinkM.S. B.S.
PN Sequence
(long code)
Allows Channalization
and privacy
2
42
possible
Amplitude
Time
Channelization – CDMA
PN Sequence
(short code)
PN Offset 1 PN Offset 2 PN Offset 512
. . .
Forward Link B.S. M.S.
1
2
3
1
2
3
1
2
3
1
2
3
Amplitude Time
Frequency
Channelization – TDMA
TX Ch...nTX Ch 1RX Ch...nRX Ch1
Figure 16. AMPS: TDMA & CDMA Freq and time domain
With FDMA Channelization (Analog AMPS), a channel is 30 kHz wide, this where all
the signal’s transmission power is concentrated. Different users are assigned
different frequency channels. FDMA is the acronym for Frequency Division Multiple
Access. Interference to and from adjacent channels is limited by the use of
bandpass filters that only pass signal’s within a specified narrow frequency band
while rejecting signals at other frequencies. The analog FM cellular system AMPS,
uses FDMA.
The US 800 MHz cellular system divides the allocated spectrum into 30 kHz
bandwidth channels. Narrowband FM modulation is used with AMPS, resulting in 1
call per 30 kHz of spectrum. Because of interference, the same frequency cannot
be used in every cell.