User Guide

Technical Documentation
NHP–4
System Overview
PAMS
Page 3–7
Issue 1 04/99
The frequency reuse factor is a number representing how often the same frequency
can be reused. To provide acceptable call quality, a Carrier–to–Interference ratio
(C/I) of at least 18 dB is needed. Practical results show that in most cases to
maintain a 18 dB (C/I) a frequency reuse factor of 7 is required. Please note that C/I
is carrier to interference, not signal to noise ratio The resulting capacity is one call
per 210 kHz of spectrum in each cell.
With TDMA, a channel consists of a time slot in a periodic train of time intervals
making up a frame. A given signal’s energy is confined to one of these time slots.
The IS–54B TDMA standard provides a basic modulation efficiency of three voice
calls per 30 kHz of bandwidth. The resulting capacity is one call per 70 kHz of
spectrum or three times that of the analog FM system.
With CDMA each signal consists of a different pseudo random binary sequence that
modulates the carrier, spreading the spectrum of the waveform. A large number of
CDMA signals share the same frequency spectrum. The signals are separated in
the receivers by using a correlator that accepts only signal energy from the selected
binary sequence and de–spreads its spectrum simultaneously. The other users’
signals, whose codes do not match, are not de–spread and as a result, contribute
only minimally to the noise and represent a self–interference generated by the
system. The forward link (B.S. to M.S.) “channels” are separated by offsets in the
short code PN sequence. Reverse link channels are separated by different long
code PN sequences. A detailed description of the forward and reverse links is
given later.
AMPS = 1.5 MHz / 30kHz = 50 Channels
Capacity = 50 Channels / 7 (1 in 7 Frequency Reuse)
AMPS = 7 calls
GSM = 1.5 MHz / 200 kHz = 7 Channels
Capacity = 7 Channels / 7 x 8 Time Slots
GSM = 8 calls
DAMPS = 1.5 MHz / 30 kHz = 50 Channels
Capacity = 50 Channels / 7 x 3 Time Slots
DAMPS = 21 calls
CDMA Capacity
Capacity varies between 30 to 40 calls per CDMA
channel. Actual capacity depends Rho, processing
gain, error correction coding gain of M.S. vs signals
in cell and external cell signals.
CDMA = 1.5 MHz 1 CDMA channel + 1.2288MHz
Figure 2. CDMA Capacity gains
Why should NOKIA go to so much trouble to develop CDMA? CAPACITY! To see
how CDMA increases capacity over present 800 MHz systems (AMPS and DAMPS)
lets look at a 1.5 MHz span of frequencies and compare. A CDMA frequency
channel is 1.2288 MHz wide however to provide guard bands in order to reduce
potential interference with adjacent analog channels a total of 1.5 MHz will be used.