ST2115
Scientech Technologies Pvt. Ltd.
21
Model of spread-spectrum digital communications system
Figure 6
Interfering signal may consist of a high-power sinusoid in the bandwidth occupied by
the information-bearing signal. Such a signal is narrowband. As a second example,
the interference generated by other users in a multiple-access channel depends on the
type of spread-spectrum signals that are employed by the various users to transmit
their information. If all users employ broadband signals, the interference may be
characterized as an equivalent broadband noise. If the users employ frequency
hopping to generate spread-spectrum signals, the interference from other users may be
characterized as narrowband.
Our discussion will focus on the performance of spread-spectrum signals for digital
communication in the presence of narrowband and broadband interference. Two types
of digital modulation are considered, namely, PSK and FSK. PSK modulation is
appropriate for applications where phase coherence between the transmitted signal
and the received signal can be maintained over a time interval that spans several
symbol (or bit) intervals. On the other hand, FSK modulation is appropriate in
applications where phase coherence of the carrier cannot be maintained due to time
variations in the transmission characteristics of the communications channel. For
example, this may be the case in a communications link between two high-speed
aircraft or between a high-speed aircraft and a ground-based terminal.
The PN sequence generated at the modulator is used in conjunction with the PSK
modulation to shift the phase of the PSK signal pseudo randomly, as described below
at a rate that is an integer multiple of the bit rate. The resulting modulated signal is
called a
direct-sequence (DS) spread-spectrum signal.
When used in conjunction with
binary or
M
-array (
M
> 2) FSK, the PN sequence is used to select the frequency of the
transmitted signal pseudo randomly. The resulting signal is called a
frequency-hopped
(FH) spread-spectrum signal
. Although other types of spread-spectrum signals can be
generated, our treatment will emphasize DS and FH spread-spectrum communication
systems, which are the ones generally used in practice.
www.hik-consulting.pl
