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Sec. 5–13 Spread Spectrum Systems 389
rejection, and covert operation, or low-probability of intercept (LPI) capability. (The latter
considerations are especially important in military applications.) These performance objectives
can be optimized by using spread spectrum techniques.
Multiple-access capability is needed in cellular telephone and personal communication
applications, where many users share a band of frequencies, because there is not enough
available bandwidth to assign a permanent frequency channel to each user. Spread spectrum
techniques can provide the simultaneous use of a wide frequency band via code-division
multiple-access (CDMA) techniques, an alternative to band sharing. Two other approaches,
time-division multiple access (TDMA) and frequency-division multiple access (FDMA), are
studied in Sections 3–9, 5–7, and 8–5.
There are many types of spread spectrum (SS) systems. To be considered an SS system,
a system must satisfy two criteria:
1. The bandwidth of the transmitted signal s(t) needs to be much greater than that of the
message m(t).
2. The relatively wide bandwidth of s(t) must be caused by an independent modulating
waveform c(t) called the spreading signal, and this signal must be known by the
receiver in order for the message signal m(t) to be detected.
The SS signal is
s(t) = Re{g(t)e jv ct }
(5–120a)
where the complex envelope is a function of both m(t) and c(t). In most cases, a product
function is used, so that
g(t) = g m (t)g c (t)
(5–120b)
where g m (t) and g c (t) are the usual types of modulation complex-envelope functions that generate
AM, PM, FM, and so on, as given in Table 4–1. The SS signals are classified by the type
of mapping functions that are used for g c (t).
The following are some of the most common types of SS signals:
• Direct Sequence (DS). Here, a DSB-SC type of spreading modulation is used
[i.e., g c (t) = c(t)], and c(t) is a polar NRZ waveform.
• Frequency Hopping (FH). Here g c (t) is of the FM type where there are M = 2 k hop
frequencies determined by the k-bit words obtained from the spreading code waveform
c(t).
• Hybrid techniques that include both DS and FH.
We illustrate exactly how DS and FH systems work in the remaining sections of the chapter.
Direct Sequence
Assume that the information waveform m(t) comes from a digital source and is a polar
waveform having values of ;1. Furthermore, let us examine the case of BPSK modulation,