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Sec. 5–15 Study-Aid Examples 397
source information is modulated onto a carrier using conventional FSK or BPSK techniques to
produce an FSK or a BPSK signal. The frequency hopping is accomplished by using a mixer
circuit wherein the LO signal is provided by the output of a frequency synthesizer that is hopped
by the PN spreading code. The serial-to-parallel converter reads k serial chips of the spreading
code and outputs a k-chip parallel word to the programmable dividers in the frequency synthesizer.
(See Fig. 4–25 and the related discussion of frequency synthesizers.) The k-chip word
specifies one of the possible M = 2 k hop frequencies, v 1 , v 2 ,..., v M .
The FH signal is decoded as shown in Fig. 5–42b. Here, the receiver has the knowledge
of the transmitter, c(t), so that the frequency synthesizer in the receiver can be hopped in
synchronism with that at the transmitter. This despreads the FH signal, and the source information
is recovered from the dehopped signal with the use of a conventional FSK or BPSK
demodulator, as appropriate.
SS Frequency Bands
In 1985, the FCC opened up three shared-frequency bands—902 to 928 MHz, 2,400 to 2,483.5
MHz, and 5,725 to 5,850 MHz—for commercial SS use with unlicensed 1-W transmitters. This
has led to the production and use of SS equipment for telemetry systems, wireless local area networks
(Wi-Fi or 802.11a, 802.11b, 802.11g, and 802.11n) for personal computers, and wireless
fire and security systems. Some SS applications have advantages over other systems. For example,
an SS cellular telephone system (i.e., CDMA) appears to be able to accommodate about 1,000
users per cell, compared to the 55 users per cell the U.S. analog cellular system accommodates
[Schilling, Pickholtz, and Milstein, 1990]. (See Sec. 8–8 for CDMA cellular telephone standards.)
For further study on SS systems, the reader is referred to books and papers that have been written
on the this subject [Cooper and McGillem, 1986; Dixon, 1994; Ipatov, 2005; McGill, Natali, and
Edwards, 1994; Peterson, Ziemer, and Borth, 1995; Rhee, 1998; Torrieri, 2004].
5–14 SUMMARY
In this chapter, a wide range of analog and digital modulation systems were examined on the
basis of the theory developed in Chapters 1 through 4. AM, SSB, PM, and FM signaling techniques
were considered in detail. Standards for AM, FM, and IBOC digital broadcasting signals
were given. Digital signaling techniques such as OOK, BPSK, FSK, MSK and OFDM
were developed. The spectra for these digital signals were evaluated in terms of the bit rate of
the digital information source. Multilevel digital signaling techniques such as QPSK, MPSK,
and QAM, were also studied, and their spectra were evaluated.
Spread spectrum signaling was examined. This technique has multiple-access capability,
antijam capability, interference rejection, and a low probability of intercept, properties
that are applicable to personal communication systems and military systems.
5–15 STUDY-AID EXAMPLES
SA5–1 Formula for SSB Power Prove that the normalized average power for an SSB signal is
8s 2 (t)9 = A 2 c 8m 2 (t)9, as given by Eq. (5–25).