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Unformatted text preview: CHAPTER 6
SIGNAL ENCODING TECHNIQUES
A N SWERS
NSWERS TO Q UESTIONS 6.1 In differential encoding, the signal is decoded by comparing the polarity of adjacent
signal elements rather than determining the absolute value of a signal element.
6.2 A modem converts digital information into an analog signal, and conversely.
6.3 Cost, capacity utilization, and security and privacy are three major advantages
enjoyed by digital transmission over analog transmission.
6.4 With amplitude-shift keying, binary values are represented by two different
amplitudes of carrier frequencies. This approach is susceptible to sudden gain
changes and is rather inefficient.
6.5 Non return-to-zero-level (NRZ-L) is a data encoding scheme in which a negative
voltage is used to represent binary one and a positive voltage is used to represent
binary zero. A disadvantage of NRZ transmission is that it is difficult to determine
where one bit ends and the next bit begins.
6.6 The difference is that offset QPSK introduces a delay of one bit time in the Q stream
6.7 QAM takes advantage of the fact that it is possible to send two different signals
simultaneously on the same carrier frequency, by using two copies of the carrier
frequency, one shifted by 90˚ with respect to the other. For QAM, each carrier is
ASK modulated. 6.8 The sampling rate must be higher than twice the highest signal frequency. 6.9 Frequency modulation (FM) and phase modulation (PM) are special cases of angle
modulation. For PM, the phase is proportional to the modulating signal. For FM,
the derivative of the phase is proportional to the modulating signal. A N SWERS
NSWERS TO PROBLEMS 6.1 s(t) = d 1 (t)cos wct + d 2 (t)sin wct
Use the following identities: cos2α = 2cos2 α – 1; sin2α = 2sinα cosα
s(t) coswct = d 1 (t)cos2 wct + d 2 (t)sinwct coswct
= (1/2)d1 (t) + (1/2)d1 (t) cos2wct + (1/2)d2 (t) sin2wct
Use the following identities: cos2α = 1 – 2 sin 2 α; sin2 α = 2sin α cosα
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- Fall '11