722 COMMUNICATION SYSTEMS
signal transmitted from a mobile subscriber to a fixed subscriber will undergo two different types
of analog-to-digital (A/D) encoding, whereas speech-signal communication between two mobiles
serviced by different base stations will undergo four translations between the analog and digital
domains.
In the conversion of analog audio signals to digital form, with the development of the compact
disc (CD) player and the digital audio tape recorder, the most dramatic changes and benefits
have been experienced by the entertainment industry. The CD system, from a systems point
of view, embodies most of the elements of a modern digital communication system: A/D and
D/A conversion, modulation/demodulation, and channel coding/decoding. Figure 15.3.12 shows
a general block diagram of the elements of a CD digital audio system. The sampling rate in a
CD system is chosen to be 44.1 kHz, which is compatible with the video recording equipment
commonly used for digital recording of audio signals on magnetic tape. The samples of both theL
andRsignals are quantized using PCM with 16 bits per sample. While the D/A conversion of the
two 16-bit signals at the 44.1-kHz sampling rate is relatively simple, the practical implementation
of a 16-bit D/A converter is very expensive. Because inexpensive D/A converters with 12-bit (or
less) precision are readily available, a method is to be devised for D/A conversion that employs
a low precision (and hence a low-cost D/A converter), while maintaining the 16-bit precision
of the digital audio signal. Without going into details, the practical solution to this problem is
to expand the bandwidth of the digital audio signal by oversampling through interpolation and
digital filtering prior to analog conversion.
Time-division multiple access (TDMA) is an important means by which each station on earth
timeshares the communication satellite in the sky, and broadcasts to all other stations during its
assigned time. Figure 15.3.13 shows the communication links of several (N) earth stations that
communicate with each other through satellite. All stations use the same up-link frequency, and
all receive a single down-link frequency from the satellite.Carrier Modulation by Digital Signals
Digitally modulated signals with low-pass spectral characteristics can be transmitted directly
throughbasebandchannels (having low-pass frequency-response characteristics) without the
need for frequency translation of the signal. However, there are many communicationbandpass
channels (telephone, radio, and satellite channels) that pass signals within a band of frequencies
(that is far removed from dc). Digital information may be transmitted through such channels by
using a sinusoidal carrier that is modulated by the information sequence in either amplitude, phase,
or frequency, or some other combination of amplitude and phase. The effect of impressing the in-
formation signal on one or more of the sinusoidal parameters is to shift the frequency content of the
transmitted signal to the appropriate frequency band that is passed by the channel. Thus, the signal
is transmitted by carrier modulation. There are several carrier-modulation methods. However, we
shall limit our discussion to the following, assuming only binary modulation in all cases:- Amplitude-shift keying (ASK)
- Phase-shift keying (PSK)
- Differential phase-shift keying (DPSK)
- Frequency-shift keying (FSK).
AMPLITUDE-SHIFTKEYING(ASK)
A carrier’s amplitude is keyed between two levels (binary 1 and 0) in binary ASK. Figure 15.3.14
shows the functions of a coherent ASK communication system. Let us consider a bit interval from