0195136047.pdf

716 COMMUNICATION SYSTEMS

not lose track of polarity while processing polar or Manchester waveforms, a technique called

differential encoding(see Problem 15.3.9) is employed so as to remove the need to maintain

polarity.

Because the digits in a typical digital sequence fluctuate randomly between 0s and 1s with

time, the formatted waveform is then a randomly fluctuating set of pulses corresponding to the

selected format. With such random waveforms, one uses thepower spectral density(with units of

V^2 /Hz) to define the spectral content. On comparing the three waveform formats, the unipolar and

polar formats both have the same bandwidth and relative side-lobe level, whereas the Manchester

waveform has no spectral component at direct current, but requires twice the bandwidth of the

other two signals.

Pulse-Code Modulation (PCM)

PCM is the simplest and oldest waveform coding scheme for processing an analog signal by

sampling, quantizing, and binary encoding. Figure 15.3.6 shows a functional block diagram of

a PCM system transmitter. In order to guarantee that the message is band-limited to the spectral

extent for which the system is designed, a low-pass filter is introduced. The compressor is rather

optional for better performance. Let us assume that the PCM signal is transmitted directly over

the baseband channel. Corrupted by the noise generated within the receiver, the PCM signal is

shown as the input to the PCM reconstruction function in Figure 15.3.7, which depicts a block

diagram of functions (including an optional expandor) needed to receive PCM. The operations of

the receiver are basically the inverse of those in the transmitter. The first and most critical receiver

operation is to reconstruct the originally transmitted PCM signal as nearly as possible from the

noise-contaminated received waveform. The effect of noise is to be minimized through a careful

selection of circuit implementation.

The only knowledge required of the receiver to reconstruct the original PCM signal is whether

the various transmitted bits are 0s and 1s, depending on the voltage levels transmitted, assuming

that the receiver is synchronized with the transmitter. The two levels associated with unipolar

0111010

(a)

(c)

(b)

(d)

Tb

A

t

t

t

A

A

−A

−A

0101100 Figure 15.3.5Waveform formats for a binary

digital sequence.(a)Binary digital sequence{bk}.

(b)Its unipolar format.(c)Its polar format.(d)Its

Manchester format.