Page 306 - Satellite Communications, Fourth Edition

286 Chapter Ten

like symbols, and bit timing can be extracted. However, dc drift still
occurs with long strings of like symbols.
In the split-phase or Manchester encoding shown in Fig. 10.2d, a tran-
sition between positive and negative levels occurs in the middle of each
bit. This ensures that transitions will always be present so that bit
timing can be extracted, and because each bit is divided equally between
positive and negative levels, there is no dc component.
A comparison of the frequency bandwidths required for digital wave-
forms can be obtained by considering the waveforms which alternate at
the highest rate between the two extreme levels. These will appear as
squarewaves. For the basic polar NRZ waveform of Fig. 10.2b, this hap-
pens when the sequence is . . . 101010 . . . The periodic time of such a
squarewave is 2T , and the fundamental frequency component is 1/2T .
b
b
For the split-phase encoding, the squarewave with the highest repeti-
tion frequency occurs with a long sequence of like symbols such as . . .
1111111 . . ., as shown in Fig. 10.2d. The periodic time of this square-
wave is T , and hence the fundamental frequency component is twice
b
that of the basic polar NRZ. Thus the split-phase encoding requires
twice the bandwidth compared with that for the basic polar NRZ, while
the bit rate remains unchanged. The utilization of bandwidth, measured
in bits per second per hertz, is therefore less efficient.
An alternate mark inversion (AMI) code is shown in Fig. 10.2e. Here,
the binary 0s are at the zero baseline level, and the binary 1s alter-
nate in polarity. In this way, the dc level is removed, while bit timing
can be extracted easily, except when a long string of zeros occurs.
Special techniques are available to counter this last problem. The
highest pulse-repetition frequency occurs with a long string of . . .
111111 . . . the periodic time of which is 2T , the same as the waveform
b
of Fig. 10.2b. The AMI waveform is also referred to as a bipolar wave-
form in North America.
Bandwidth requirements may be reduced by utilizing multilevel dig-
ital waveforms. Figure 10.3a shows a polar NRZ signal for the sequence
11010010. By arranging the bits in groups of two, four levels can be used.
For example, these may be

11 3A
10 A
01 −A
00 −3A

This is referred to as quaternary encoding, and the waveform is shown
in Fig. 10.3b. The encoding is symmetrical about the zero axis, the spac-
ing between adjacent levels being 2A. Each level represents a symbol,the
duration of which is the symbol period. For the quaternary waveform the