SIGNAL ENCODING CRITERIA

SIGNAL ENCODING CRITERIA


To begin, we need to define some terms. Recall that a digital signal is a sequence
of discrete, discontinuous voltage pulses. Each pulse is a signal element. Binary
data are transmitted by encoding each data bit into signal elements. In the simplest
case, there is a one-to-one correspondence between bits and signal elements.
An example is shown in Figure 2.9, in which binary 0 is represented by a
higher voltage level and binary 1 by a lower voltage level. Similarly, a digital bit
stream can be encoded onto an analog signal as a sequence of signal elements,
with each signal element being a pulse of constant frequency, phase, and amplitude.
There may be a one-to-one correspondence between data elements (bits)
and analog signal elements. For both analog and digital signals, there may be a
one-to-multiple or multiple-to-one correspondence between data elements and
signal elements, as will be shown.
The data signaling rate, or just data rate, of a signal is the rate, in bits per second,
that data are transmitted. The duration or length of a bit is the amount of
time it takes for the transmitter to emit the bit; for a data rate R, the bit duration
is 11R. The modulation rate, in contrast, is the rate at which the signal level is
changed. This will depend on the nature of the encoding, as explained later. The
modulation rate is expressed in baud, which means signal elements per second.
Table 6.1 summarizes key terms; these should be clearer when we see an example
later in this chapter.

The tasks involved in interpreting digital signals at the receiver can be summarized
by again referring to Figure 2.9. First, the receiver must know the timing of
each bit. That is, the receiver must know with some accuracy when a bit begins and
ends. Second, the receiver must determine whether the signal level for each bit
position is high (0) or low (1). In Figure 2.9, these tasks are performed by sampling
each bit position in the middle of the interval and comparing the value to a threshold.
Because of noise and other impairments, there will be errors, as shown.
What factors determine how successful the receiver will be in interpreting the
incoming signal? We saw in Chapter 2 that three factors are important: the signal-tonoise
ratio (or, better, Eb/No), the data rate, and the bandwidth. With other factors
held constant, the following statements are true:
• An increase in data rate increases bit error rate (BERV
• An increase in SNR decreases bit error rate.
• An increase in bandwidth allows an increase in data rate.