when the channel bit rate R is fixed, the amount of time available to transmit one of
the M waveforms corresponding to a 6-bit sequence is b times the time period in a
system that uses binary modulation.
The communication channel is the physical medium that is used to send the
signal from the transmitter to the receiver. In wireless transmission, the channel may
be the atmosphere (free space). On the other hand, telephone channels usually employ
a variety of physical media, including wire lines,optical fiber cables, and wireless
(microwave radio). Whatever the physical medium used for transmission of the
information, the essential feature is that the transmitted signal is corrupted in a
random manner by a variety of possible mechanisms, such as additive thermal noise
generated by electronic devices,man-made noise, e.g., automobile ignition noise,and
atmospheric noise,e.g..electrical lightning discharges during thunderstorms.
At the receiving end of a digital communications system, the digital demodulator
processes the channel-corrupted transmitted waveform and reduces the waveforms to
a sequence of numbers that represent estimates of the transmitted data symbols
(binary or M-ary). This sequence of numbers is passed to the channel decoder, which
attempts to reconstruct the original information sequence from knowledge of the code
used by the channel encoder and the redundancy contained in the received data.
A measure of how well the demodulator and decoder perform is thefrequency
with which errors occur in the decoded sequence. More precisely,the average
probability of a bit-error at the output of the decoder is a measure of the performance
of the demodulator-decoder combination. In general, the probability of error is a
function of the codc characteristics, the types of waveforms used to transmit the
information over the channci, the transmitter power, the characteristics of the channel,
i.e., the amount of noise, the nature of the interference, etc., and the method of
demodulation and decoding. These items and their effect on performance will be
discussed in detail in subsequent chapters.
As a final step, when an analog output is desired, the source decoder accepts
the output sequence from the channel decoder and, from knovtledge of the source
encoding method used, attempts to reconstruct the original signal from the source.
