SIGNAL ENCODING CRITERIA
There is another factor that can be used to improve performance, and that is the
encoding scheme. The encoding scheme is simply the mapping from data bits to signal
elements.A variety of approaches are in use. Before describing these techniques, let us
consider the following ways of evaluating or comparing the various techniques.
• Signal spectrum: Several aspects of the signal spectrum are important. A lack
of high-frequency components means that less bandwidth is required for
transmission. In addition, lack of a direct current (dc) component is also
desirable. With a dc component to the signal, there must be direct physical
attachment of transmission components. With no dc component, alternating
current (ac) coupling via transformer is possible; this provides excellent
electrical isolation, reducing interference. Finally, the magnitude of the effects
of signal distortion and interference depend on the spectral properties of the
transmitted signal. In practice, it usually happens that the transfer function of
a channel is worse near the band edges. Therefore, a good signal design
should concentrate the transmitted power in the middle of the transmission
bandwidth. In such a case, less distortion should be present in the received
signal. To meet this objective, codes can be designed with the aim of shaping
the spectrum of the transmitted signal.
• Clocking: The receiver must determine the beginning and end of each bit
position. This is no easy task. One rather expensive approach is to provide a
separate clock channel to synchronize the transmitter and receiver. The alternative
is to provide some synchronization mechanism that is based on the
transmitted signal. This can be achieved with suitable encoding.
• Signal interference and noise immunity: Certain codes exhibit superior performance
in the presence of noise. This is usually expressed in terms of a BER.
• Cost and complexity: Although digital logic continues to drop in price, this
factor should not be ignored. In particular, the higher the signaling rate to
achieve a given data rate, the greater the cost. We will see that some codes
require a signaling rate that is in fact greater than the actual data rate.
We now turn to a discussion of various techniques.
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