The tasks involved in interpreting digital signals at the receiver can be summarized as follows. First, the receiver must know the timing of each bit, knowing 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). These tasks can be 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 was shown in Chapter 3, three factors are important: the signal-to-noise ratio, 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 (BER).
• An increase in SNR decreases bit error rate.
• An increase in bandwidth allows an increase in data rate.
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 have been tried. In what follows, we describe some of the more common ones.
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