TABLE OF CONTENTS In digital communications the near universal figure of merit for a system is the BER. As a function of the signal to noise or interference, what is the probability the data bit sent was received in error (i.e., 0 ?1, or 1 ?0)? A related measure that is sometimes used is the message error rate (MER). For example, the sent message STOP is not decoded correctly, but decoded to another possible message like GO. In this chapter we will develop the concept of a bit error and how to set up accurate BER measurements that run in the least time possible.
Figure 9.1 shows a simple overall statement of the BER process.
A data source X 1 (k) at R bps is encoded, modulated, and transmitted. The medium between the transmitter and receiver is known as the channel. The channel has two effects on the signal. First, it can simply add noise and other interference, and second, the signal can bounce off of objects and create a fade. The receiver demodulates the signal, establishes the proper timing for detection, and finally puts out a second data stream X 2 (k) of rate R, which the receiver believes to be the actual message. The BER is the bit-by-bit comparison of this data stream with the input stream. The measurement algorithm is the XOR logical operation
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