 the other. In the absence of noise, only one correlator will transmission link is inadequate, it is necessary to use an error produce an output. If noise is present, other correlators will also detecting or error correcting code for the data (Lucky et al, 1968; produce some output. However, that corresponding to the symbol Taub and Schilling, 1986). actually received should produce the largest output and this is selected as being the one most likely to be present. This maximum likelihood strategy has been shown to be the optimum for additive Gaussian noise when the symbols are equally probable (Harman, 1963). Figure 19.4 Correlation receiver Signal to noise ratio (dB) Figure 19.3 Error rate for transmission of unipolar binary Since an integrator is a low pass filter, each of the correlators in signal disturbed by white noise Figure 19.4 is equivalent to a coherent demodulator. Thus, a coher- ent demodulator with the appropriate waveform applied as its local carrier acts as a correlation detector. On a long transmission link, it is possible to use regenerative repeaters instead of analogue amplifiers. A regenerative repeater It may be noted that the incoming pulses to a correlation receiver (Dorward, 1991) samples the received waveform at intervals corre- do not need to be a baseband signal. They can be pulses of a modulated carrier. sponding to the digit rate. If the received voltage at the sampling instant exceeds a threshold voltage, this triggers a pulse generator Another approach is to postulate that there can be a filter which which transmits a pulse to the next section of the link. If the received enhances the signal and reduces the noise voltage as much as voltage is below the threshold, no pulse is generated. If both positive possible. Such a filter is called a matched filter, because its transfer and negative pulses are transmitted, the regenerator is required to function is tailored to the particular input signal to be detected. determine whether the received voltage is positive or negative and It has 