In our concluding chapter we will combine our photodetector and receiver-noise modeling techniques with front-end and demodulator designs to construct complete receiver structures. Our goal is to predict the overall performance of an optical receiver. We begin this chapter with a short introduction to the demodulation techniques used to recover digital information from the electrical signals that are present at the output of the receiver front-end.
7.1 Digital Demodulation
The functional components of a digital receiver are illustrated in Fig. 7.1. The optical signal received from the channel may be optically processed prior to photodetection. The electrical signal resulting from photodetection is then amplified and demodulated to recover the digital information. The electrical amplifier is usually configured as either an automatic-gain-control amplifier or as a limiting amplifier [1â3]. This provides a relatively constant electrical signal level to the demodulator regardless of the amount of optical power received.
The digital receiver must recover both the digital information present in the received signal and a corresponding digital clock waveform. The clock must have a stable, well defined timing relationship with the boundaries between adjacent symbols in the information waveform because it is used by the decision circuitry to examine the information carrying waveform at precise times to determine the symbol being received.