Optical Communications, 2nd Edition

4.1 THE NONCOHERENT COMMUNICATION SYSTEM MODEL

The typical model of a noncoherent (direct detecting) communication system is shown in Figure 4.1. The desired information is intensity modulated onto an optical source and transmitted to the receiver over a space or fiber link. The optical receiver collects the field with the receiver area. After preprocessing in the receiver front end (focusing and filtering), the collected field is imaged through the receiver lens system onto the photodetecting surface in the focal plane. In a space system, background radiation is also collected by the lens and focused along with the transmitted field. The photodetector, being basically a power detecting device, responds to the instantaneous field count rate process produced from the receiver area. Its output appears as a shot noise process whose count rate is proportional to the instantaneous receiver power. This output shot noise represents the demodulated optical signal. For the receiver to recover the desired signal, it is necessary that the transmitted information be associated with the intensity variation of the transmitted field. The information waveform may be directly modulated onto the intensity of the transmitted field, or it may be subcarrier-modulated prior to optical intensity modulation. In the latter case, further receiver processing is necessary, following optical detection, to recover the information waveform. To design the proper post-detection processing it is first necessary ...