Studies of Rectangular-Wave Modulation and Pulse-Interval Modulation Systems

Abstract

Rectangular-wave modulation (RWM) and pulse-interval modulation (PIM) systems were first reported in 1966 and 1967 respectively. Both RWM and PIM are asynchronous pulse modulation schemes. The two modulators are self-oscillating and produce a zero-signal output in the absence of any modulation. The zero-signal output is a periodic square wave in RWM, whereas in PIM, it is a periodic pulse train, Under modulation, the RWM output produces a large variation in pulse durations and a relatively smaller change in the time period of the square wave; whereas in PIM, the interval between the pulses varies and the average pulse-repetition frequency remains constant. It is the objective of the present research to understand and to analyse the RWM and PIM systems from a communication point of view. The RWM and PIM systems are studied almost on parallel lines. The performance criteria of an analogue communication system are usually specified in terms of its signal-to-noise ratio, bandwidth, multichannel capability, spectrum and autocorrelation function. Therefore, a theoretical study of RWM and PIM systems based on these aspects was undertaken and the corresponding expressions were obtained. Experimental spectrum measurements made on RWM and PIM justify the validity of theoretical expressions. Above all, the experimental determination of signal-to-noise ratio of RWM and PIM systems confirm the theoretical results. Both RWM and PIM are suitable for modulation by sinusoidal, speech and data signals. The performance criterion of a digital communication system is often stated in terms of its error probability. Communication of digital RWM and PIM waveforms is considered over an additive, white Gaussian noisy channel. The RWM and PIM waveforms are replaced by the signal vectors, the decision regions are formed and the probability of correct detection under the worst case is evaluated.