Almost-Coherent Detection of Phase-Shift-Keyed Signals Using an Injection-Locked Oscillator

The two generally recognized methods of detection of phase-shiftkeyed (PSK) signals are coherent detection and differential detection. Coherent detection has been shown to be optimum in the presence of Gaussian noise,1 but, due to the difficulty of storing an absolute phase reference at the receiver, it is seldom used in practice. In a recent paper,2 B. Glance showed that an injection-locked oscillator, the locking bandwidth of which is much less than the modulation rate can, under certain conditions, be used to derive a phase reference from the input signal itself. This is actually a form of a quadrature reference system, where the phase of one quadrature remains essentially unkeyed, and is used to provide the reference tone. 3,4 In this paper, we examine the effectiveness of this scheme for a two-phase P S K system where the modulation is a random digital signal and additive Gaussian noise is present. We derive an expression for the probability distribution of the reference phase, and from this calculate the average probability of a detection error. We find that if the modulation rate is much greater than the bandwidth of the oscillator, and if a suitable encoder and decoder are used, the method very nearly approaches the ideal performance of coherent detection. 1867