Real-time signal-to-noise ratio (snr) estimation for bpsk and qpsk modulation using the active communications channel

Abstract

Method and apparatus for estimating signal-to-noise ratio (SNR) γ of a composite input signal e(t) on a phase-modulated (e.g., BPSK) communications link. A first demodulator receives the composite input signal and a stable carrier signal and outputs an in-phase output signal; a second demodulator receives the composite input signal and a phase-shifted version of the carrier signal and outputs a quadrature-phase output signal; and phase error θ(t) contained within the composite input signal e(t) is calculated from the outputs of the first and second demodulators. A time series of statistically independent phase error measurements θ(t), θ(t), . . . , θ(t) is obtained from the composite input signal subtending a time interval Δt=t−twhose value is small enough such that γ(t) and σ(t) can be taken to be constant in Δt. A biased estimate γ* for the signal-to-noise ratio (SNR) γ of the composite input signal is calculated using maximum likelihood (ML) estimation techniques, and an unbiased estimate {circumflex over (γ)} for the signal-to-noise ratio (SNR) γ of the composite input signal is determined from the biased estimate γ*, such as by use of a look-up table.