Two-stage carrier synchronization techniques for nonselective fading

Transmitted reference carrier synchronization techniques can potentially provide near-coherent performance in the presence of time-varying fading. Such techniques are spectrally not as efficient as nonreference-based architectures and can have performance several dB worse than ideal coherent in higher order modulations (e.g., 64-QAM). This paper presents a two-stage demodulator that facilitates improved spectral efficiency or provides better performance (or sometimes both). The two-stage architecture results from examining the optimal detector for M-QAM. The first stage produces tentative decisions and removes the modulation from the received signal. The second stage implements a smoothing filter for the resulting signal, providing an improved carrier estimate. Monte Carlo simulation results demonstrate the improved performance For 4-QAM, 16-QAM, and 64-QAM