Maximum likelihood sequence detection using a pilot tone

摘要

This paper derives, analyzes, and simulates a maximum likelihoodn(ML) sequence detector (MLSD) for a linearly modulated signalntransmitted with a pilot tone (PT-MLSD). The transmitted signal isndistorted by a time-varying frequency-selective Rayleigh fading channelnand corrupted by additive Gaussian noise. The received signal isnunsynchronized in that there are residual carrier frequency, carriernphase, and symbol timing offsets. The PT-MLSD treats the channel as anstochastic process, and so symbol sequences are distinguished by theirnautocovariances. Coherent communication is possible even in overspreadnchannels. As the sequences' autocovariances explicitly account for thenchannel's time variation, the PT-MLSD's bit error rate (BER) floor isnnormally lower than the BER floor suffered by receivers that estimatenthe channel impulse response conventionally. Both the data-bearingnsignal and pilot tone are used together for synchronization,nequalization, and detection. The pilot tone is only needed to remove thenconstellation's phase ambiguity and provide a stable amplitude referencenfor QAM constellations. It is not needed for estimating the channelnimpulse response. The pilot tone does not require a spectral null fornits insertion, and it does not significantly degrade the peak-to-averagenor maximum-to-minimum power ratios. Thus, many of the disadvantages ofnother pilot tone systems are avoided, as there is no bandwidthnexpansion, and linear amplification is not made appreciably morendifficult