Two-ray Fading Channel Research Articles

On Optimum and Suboptimum Coherent Detection of Continuous Phase Modulation on a Two-Ray Multipath Fading Channel

In this paper, the performance of continuous phase modulation (CPM) transmitted on a two-ray fading channel and received in white Gaussian noise is studied. The optimum coherent maximum likelihood (ML) detector and approximations thereof and their performance are studied by means of minimum Euclidean distance and simulated symbol error probability. A linear detector optimum at large signal-to-noise ratios is also studied and the performance is given by means of error probability. It is assumed that measurements on the channel provide information about the channel parameters. It is found that the loss in signal power due to the channel is small when an ML detector or an approximation thereof is used for binary schemes with modulation index h =1/2 . The loss for these schemes with a linear detector becomes significantly larger, especially when MSK is transmitted. The performance for this linear detector can, however, be improved significantly by using decision feedback, but still, the performance of the ML detector is superior.

Digital Radio Receiver Responses for Combating Frequency-Selective Fading

Frequency-selective fading in a radio channel, caused by multipath propagation or any other phenomenon, can seriously degrade the effectiveness of digital transmission. Receiver processing that adapts in an appropriate way to the prevailing channel response can strongly reduce the harmful effects of such fading. We examine here the theoretical possibilities of adaptive processing by deriving and analyzing receiver responses for three different criteria. The criteria used permit an analytical approach that is simple and exact. The performance characteristics derived for the three receivers bracket the "best" performance that is possible in practice and reveal possible tradeoffs between performance and practicality. The analysis applies generally to the broad class of quadrature amplitude modulation (QAM) signals, which are assumed to be filtered in the transmitter to enforce spectral emission requirements. For each receiver response we derive a formula for the "power penalty", defined as the increase in transmitter power (over some theoretical minimum) needed to compensate for transmit filtering and channel fading. The power penalty formulas are evaluated for a two-ray multipath fading channel and for each of two common forms of modulation/transmit filtering. Graphical results are given for numerous combinations of the multipath parameters and the transmitter bandwidth-to-symbol rate ratio. The results of this study can be used to predict the probability (time fraction) of multipath outage, provided that the statistics of the channel parameters are known. Some cursory comparisons are made between the new results and those of a previous analysis for fixed receivers. The outage reductions made possible by using appropriate receiver responses are found to be quite large, possibly two or more orders of magnitude.