Digital Radio Receiver Research Articles

Multipath Outage Performance of Digital Radio Receivers Using Finite-Tap Adaptive Equalizers

Recent analysis/simulation studies have quantified the multipath outage statistics of digital radio systems using ideal adaptive equalization. In this paper, we consider the use of finite-tap delay line equalizers, with the aim of determining how many taps are needed to approximate ideal performance. To this end, we assume an <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> -level QAM system using cosine rolloff spectral shaping and an adaptive equalizer with either fractionally spaced or synchronously spaced taps. We invoke a widely used statistical model for the fading channel and computer-simulate thousands of responses from its ensemble. For each trial, we compute a detection signal-to-distortion measure, suitably maximized with respect to the tap gains. We can thereby obtain probability distributions of this measure for specified combinations of system parameters. These distributions, in turn, can be interpreted as outage probabilities (or outage seconds) versus the number of modulation levels. A major finding of this study is that, for the assumed multipath fading model, very few taps (the order of five) are needed to approximate the performance of an ideal infinite-tap equalizer. We also find that a simple, suboptimal form of timing recovery is generally quite adequate, and that fractionally spaced equalizers are more advantageous than synchronously spaced equalizers with the same number of taps. This advantage is minor for rolloff factors of 0.5 and larger but increases dramatically as the rolloff factor approaches zero.

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.

Effect of Carrier Mistracking and Phase Jitter in a QPRS Receiver

A straightforward procedure is described for estimating the effect of carrier mistracking and phase jitter on the error rate of a digital microwave radio receiver using a Quadrature Partial Response Scheme (QPRS) for modulation. Graphical results presented can be readily used to gain insight into the performance of a QPRS receiver.