Time-varying Frequency-selective Channels Research Articles

Maximum-likelihood synchronization, equalization, and sequence estimation for unknown time-varying frequency-selective Rician channels

This paper develops a receiver structure to perform jointly maximum-likelihood (ML) synchronization, equalization, and detection of a linearly modulated signal transmitted over a time-varying frequency-selective Rician-faded channel, corrupted by additive white Gaussian noise (AWGN). The receiver is particularly suited to a fast-fading channel, where other receivers that rely on estimating the channel cannot track it quickly enough. The signal mean and autocovariance are needed, and a scheme is proposed for estimating these quantities adaptively. The receiver processes the specular and diffuse components (corresponding to the signal mean and autocovariance) separately. Processing the known specular component is the classical detection problem. The unknown diffuse component is processed by predictors. We show that the predictors can achieve synchronization in a novel manner, if synchronization is required. A union bound on the receiver's bit-error rate (BER) is derived, and it tightly bounds simulated BERs in fast-fading at high signal-to-noise ratios (SNRs).

MLSE for correlated diversity sources and unknown time-varying frequency-selective Rayleigh-fading channels

Yu and Pasupathy (see ibid., vol.43, p. 1534-44, 1995) derived a maximum-likelihood sequence estimation (MLSE) receiver structure for unknown time varying frequency-selective Rayleigh-fading channels and uncorrelated diversity sources. This receiver design is extended to the case of correlated diversity sources. Correlated diversity sources typically arise with space diversity, where constraints on antenna volume require that diversity antennae be placed too closely together. Analytic and simulated bit-error rate (BER) curves are presented for receivers which exploit and ignore the correlation. In the former case, we find a small BER improvement that reduces with decreasing correlation. However, for a fixed receiver complexity, superior performance is achieved when the correlation is ignored.

Performance of adaptive multisensor decision feedbackequaliserfor time-varying frequency selective radio channels

A multisensor decision feedback equaliser based on the minimum mean squared error (MMSE) criterion is studied. The superiority of the performance of the multisensor equaliser is shown by simulation of a whole communication system in which the adaptive equaliser is incorporated. The recursive least squares (RLS) algorithm is used to update the coefficients. From the results obtained for a time-varying urban terrain channel model, the extremely interesting tracking capability of the multisensor equaliser is shown.

A spread spectrum communications channel sounder

The design of a microwave channel probing analyzer is presented and discussed. Special attention is paid to the characteristics of the transmission path to be measured; the choice of spread spectrum probing signal; the signal processing methods used; modulation, demodulation, and synchronization at the transmitter and receiver sites; the path model to be fitted to the measured data and finally, the impact of all of these issues upon the necessary hardware and software. Following a formulation of the channel-probing problem, transfer function regression techniques are reviewed to illustrate the advantages of periodic averaging over established correlation methods. The construction of an experimental sounder based on these methods is described. The equipment has been used to measure the time-variant frequency-selective channel dynamics of a short (158 km) troposcatter transhorizon link at a frequency of 11.647 GHz over a 31.25-MHz bandwidth. Off-line, nonlinear spectrum analysis, based on Prony's method and singular value decomposition, is used to estimate parameters of a high-resolution delay model that accurately reflects the underlying ray structure of the transmission path.