Frequency-selective Multipath Fading Channels Research Articles

Adaptive MMSE maximum likelihood CDMA multiuser detection

The well-known code division multiple access maximum likelihood receiver (MF-ML) uses a bank of matched filters as a generator of sufficient statistics for maximum likelihood detection of users transmitted symbols. In this paper, the bank of matched filters is replaced by a bank of adaptive minimum mean squared error (MMSE) filters as the generator of sufficient statistics. This formal replacement of the MF bank by the adaptive MMSE filter bank has significant conceptual consequences and provides improvement by several performance measures. The adaptive MMSE-ML receiver's digital implementation is significantly computationally simplified. The advantages of the proposed adaptive MMSE-ML receiver over the MF-ML receiver are: (1) ability to perform joint synchronization, channel parameter estimation, and signal detection where the signal is sent over an unknown, slowly time-varying, frequency-selective multipath fading channel; (2) increased information capacity in a multicellular environment; and (3) significantly improved bit error rate (BER) performance in a multicellular mobile communications environment. The information capacity and the BER of the proposed MMSE-ML receiver are analyzed. Numerical results showing the BER performance of the MMSE-ML receiver in a multipath channel environment are presented.

Multistage interference cancellation with diversity reception for asynchronous QPSK DS/CDMA systems over multipath fading channels

This paper introduces a multistage interference cancellation (MIC) technique with diversity reception for quadrature phase shift keying (QPSK) asynchronous direct-sequence code division multiple access (DS/CDMA) systems over frequency-selective multipath Rayleigh fading channels. Unlike the previous MIC, which tries to remove the lump sum of the multiple-access interference (MAI) and self-interference (SI), this introduced MIC attempts to cancel only the MAI and part of the SI due to the intersymbol interference, while treating the remaining SI created by the current symbol as useful information for symbol decision. In this technique, the RAKE combining is used to collect signal replicas over multiple fading paths. Upper and lower bounds on the bit error probability are derived using a Gaussian approximation and the characteristic function method. Furthermore, effects of channel estimation error on the performance are studied. Analytical and simulation results show that the introduced MIC can provide a performance extremely close to that in an ideal single-user environment and outperforms the previous MIC even in the presence of channel estimation error.

The effect of diversity on a burst-mode carrier-frequency estimator in the frequency-selective multipath channel

This paper describes a carrier-frequency offset estimator derived using maximum-likelihood techniques. The estimator is designed for a digital space-diversity receiver, operating in a wide-band frequency-selective multipath fading channel. The estimator is suited to a burst-mode time-division-multiple-access system, because the estimate is formed in an open-loop manner, and relies on a training sequence normally used in burst-mode systems. The main advantage is that it does not require channel state information; rather, it only requires knowledge of the auto-correlation of the channel. Simulation results show the estimator to be unbiased over a wide frequency range. The normalized error standard deviation is shown to be 0.0015 across the frequency offset range for a receiver with four diversity branches, when the normalized channel delay spread is 0.1, the signal-to-noise ratio (SNR) is 10 dB, and the training sequence length is 23 symbols. It is found that, for most parameter configurations, there is approximately a factor of two improvement in the estimate error standard deviation when the antenna diversity is doubled.

A noncoherent tracking loop with diversity and multipath interference cancellation for direct-sequence spread-spectrum systems

A noncoherent tracking loop with diversity reception and multipath interference cancellation is proposed for direct-sequence spread-spectrum systems on frequency-selective multipath fading channels. The basic idea of the loop is to reproduce the multipath interference and remove it from the received signal, and then a path diversity is embedded in the loop to improve the loop performance. The tracking error performance of the loop is evaluated by linear analyses and computer simulations. Numerical results show that the proposed tracking loop outperforms the traditional delay locked loop and performs very well under all considered fading conditions.