Varying Frequency Selective Fading Channels Research Articles

Adaptive Selection Combining Receiver over Time Varying Frequency Selective Fading Channel in Class-A Noise

An adaptive selection combining (SC) scheme is proposed for time varying mobile communication channel in Class-A impulsive noise. The receiver adaptively selects a diversity branch out of the available branches and discards the others. This is performed by computing the maximum likelihood (ML) metric of each diversity branch and selects the branch with the maximum metric. The proposed adaptive SC scheme dynamically adjusts the threshold value according to the time variations of the channel. Equalization and data detection are performed after combining using maximum likelihood sequence estimation implemented by Viterbi algorithm (MLSE-VA). The minimum survivor technique is employed to reduce the complexity of the receiver.

Interleaver Structures for Channel Estimation and Decoding on the Frequency Selective Fading Channel

This paper examines different designs for combined channel estimation and decoding for a point to point time varying frequency selective fading channel. The channel state information (CSI) is not known a priori at either the transmitter or the receiver. Channel estimation is performed through a combination of conventional pilots and manufactured pilots from previously decoded data. The paper examines how different interleaver structures affect the tradeoff between achievable rates, delay and complexity. When there is no delay constraint at all, the conventional pilots can be eliminated completely and a proper interleaving scheme can convert half the symbols into manufactured pilots. When there is a delay constraint, the paper explores various interleaver structures that combine the conventional pilots with the manufactured pilots in different ways to get much better achievable rates than conventional pilot-symbol aided channel estimation.

Optimal Superimposed Training Sequences for Channel Estimation in MIMO-OFDM Systems

In this work an iterative time domain Least Squares (LS) based channel estimation method using superimposed training (ST) for a Multiple Input Multiple Output Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system over time varying frequency selective fading channels is proposed. The performance of the channel estimator is analyzed in terms of the Mean Square Estimation Error (MSEE) and its impact on the uncoded Bit Error Rate (BER) of the MIMO-OFDM system is studied. A new selection criterion for the training sequences that jointly optimizes the MSEE and the BER of the OFDM system is proposed. Chirp based sequences are proposed and shown to satisfy the same. These are compared with the other sequences proposed in the literature and are found to yield a superior performance. The sequences, one for each transmitting antenna, offers fairness through providing equal interference in all the data carriers unlike earlier proposals. The effectiveness of the mathematical analysis presented is demonstrated through a comparison with the simulation studies. Experimental studies are carried out to study and validate the improved performance of the proposed scheme. The scheme is applied to the IEEE 802.16e OFDM standard and a case is made with the required design of the sequence.

Adaptive MMSE decision feedback equalizer for asynchronous CDMA with erasure algorithm

Adaptive decision feedback equalizer with erasure algorithm (E-DFE) is proposed for asynchronous code division multiple access (CDMA) transmission, which not only combats intersymbol interference (ISI) and multiple access interference (MAI) but also reduces the effects of error propagation in the presence of Gaussian background noise. To reduce the possibility of feeding back the wrong decisions, the output of the feedforward filter of the E-DFE is processed before it is fed back to the feedback filter. Specifically, the focus is on the performance of E-DFE using soft-slicer based on a novel erasure algorithm. In addition, the fully connected feedback filter of E-DFE has been used to eliminate ISI due to other active users. Comparison of the performance of conventional decision feedback equalizer (C-DFE) and E-DFE over slowly varying frequency selective fading channel is presented to show the advantages of E-DFE in terms of reduced average bit error rate performance. Simulation results are also presented to show the substantial improvement in its performance under near-far situations and the sudden change in the signal power of the desired user.