Hafizal Mohamad

Subband decomposition techniques for adaptive channel equalisation

In this contribution, the convergence behaviour of the adaptive linear equaliser based on subband decomposition technique is investigated. Two different subband-based linear equalisers are employed, with the aim of improving the equalisers convergence performance. Simulation results over three channel models having different spectral characteristic are presented. Computer simulations indicate that subband-based equalisers outperform the conventional fullband linear equaliser when channel exhibit severe spectral dynamic. Convergence rate of subband equalisers are governed by the slowest subband, whereby different convergence behaviour in each individual subband is observed. Finally, the complexity of fullband and subband equalisers is discussed.

A performance comparison of fullband and different subband adaptive equalisers

We present two different fractionally spaced (FS) equalisers based on subband methods, with the aim of reducing the computational complexity and increasing the convergence rate of a standard fullband FS equaliser. This is achieved by operating in decimated subbands; at a considerably lower update rate and by exploiting the prewhitening effect that a filter bank has on the considerable spectral dynamics of a signal received through a severely distorting channel. The two presented subband structures differ in their level of realising the feedforward and feedback part of the equaliser in the subband domain, with distinct impacts on the updating. Simulation results pinpoint the faster convergence at lower cost for the proposed subband equalisers.

On sub-band adaptive chip equalisation for direct sequence code division multiple access downlink over dispersive channels

In the downlink of a synchronous direct sequence code division multiple access (DS-CDMA) system utilising orthogonal codes, intersymbol interference (ISI) arises due to multi-path propagation, which destroys the code orthogonality and in turn, gives rise to multiple access interference (MAI). Therefore if the ISI can be mitigated through channel equalisation, the MAI can be effectively reduced. However, existing channel equalisers are plagued with different performance and complexity issues. A sub-band adaptive chip equalising (SACE) receiver is proposed as an effective solution for performance enhancement over time-dispersive DS-CDMA downlinks. The SACE receiver exploits the pre-whitening effect of the filter banks to reduce the eigenvalue spread of the input signal covariance matrix. Results based on the popular COST-207 channel models for bad urban and hilly terrain areas are presented with varying loaded conditions. It is observed that in highly dispersive channels, the proposed SACE receiver significantly outperforms the correlation receiver and the full-band equaliser while retaining the desirable attribute of graceful degradation in CDMA networks.

Subband adaptive chip equalization for interference suppression in DS-UWB

We propose a subband adaptive chip equalizing (SACE) receiver for high-speed direct-sequence ultra-wideband (DS-UWB) communications to efficiently suppress intersymbol interference (ISI) and multiple-access interference (MAI) in short-range indoor networks. The SACE receiver exploits the pre-whitening effect of the filter banks to reduce the eigenvalue spread of the input signal covariance matrix. Numerical results show that the proposed receiver significantly outperforms the fullband adaptive chip equalizing (FACE) receiver while maintaining a lower complexity.

A fast converging fractionally spaced equaliser

We present subband adaptive structures that can counteract the slow convergence which is imposed on the feed-forward part of a fractionally spaced equaliser by the spectral dynamics of the oversampled channel output. While the prewhitening of the subband approach is therefore beneficial to the feed-forward part, additionally operating the feedback part in subbands results in a more balanced and faster convergence, which is demonstrated in simulations.

Code rate-diversity tradeoff with linear block codes for MIMO wireless systems

We propose a code rate-diversity tradeoff scheme for wireless communications in which we combine cooperation with existing G2 orthogonal space-time block codes (OSTBCs) utilizing linear block codes for MIMO slow Rayleigh fading channels. Achieving the substantial diversity gains of the MIMO channel requires both linear block codes and OSTBCs that can take advantage of these channel resources. Simulation results confirm a significant improvement in the BER for both linear block codes such as Cyclic codes and Hamming codes.