MIMO equalization.

Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2005.In recent years, space-time block co'des (STBC) for multi-antenna wireless systems have emerged as attractive encoding schemes for wireless communications. These codes provide full diversity gain and achieve good performance with simple receiver structures without the additional increase in bandwidth or power requirements. When implemented over broadband channels, STBCs can be combined with orthogonal frequency division multiplexing (OFDM) or single carrier frequency domain (SC-FD) transmission schemes to achieve multi-path diversity and to decouple the broadband frequency selective channel into independent flat fading channels. This dissertation focuses on the SC-FD transmission schemes that exploit the STBC structure to provide computationally cost efficient receivers in terms of equalization and channel estimation. The main contributions in this dissertation are as follows: • The original SC-FD STBC receiver that bench marks STBC in a frequency selective channel is limited to coherent detection where the knowledge of the channel state information (CSI) is assumed at the receiver. We extend this receiver to a multiple access system. Through analysis and simulations we prove that the extended system does not incur any performance penalty. This key result implies that the SC-FD STBC scheme is suitable for multiple-user systems where higher data rates are possible. • The problem of channel estimation is considered in a time and frequency selective environment. The existing receiver is based on a recursive least squares (RLS) adaptive algorithm and provides joint equalization and interference suppression. We utilize a system with perfect channel state information (CSI) to show from simulations how various design parameters for the RLS algorithm can be selected in order to get near perfect CSI performance. • The RLS receiver has two modes of operation viz. training mode and direct decision mode. In training mode, a block of known symbols is used to make the initial estimate. To ensure convergence of the algorithm a re-training interval must be predefined. This results in an increase in the system overhead. A linear predictor that utilizes the knowled~e of the autocorrelation function for a Rayleigh fading channel is developed. The predictor is combined with. the adaptive receiver to provide a bandwidth efficient receiver by decreasing the training block size.· The simulation results show that the performance penalty for the new system is negligible. • Finally, a new Q-R based receiver is developed to provide a more robust solution to the RLS adaptive receiver. The simulation results clearly show that the new receiver outperforms the RLS based receiver at higher Doppler frequencies, where rapid channel variations result in numerical instability of the RLS algorithm. The linear predictor is also added to the new receiver which results in a more robust and bandwidth efficient receiver

Performance Improvement of Neural Network Based RLS Channel Estimators in MIMO-OFDM Systems

The objective of this study was tointroduce a recursive least squares (RLS) parameter estimatorenhanced by using a neural network (NN) to facilitate the computing of a bit error rate (BER) (error reduction) during channels estimation of a multiple input-multiple output orthogonal frequency division multiplexing (MIMO-OFDM) system over a Rayleigh multipath fading channel.Recursive least square is an efficient approach to neural network training:first, the neural network estimator learns to adapt to the channel variations then it estimates the channel frequency response. Simulation results show that the proposed method has better performance compared to the conventional methods least square (LS) and the original RLS and it is more robust at high speed mobility

Chip level decision feedback equalizer for CDMA downlink channel

In most commercial wideband code division multiple access (W-CDMA) systems, the transmitted signal in the downlink channel is spread by orthogonal codes to accommodate different users. However, frequency selective fading destroys the orthogonality and causes multiple access interference (MAI). The rake receiver has been proposed for a receiver in the downlink channel. Although providing reasonable performance due to path diversity, the rake receiver does not restore the orthogonality. As a result, the MAI is still present at the output of the rake receiver and CDMA system becomes an interference limited system. Thus, a better approach to provide MAI suppression shall be considered. The chip level linear equalizer followed by a despreader is an attractive alternative receiver to restore the orthogonality and to suppress the MAI. However, the performance of the chip level linear equalizer depends on the spectral characteristic of the channel and may not be satisfactory for some channels. To overcome this difficulty, the chip level decision feedback equalizer can be used. To improve the performance further, multiple-input multiple-output (MIMO) channels by using multiple antennas can also be employed. Recent research in information theory shows that multiple antennas can mitigate multipath fading in a wireless channel, increase the information capacity of wireless communication systems dramatically, and improve the reliability of the communications over wireless channel.In this thesis, the chip level decision feedback equalizer for CDMA downlink channel with multþle antennas is investigated. The work includes the design of the chip level decision feedback equalizer when the space time spreading scheme and the Alamouti scheme are employed at the transmitter. Theoretical and simulation results show significant performance gains compared to the rake receiver and the chip level linear equalizer.Thesis (M.Eng.Sc.) -- University of Adelaide, School of Electrical and Electronic Engineering, 2004