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

Semi-blind channel estimation for multiuser OFDM-IDMA systems.

M. Sc. Eng. University of KwaZulu-Natal, Durban 2014.Over the last decade, the data rate and spectral efficiency of wireless mobile communications have been significantly enhanced. OFDM technology has been used in the development of advanced systems such as 3GPP LTE and terrestrial digital TV broadcasting. In general, bits of information in mobile communication systems are conveyed through radio links to receivers. The radio channels in mobile radio systems are usually multipath fading channels, which cause inter-symbol interference (ISI) in the received signal. The ability to know the channel impulse response (CIR) and Channel State Information (CSI) helps to remove the ISI from the signal and make coherent detection of the transmitted signal at the receiver end of the system easy and simple. The information about CIR and CSI are primarily provided by channel estimation. This thesis is focused on the development of multiple access communication technique, Multicarrier Interleave Division Multiple Access (MC-IDMA) and the corresponding estimation of the system channel. It compares various efficient channel estimation algorithms. Channel estimation of OFDM-IDMA scheme is important because the emphasis from previous studies assumed the implementation of MC-IDMA in a perfect scenario, where Channel State Information (CSI) is known. MC-IDMA technique incorporates three key features that will be common to the next generation communication systems; multiple access capability, resistance to multipath fading and high bandwidth efficiency. OFDM is almost completely immune to multipath fading effects and IDMA has a recently proposed multiuser capability scheme which employs random interleavers as the only method for user separation. MC-IDMA combines the features of OFDM and IDMA to produce a system that is Inter Symbol Interference (ISI) free and has higher data rate capabilities for multiple users simultaneously. The interleaver property of IDMA is used by MC-IDMA as the only means by which users are separated at the receiver and also its entire bandwidth expansion is devoted to low rate Forward Error Correction (FEC). This provides additional coding gain which is not present in conventional Multicarrier Multiuser systems, (MC-MU) such as Code Division Multiple Access (CDMA), Multicarrier-Code Division Multiple Access (MC-CDMA) systems, and others. The effect of channel fading and both cross-cell and intra-cell Multiple Access Interference (MAI) in MC-IDMA is suppressed efficiently by its low-cost turbo-type Chip-by-Chip (CBC) multiuser detection algorithm. We present the basic principles of OFDM-IDMA transmitter and receiver. Comparative studies between Multiple Access Scheme such as Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA), CDMA and IDMA are carried out. A linear Minimum Mean Square Error (MMSE)-based estimation algorithm is adopted and implemented. This proposed algorithm is a non-data aided method that focuses on obtaining the CSI, remove ISI and reduce the complexity of the MMSE algorithm. However, to obtain a better and improved system performance, an improved MMSE algorithm and simplified MMSE using the structured correlation and reduced auto-covariance matrix are developed in this thesis and proposed for implementation of semi-blind channel estimation in OFDM-IDMA communication systems. The effectiveness of the adopted and proposed algorithms are implemented in a Rayleigh fading multipath channel with varying mobile speeds thus demonstrating the performance of the system in a practical scenario. Also, the implemented algorithms are compared to ascertain which of these algorithms offers a better and more efficient system performance, and with less complexity. The performance of the channel estimation algorithm is presented in terms of the mean square error (MSE) and bit error rate (BER) in both slow fading and fast fading multipath scenarios and the results are documented as well

Particle filtering for joint symbol and code delay estimation in DS spread spectrum systems in multipath environment

We develop a new receiver for joint symbol, channel characteristics, and code delay estimation for DS spread spectrum systems under conditions of multipath fading. The approach is based on particle filtering techniques and combines sequential importance sampling, a selection scheme, and a variance reduction technique. Several algorithms involving both deterministic and randomized schemes are considered and an extensive simulation study is carried out in order to demonstrate the performance of the proposed methods.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Performance of a space-time coded multicarrier CDMA system in frequency-selective Rayleigh channel.

Ph. D. University of KwaZulu-Natal, Durban 2014.The increasing demand for wireless services requires fast and robust broadband wireless communication for efficient utilisation of the scarce electromagnetic spectrum. One of the promising techniques for future wireless communication is the deployment of multi-input multi-output (MIMO) antenna system with orthogonal frequency division multiplexing (OFDM) coupled with multiple-access techniques. The combination of these techniques guarantees a much more reliable and robust transmission over the hostile wireless channel. This thesis investigates the performance of a multi-antenna space-time coded (STC) multi-carrier code-division multiple-access (MC-CDMA) system in a frequency-selective channel using Gold codes as spreading sequences. Spreading codes are known to be central to the performance of spread spectrum systems, STC MC-CDMA systems inclusive. Initial phase of this research work investigates multiple-access performance of spreading codes for the communication system. The performance of different sets of Gold codes for increasing number of interfering users for up to a thousand users and eight different code lengths, ranging from 31 to 4095-chip Gold codes, were considered. Simulation results show that odd-degree Gold codes give better bit-error-rate performance than even-degree Gold codes. Whereas the odd-degree codes exhibited relatively marginal loss in performance when the system was loaded, their even-degree counterparts degraded rapidly in performance, resulting in early emergence of an error floor, culminating in premature system saturation. Furthermore in this thesis, software simulations were carried to investigate the performance of a direct-sequence (DS) CDMA system in a flat-fading Rayleigh channel, and a multi-carrier (MC) CDMA system in a frequency-selective channel using different sets of Gold. The results showed that in a flat-fading channel, the Gold codes provide a constant coding gain close to that obtainable in a Gaussian channel. The results also showed that the impact of longer spreading codes was more pronounced for the MC-CDMA system in a frequency-selective channel as indicated by significant lowering of error floors. Also, frequency diversity associated with the use of longer codes coupled with multi-carrier modulation makes the MC-CDMA system resilient to multi-path effects. Further still, this thesis investigated the performance of a space-time block-coded (STBC) CDMA system in a flat-fading channel. Results showed that at low signal-to-noise ratio, the coding gain provided by the codes surpasses the diversity advantage provided by the use of the multiple antennas. The results also showed that coding gain between no-diversity link and its Gold-coded counterpart is the same as that between the transmit-diversity link and its Gold–coded counterpart. The independence of the diversity advantage provided by multiple transmit antennas and the coding gain obtainable from the use of the spreading sequences enables the prediction of the performance of composite space-time block-coded CDMA systems. Performance of a STBC OFDM system as well as a STBC MC-CDMA system in frequency-selective channel was also investigated. Results showed that the combination of diversity gain from the use of multiple antennas, coupled with coding gain provided by the Gold codes of the CDMA system, plus the diversity gain resulting from frequency diversity of multi-carrier transmission and the spectrum-spreading by the CDMA makes the composite STBC MC-CDMA system resilient to channel fading. This fact is particularly the case for long codes. For example, with reference to the OFDM transmission, the results showed that a 511-chip Gold-coded STC MC-CDMA system provided a factor of about 3,786 reduction in error floor

Signal Detection in Space-Time Coded Communication Systems with Imperfect Channel Estimation and Carrier Frequency Offset

In multi-antenna communication systems, signal detection is significantly affected by the presence of channel fading and the introduction of Carrier Frequency Offset (CFO) during signal demodulation. The conventional solution is to estimate the Channel State Information (CSI) and CFO and apply estimates in a detector metric that assumes perfect knowledge of CSI and CFO. This thesis proposes new metrics for Space-Time Block decoding with noisy CSI and CFO estimates by including the error variance of CSI and CFO estimates in the metric derivation.The BER performance of the conventional metric and proposed metrics, both using Joint Maximum A Posteriori (MAP) CSI/CFO estimates shows that the former slightly outperforms the latter and their performances converge at high SNR values. However, under worse-case scenarios, the proposed metrics outperform the conventional metric.We conclude that the joint MAP estimator/conventional metric combination is more appropriate for signal detection due to its relatively low complexity