Performance evaluation of detection algorithms for MOMI OFDM systems

Includes abstract.Includes bibliographical references (leaves 79-86).Introduction of Multi Input Multi Output (MIMO) Orthogonal Frequency Division Multiplexing (OFDM) as the base air interface method for Next Generation Network (NGN) will face a number of challenges from hostile channel conditions to interference from other users. This would result in an increase of detection complexity required for mobile systems. Complex detection will reduce the battery life of mobile devices because of the many calculations that have to be done to decode the signal. Very powerful detection algorithms exist but they introduce high detection complexity. NGN will employ different MIMO systems, but this research will consider spatially multiplexed MIMO which is used to improve the data rate and network capacity. In NGN different multi access modulation schemes will be used for uplink and downlink but they both have OFDM as the basic building block. In this work performance of MIMO OFDM is investigated in different channels models and detection algorithms. A low complexity detection scheme is proposed in this research to improve performance of MIMO OFDM. The proposed detection scheme is investigated for different channel characteristics. Realistic channels conditions are introduced to evaluate the performance of the proposed detection scheme. We analyze weaknesses of existing linear detectors and the enhancements that can be done to improve their performance in different channel conditions. Performance of the detectors is evaluated by comparison of Bit Error Rate (BER) and Symbol Error Rate (SER) against signal to noise ratio (SNR). This thesis proposes a detector which shows a higher complexity than linear detectors but less than Maximum Likelihood Detector (MLD). The proposed detector shows significant BER improvement in all channel conditions. For better performance evaluation this work also investigates performance of MIMO OFDM detectors in realistic channels like Kronecker and Weichselberger channel models

PERFORMANCE ANALYSIS OF SELECTIVE WEIGHTED LINEAR PARALLEL INTERFERENCE CANCELLATION SCHEME FOR MC-CDMA SYSTEM

ABSTRACT A novel Selective Weighted Linear Parallel Interference Cancellation (SWLPIC) technique is proposed to reduce the interference (degrading effect) on a desired user caused by other users sharing the same channel in a Multicarrier Code Division Multiple Access (MC-CDMA) system. The SWLPIC technique cancels the interference using WLPIC on selected users whose instantaneous Signal to Interference Ratio (SIR) exceed certain threshold on Rayleigh fading channels in each subcarrier. In WLPIC the multi access interference (MAI) are estimated and cancelled based on the soft outputs of individual subcarriers. The MAI estimate of individual subcarriers is scaled by a weight before cancellation. Also, the SIR for each subcarrier is determined and an expression for the optimum threshold is derived by optimizing the approximate expression for the SIR. The interference cancelled outputs of different subcarriers are then combined to form the final decision statistic. The simulation results show that the SWLPIC technique outperforms over matched filter (MF) detector and conventional Linear PIC (LPIC). Also the performance of SWLPIC approaches weighted LPIC (WLPIC)

Joint Unitary Triangularization for MIMO Networks

This work considers communication networks where individual links can be described as MIMO channels. Unlike orthogonal modulation methods (such as the singular-value decomposition), we allow interference between sub-channels, which can be removed by the receivers via successive cancellation. The degrees of freedom earned by this relaxation are used for obtaining a basis which is simultaneously good for more than one link. Specifically, we derive necessary and sufficient conditions for shaping the ratio vector of sub-channel gains of two broadcast-channel receivers. We then apply this to two scenarios: First, in digital multicasting we present a practical capacity-achieving scheme which only uses scalar codes and linear processing. Then, we consider the joint source-channel problem of transmitting a Gaussian source over a two-user MIMO channel, where we show the existence of non-trivial cases, where the optimal distortion pair (which for high signal-to-noise ratios equals the optimal point-to-point distortions of the individual users) may be achieved by employing a hybrid digital-analog scheme over the induced equivalent channel. These scenarios demonstrate the advantage of choosing a modulation basis based upon multiple links in the network, thus we coin the approach "network modulation".Comment: Submitted to IEEE Tran. Signal Processing. Revised versio

Performance of turbo multi-user detectors in space-time coded DS-CDMA systems

Includes bibliographical references (leaves 118-123).In this thesis we address the problem of improving the uplink capacity and the performance of a DS-CDMA system by combining MUD and turbo decoding. These two are combined following the turbo principle. Depending on the concatenation scheme used, we divide these receivers into the Partitioned Approach (PA) and the Iterative Approach (IA) receivers. To enable the iterative exchange of information, these receivers employ a Parallel Interference Cancellation (PIC) detector as the first receiver stage

An investigation into the improvement in WCDMA system performance using multiuser detection and interference cancellation

WCDMA is typically characterised as a system capable of providing mobile users with data rates up to 2 Mb/s and beyond. It has been termed an ultra high-speed, ultra high-capacity radio technology that will be able to carry a new range of fast, colourful media, such as colour graphics, video, animations, digital audio, Internet and e-mail that consumers will be able to access over their mobiles devices. This current study has researched on the various existing Multiuser detection (MUD) processes or proposals conducted by various research institutions around the world. It has identified the advantages that the past work offers, and it is these advantages that form the basis of the current research into the improvement techniques. The proposed Partial Parallel Pipelined Multiuser Detector (PPPMUD or P3MUD) has come about from two main flavours or directions of research. The first one seeks to promote the Soft Parallel Interference Cancellation technique as an effective bias mitigation technique. This bias occurring in the second stage decision statistics, exhibits a very harmful effect on system Bit Error Rate, (BER), particularly for large system loads. This current study goes further by carefully analysing the Soft Cancellation Factor, SCF behaviour to eventually derive and determine the optimum SCF value which exhibits positive characteristics when varied with the increasing system load (number of users). This optimum SCF value is called the universal SCF or SCFUNV, as it is theoretically supposed to perform favourably under various system loads. A favourable or acceptable performance would be characterised by low observed or measured BER during the system processing stages. A further enhancement to the operational performance of the SCFUNV algorithm is the SCFUNV Compensator, which is basically a compensation mechanism created by modelling the behaviour of the SCF values, and adjusts the SCFUNV depending on the system load, (number of simultaneous users). Thus, the SCFUNV is adaptively adjusted in order to perform acceptably under all load conditions. The second direction of research, as regards improvements in MUD techniques, involves the conventional Bit-Streaming, Pipelined Multiuser Detector. This came about due to the computational complexity as well as matrix inversions which affected earlier asynchronous multiuser detection techniques. This detector has a pipelined architecture which avoids multishot (block-based) detection and instead, processes the bits in a streaming fashion. The architecture consists of a matched filter followed by three stages of parallel interference cancellation, (PIC). This present study extends that research by outlining the advantages of incorporating the soft parallel interference cancellation technique, by way of the universal soft cancellation factor, (SCFUNV), into the conventional pipelined multiuser detector architecture to form the P3MUD architecture, which includes the compensator. The contributions of the proposed P3MUD system is that the observed BER output simulations are promising, with an observed overall decrease in the error rate for the P3MUD process, as compared to the conventional pipelined detection method. This decrease signifies an improvement offered by the proposed (P3MUD) algorithm. Further observed analysis indicates the possibility of decreasing the number of parallel interference cancellation stages from three to at least two, after the matched filter detection stage, without an observable change in system BER. Hence, the proposal of the two-stage P3MUD.Dissertation (MEng (Electronic))--University of Pretoria, 2006.Electrical, Electronic and Computer Engineeringunrestricte

Physical-Layer Cooperation in Coded OFDM Relaying Systems

Mobile communication systems nowadays require ever-increasing data rate and coverage of wide areas. One promising approach to achieve this goal is the application of cooperative communications enabled by introducing intermediate nodes known as relays to support the transmission between terminals. By processing and forwarding the receive message at the relays, the path-loss effect between the source and the destination is mitigated. One major limit factor for relay assisted communications is that a relay cannot transmit and receive using the same physical resources. Therefore, a half-duplex constraint is commonly assumed resulting in halved spectral efficiency. To combat this drawback, two-way relaying is introduced, where two sources exchange information with each. On the other hand, due to the physical limitation of the relays, e.g., wireless sensor nodes, it's not possible to implement multiple antennas at one relay, which prohibits the application of multiple-input multiple-output (MIMO) techniques. However, when treating multiple relays as a cluster, a virtual antenna array is formed to perform MIMO techniques in a distributed manner. %This thesis aims at designing efficient one-way and two-way relaying schemes. Specifically, existing schemes from the literature are improved and new schemes are developed with the emphasis on coded orthogonal frequency division multiplexing (OFDM) transmissions. Of special interest is the application of physical-layer network coding (PLNC) for two-phase two-way relaying. In this case, a network coded message is estimated from the superimposed receive signal at the relay using PLNC schemes. The schemes are investigated based on a mutual information analysis and their performance are improved by a newly proposed phase control strategy. Furthermore, performance degradation due to system asynchrony is mitigated depending on different PLNC schemes. When multiple relays are available, novel cooperation schemes allowing information exchange within the relay cluster are proposed that facilitate distributed MIMO reception and transmission. Additionally, smart signaling approaches are presented to enable the cooperation at different levels with the cooperation overhead taken into account adequately in system performance evaluation