Intelligent joint channel parameter estimation techniques for mobile wireless positioning applications

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.Mobile wireless positioning has recently received great attention. For mobile wireless communication networks, an inherently suitable approach is to obtain the parameters that are used for positioning estimates from the radio signal measurements between a mobile device and one or more xed base stations. However, obtaining accurate estimates of these location-dependent channel parameters is a challenging task. The focus of this thesis is on the estimation of these channel parameters for mobile wireless positioning applications. In particular, we investigate novel estimators that jointly estimate more than one type of channel parameters. We rst perform a comprehensive critical review on the most recent and popular joint channel parameter estimation techniques. Secondly, we improve a state-of-the-art technique, namely the Space Alternating Generalised Expectation maximisation (SAGE) algorithm by employing adaptive interference cancellation to improve the estimation accuracy of weaker paths. Thirdly, a novel intelligent channel parameter estimation technique using Evolution Strategy (ES) is proposed to overcome the drawbacks of the existing iterative maximum likelihood methods. Furthermore, given that in reality it is di cult to obtain the number of multipath in advance, we propose a two tier Hierarchically Organised ES to jointly estimate the number of multipath as well as the channel parameters. Finally, we extend the proposed ES method to further estimate the Doppler shift in mobile environments. Our proposed intelligent joint channel estimation techniques are shown to exhibit excellent performance even with low Signal to Noise Ratio (SNR) channel conditions as well as robust against uncertainties in initialisations.EPSRC and Cambridge Silicon Radi

High Capacity CDMA and Collaborative Techniques

The thesis investigates new approaches to increase the user capacity and improve the error performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation and collaborative spreading and space diversity techniques. Collaborative Coding Multiple Access (CCMA) is also investigated as a separate technique and combined with CDMA. The advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the uplink and downlink are proposed and evaluated. Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM) property of digital signals to blindly suppress interference during the despreading process and obtain amplitude estimation with minimum mean squared error for use in cancellation stages. Two new blind adaptive receiver designs employing successive and parallel interference cancellation architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respectively, are presented. These techniques have shown to offer near single user performance for large number of users. It is shown to increase the user capacity by approximately two fold compared with conventional IC receivers. The spectral efficiency analysis of the techniques based on output signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore, an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI without any knowledge of channel amplitudes and allows large number of users compared with equal gain and maximum ratio combining techniques normally used in practice. New user collaborative schemes are proposed and analysed theoretically and by simulations in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First, a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA are designed and analysed. Next, a new user collaborative scheme with successive interference cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce MAI and achieve improved diversity. To further improve the performance of C-SIC under high system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed. It is shown to minimize the residual MAI, leading to improved user capacity and a more robust system. It is known that collaborative diversity schemes incur loss in throughput due to the need of orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time encoding methods. At the receiver collaborative joint detection is performed to separate each paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth as inter-user channel SNR becomes high. A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA for both the downlink and uplink. First, collaborative group spreading technique for the downlink of overloaded CDMA system is introduced. It allows the sharing of the same single spreading sequence for more than one user belonging to the same group. This technique is referred to as Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative coding is used for each group to form a composite codeword signal of the users and then a single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite codeword is carried out to extract the user’s own information while maintaining a high SINR performance. To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions, Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative coding multiple access and space-time coding principles. A new scheme for uplink of CDMA using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved by grouping multiple users to share the same spreading sequence and performing MUD on per group basis followed by a low complexity ML decoding at the receiver. This approach has shown to support much higher number of users than the available sequences while also maintaining the low receiver complexity. For improved performance under highly correlated channel conditions, T-user collaborative coding is also investigated within the CS-CDMA-UL system

Técnicas de cancelamento PIC para o sistema UMTS-TDD

Mestrado em Engenharia Electrónica e TelecomunicaçõesO objectivo desta dissertação é estender o conceito das técnicas PIC (Parallel Interference Cancellation) a receptores com agregados de antenas e avaliar o desempenho das diferentes estruturas PIC em vários cenários no sistema UMTS-TDD (Universal Mobile Telecommunication System). Para tal, a dissertação inclui secções de preparação, onde é efectuado uma revisão da evolução das comunicações móveis celulares, de alguns aspectos fundamentais das técnicas de espalhamento de espectro, das especificações do sistema UMTS-TDD relevantes para o trabalho e a nível mais avançado uma síntese das principais técnicas MUD (Multiuser Detection) propostas na literatura. Após a apresentação deste material preparatório necessário para compreender o trabalho realizado, são descritos, caracterizados, avaliados e comparados os vários algoritmos. Foram implementadas numa cadeia de simulação vários tipos de estruturas PIC, designadamente, com um e dois estágios, com cancelamento parcial e completo, com estimativas hard e soft, com uma antena e com um agregado circular uniforme de M elementos. De forma a avaliar o desempenho destas estruturas PIC foram obtidos resultados em vários cenários, nomeadamente, com estimativas perfeitas dos parâmetros do canal e controlo de potência perfeito; com estimativas imperfeitas dos parâmetros do canal e controlo de potência perfeito; com factores de espalhamento variáveis e estimativas perfeitas dos parâmetros do canal. Os parâmetros necessários para efectuar simulações ao nível do sistema foram, também, calculados. Como conclusão principal deste trabalho, ressalta que a estrutura HD_PIC (hard decision - PIC) com um estágio é a que apresenta a melhor relação desempenho/complexidade, verificando-se que a combinação desta estrutura com um agregado de antenas é uma técnica extremamente promissora para o UMTS_TDD. Palavras Chave: detecção multi-utilizador, cancelamento de interferência, agregados de antenas, filtragem espacio-temporal, DS_CDMA, PIC e UMTSTD

Iterative Detection for Overloaded Multiuser MIMO OFDM Systems

Inspired by multiuser detection (MUD) and the ‘Turbo principle’, this thesis deals with iterative interference cancellation (IIC) in overloaded multiuser multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) systems. Linear detection schemes, such as zero forcing (ZF) and minimum mean square error (MMSE) cannot be used for the overloaded system because of the rank deficiency of channel matrix, while the optimal approach, the maximum likelihood (ML) detection has high computational complexity. In this thesis, an iterative interference cancellation (IIC) multiuser detection scheme with matched filter and convolutional codes is considered. The main idea of this combination is a low complexity receiver. Parallel interference cancellation (PIC) is employed to improve the multiuser receiver performance for overloaded systems. A log-likelihood ratio (LLR) converter is proposed to further improve the reliability of the soft value converted from the output of the matched filter. Simulation results show that the bit error rate (BER) performance of this method is close to the optimal approach for a two user system. However, for the four user or more user system, it has an error floor of the BER performance. For this case, a channel selection scheme is proposed to distinguish whether the channel is good or bad by using the mutual information based on the extrinsic information transfer (EXIT) chart. The mutual information can be predicted in a look-up table which greatly reduces the complexity. For those ‘bad’ channels identified by the channel selection, we introduce two adaptive transmission methods to deal with such channels: one uses a lower code rate, and the other is multiple transmissions. The use of an IIC receiver with the interleave-division multiple access (IDMA) to further improve the BER performance without any channel selection is also investigated. It has been shown that this approach can remove the error floor. Finally, the influence of channel accuracy on the IIC is investigated. Pilot-based Wiener filter channel estimation is used to test and verify how much the IIC is influenced by the channel accuracy

Signal processing for future MIMO-OFDM wireless communication systems

The combination of multiple-input multiple-output (MIMO) technology and orthogonal frequency division multiplexing (OFDM) is likely to provide the air-interface solution for future broadband wireless systems. A major challenge for MIMO-OFDM systems is the problem of multi-access interference (MAI) induced by the presence of multiple users transmitting over the same bandwidth. Novel signal processing techniques are therefore required to mitigate MAI and thereby increase link performance. A background review of space-time block codes (STBCs) to lever age diversity gain in MIMO systems is provided together with an introduction to OFDM. The link performance of an OFDM system is also shown to be sensitive to time-variation of the channel. Iterative minimum mean square error (MMSE) receivers are therefore proposed to overcome such time-variation. In the context of synchronous uplink transmission, a new two-step hard-decision interference cancellation receiver for STBC MIMO-OFDM is shown to have robust performance and relatively low complexity. Further improvement is obtained through employing error control coding methods and iterative algorithms. A soft output multiuser detector based on MMSE interference suppression and error correction coding at the first stage is shown by frame error rate simulations to provide significant performance improvement over the classical linear scheme. Finally, building on the "turbo principle", a low-complexity iterative interference cancellation and detection scheme is designed to provide a good compromise between the exponential computational complexity of the soft interference cancellation linear MMSE algorithm and the near-capacity performance of a scheme which uses iterative turbo processing for soft interference suppression in combination with multiuser detection.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Signal processing for future MIMO-OFDM wireless communication systems

The combination of multiple-input multiple-output (MIMO) technology and orthogonal frequency division multiplexing (OFDM) is likely to provide the air-interface solution for future broadband wireless systems. A major challenge for MIMO-OFDM systems is the problem of multi-access interference (MAI) induced by the presence of multiple users transmitting over the same bandwidth. Novel signal processing techniques are therefore required to mitigate MAI and thereby increase link performance. A background review of space-time block codes (STBCs) to lever age diversity gain in MIMO systems is provided together with an introduction to OFDM. The link performance of an OFDM system is also shown to be sensitive to time-variation of the channel. Iterative minimum mean square error (MMSE) receivers are therefore proposed to overcome such time-variation. In the context of synchronous uplink transmission, a new two-step hard-decision interference cancellation receiver for STBC MIMO-OFDM is shown to have robust performance and relatively low complexity. Further improvement is obtained through employing error control coding methods and iterative algorithms. A soft output multiuser detector based on MMSE interference suppression and error correction coding at the first stage is shown by frame error rate simulations to provide significant performance improvement over the classical linear scheme. Finally, building on the "turbo principle", a low-complexity iterative interference cancellation and detection scheme is designed to provide a good compromise between the exponential computational complexity of the soft interference cancellation linear MMSE algorithm and the near-capacity performance of a scheme which uses iterative turbo processing for soft interference suppression in combination with multiuser detection

Pré-filtragem no espaço-frequência para o sistema MC-CDMA

Doutoramento em Engenharia ElectrotécnicaO trabalho desta tese enquadra-se na área de comunicações móveis, mais especificamente em sistemas de portadora múltipla. O MC-CDMA, que combina a modulação OFDM com o espalhamento na frequência, é um dos candidatos mais promissores para a interface-ar dos futuros sistemas de comunicações móveis – 4G. O objectivo desta tese é propor e avaliar técnicas de pré-filtragem e de codificação, projectadas no espaço-frequência/tempo para o sistema MC-CDMA, no sentido descendente (Downlink). Inicialmente, são discutidos conceitos teóricos fundamentais para se compreender o mecanismo físico de propagação inerente às comunicações móveis, apresentando-se depois vários sistemas de portadora múltipla. É dada especial atenção ao sistema convencional MC-CDMA. Este sistema é importante porque serve de referência ao desempenho obtido com as técnicas de transmissão propostas nesta tese. Estas técnicas são projectadas tendo em conta as restrições em termos de complexidade do terminal móvel. Assim, a estação base é equipada com um agregado de antenas e o terminal móvel com uma antena, sendo neste último, apenas implementadas técnicas de detecção mono-utilizador. Assumindo, que a estação base conhece a resposta do canal antes da transmissão, são propostas diferentes estratégias de transmissão: os filtros são projectados no espaço-frequência para o sistema MC-CDMA combinados com ou sem equalização no terminal móvel; e o filtro é projectado apenas na frequência para o sistema MC-CDMA com codificação no espaço-frequência/tempo. O algoritmo é baseado na minimização da potência transmitida sujeita à total eliminação da interferência de acesso múltiplo e das distorções provocadas pelo canal rádio móvel. Todos os esquemas propostos são validados e comparados, através de simulações, em cenários típicos de interiores e exteriores. Como principal conclusão desta tese, destaca-se a significativa melhoria de desempenho obtido com estas técnicas, relativamente ao sistema convencional MC-CDMA. Além disso, este desempenho é conseguido com um terminal móvel de reduzida complexidade. Assim, estas técnicas permitem aumentar significativamente a capacidade do sistema e, simultaneamente, transferir grande parte da complexidade do terminal móvel para a estação base.The scope of this thesis targets multi-carrier modulation techniques for mobile radio communications system. MC-CDMA combining multi-carrier modulation and spreading in the frequency domain is widely viewed as a promising candidate for 4G air interfaces. The aim of this thesis is to propose and evaluate pre-filtering and coding techniques designed in space and frequency/time for the downlink MC-CDMA system. Initially, the fundamental propagation mechanisms inherent to mobile radio communications are discussed and then several multi-carrier schemes are presented. Furthermore, special attention is given to the conventional MCCDMA system, since it can be used as the reference benchmark performance for the advanced transmission techniques proposed in this thesis. These transmission schemes are designed taking into account the complexity constraints at the mobile terminal. Hence, the basestation is equipped with an antenna array and the mobile terminal comprises a single antenna and single user detection scheme. Based on the assumption that the basestation has prior channel knowledge, different transmission strategies are proposed: spacefrequency pre-filtering schemes combined with single user equalizers at the MT for the MC-CDMA system; frequency pre-filtering scheme for spacefrequency/ time coding MC-CDMA system. The algorithm is based on the minimization of the transmitted power subject to MAI and channel distortion elimination. The proposed pre-filtering schemes are assessed and compared through simulations in typical indoor and pedestrian scenarios. This work concluded that with the proposed pre-filtering schemes, we obtain a considerable performance improvement in typical indoor and outdoor scenarios with a low complexity mobile terminal design and allow a transfer of implementation complexity from the mobile to the basestation