In order to improve communication over a dispersive channel in a CDMA system, we have to re-establish the orthogonally of codes which are used when combining input signals from many users onto a single communication path, as otherwise the performance of such system is limited significantly by inter-symbol interference (ISI) and multiuser access interference (MAI). In order to achieve this, adaptive filters are employed. A variety of adaptive schemes to remove ISI and MAI have been reported in the literature, some of which rely on training sequences, such as the Least Mean Squares (LMS) and Recursive Least Squares (RLS) algorithms, or on blind adaptation, such as the Constant Modulus Algorithm (CMA) or the Decision Directed algorithm (DD), which has similar convergence properties as the LMS in the absence of decision errors, the CMA is relatively slow compared to the DD ...

Transmit diversity is a powerful technique for enhancing the channel capacity and reliability of multiple-input and multiple-output (MIMO) wireless systems. This thesis considers extended orthogonal space-time block coding (EO-STBC) with beamsteering angles, which have previously been shown to potentially achieve full diversity and array gain with four transmit and one receive antenna. The optimum setting of beamsteering angles applied in the transmitter, which has to be calculated based on channel state information (CSI) at the receiver side, must be quantised and feed back to the transmitter via a reverse feedback link. When operating in a fading scenario, channel coefficients vary smoothly with time. This smooth evolution of channel coefficients motivates the investigation of differential feedback, which can reduce the number of feedback bits, while potentially maintaining near optimum performance. The hypothesis that the smooth evolution of channel coefficients translates into ...

Recently, many works have been accomplished on transmit diversity for a high-speed data transmission through the wireless channel. A Multiple Input Multiple Output (MIMO) system which employs multiple antennas at transmitter and receiver has been shown to be able to improve transmission data rate and capacity of the system. When the channel state information (CSI) is unknown at the transmitter, an multiple input single output (MISO) system combined with the transmit diversity of space time coding modulation known as space-time block coding (STBC) has taken a great attention. However, the performance of STBC is deteriorated under frequency selective fading due to inter symbol interference (ISI). An STBC employing tapped delay line adaptive array (STBC-TDLAA) is known as a solution for this problem since it utilizes the delayed signals to enhance the desired signal instead of excluding them as interferences. However, ...

Multiple-input multiple-output (MIMO) systems are encountered for example in communications if several transmit and receive antennas are empoyed, such that a separate transmit channel exists between every possible pairing of transmitter and receiver antennas. As a results if this spatial diversity, the channel capacity is dramatically increased over the single-inout single-output (SISO) case. While this increase is desired, the use of high data rates requires sophistiocated equalisation and/or detection schemes in the receiver to compensate for spatial and temporal dispersion in broadband MIMO channels, since a time-dispersive, in addition ot spatially-dispersice channel, must be assumed. The estimation of the broadband MIMO channel or its inverse is in general difficult and calls for training sequences that reduce the slot time for the transmission of actual data, which may counteract the promised gain in channel capacity. Another problem can be the computational ...

Code division multiple access (CDMA) techniques have been widely employed by different wireless systems with many advantages. However, the performance of these systems is limited by interference. A number of different interference suppression techniques have been proposed, including multiuser detection, beamforming, adaptive supervised and blind algorithms, and transmit processing techniques requiring a limited feedback channel. Recently, CDMA techniques have also been combined with multicarrier and multiantenna schemes to further increase the system capacity and performance. This thesis investigates the existing algorithms and structures and proposes novel interference suppression algorithms for spread spectrum systems. Firstly we investigate blind constrained constant modulus (CCM) stochastic gradient (SG) receivers with a low-complexity variable step-size mechanism for downlink direct sequence CDMA (DS-CDMA) systems. This algorithm provides better performance than existing blind schemes in non-stationary scenarios. Convergence and tracking analyses of the proposed adaptation techniques are ...

In the last decade, cooperation among multiple terminals has been seen as one of the more promising strategies to improve transmission speed in wireless communications networks. Basically, the idea is to mimic an antenna array and apply distributed versions of well-known space-diversity techniques. In this context, the simplest cooperative scheme is the relay channel: all the terminals (relays) that overhear a point-to-point communication between a source and a destination may decide to aid the source by forwarding (relaying) its message. In a mobile system, it is common to assume that the relays do not have any information about the channel between them and the destination. Under this hypothesis, the best solution to exploit the diversity offered by multiple transmitting antennas is to use space-time coding (STC). However, classical STC's are designed for systems with a fixed and usually low number ...

Since several years, multi-antenna systems are foreseen as a potential solution for increasing the throughput of future wireless communication systems. The aim of this thesis is to study and to improve the transmitter and receiver's techniques of these MIMO (Multiple Input Multiple Output) systems in the context of a multi-carrier transmission. On the one hand, the OFDM (Orthogonal Frequency Division Multiplex) modulation, which transform a frequency selective channel into multiple non frequency selective channels, is particularly well adapted to the conception of MIMO receivers with low complexity. On the other hand, two techniques allowing to improve the exploitation of frequential and/or temporal diversities are associated with OFDM, namely linear precoding (LP-OFDM) and CDMA in a MC-CDMA (Multicarrier Code division Multiplex Access) scheme. We have associated LP-OFDM and MC-CDMA with two MIMO techniques which require no channel state information at the ...

The majority of communication systems need the previous knowledge of the channel, which is usually estimated by means of a training sequence. However, the transmission of pilot symbols provokes a reduction in bandwidth efficiency, which precludes the system from reaching the limits predicted by the Information Theory. This problem has motivated the development of a large number of blind channel estimation and equalization techniques, which are able to obtain the channel or the source without the need of transmitting a training signal. Usually, these techniques are based on the previous knowledge of certain properties of the signal, such as its belonging to a finite alphabet, or its higher-order statistics. However, in the case of multiple-input multipleoutput (MIMO) systems, it has been proven that the second-order statistics of the observations provide the sufficient information for solving the blind problem. The aim ...

The third generation mobile system Universal Mobile Telecommunications System (UMTS) Terrestrial Radio Access (UTRA) has been mainly specified to provide various multimedia capabilities and good service quality. However, since UMTS is based on direct sequence CDMA (DS-CDMA) techniques the performance and the capacity of such systems is significantly limited by multiuser access interference (MAI) and inter-symbol interference (ISI). Therefore, robust and reliable detectors are required to mitigate these effects. Specifically, the multi-user detector exhibits a significant improvement in capacity and spectrum efficiency compared with the conventional matched filter receiver and single-user detector. Nevertheless, its complexity and prior knowledge requirement render it unsuitable for application in the downlink due to handset constraints. In this thesis, we propose a new robust and simple blind multiuser equaliser for downlink DS-CDMA systems, the so-called filtered-R multiple error CM algorithm (FIRMER-CMA) equaliser. The latter has ...

Joint precoding and equalisation can help to effectively exploit the advantages of multi-input multi-output (MIMO) wireless communications systems. For broadband MIMO channels with channel state information (CSI) such techniques to date rely on block transmission where guard intervals are applied to mitigate inter-block (IBI) and inter-symbol interference (ISI) but reduce spectral efficiency. Therefore, this thesis investigates novel MIMO transceiver designs to improve the transmission rate and error performance. Firstly, a broadband MIMO precoding and equalisation design is proposed which combines a recently proposed broadband singular value decomposition (BSVD) algorithm for MIMO decoupling with conventional block transmission techniques to address the remaining broadband SISO subchannels. It is demonstrated that the BSVD helps not only to remove co-channel interference within a MIMO channel, but also reduces ISI at a very small loss in channel energy, leading to an improved error performance and ...

Multiple-input multiple-output (MIMO) technology promises significant capacity improvements in order to more efficiently utilise the radio frequency spectrum. To achieve its anticipated multiplexing gain as well as meet the requirements for high data rate services, proposed broadband systems are based on OFDM or similar block based techniques, which are afflicted by poor design freedom at low redundancy, and are known to suffer badly from co-channel interference (CCI) in the presence of synchronisation errors. Non-block based approaches are scarce and use mostly decision feedback equalisation (DFE) or V-BLAST approaches adopted for the broadband case, as well as Tomlinson-Harashima precoding (THP). These methods do not require a guard interval and can therefore potentially achieve a higher spectral efficiency. The drawback of these schemes is the large effort in determining the optimum detection order in both space and time, often motivating the adoption ...

This thesis is concerned with the use of multiple antenna elements in wireless communication over frequency non-selective radio channels. Both measurement results and theoretical analysis are presented. New transmit strategies are derived and compared to existing transmit strategies, such as beamforming and space time block coding (STBC). It is found that the best transmission algorithm is largely dependent on the channel characteristics, such as the number of transmit and receive antennas and the existence of a line of sight component. Rayleigh fading multiple input multiple output (MIMO) channels are studied using an eigenvalue analysis and exact expressions for the bit error rates and outage capacities for beamforming and STBC is found. In general are MIMO fading channels correlated and there exists a mutual coupling between antenna elements. These findings are supported by indoor MIMO measurements. It is found that the ...

Wireless broadband communications for users moving at vehicular speed is a cor- nerstone of future fourth generation (4G) mobile communication systems. We inves- tigate a multi-carrier (MC) code division multiple access (CDMA) system which is based on orthogonal frequency division multiplexing (OFDM). A spreading sequence is used in the frequency domain in order to distinguish individual users and to take advantage of the multipath diversity of the wireless channel. The transmission is block oriented. A block consists of OFDM pilot and OFDM data symbols. At pedestrian velocities the channel can be modelled as block fading. We ap- ply iterative multi-user detection and channel estimation. In iterative receivers soft symbols are derived from the output of an soft-input soft-output decoder. These soft symbols are used in order to reduce the interference from other users and to enhance the channel estimates. We ...

Multi-input multi-output (MIMO) wireless networks and distributed MIMO relaying wireless networks have attracted significant attention in current generation of wireless communication networks, and will play a key role in the next generation of wireless net- works. The improvement of network capacity, data rate and reliability can be achieved at the cost of increasing computational complexity of employing space-time coding (STC) and distributed STC (DSTC) in MIMO and distributed MIMO relaying networks, respectively. Efficient designs and algorithms to achieve high diversity and coding gains with low computational complexity in encoding and decoding of STC and DSTC schemes are essential. In this thesis, DSTC designs with high diversity and coding gains and efficient detection and code matrices optimization algorithms in cooperative MIMO networks are proposed. Firstly, adaptive power allocation (PA) algorithms with different criteria for a coop- erative MIMO network equipped with ...

The demand for mobile communication systems with high data rates has dramatically increased in recent years. New methods are necessary in order to satisfy this huge communications demand, exploiting the limited resources such as bandwidth and power as efficient as possible. MIMO systems with multiple an- tenna elements at both link ends are an efficient solution for future wireless communications systems as they provide high data rates by exploiting the spatial domain under the constraints of limited bandwidth and transmit power. Space-Time Block Coding (STBC) is a MIMO transmit strategy which exploits transmit diversity and high reliability. STBCs can be divided into two main classes, namely, Orthogonal Space-Time Block Codes (OSTBCs) and Non-Orthogonal Space-Time Block Codes (NOSTBCs). The Quasi-Orthogonal Space-Time Block Codes (QSTBCs) belong to class of NOSTBCs and have been an intensive area of research. The OSTBCs achieve full ...