Dissertations / Theses on the topic 'Antennas (Electronics) Code division multiple access'

This thesis focuses on new strategies of designing Optical Code-Division Multiple Access (OCDMA) networks. Specifically, two new spreading code families of 2-dimensional (2D) wavelength-time system are considered: Depth-First Search Codes (DFSC) and Balanced Codes for Differential Detection (BCDD). DFSC utilizes a depth-first search algorithm to generate unipodal codes with maximum unit auto- and cross-correlation properties that are suitable for direct detection. These codes have similar interference-limited bit error rate (BER) performance as most 2D wavelength-time codes, but the algorithm can generate more codes, enabling the full potential of Forward Error Correction (FEC). BCDD defines a new set of high weight antipodal codes with relaxed correlation constraints that is suitable for differential detection. These codes can support approximately twice as many users as the other previously published OCDMA systems. Using a system with 32 wavelengths and 16 time chips operating at OC-12 transmission rates (622Mbps), BCDD can support an aggregate throughput of approximately 136Gbps when proper FEC is applied.<br>Furthermore, studies on the information theoretical capacity of chip synchronous OCDMA channel with Single User Detection (SUD) is conducted to obtain the ultimate throughput that can be achieved. Calculations are done under three assumptions: (i) interference-limited channel, (ii) interference-limited channel with Gaussian noise; or (iii) Gaussian approximated interference channel. In additions, system specific DFSC and BCDD capacity is obtained. These results are used as the basis for comparison among DFSC, BCDD and other previously proposed OCDMA systems. It is found that the maximum throughput of an OCDMA system is limited to about 0.7 bits per OCDMA chip. With the application of turbo code, BCDD can support an aggregate throughput of about 0.42 bits per OCDMA chip.

In this thesis few new code sets and a multi-user interference (MUI) cancellation scheme have been proposed for Optical Code Division Multiple Access (OCDMA) systems, which can be employed in the next generation of global communication networks to enhance their existing systems’ performance dramatically. The initial evaluation of the proposed code sets shows that their implementation improves the performance, decreases the BER and increases security considerably. Also the proposed MUI cancellation scheme totally removes all the cross-talk and interference between the active users within the network. These novel schemes and codes can be easily implemented in the optical packet switched networks. Optical switching has the ability of bandwidth manipulation at the wavelength level (e.g. with optical circuit/packet/burst switching); the capability to accommodate a wide range of traffic distributions, and also to make dynamic resource reservations possible. This thesis first gives a brief overview of co-channel interference reduction in OCDMA networks, then proposes two novel code sets, Uniform Cross-Correlation Modified Prime Code (UC-MPC) and Transposed UC-MPC (T-UCMPC), along with their evaluation and analysis in various systems, including IP routing over an OCDMA network. Thereafter, the new MUI cancellation scheme is proposed and then the proposed code sets and the MUI cancellation scheme are implemented and analysed in a laboratory-based experimental test bed. Finally the conclusion of this research is discussed.

This thesis describes the work done in evaluating the capacity of a multi-service third generation (3G) wireless communication system based on the Universal Mobile Telecommunication System (UNITS). More specifically, the study presented in these pages examines the Time Division Duplex (TDD) paradigm supported by the 3G Partnership Project (3GPP).<br>The goal is to establish the probability that a new connection attempt be successful as a function of the instantaneous load on the network. The sought capacity is evaluated with the help of an analytical model and a simulation tool. Both tools used a so-called system-level approach. The model and the simulator were developed based on 3GPP's standard and available data from link-level simulations.<br>Some of the significant results presented are: (1) A maximum of around 7 to 9 orthogonal codes can be used per timeslot. (2) A UMTS TDD CDMA system should be used in pico- and micro-cellular environments in order to support efficient data transmissions. (3) The multicode interference is often wrongfully neglected in capacity studies. (4) Capacity is downlink interference-limited, even with a modest MUD efficiency.

Systematic block codes compute (n-k) parity bits for an information block of k bits, and append the parity bits to the end of the information bits to form an n-bit codeword. This redundancy provides certain ability for the receiver to detect or correct a limited number of errors caused by noise and interference on the channel. In direct sequence spread spectrum (DS-SS) communication systems, the information is spread over a large bandwidth by multiplying the information bits with a high rate pseudonoise (PN) sequence. In this thesis, these two techniques are combined to create a new technique, named parity bit selected direct sequence code division multiple access (parity bit selected DS-CDMA). The proposed scheme uses the (n-k) parity bits to select one of 2 (n-k) mutually orthogonal PN sequences. Then the selected PN sequence is used to spread each bit in the information block of k bits. But the (n-k) parity bits are not appended to the end of the information bits, and are not transmitted to the receiver. The receiver can obtain the parity bits by using 2(n-k) matched filters to determine which PN sequence is used for an information block of k bits. Then the errors can be corrected by employing a decoding algorithm. Block codes with different lengths are considered in our simulation in order to find which one is best for the proposed scheme at a given Eb/No. The simulation results in a multi-user environment show that the proposed parity bit selected DS-CDMA system can provide better BER performance than both the uncoded DS-CDMA and the block coded DS-CDMA systems.

Commutation signalling is an antimultipath technique employing direct-sequence bandwidth spreading for combatting multipath induced ISI. We propose a CDMA-like multiple-access scheme which utilizes this inherent spreading for broadband cellular indoor wireless communications. The intra-cell multiple-access is by time division while the inter-cell interference is reduced via combined frequency and code division. The frequency bands of adjacent cells overlap to increase bandwidth efficiency. We consider the interference between spread-spectrum signals with partially overlapping spectra and apply the results to the analysis of this cellular system. The uplink and downlink are considered with and without cell sectorization. The downlink is used for obtaining certain system design parameters and for comparing the merits of different pulse shapes. We finally compare the proposed scheme with the usual TDMA/FDMA and CDMA schemes. The proposed scheme is found to require less bandwidth compared to the usual TDMA/FDMA scheme. However, CDMA requires a lesser bandwidth than the proposed scheme since it employs powerful very low rate channel coding techniques.

The development and growth of new communication services and emerging applications requires high-performance access networks capable of providing high-bandwidth interconnections to end-users. In recent years, optical code-division multiple access (OCDMA) has been proposed as a means for providing flexible access to high-bandwidth applications and offering different levels of quality of service. This thesis explores the impact of encoder/decoder mismatch on system performance for 2 dimensional wavelength-time (2D lambda-t) OCDMA and the implementation of a multi-user 2D lambda-t OCDMA direct detection demonstrator system. Our analysis and experimental demonstrations are based on depth-first search codes (DFSCs) which have previously been shown to be attractive in OCDMA applications. We have developed an OCDMA system simulation model that quantifies the BER performance as wavelength and/or time misalignments (which cause mismatch in the encoder/decoder) increase. Furthermore, we have succeeded in encoding and decoding DFSCs as well as in demonstrating a 4-user system which has been implemented using standard commercially available off-the-shelf optical components.

Thesis (Ph. D.)--Hong Kong University of Science and Technology, 2002.<br>Includes bibliographical references (leaves 111-117). Also available in electronic version. Access restricted to campus users.

In recent years, there has been an increasing demand for high-performance networks to support emerging technologies, new applications, and innovative communication services. Optical code division multiple-access (OCDMA) has been proposed as a network solution offering flexible access to high-bandwidth applications while supporting different levels of quality of service. In this thesis, we provide the first demonstration of successful encoding and decoding of two-dimensional wavelength-time (2D lambda-t) OCDMA bipolar codes. Furthermore, we develop a generic simulation platform for investigating the impact of component mismatch, caused by wavelength and/or time misalignments in the optical encoders/decoders or misbalance in the photodetectors, on the bit error rate (BER) system performance of 2D lambda-t bipolar OCDMA systems. In particular we apply the simulation model to compare the performance of various 2D lambda-t bipolar codes.

This thesis describes the research on the implementation of all-optical code generation and recognition based on superstructure fibre Bragg grating (SSFBG) for use in Optical Code Division Multiple Access (OCDMA) systems and also in high-speed all-optical packet switched networks.These results highlight the precision and flexibility of the continuous grating writing process and show that the SSFBG technology represents a promising technology not just for OCDMA but an extended range of other pulse shaping, and associated optical processing applications such as required within optical packet switched networks

Alternative methods to minimum-variance distortionless-response (MVDR) detection under small data records in code-division multiple-access binary phase-shift keying systems are explored. A comprehensive review of the conventional and optimal detectors as well as the principal component, cross-spectral and auxiliary vector (AV) methods are presented. The concept of widely linear (WL) filtering is introduced. Two different versions of the WL MVDR filters are proved to be equivalent; implementing the WL MVDR detector using the AV structure is proposed and shown to provide significant performance gain. We also propose two novel methods to implement the WL and plain MVDR detectors. One, based on the idea of limiting the eigenvalues of the sample correlation matrix, is outperformed by the AV implementation. The other method approximates the inverse of sample correlation matrix by inverting its eigenvalues using a polynomial function; it outperforms the AV implementation in terms of complexity while providing similar performances.<br>Cette thèse explore de nouvelles méthodes d'amélioration de la performance du détecteur MVDR dans des systèmes DS-CDMA BPSK lorsque le nombre de vecteurs reçus utilisés pour estimer la matrice de corrélation est peu élevé. Une analyse des détecteurs conventionnel et optimal ainsi que les méthodes principal component (PC), cross-spectral (CS) et auxiliary vector(AV) sont présentés. Le concept de filtrage widely linear (WL) est introduit. L'équivalence de deux versions différentes du filtre WL MVDR est démontrée. La réalisation du détecteur WL MVDR utilisant une structure AV est proposée et le gain de performance significatif accompli par ce détecteur est prouvé. Nous proposons aussi deux nouvelles méthodes de réalisation des filtres MVDR WL et régulier. La première méthode, basé sur l'idée de limiter les valeurs propres de la matrice de corrélation estimée, est moins performante que la méthode AV. L'autre méthode obtient une approximation de l'inverse de la matrice de corrélation estimée en inversant ses valeurs propres l'aide d'une fonction polynomiale. Cette méthode est meilleure que la méthode AV en terme de complexité tout en présentant des performances similaires.

This thesis documents my work in the telecommunication system laboratory at the Optoelectronics Research Centre, towards the implementation of code reconfigurable OCDMA and all-optical packet switching nodes based on fibre Bragg grating (FBG) technology. My research work involves characterizing the performance of various gratings, specifically high reflectivity, short chip duration, long code sequences, multiple phase level and tunable superstructured fiber Bragg gratings (SSFBGs), by using the recently proposed Frequency-Resolved Optical Gating technique based on Electro-Absorption Modulator (EAM-FROG). This technology can obtain the complex code profile along the grating, making it a powerful method to understand the thermally-induced code-reconfigurable grating. Efforts have been made to improve the grating design to achieve better system performance. Three different types of FBGs optical encoder/decoder, e.g. conventional discrete phaseshift SSFBGs, code-reconfigurable gratings, and novel continuous phase-shift SSFBGs, have been investigated comparatively, as well as their performance in various optical coding/decoding systems. This thesis also discusses the possibility of reducing multiple access interference (MAI) using a Two-Photon Absorption (TPA) process. The advanced grating devices enable the improvement of system performance. A dynamically reconfigurable optical packet processing system and a 16-channel reconfigurable OCDMA/DWDM system with 50GHz DWDM intervals has been demonstrated. These results highlight the feasibility of FBG-based optical coding/decoding techniques, with improved system flexibility and sustainability.

Power consumption requires critical consideration during system design for portable wireless communication devices as it has a direct influence on the battery weight and volume required for operation. Wideband Code Division Multiple Access (W-CDMA) techniques are favoured for use in future generation mobile communication systems. This thesis investigates novel low power techniques for use in system blocks within a W-CDMA adaptive linear minimum mean squared error (LMMSE) receiver architecture. Two low power techniques are presented for reducing power dissipation in the LMS adaptive filter, this being the main power consuming block within this receiver. These low power techniques are namely the decorrelating transform, this is a differential coefficient technique, and the variable length update algorithm which is a dynamic tap-length optimisation technique. The decorrelating transform is based on the principle of reducing the wordlength of filter coefficients by using the computed difference between adjacent coefficients in calculation of the filter output. The effect of reducing the wordlength of filter coefficients being presented to multipliers in the filter is a reduction in switching activity within the multiplier thus reducing power consumed. In the case of the LMS adaptive filter, with coefficients being continuously updated, the decorrelating transform is applied to these calculated coefficients with minimal hardware or computational overhead. The correlation between filter coefficients is exploited to achieve a wordlength reduction from 16 bits down to 10 bits in the FIR filter block. The variable length update algorithm is based on the principle of optimising the number of operational filter taps in the LMS adaptive filter according to operating conditions. The number of taps in operation can be increased or decreased dynamically according to the mean squared error at the output of the filter. This algorithm is used to exploit the fact that when the SNR in the channel is low the minimum mean squared error of the short equaliser is almost the same as that of the longer equaliser. Therefore, minimising the length of the equaliser will not result in poorer MSE performance and there is no disadvantage in having fewer taps in operation. If fewer taps are in operation then switching will not only be reduced in the arithmetic blocks but also in the memory blocks required by the LMS algorithm and FIR filter process. This reduces the power consumed by both these computation intensive functional blocks. Power results are obtained for equaliser lengths from 73 to 16 taps and for operation with varying input SNR. This thesis then proposes that the variable length LMS adaptive filter is applied in the adaptive LMMSE receiver to create a low power implementation. Power consumption in the receiver is reduced by the dynamic optimisation of the LMS receiver coefficient calculation. A considerable power saving is seen to be achieved when moving from a fixed length LMS implementation to the variable length design. All design architectures are coded in Verilog hardware description language at register transfer level (RTL). Once functional specification of the design is verified, synthesis is carried out using either Synopsys DesignCompiler or Cadence BuildGates to create a gate level netlist. Power consumption results are determined at the gate level and estimated using the Synopsys DesignPower tool.

Thesis (Ph.D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009.<br>Committee Chair: James Stevenson Kenney; Committee Member: Gregory David Durgin; Committee Member: Madhavan Swaminathan

by Siu-Lung Hui, Freeman.<br>Thesis (M.Phil.)--Chinese University of Hong Kong, 1998.<br>Includes bibliographical references (leaves 64-[67]).<br>Abstract also in Chinese.<br>Chapter 1 --- Introduction --- p.1<br>Chapter 1.1 --- Use of Antenna Arrays in Mobile Communications --- p.1<br>Chapter 1.1.1 --- Overview --- p.1<br>Chapter 1.1.2 --- Beamforming --- p.2<br>Chapter 1.2 --- DS/CDMA System s and Multiple Access Interferences --- p.4<br>Chapter 1.3 --- Multiuser Detection Schemes --- p.7<br>Chapter 1.4 --- Outline of Thesis --- p.11<br>Chapter 2 --- A Blind Adaptive Receiver with Antenna Arrays and M-ary Orthogonal Data Signals --- p.13<br>Chapter 2.1 --- Introduction --- p.13<br>Chapter 2.2 --- System Model --- p.15<br>Chapter 2.3 --- Eigen-Analysis Algorithm --- p.21<br>Chapter 2.4 --- Simulation Results --- p.92<br>Chapter 2.5 --- Adaptive Algorithm --- p.27<br>Chapter 2.6 --- Summary --- p.30<br>Chapter 3 --- Detection with the Use of the Two-Stage Spreading Scheme --- p.32<br>Chapter 3.1 --- Introduction --- p.32<br>Chapter 3.2 --- System Model --- p.34<br>Chapter 3.3 --- Blind Beamforming --- p.36<br>Chapter 3.4 --- Blind Adaptive Multiuser Detection without Antenna Arrays --- p.38<br>Chapter 3.4.1 --- Stochastic Gradient Descent Algorithm --- p.40<br>Chapter 3.4.2 --- Alternative Matrix Approach --- p.41<br>Chapter 3.5 --- Theoretical Combined Receiver Model --- p.41<br>Chapter 3.6 --- Practical Implementation of the Receiver --- p.50<br>Chapter 3.6.1 --- Combined Scheme with Adaptive Algorithms --- p.50<br>Chapter 3.6.2 --- Simplified Structure --- p.52<br>Chapter 3.7 --- Summary --- p.54<br>Chapter 4 --- Conclusions and Future Work --- p.55<br>Chapter A --- Correlation Properties --- p.58<br>Chapter B --- Adaptive Algorithm --- p.62<br>Bibliography --- p.64

Recent studies have shown that considerable system capacity gains in mobile communication systems can be obtained by exploiting the use of antenna arrays at the base station. Unfortunately, these studies make little mention of practical issues concerning implementation. It is thus one of the objectives of the Centre of Excellence (CoE) in Radio Access Technologies at the University of Natal to investigate the development of a widehand CDMA adaptive array transceiver using Alcatel software radios as the transceiver platforms. Such a transceiver system can be subdivided into three major sections: RF front-end, signal digitization and baseband processing stages. Due to the enormity of such an undertaking, the research outlined in this thesis is focused on (but not isolated to) some aspects of the RF front-end implementation for the proposed system. The work in this thesis can be catergorized into two sections. The first section focuses on the theoretical and practical (or implementation) aspects of antenna arrays and beamforming. In particular, it is evident that digital (rather than analogue) beamforming in a multi user environment, is a more viable option from both a cost and implementation standpoint. The second section evaluates the impact of RF component noise and local oscillator generated phase noise in a DS-CDMA system. The implementation of a RP front-end for a BPSK transceiver also forms part of the work in this section. LO phase noise and Error Vector Magnitude (EVM) measurements are performed on this system to support relevant theory. By use of the HP89410A phase noise measurement utility and the phase noise theory developed in this thesis, a quantitative phase noise comparison between two frequency sources used in the system were made. EVM measurement results conclusively verified the importance of an LNA in the system. It has also been shown that the DS-CDMA simulated system exhibits superior performance to the implemented BPSK system. Furthermore, an EVM troubleshooting methodology is introduced to identify possible impairments within the BPSK receiver RF front-end. However, this thesis was written with the intention of bridging the gap between the theoretical and practical implementation aspects of RP wireless communication systems. It is the author's opinion that this has been achieved to a certain extent.<br>Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.

Multiuser detection strategies are derived and analyzed for several signal detection problems arising in Code-Division Multiple-Access channels. Modulation schemes which lend themselves to coherent and noncoherent demodulation are considered. For coherent communication, a suboptimum multistage detection strategy based on successive multiple-access interference rejection is proposed for both synchronous and asynchronous Gaussian CDMA channels. The resulting multistage detectors process the sufficient statistics via a nonlinear multistage decision algorithm. An efficient, fixed decoding delay implementation of the multistage detector for the asynchronous channel is obtained and is shown to require a computational and storage complexity per binary decision which is linear in the number of users $K$. The multistage detector therefore contrasts the optimum demodulator, which is based on a dynamic programming algorithm; has a variable decoding delay; and a software complexity which is exponential in $K$. Performance analysis of the multistage detector is characterized in terms of bit-error probability. A comparative error probability study is undertaken between the conventional, optimum and the multistage detectors along with the decorrelating suboptimum detectors. Results obtained here indicate that significant improvements over the conventional single-user detector in bandwidth efficiency and near-far immunity are afforded by the multistage and decorrelating detectors. The multistage detector is particularly well suited for the demodulation of signals of widely dissimilar energies, corresponding to near-far scenarios. The noncoherent multiuser detection problem is studied for the synchronous CDMA channel where the modulation scheme considered is differential phase-shift keying. A class of quadratic detectors is defined with the objective of chosing the optimal quadratic detector. Asymptotic efficiency is considered as the performance measure of interest. It is equivalent to error probability in the low background noise region and quantifies performance loss due to presence of interfering signals. Since the receiver does not attempt to estimate signal energies and phases in this noncoherent system, the optimal detector is chosen as the one which optimizes the worst-case asymptotic efficiency over interfering signal uncertainties. This minimax approach yields an optimal detector, the bit-error probability of which is invariant to interfering signal uncertainties, thereby alleviating the near-far problem associated with the conventional single-user detection scheme.

We consider the estimation of channel parameters for code division multiple access (CDMA) communication systems operating over channels with either single or multiple propagation paths. We present two approaches for decomposing this multiuser channel estimation problem into a series of single user problems. In the first method, the interfering users are treated as colored, non-Gaussian noise, and the maximum likelihood estimate is formed from the sample mean and sample covariance matrix of the received signal. In the second method, we exploit the eigenstructure of the sample correlation matrix to partition the observation space into a signal subspace and a noise subspace. The channel estimate is formed by projecting a given user's spreading waveform into the estimated noise subspace and then either maximizing the likelihood or minimizing the Euclidean norm of this projection. Both of these approaches yield algorithms which are near-far resistant and are capable of tracking slowly varying channels.

Optical communication systems offer a bandwidth orders of magnitude larger than any electric or radio-frequency system. Yet, historically, optical communication systems have been designed by adapting existing conventional low frequency systems to operate at light frequencies, thus limiting the rate of information transfer to those previously achievable at lower frequencies. The purpose of this study is to show that it is possible, through the design of the optical communication system according to its own statistical characteristics, to achieve reliable information transfer rates commensurate with the optical bandwidth, that is, orders of magnitude higher than any conventional systems. Two optical code-division multiple-access systems are proposed, one a conventional time encoded system, and one an innovative spectral amplitude encoded system. The spectral amplitude encoded system provides many advantages including the ability to implement efficient symbol detectors, insensitivity to chromatic dispersion, and availability of large codes. An optimized single-user detector, both hard-decision and soft-decision multistage detectors, and a local search detector are proposed as alternatives to the weak conventional detector and the complex optimal detector. A full performance analysis of the two systems with each detector is performed. The main analysis tool is an approximation based on large deviation theory. The approximation to the error probability is verified by comparing the results to values obtained using a characteristic function method for small problems. Some simplifying assumptions are made to evaluate the moment generating function and the validity of these is confirmed by comparing to simulation results. Both random user sequences and well-designed deterministic codes are investigated. The optimized single-user detector performed considerably better than the conventional correlation detector in all cases. All multiuser detectors error probabilities compared favorably to the theoretical lower bound. The soft-decision multistage detector showed significant improvement over the hard-decision counterpart. The local search detector outperformed them all, at the expense of added complexity. The asymptotic multiuser efficiency of the system with each detector was also computed using large deviation theory. All proposed detectors are shown to be robust against multiple-access interference.

This research examines the theoretical and experimental aspects of code-division multiple access (CDMA) communication in the incoherent, or direct detection, optical domain. We develop new modulation and detection principles that permit all-optical implementation of the bipolar, +1/$-$1, code and correlation detection available in the radio frequency (RF) CDMA systems. This is possible in spite of the non-negative, or unipolar, +1/0, nature of the incoherent optical system that only detects and processes the signal intensity. The unipolar optical system using the new principles is equivalent to the bipolar RF system in that the correlation properties of the codes are completely preserved. The optical CDMA system can be realized either in time or frequency domain encoding with all-optical components. All-optical implementation is extremely important in practice because the symbol rates of the individual users are far less than the bandwidth of the optical fiber. Frequency domain or spectral amplitude encoding significantly increases the number of available codewords that can be assigned to the subscribers, and is the focus of this work. The spectral amplitude encoding uses incoherent, broad bandwidth superfluorescent fiber source (SFS) that has the limiting signal-to-noise ratio characteristics associated with thermal light. Measurements of the photodetector illuminated by an erbium-doped SFS confirm that the noise distribution follows a negative binomial probability mass function as predicted for the photoelectron counting statistics of thermal light. The analysis based on this statistical model results in improved performance over the Gaussian assumption, which predicts a performance floor. The analysis also shows that optical CDMA system, even with the use of orthogonal codes, is interference limited. We implement an experimental prototype system that demonstrates the first use of bipolar codes for spectral amplitude CDMA. The experimental results validate the modulation and detection principles and demonstrate the feasibility of optical CDMA systems based on those principles.

Efforts in performance analysis of Code Division Multiple Access (CDMA) systems have concentrated on obtaining asymptotic approximations and bounds for system error probabilities. As such, these cannot capture the sensitivities of the system performance to any class of parameters, and the optimization of such systems (with respect to any class of parameters) presents itself to no analytical solutions. A discrete event dynamic systems (DEDS) formulation is developed for CDMA systems whereby the sensitivity of the average probability of error can be evaluated with respect to a wide class of system parameters via sample path based gradient estimation techniques like infinitesimal perturbation analysis (IPA) and the likelihood ratio (LR) method. Appropriate choice of the sample path and the corresponding sample performance function leads to analyzing the sensitivity of the average probability of error to near-far effects, power control, and code parameters. Further, these sensitivity analysis methods are incorporated in gradient algorithms for optimizing system performance in terms of the minimum probability of detection error. Specifically, for direct-sequence CDMA systems, IPA based stochastic gradient algorithms are used to develop a class of adaptive linear detectors that are optimum in that they minimize the average probability of bit-error. These detectors outperform both the matched filter and MMSE detectors, and also alleviate the disadvantage of multiuser detection schemes that require implicit information on the multiple access interference. For CDMA systems in the optical domain, IPA based stochastic algorithms are used to develop a class of adaptive threshold detectors that minimize the average probability of bit-error. These detectors outperform the correlation detector and also preclude the need for assumptions on the interference statistics required by existing optimum one-shot detectors. All adaptive detection schemes developed here are easily implementable owing to the simple recursive structures that arise out of our sample realization based approach. The sequential versions of the adaptive detectors developed in here require no preamble, which makes them a viable choice for CDMA channels subject to temporal variations due to dispersion effects and variable number of users in the channel.

In this thesis, the method of "classification with empirically observed statistics"--also known as empirical classification, empirical detection, universal classification, and type-based detection--is configured and applied to the despreading/detection receiver operation of a spread-spectrum (SS), code division multiple access (CDMA) communications system. In static and Rayleigh-fading environments, the empirical detector is capable of adapting to unknown noise environments in a superior manner than the linear matched-filter despreader/detector, and done with reasonable amounts of training. Compared to the optimum detector, when known, the empirical detector always approaches optimal performance, again, with reasonable amounts of training. In an interference-limited channel, we show that the single-user likelihood-ratio detector, which is the optimum single-user detector, can greatly outperform the matched filter in certain imperfect power-control situations. The near optimality of the empirical detector implies that it, too, will outperform the matched filter in these situations. Although the empirical detector has the added cost of requiring chip-based phase synchronization, its consistent and superior performance in all environments strongly suggests its application in lieu of the linear detector for SS/CDMA systems employing long, pseudo-random spreading. In order to apply empirical classification to digital communications, we derive the empirical forced-decision detector and show that it is asymptotically optimal over a large class of empirical classifiers.

The proliferation of wireless communications services combined with the limited spectrum availability have placed the bandwidth utilization as a major performance measure. Consequently, the bandwidth allocation technique to multiple signals and the bandwidth occupied by each signal are issues of paramount importance. For voice and bursty data communications, code-division multiple-access provides excellent bandwidth management. The objective to produce constant-envelope signals with compact spectral characteristics is most effectively accomplished using continuous phase modulation. The purpose of this study is to examine detection issues for signals that combine the above techniques. For a synchronous system, the reliable operation of a single-user receiver without power control requires spreading codes that exhibit minimal mutual interference. Signal memory is essential for good performance and precludes the existence of orthogonal codes. Code design is examined for two signal formats that offer different spectral and error rate characteristics. A recursive algorithm that provides the structure and maximum number of codes is presented for both signal formats. Moreover, the code performance is evaluated for an asynchronous system with power control. To avoid the performance limitations of the single-user receiver in the presence of interference and the disadvantages of power control, multiuser detectors are considered for both synchronous and asynchronous systems. The optimum coherent multiuser detector is briefly analyzed and its computational complexity is shown to be prohibitively large for practical applications. For this reason, the emphasis is placed on suboptimum detectors with linear complexity and near-optimum performance. The choice of an appropriate set of decision statistics is crucial for this objective and conventional detectors, if applicable, perform poorly. Two linear complexity detection methods that can be applied to both signal formats are proposed for each system. The individual code design to optimize the error rate for a specific receiver complexity is determined and substantial gains are achieved over antipodal signaling. Moreover, the spectral and error rate performance are largely independent and impressive capacity improvements are obtained over conventional systems for a modest increase in the complexity of the receiver.

Ma Wing Kin.<br>Thesis (Ph.D.)--Chinese University of Hong Kong, 2001.<br>Includes bibliographical references (p. 150-155).<br>Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web.<br>Mode of access: World Wide Web.<br>Abstracts in English and Chinese.

WCDMA promises to achieve high-speed internet, high quality image transmission and high-speed data services with larger system capacity. However, Multiple Access Interference is one of the major causes of transmission impairment, which reduces the link capacity in WCDMA systems. The Adaptive Antenna Array (AAA) technique reduces multiple access interference by directing antenna beam nulls towards the interfering signals by weighting the received signals from all antennas before combining the signals. With the very rapid advancement of wireless personal communications services, a new challenge to the cellular industry is the integration of multiple systems and applications on a single device. A software radio technique offers a possible solution to achieve this goal including international roaming and multiple standard operations within the same geographical area. The main attraction of a software radio is it's flexibility, in that it can be programmed for emerging cellular standards allowing it to be updated with new software without any changes in the hardware infrastructure. A software radio incorporating adaptive array beamforming at the receiver can increase the total carried traffic in a system and transmit power while the probability of call blocking and forced termination can also be decreased. This dissertation examines WCDMA, AAA, power control and software radio techniques in the world of wireless communication systems. Once the theoretical background of CDMA and AAA has been substantiated, the thesis establishes the need for power control in mobile systems by examining simulation results. An AAA receiver with six antenna elements is proposed and evaluated in different environments as a precursor to implementation. It can be inferred that when the link is interference limited, the link capacity can be increased and it has been shown that the AAA receiver with six antenna elements increases the link capacity to about 2.9 times that of the single antenna RAKE receiver. This thesis also examines the basic concepts of VHDL and considers this as the principle means to program reconfigurable core FPGA's in the software radio. A three-layered (PC/DSP/FPGA) software radio test bed is used to implement an AAA receiver. The architecture of the test bed is designed in such a way that it can be used to evaluate the performance of various FPGA based transceivers and coding schemes etc. Many of the desirable features and flexibilities inherent in the software radio concept are available on this test bed and the system has proved to be capable of high speed digital processing and is ideally suited to the development of time critical system components. The bit error rate achieved using the implemented receiver is assessed and compared to simulation results in an environment incorporating Rayleigh fading and AWGN.<br>Thesis (M.Sc.Eng.)-University of Natal, Durban, 2001.

The problems of optimal as well as suboptimal detection for CDMA transmissions over an additive white Gaussian noise (AWGN) channel, have been the focus of study in the recent past. However, CDMA transmissions are frequently made over channels which exhibit fading and/or dispersion; hence receivers need to be designed which take into account this behaviour. In spite of the major research effort invested in multiuser demodulation techniques, several practical as well as theoretical open problems still exist. Some of them are considered in more detail in this thesis. The aim of the thesis is to develop multiuser demodulation algorithms for mobile communication systems in frequency-selective fading channels, and to analyze their implementation complexity. The emphasis is restricted to the uplink of an asynchronous DS-CDMA system where the users transmit in an uncoordinated manner and are received by one centralized receiver. The original work that is undertaken for the MScEng study is the evaluation of a multiuser receiver structure for a frequency-selective fading channel, where there exists a steady specular path and two fading paths. Furthermore, the effect of using selection diversity is investigated by examining the bit error rate, asymptotic multi user efficiency and near-far resistance of the proposed detector structure. These results are confirmed both analytically and by simulation in the thesis. An investigation is also conducted into the application of neural networks to the problem of multiuser detection in code division multiple access systems. The neural network will be used as a classifier in an adaptive receiver which incorporates an extended Kalman filter for joint amplitude and delay estimation. Finally, some open problems for future research will be pointed out in the thesis. Keywords: AWGN channel , DS-CDMA system, frequency-selective, multi user demodulation, asymptotic multiuser efficiency, near-far resistance, neural network, Kalman filter.<br>Thesis (M.Sc.Eng.)-University of Natal, Durban, 2000.

Τα τελευταία χρόνια χαρακτηρίζονται από μια αλματώδη ανάπτυξη των προϊόντων και υπηρεσιών που βασίζονται στα δίκτυα ασύρματης επικοινωνίας, ενώ προκύπτουν σημαντικές ερευνητικές προκλήσεις. Τα συστήματα πολλαπλών κεραιών στον πομπό και στο δέκτη, γνωστά και ως συστήματα MIMO (multi-input multi-output), καθώς και η τεχνολογία πολλαπλής προσπέλασης με χρήση κωδικών (code division multiple access, CDMA) αποτελούν δύο από τα βασικά μέτωπα ανάπτυξης των ασύρματων τηλεπικοινωνιών. Στα πλαίσια της παρούσας διδακτορικής διατριβής, ασχοληθήκαμε με την ανάπτυξη και μελέτη αλγορίθμων επεξεργασίας σήματος για τα δύο παραπάνω συστήματα, όπως περιγράφεται αναλυτικά παρακάτω. Σχετικά με τα συστήματα MIMO, η πρωτοποριακή έρευνα που πραγματοποιήθηκε στα Bell Labs γύρω στα 1996, όπου αναπτύχθηκε η αρχιτεκτονική BLAST (Bell Labs Layered Space-Time), απέδειξε ότι η χρήση πολλαπλών κεραιών μπορεί να οδηγήσει σε σημαντική αύξηση της χωρητικότητας των ασύρματων συστημάτων. Προκειμένου να αξιοποιηθούν οι παραπάνω δυνατότητες, απαιτείται η σχεδίαση σύνθετων δεκτών MIMO. Προς αυτήν την κατεύθυνση, έχει προταθεί ένας μεγάλος αριθμός μεθόδων ισοστάθμισης του καναλιού. Ωστόσο, οι περισσότερες από αυτές υποθέτουν ότι το ασύρματο κανάλι είναι: 1) χρονικά σταθερό, 2) συχνοτικά επίπεδο (δεν εισάγει διασυμβολική παρεμβολή), και κυρίως 3) ότι είναι γνωστό στο δέκτη. Δεδομένου ότι σε ευρυζωνικά συστήματα μονής φέρουσας οι παραπάνω υποθέσεις είναι δύσκολο να ικανοποιηθούν, στραφήκαμε προς τις προσαρμοστικές μεθόδους ισοστάθμισης. Συγκεκριμένα, αναπτύξαμε τρεις βασικούς αλγορίθμους. Ο πρώτος αλγόριθμος αποτελεί έναν προσαρμοστικό ισοσταθμιστή ανάδρασης αποφάσεων (decision feedback equalizer, DFE) για συχνοτικά επίπεδα κανάλια ΜΙΜΟ. Ο προτεινόμενος MIMO DFE ακολουθεί την αρχιτεκτονική BLAST, και ανανεώνεται με βάση τον αλγόριθμο αναδρομικών ελαχίστων τετραγώνων (RLS) τετραγωνικής ρίζας. Ο ισοσταθμιστής μπορεί να παρακολουθήσει ένα χρονικά μεταβαλλόμενο κανάλι, και, από όσο γνωρίζουμε, έχει τη χαμηλότερη πολυπλοκότητα από όλους τους δέκτες BLAST που έχουν προταθεί έως σήμερα. Ο δεύτερος αλγόριθμος αποτελεί την επέκταση του προηγούμενου σε συχνοτικά επιλεκτικά κανάλια. Μέσω κατάλληλης μοντελοποίησης του προβλήματος ισοστάθμισης, οδηγηθήκαμε σε έναν αποδοτικό DFE για ευρυζωνικά κανάλια MIMO. Τότε, η διαδικασία της ισοστάθμισης εμφανίζει προβλήματα αριθμητικής ευστάθειας, που λόγω της υλοποίησης RLS τετραγωνικής ρίζας αντιμετωπίστηκαν επιτυχώς. Κινούμενοι προς την κατεύθυνση περαιτέρω μείωσης της πολυπλοκότητας, προτείναμε έναν προσαρμοστικό MIMO DFE που ανανεώνεται με βάση τον αλγόριθμο ελαχίστων μέσων τετραγώνων (LMS) υλοποιημένο εξ ολοκλήρου στο πεδίο της συχνότητας. Με χρήση του ταχύ μετασχηματισμού Fourier (FFT), μειώνεται η απαιτούμενη πολυπλοκότητα. Παράλληλα, η μετάβαση στο πεδίο των συχνοτήτων έχει ως αποτέλεσμα την προσεγγιστική διαγωνοποίηση του συστήματος, προσφέροντας ανεξάρτητη ανανέωση των φίλτρων ανά συχνοτική συνιστώσα και επιτάχυνση της σύγκλισης του αλγορίθμου. Ο προτεινόμενος ισοσταθμιστής πετυχαίνει μια καλή ανταλλαγή μεταξύ απόδοσης και πολυπλοκότητας. Παράλληλα με τα παραπάνω, ασχοληθήκαμε με την εκτίμηση του ασύρματου καναλιού σε ένα ασύγχρονο σύστημα CDMA. Το βασικό σενάριο είναι ότι ο σταθμός βάσης γνωρίζει ήδη τους ενεργούς χρήστες, και καλείται να εκτιμήσει τις παραμέτρους του καναλιού ανερχόμενης ζεύξης ενός νέου χρήστη που εισέρχεται στο σύστημα. Το πρόβλημα περιγράφεται από μια συνάρτηση ελαχίστων τετραγώνων, η οποία είναι γραμμική ως προς τα κέρδη του καναλιού, και μη γραμμική ως προς τις καθυστερήσεις του. Αποδείξαμε ότι το πρόβλημα έχει μια προσεγγιστικά διαχωρίσιμη μορφή, και προτείναμε μια επαναληπτική μέθοδο υπολογισμού των παραμέτρων. Ο προτεινόμενος αλγόριθμος δεν απαιτεί κάποια ειδική ακολουθία διάχυσης και λειτουργεί αποδοτικά ακόμη και για περιορισμένη ακολουθία εκπαίδευσης. Είναι εύρωστος στην παρεμβολή πολλαπλών χρηστών και περισσότερο ακριβής από μια υπάρχουσα μέθοδο εις βάρος μιας ασήμαντης αύξησης στην υπολογιστική πολυπλοκότητα.<br>Over the last decades, a dramatic progress in the products and services based on wireless communication networks has been observed, while, at the same time, new research challenges arise. The systems employing multiple antennas at the transmitter and the receiver, known as MIMO (multi-input multi-output) systems, as well as code division multiple access (CDMA) systems, are two of the main technologies employed for the evolution of wireless communications. During this PhD thesis, we worked on the design and analysis of signal processing algorithms for the two above systems, as it is described in detail next. Concerning the MIMO systems, the pioneering work performed at Bell Labs around 1996, where the BLAST (Bell Labs Layered Space-Time) architecture has been developed, proved that by using multiple antennas can lead to a significant increase in wireless systems capacity. To exploit this potential, sophisticated MIMO receivers should be designed. To this end, a large amount of channel equalizers has been proposed. However, most of these methods assume that the wireless channel is: 1) static, 2) frequency flat (no intersymbol interference is introduced), and mainly 3) it is perfectly known at the receiver. Provided that in high rate single carrier systems these assumptions are difficult to be met, we focused our attention on adaptive equalization methods. More specifically, three basic algorithms have been developed. The first algorithm is an adaptive decision feedback equalizer (DFE) for frequency flat MIMO channels. The proposed MIMO DFE implements the BLAST architecture, and it is updated by the recursive least squares (RLS) algorithm in its square root form. The new equalizer can track time varying channels, and, to the best of our knowledge, it has the lowest computational complexity among the BLAST receivers that have been proposed up to now. The second algorithm is an extension of the previous one to the frequency selective channel case. By proper modeling of the equalization problem, we arrived at an efficient DFE for wideband MIMO channels. In this case, the equalization process encounters numerical instability problems, which were successfully treated by the square root RLS implementation employed. To further reduce complexity, we proposed an adaptive MIMO DFE that is updated by the least mean square (LMS) algorithm, fully implemented in the frequency domain. By using the fast Fourier transform (FFT), the complexity required is considerably reduced. Moreover, the frequency domain implementation leads to an approximate decoupling of the equalization problem at each frequency bin. Thus, an independent update of the filters at each frequency bin allows for a faster convergence of the algorithm. The proposed equalizer offers a good performance - complexity tradeoff. Furthermore, we worked on channel estimation for an asynchronous CDMA system. The assumed scenario is that the base station has already acquired all the active users, while the uplink channel parameters of a new user entering the system should be estimated. The problem can be described via a least squares cost function, which is linear with respect to the channel gains, and non linear to its delays. We proved that the problem is approximately decoupled, and a new iterative parameter estimation method has been proposed. The suggested method does not require any specific pilot sequence and performs well even for a short training interval. It is robust to multiple access interference and more accurate compared to an existing method, at the expense of an insignificant increase in computational complexity.