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Spread-spectrum (SS) communications; with its inherent interference rejection and multiple- access capability, has become a bandwidth-efficient and popular technique for use in multi-user systems. In particular, direct-sequence code-division multiple access (DS- CDMA) can potentially accommodate more users than other multiple-access techniques. However, the efficiency of DS-CDMA systems depends heavily on the synchronization capability of the receiver. The receiver must maintain continuous synchronization be- tween the incoming DS/SS signal and the locally generated pseudo-noise (PN) codes. This research project, entitled Multi-User Synchronization in DS-CDMA, studies three important synchronization issues of a DS-CDMA system. They are all related to PN code tracking for multi-user wireless communications.

This research project studies three important issues of a direct-sequence code-division multiple-access (DS/CDMA) system. RP 36/90

In recent times, heterogeneous cellular networks (HCNs) have been proposed as an attractive solution to cater to the increased demands in mobile user capacity and could have widespread application in cellular networks deployed under 5th generation (5G) mobile standards. Deploying small cells to coexist with macrocell base transmitting stations (BTSs) from traditional cellular networks enables increased area spectral efficiency (ASE) and throughput through the enhanced reuse of available bandwidth. However, deployment of HCNs is a complex operation and ongoing research aims to solve some of the problems relating to it. In particular, the aggregation of carrier interference can create coverage holes if cell planning is not optimized. While increased ASE may still be achieved by simply deploying small cells in large numbers, the coverage of the system can suffer due to overwhelming interference at mobile terminals, with reduced transmission efficiency per BTS. Therefore, it is important to consider strategies to mitigate interference and increase BTS efficiency when designing BTS location and resource allocation. Stochastic geometric models such as the Poisson Point Process (PPP) have been researched in detail to model the random nature of highly densified HCN BTS locations and uniformly distributed users. In most of this research, the emphasis is placed on the overall spatial performance metrics such as coverage and spectral efficiency. However, using the PPP to model users and BTSs in an HCN ignores practical factors such as the correlation between BTS and user locations, as well as the separation that usually exists between BTSs, where the latter is desirable for interference mitigation. In this thesis, we propose that these factors should be analyzed and accounted for when planning the locations of BTSs in two-tier HCNs and allocating resources for each BTS. Following a comprehensive literature review of stochastic geometric models, we introduce a novel framework that models user correlation to BTSs based on the Poisson Cluster Process (PCP). Furthermore, we incorporate a cross-tier separation rule between tier-1 macrocell BTSs and tier-2 picocell BTSs, by modelling the latter as a Poisson Hole Process (PHP). The first part of the analysis is a comprehensive study of distance distributions and the association of different user groups to BTSs from each tier, as well as the utilization of BTSs under constrained resources. These theoretical models are reused for the analysis of coverage, with some detail on how this affects the rate and ASE. Using numerical results, we analyze the effect of centralizing BTSs to user hotspots and demonstrate what the optimal separation between tier-1 and tier-2 BTSs must be in situations when the network has enough resources for users, and conversely, when resources are limited. The HCN model is extended by incorporating a co-tier separation rule that prohibits tier-2 BTSs from existing within a minimum distance from each other. The proposed model is analyzed for the case of uniform users, such that we measure the effectiveness of deploying tier-2 BTSs to fill coverage holes that exist naturally. We begin the analysis by focusing on distance PDFs and association probability, while showing that a minimum radius of association can be guaranteed based on the setting of co-tier and cross-tier separations. This is followed by a comprehensive analysis of coverage probability, and its relationship to ASE. Limits on both separation categories are explored for optimizing spatial coverage and ASE, while maintaining a viable tier-2 BTS density to form an HCN. The discussion highlights trade-offs that exist between coverage and ASE according to different separation settings and suggests the use of thresholds on each performance parameter to determine the optimal combination of co-tier and cross-tier separation. We showcase how this proposed planning technique works better as a rule of thumb in deploying HCNs when compared to benchmark results and include practical measures of throughput to support this claim. The work is concluded with a summary of contributions and recommendations for future work. One major conclusion is that careful placement of tier-2 BTSs is needed in relation to hotspots modelled by a PCP, and that the coverage of an HCN is maximized when the BTSs are near the centers of hotspots. Furthermore, optimally separating tier-2 BTSs from each other and tier-1 BTSs results in optimized coverage and ASE for the HCN. The work can be extended towards incorporating the mixed user distribution for the analysis of the HCN modelled according to both co-tier and cross-tier separations. Extension towards the use of alternate path loss and fading models such as the ones used to measure path loss indoors are also recommended. Doctor of Philosophy

In this dissertation, we look at selected topics in the latter two problems. In particular, we present near-optimum MIMO differential decoding algorithms, aiming at high decoding speed or improved performance in continuous fading, and derive tight performance bounds for space-time codes. DOCTOR OF PHILOSOPHY (EEE)

In this thesis, we study beamforming strategies for downlink transmission in different wireless communication systems. The major objective is to properly design the beamformer at the transmitter to mitigate the interference at the receivers such that the overall system performance can be enhanced, e.g., the network throughput is maximized. Firstly, the sum-rate optimization problem in a multicell system is considered. The original problem is transformed to a convex semidefinite programming problem by applying the uplink-downlink duality followed by semidefinite relaxation, which can be solved by using convex optimization techniques. Moreover, beamforming strategies in the massive multiple-input multiple-output (MIMO) system of the fifth-generation (5G) network are investigated. Assuming that the base station (BS) is equipped with two-dimensional (2D) massive antenna arrays, we analyze the condition in which the pilot contamination can be mitigated by applying the Von Mises Fisher (VMF) channel model and exploiting the property of the channel covariance matrix. It is shown that the pilot contamination can be mitigated if the angle of arrivals (AoAs) of the interfering users have a nonoverlapping region with the AoA of the desired user. We propose two realistic three-dimensional (3D) beamforming strategies, namely, region-partition beamforming when the BS has perfect channel state information (CSI) and statistic-based beamforming when the BS only knows the statistical knowledge of a two-tier clustered user distribution. The probability density function (pdf) of the two-tier user distribution is derived accordingly. When the full CSI is not available at the BS, a novel limited feedback scheme based on the compressive sensing (CS) technique is proposed to offer accurate channel knowledge. The channel is compressed through linear measurement at the receiver and fed back to the BS. The CS measurement is modelled as a quantization procedure in which quantization noises are generated according to the number of feedback bits. The original channel can be recovered with high accuracy by solving an optimization problem at the BS. Lastly, the secrecy sum-rate maximization problem in Gaussian wiretap channels is considered. To guarantee the secure communication in an interference wiretap channel, we apply a novel decomposition framework to transform the original secrecy sum-rate function to decoupled convex approximation. The decomposition framework is based on Taylor-series expansion and is able to preserve the convexity of the secrecy sum-rate function and linearize the nonconvex part. Following that, we propose a distributed iterative optimization algorithm where the BSs iteratively solve a sequence of convex optimization subproblems in parallel until the optimal beamformers are obtained. Numerical results are presented to validate the theoretical analysis and effectiveness of the proposed approaches. DOCTOR OF PHILOSOPHY (EEE)

Impulse-radio (IR) ultra-wideband (UWB) has drawn much attention as a promising technology for low-data-rate applications such as ranging, identification and low-rate communication in sensor networks. In this research work, we focus on the communication perspective of the IR-UWB applications. To begin with, we investigate various UWB transmission and reception schemes. With the criteria of low complexity, low power consumption and reliable data transmission, we select two systems for detailed study. They are transmitted reference (TR) signaling with auto-correlation receiver (AcR) and on-off keying (OOK) transmission with energy detection receiver. After studying the systems’ characteristics, we observe that the system performance suffers mainly from two types of interference. They are narrowband interference (NBI) from existing in-band wireless services, and multiple access interference (MAI). TR systems have another source of interference which is interpulse interference (IPI) from the overlapping of reference and data waveforms after passing through the highly dispersive propagation channel. Various suppression schemes have been investigated for all these types of interference. For NBI mitigation, notch filtering is adopted in this work. IPI can be mitigated by applying the statistic averaging concept. Our study shows that with these two techniques, the overall performance of TR systems can be improved significantly. Theoretical analysis is provided to evaluate the lower-bound performance for the single-user case. It shows that the system performance is able to approach that under the additive-white-Gaussian-noise (AWGN) environment. We also observe that OOK can be seen as a special case of TR when the delay between reference and amplitude-modulated data pulses is zero. Inspired by the AcR used for TR signals, we then propose a pseudo-coherent detector for OOK signals which is able to provide significant performance improvement as compared with the conventional energy detector. We also look into implementation issues of the proposed receiver. The high sampling rate required by the digital implementation motivates the investigation on techniques to reduce the sampling rate. We then develop a novel subsampling technique, namely bandpass down sampling (BPDS) technique from the standpoint of signal detection rather than signal reconstruction. Our proposed BPDS is capable of reducing the sampling rate to one tenth of the Nyquist rate with small performance degradation introduced to the receivers. Our analysis work is subsequently extended to multiple access system or multiuser system under quasi-synchronous condition. Extended from OOK signaling for single user system, unipolarWalsh code (UWC) are adopted in our proposed multiuser system to distinguish different users. Over bipolar coding, unipolar coding has the advantages of providing better correlation template, and hence, lower detection error rate for the receiver. BPDS technique is also considered in our multi-user system for the ease of digital implementation with significant reduction in the required sampling rate. Theoretical analysis and numerical results support that our system can provide near-optimal performance under perfect timing acquisition and satisfactory performance with certain amount of timing acquisition error. DOCTOR OF PHILOSOPHY (EEE)

In this thesis, we consider interference suppression for fast frequency-hopped (FFH) communication systems over fading channels. Three types of interference, namely, partial-band noise jamming (PBNJ), multitone jamming (MTJ) and multiple-access interference (MAI), are of interest. To suppress the effect of PBNJ, we propose a maximum-likelihood (ML) receiver for frequency-selective Rayleigh-fading channels and derive the corresponding analytical bit-error rate (BER) expressions. We then focus on a more general frequency-selective Rician-fading channel, in which an optimum ML receiver structure is proposed. In addition, two suboptimum ML receivers and their corresponding analytical BER expressions are presented. Furthermore, the linear-combining and product-combining receivers are also investigated under the same channel conditions. To mitigate the composite effect of MTJ and PBNJ, we present analytical BER expressions for an ML receiver under frequency non-selective Rayleigh-fading channel conditions. Following that, frequency-selective Rayleigh-fading channels are considered. System performance of the linear-combining, product-combining and ML receivers is studied and compared. We then investigate the effects of timing and frequency offsets on system performance under frequency non-selective Rician-fading channel conditions. Numerical results show that the FFH system is very sensitive to the timing and frequency offsets. Lastly, we focus on the suppression of MAI. An optimum ML receiver and two suboptimum ML receivers, namely, the ML-A and ML-B receivers, are proposed for synchronous systems over frequency non-selective Rician-fading channels. Analytical BER expressions of these two proposed suboptimum ML receivers are presented. In addition, we study the system performance of the linear-combining, product-combining and clipper receivers, and compare their BER performance with the proposed optimum and two suboptimum ML receivers. Performance comparisons show that the proposed optimum ML receiver has the best BER performance, and the ML-A receiver outperforms the other four receivers. DOCTOR OF PHILOSOPHY (EEE)

This dissertation is aimed at research on mitigation techniques to reduce the negative effects in a wireless communication system. We analyze and calculate the numerical relationship between bit error rate and signal-to-noise ratio. We also simulate and observe various multipath propagations over a wireless fading channel. Horizontal and vertical comparison of the improvements brought about by different diversity combining techniques is listed. Firstly, conventional diversity combining techniques including selection combining, maximum-ratio combining, and equal-gain combining, are applied. Meanwhile, we state the optimum combining technique for different modulation and demodulation schemes, especially refers to the coherent and non-coherent modulation. Then the generalization selection combining is proposed, by plotting the second-order selection combining, both in theoretical and simulated results, to achieve the sub-optimum performance with less system complexity. Based on the theoretical analysis which was published by the recent researchers, we do a complete comparison in this dissertation depending on the simulations, and as a result, this dissertation investigates the feasibility of the second-order selection combining as an alternative of the optimum diversity combining in simulated results. Master of Science (Communications Engineering)

This report mainly involves the study of full-duplex (FD) transmission in the fifth-generation (5G) communication. The detailed study of the implementation of FD in 5G is elaborated. FD transmission is a well-known technique to double the spectral efficiency in wireless communications. However, this technique is hindered by self-interference. Recent research reveals that self-interference can be completely eliminated, which enables FD transmission on transceivers with high spectral efficiency for small-cell base stations. Accordingly, FD transmission is regarded as a strong candidate enabling 5G cellular networks. In this project, the FD technology is investigated in various environments and an effective application relating to it is also discussed. Inclusive of FD, the remaining types of communication channels are also discussed in detail. Moreover, the list of mobile phone generation along with its evolution is described with practical examples. Furthermore, advanced mitigation techniques are also studied. It is aimed to determine a feasible detection of a scheme that eliminates the loopback-interference caused within the antenna circuit. The performance parameters are explained theoretically for these advanced mitigation techniques and used for future analysis. Each method is deeply discussed with all the possible drawbacks and advantages to short list an ideal technique which eliminates the occurred interference signal circulating within the transmitter antenna Master of Science (Communications Engineering)

Security has become an increasingly significant and urgent issue in wireless networks. The growing computational capability of the eavesdropper (Eve) and more and more complicated secure key management in large scale and heterogeneous networks, pose more and more stringent requirement to the cryptographic protocols. As a new design paradigm, physical layer security has been recognised as a promising solution to enhance the security of wireless links relying on the channel properties and advanced signal processing, and can act as either an alternative or a complementary solution to the conventional cryptographic methods. Without complicated secure key generation and management, physical layer security is quite suitable for the large scale distributed networks. In this thesis, we aim to improve the secrecy performance in emerging wireless communication systems relying on different physical layer security techniques. Firstly, the security of an amplify-and-forward successive relaying network with multiple untrusted relay nodes is investigated, where the conventional detrimental inter-relay interference is exploited to jam the untrusted nodes without assistance of external helpers. Considering different complexity requirements, several relay selection schemes are proposed, and the closed-form expressions of the lower bound of the secrecy outage probability and the maximum secrecy diversity order are derived accordingly. It is shown that the maximum secrecy diversity order of N-1 can be achieved for an N relay nodes network. Secondly, the artificial noise (AN) aided secure transmission strategy for a multiple-input single-output multiple-Eve (MISOME) system with a secure user (Bob) and a normal user (NU) is investigated. The power allocations among Bob, NU and AN, as well as the wiretap code rates, are jointly optimized to maximize the effective secrecy throughput (EST), under the average throughput constraint of NU. An alternative optimization algorithm with guaranteed convergence is proposed to obtain the optimal parameters. It is shown that the EST increases with the transmitting power and the number of transmit antennas, and decreases with the throughput constraint of NU, and the EST can be improved through injecting AN and concurrent transmission of Bob and NU. Thirdly, physical layer security in a multi-antenna small-cell network is investigated, where the multi-antenna base stations (BSs), cellular users, and Eves are all randomly distributed according to independent Poisson point processes. Stochastic geometry is applied to analyze the connection and secrecy outage probabilities and the average achievable secrecy rate. The impact of different parameters, including power allocation, BS and Eve density, and the adaptive eavesdropping or jamming on the secrecy performance is analyzed. It is shown that the average secrecy rate is a quasi-concave function of the power allocation factor and monotonically decreases with the ratio of the Eve-BS density. Finally, we study the physical layer security in a large-scale heterogeneous network consisting of both sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot spoofing attacks from the Eves, the coverage and secrecy probabilities are derived using stochastic geometry and the conditions under which the millimeter wave tier outperforms the sub-6 GHz counterpart are discussed in terms of both coverage and secrecy.It is shown that the mmWave small cell can provide better coverage performance in the high transmission code-rate region, and the secrecy performance of the mmWave system outperforms the sub-6 GHz counterpart in the low redundant rate region, which reveals the advantage of using mmWave for secure communication. Doctor of Philosophy (EEE)