Performance analysis of spectrum sensing techniques for future wireless networks

In this thesis, spectrum sensing techniques are investigated for cognitive radio (CR) networks in order to improve the sensing and transmission performance of secondary networks. Specifically, the detailed exploration comprises of three areas, including single-node spectrum sensing based on eigenvalue-based detection, cooperative spectrum sensing under random secondary networks and full-duplex (FD) spectrum sensing and sharing techniques. In the first technical chapter of this thesis, eigenvalue-based spectrum sensing techniques, including maximum eigenvalue detection (MED), maximum minimum eigenvalue (MME) detection, energy with minimum eigenvalue (EME) detection and the generalized likelihood ratio test (GLRT) eigenvalue detector, are investigated in terms of total error rates and achievable throughput. Firstly, in order to consider the benefits of primary users (PUs) and secondary users (SUs) simultaneously, the optimal decision thresholds are investigated to minimize the total error rate, i.e. the summation of missed detection and false alarm rate. Secondly, the sensing-throughput trade-off is studied based on the GLRT detector and the optimal sensing time is obtained for maximizing the achievable throughput of secondary communications when the target probability of detection is achieved. In the second technical chapter, the centralized GLRT-based cooperative sensing technique is evaluated by utilizing a homogeneous Poisson point process (PPP). Firstly, since collaborating all the available SUs does not always achieve the best sensing performance under a random secondary network, the optimal number of cooperating SUs is investigated to minimize the total error rate of the final decision. Secondly, the achievable ergodic capacity and throughput of SUs are studied and the technique of determining an appropriate number of cooperating SUs is proposed to optimize the secondary transmission performance based on a target total error rate requirement. In the last technical chapter, FD spectrum sensing (FDSS) and sensing-based spectrum sharing (FD-SBSS) are investigated. There exists a threshold pair, not a single threshold, due to the self-interference caused by the simultaneous sensing and transmission. Firstly, by utilizing the derived expressions of false alarm and detection rates, the optimal decision threshold pair is obtained to minimize total error rate for the FDSS scheme. Secondly, in order to further improve the secondary transmission performance, the FD-SBSS scheme is proposed and the collision and spectrum waste probabilities are studied. Furthermore, different antenna partitioning methods are proposed to maximize the achievable throughput of SUs under both FDSS and FD-SBSS schemes

Performance analysis of multi-antenna wireless systems

In this thesis we apply results from multivariate probability, random matrix theory (RMT) and free probability theory (FPT) to analyse the theoretical performance limits of future-generation wireless communication systems which implement multiple-antenna technologies. Motivated by the capacity targets for fifth generation wireless communications, our work focuses on quantifying the performance of these systems in terms of several relevant metrics, including ergodic rate and capacity, secrecy rate and capacity, asymptotic capacity, outage probability, secrecy outage probability and diversity order. Initially, we investigate the secrecy performance of a wirelessly powered, wiretap channel which incorporates a relatively small number of transmit antennas in a multiple-input single-output scenario. We consider two different transmission protocols which utilise physical layer security. Using traditional multivariate probability techniques we compute closed-form expressions for the outage probability and secrecy outage probability of the system under both protocols, based on the statistical properties of the channel. We use these expressions to compute approximations of the connection outage probability, secrecy outage probability and diversity orders in the high signal-to-noise ratio (SNR) regime which enables us to find candidates for the optimal time-switching ratio and power allocation coefficients. We show that it is possible to achieve a positive secrecy throughput, even in the case where the destination is further away from the source than the eavesdropper, for both protocols and compare their relative merits. We then progress to considering small-scale multiple-input multiple-output (MIMO) channels, which can be modelled as random matrices. We consider a relay system that enables communication between a remote source and destination in the presence of an eavesdropper and describe a decode-and-forward (DF) protocol which uses physical layer security techniques. A new result on the joint probability density function of the largest eigenvalues of the channel matrix is derived using results from RMT. The result enables us to compute the legitimate outage probability and diversity order of the proposed protocol and to quantify the effect of increasing the number of relays and antennas of the system. Next, we consider much larger-scale massive MIMO arrays, for which analysis using finite results becomes impractical. First we investigate the ergodic capacity of a massive MIMO, non-orthogonal multiple access system with unlimited numbers of antennas. Employing asymptotic results from RMT, we provide closed-form solutions for the asymptotic capacities of this scenario. This enables us to derive the optimal power allocation coefficients for the system. We demonstrate that our approach has low computational complexity and provides results much closer to optimality when compared with existing, suboptimal methods, particularly for the case where nodes are equipped with very large antenna arrays. Finally, we analyse the ergodic capacity of a single-hop, massive MIMO, multi-relay system having more complex properties, by applying results in FPT. Our method allows for an arbitrary number of relays, arbitrarily large antenna arrays and also asymmetric characteristics between channels, which is a situation that cannot typically be analysed using traditional RMT methods. We compute the asymptotic capacity across the system for the case when the relays employ a DF protocol and no direct link exists between the endpoints. We are able to calculate the overall capacity, to a high degree of accuracy, for systems incorporating channels greater than $128\times 128$ in dimension for which existing methods fail due to excessive computational demands. Finally, the comparative computational complexities of the methods are analysed and we see the advantages of applying the FPT method

Reliable and Efficient Cognitive Radio Communications Using Directional Antennas

Cognitive Radio (CR) is a promising solution that enhances spectrum utilization by allowing an unlicensed or Secondary User (SU) to access licensed bands in a such way that its imposed interference on a license holder Primary User (PU) is limited, and hence fills the spectrum holes in time and/or frequency domains. Resource allocation, which involves scheduling of available time and transmit power, represents a crucial problem for the performance evaluation of CR systems. In this dissertation, we study the spectral efficiency maximization problem in an opportunistic CR system. Specifically, in the first part of the dissertation, we consider an opportunistic CR system where the SU transmitter (SUtx) is equipped to a Reconfigurable Antenna (RA). RA, with the capabilities of dynamically modifying their characteristics can improve the spectral efficiency, via beam steering and utilizing the spectrum white spaces in spatial (angular) domain. In our opportunistic CR system, SUtx relies on the beam steering capability of RA to detect the direction of PU\u27s activity and also to select the strongest beam for data transmission to SU receiver (SUrx). We study the combined effects of spectrum sensing error and channel training error as well as the beam detection error and beam selection error on the achievable rates of an opportunistic CR system with a RA at SUtx. We also find the best duration for spectrum sensing and channel training as well as the best transmit power at SUtx such that the throughput of our CR system is maximized subject to the Average Transmit Power Constraint (ATPC) and Average Interference Constraint (AIC). In the second part of the dissertation, we consider an opportunistic Energy Harvesting (EH)-enabled CR network, consisting of multiple SUs and an Access Point (AP), that can access a wideband spectrum licensed to a primary network. Assuming that each SU is equipped with a finite size rechargeable battery, we study how the achievable sum-rate of SUs is impacted by the combined effects of spectrum sensing error and imperfect Channel State Information (CSI) of SUs–AP links. We also design an energy management strategy that maximizes the achievable sum-rate of SUs, subject to a constraint on the average interference that SUs can impose on the PU

Safety and Reliability - Safe Societies in a Changing World

The contributions cover a wide range of methodologies and application areas for safety and reliability that contribute to safe societies in a changing world. These methodologies and applications include: - foundations of risk and reliability assessment and management - mathematical methods in reliability and safety - risk assessment - risk management - system reliability - uncertainty analysis - digitalization and big data - prognostics and system health management - occupational safety - accident and incident modeling - maintenance modeling and applications - simulation for safety and reliability analysis - dynamic risk and barrier management - organizational factors and safety culture - human factors and human reliability - resilience engineering - structural reliability - natural hazards - security - economic analysis in risk managemen