Cognitive Radio Systems: Performance Analysis and Optimal Resource Allocation

Rapid growth in the use of wireless services coupled with inefficient utilization of scarce spectrum resources has led to the analysis and development of cognitive radio systems. Cognitive radio systems provide dynamic and more efficient utilization of the available spectrum by allowing unlicensed users (i.e., cognitive or secondary users) to access the frequency bands allocated to the licensed users (i.e., primary users) without causing harmful interference to the primary user transmissions. The central goal of this thesis is to conduct a performance analysis and obtain throughput- and energy-efficient optimal resource allocation strategies for cognitive radio systems. Cognitive radio systems, which employ spectrum sensing mechanisms to learn the channel occupancy by primary users, generally operate under sensing uncertainty arising due to false alarms and miss-detections. This thesis analyzes the performance of cognitive radio systems in a practical setting with imperfect spectrum sensing. In the first part of the thesis, optimal power adaptation schemes that maximize the achievable rates of cognitive users with arbitrary input distributions in underlay cognitive radio systems subject to transmit and interference power constraints are studied. Simpler approximations of optimal power control policies in the low-power regime are determined. Low-complexity optimal power control algorithms are proposed. Next, energy efficiency is considered as the performance metric and power allocation strategies that maximize the energy efficiency of cognitive users in the presence of time-slotted primary users are identified. The impact of different levels of channel knowledge regarding the transmission link between the secondary transmitter and secondary receiver, and the interference link between the secondary transmitter and primary receiver on the optimal power allocation is addressed. In practice, the primary user may change its status during the transmission phase of the secondary users. In such cases, the assumption of time-slotted primary user transmission no longer holds. With this motivation, the spectral and energy efficiency in cognitive radio systems with unslotted primary users are analyzed and the optimal frame duration and energy-efficient optimal power control schemes subject to a collision constraint are jointly determined. The second line of research in this thesis focuses on symbol error rate performance of cognitive radio transmissions in the presence of imperfect sensing decisions. General formulations for the optimal decision rule and error probabilities for arbitrary modulation schemes are provided. The optimal decision rule for rectangular quadrature amplitude modulation (QAM) is characterized, and closed-form expressions for the average symbol error probability attained with the optimal detector under both transmit power and interference constraints are derived. Furthermore, throughput of cognitive radio systems for both fixed-rate and variable-rate transmissions in the finite-blocklength regime is studied. The maximum constant arrival rates that the cognitive radio channel can support with finite blocklength codes while satisfying statistical quality of service (QoS) constraints imposed as limitations on the buffer violation probability are characterized. In the final part of the thesis, performance analysis in the presence of QoS requirements is extended to general wireless systems, and energy efficiency and throughput optimization with arbitrary input signaling are studied when statistical QoS constraints are imposed as limitations on the buffer violation probability. Effective capacity is chosen as the performance metric to characterize the maximum throughput subject to such buffer constraints by capturing the asymptotic decay-rate of buffer occupancy. Initially, constant-rate source is considered and subsequently random arrivals are taken into account

High speed protocols for dual bus and dual ring network architectures

In this dissertation, two channel access mechanisms providing fair and bandwidth efficient transmission on dual bus and dual ring networks with high bandwidth-latency product are proposed. In addition, two effective priority mechanisms are introduced to meet the throughput and delay requirements of the diverse arrays of applications that future high speed networks must support. For dual bus architectures, the Buffer Insertion Bandwidth Balancing (BI_BWB) mechanism and the Preemptive priority Bandwidth Balancing (P_BI_BWB) mechanism are proposed. BI_BWB can significantly improve the delay performance of remote stations. It achieves that by providing each station with a shift register into which the station can temporarily store the upstream stations\u27 transmitted packets and replace these packets with its own transmissions. P_BI_BWB, an enhancement of BI_BWB, is designed to introduce effective preemptive priorities. This mechanism eliminates the effect of low priority on high priority by buffering the low priority traffic into a shift register until the transmission of the high priority traffic is complete. For dual ring architectures, the Fair Bandwidth Allocation Mechanism (FBAM) and the Effective Priority Bandwidth Balancing (EP_BWB) mechanism are introduced. FBAM allows stations to reserve channel bandwidth on a continuous basis rather than wait until bandwidth starvation is observed. Consequently, FBAM does not have to deal with the difficult issue of identifying starvation, a serious drawback of other access mechanisms such as the Local and Global Fairness Algorithms (LFA and GFA, respectively). In addition, its operation requires a significantly smaller number of control bits in the access control field of the slot and its performance is less sensitive to system parameters. Moreover, FBAM demonstrates Max-Min flow control properties with respect to the allocation of bandwidth among competing traffic streams, which is a significant advantage of FBAM over all the previously proposed channel access mechanisms. EP_BWB, an enhancement of FBAM to support preemptive priorities, minimizes the effect of low priority on high priority and supports delay-sensitive traffic by enabling higher priority classes to preempt the transmissions of lower priority classes. Finally, the great potential of EP_BWB to support the interconnection of base stations on a distributed control wireless PCN carrying voice and data traffic is demonstrated

Performance analysis of handoff resource allocation strategies through the state-dependent rejection scheme

The state-dependent rejection scheme (SRS) provides a common framework for analysing existing handoff schemes and for designing new ones easily. Designing new schemes is made simple by determining the appropriate set of state-dependent probabilities. The Markov analysis of SRS is simple and useful for drawing initial conclusions on handoff strategies. The analysis and simulations carried out here demonstrate the capability of SRS to adapt to different mobility and load scenarios and to achieve good performance while targeting quality of service performance metrics.Peer Reviewe

Proportional Fair Coding for Wireless Mesh Networks

We consider multi–hop wireless networks carrying unicast flows for multiple users. Each flow has a specified delay deadline, and the lossy wireless links are modelled as binary symmetric channels (BSCs). Since transmission time, also called airtime, on the links is shared amongst flows, increasing the airtime for one flow comes at the cost of reducing the airtime available to other flows sharing the same link. We derive the joint allocation of flow airtimes and coding rates that achieves the proportionally fair throughput allocation. This utility optimisation problem is non–convex, and one of the technical contributions of this paper is to show that the proportional fair utility optimisation can nevertheless be decomposed into a sequence of convex optimisation problems. The solution to this sequence of convex problems is the unique solution to the original non–convex optimisation. Surprisingly, this solution can be written in an explicit form that yields considerable insight into the nature of the proportional fair joint airtime/coding rate allocation. To our knowledge, this is the first time that the utility fair joint allocation of airtime/coding rate has been analysed, and also, one of the first times that utility fairness with delay deadlines has been considered

Application of advanced on-board processing concepts to future satellite communications systems: Bibliography

Abstracts are presented of a literature survey of reports concerning the application of signal processing concepts. Approximately 300 references are included