Opportunistic Beamforming is a promising scheme with potential usage in a variety of applications ranging from cellular audio/video communications to wireless mesh-networks. However, some aspects in this scheme are open to further improvements, such as effective quality of service (QoS) provisioning, and efficient handling of sparse/realistic networks. We focus on the downlink channel of a multi-rate time-division multiplexing MISO (multiple input single output) point-to-multi-point wireless communication system, and design a cross-layer scheme. We employ a two-stage opportunistic scheduler. The scheduler receives channel state information in the form of SNR (signal to noise ratio) measurements from each terminal, and schedules one terminal in each time-slot. At the first stage in a time-slot a terminal is chosen, and our gradient descent channel estimator finds an estimation of its channel vector. Then the beamformer produces a beamforming vector in an adjacent pilot channel, or in the first part of the time-slot in the data channel. Our beamforming is based on the estimation of the channel, as opposed to the random beamformer used in the opportunistic beamforming. Each terminal measures its perceived SNR even when the channel is assigned to another terminal, and sends this information back to the basestation subject to a feedback-reduction policy. The scheduler uses the new channel state information for its second stage and assigns a terminal for the succeeding time-slot. The channel estimation procedure uses the reported SNRs for each terminal, which is a significantly lower feedback rate than the amount required for coherent beamforming. Our approach efficiently handles sparse/realistic networks, and also provides a better QoS than the opportunistic beamforming scheme. / Thesis (Master, Electrical & Computer Engineering) -- Queen's University, 2007-06-30 07:00:20.468

Scheduling in wireless networks plays an important role. The undeterministic nature of the wireless channel is usually considered as an undesirable property. Recently, the idea of opportunistic scheduling is introduced and it takes advantage of the time-varying channel for performance improvement such as throughput and delay. Since the introduction of opportunistic scheduling, there are two main bodies of works. The first body of works assume that each user is greedy and has infinite backlog for transfer. With this assumption, fairness objective becomes an important factor in designing a scheduling algorithm to avoid severe starvation of certain users. Typical fairness involve processor sharing time fairness, proportional fairness, and minimum performance guarantee. On the other hand, delay performance is not a appropriate factor to evaluate the effectiveness of a scheduling algorithm because of the infinite backlog assumption. In reality, this assumption is not true as data arrives and leaves the network randomly in practice. The second body of works deal with the relaxation of the infinite backlog assumption. Thus, the notion of stability region arises. The definition of stability is that the queue at each source node remains finite. Stability region can be defined as the set of traffic intensities which can all be stabilized by the network. The well known throughput optimal algorithm is proven capable of achieving the largest stability region. In this thesis, two innovative opportunistic scheduling algorithms which aim to minimize the amount of resources used to stabilize the current traffics are proposed. The key feature of our algorithm is that the incoming traffic rates are available to the scheduler, whereas the throughput optimal algorithm has no such prior traffic knowledge. Performance comparisons are made by means of simulation to demonstrate that the proposed algorithms can achieve the same stability region as the throughput optimal algorithm. Moreover, the delay performance is better than that of the throughput optimal algorithm, especially under heavy traffic conditions.

Scheduling in wireless networks plays an important role. The undeterministic nature of the wireless channel is usually considered as an undesirable property. Recently, the idea of opportunistic scheduling is introduced and it takes advantage of the time-varying channel for performance improvement such as throughput and delay. Since the introduction of opportunistic scheduling, there are two main bodies of works. The first body of works assume that each user is greedy and has infinite backlog for transfer. With this assumption, fairness objective becomes an important factor in designing a scheduling algorithm to avoid severe starvation of certain users. Typical fairness involve processor sharing time fairness, proportional fairness, and minimum performance guarantee. On the other hand, delay performance is not a appropriate factor to evaluate the effectiveness of a scheduling algorithm because of the infinite backlog assumption. In reality, this assumption is not true as data arrives and leaves the network randomly in practice. The second body of works deal with the relaxation of the infinite backlog assumption. Thus, the notion of stability region arises. The definition of stability is that the queue at each source node remains finite. Stability region can be defined as the set of traffic intensities which can all be stabilized by the network. The well known throughput optimal algorithm is proven capable of achieving the largest stability region. In this thesis, two innovative opportunistic scheduling algorithms which aim to minimize the amount of resources used to stabilize the current traffics are proposed. The key feature of our algorithm is that the incoming traffic rates are available to the scheduler, whereas the throughput optimal algorithm has no such prior traffic knowledge. Performance comparisons are made by means of simulation to demonstrate that the proposed algorithms can achieve the same stability region as the throughput optimal algorithm. Moreover, the delay performance is better than that of the throughput optimal algorithm, especially under heavy traffic conditions.

In this dissertation, we explore the issues related to opportunistic and cooperative communications in a multiuser environment. In the first part of the dissertation, we consider opportunistic scheduling for delay limited systems. Multiuser communication over fading channels is a challenging problem due to fast varying channel conditions. On the other hand, it provides opportunities to exploit the varying nature of the channel and maximize the throughput by scheduling the user (or users) with good channel. This gain is termed as multiuser diversity. The larger the number of users, the greater is the multiuser diversity gain. However, there is an inherent scheduling delay in exploiting multiuser diversity. The objective of this work is to design the scheduling schemes which use multiuser diversity to minimize the system transmit energy. We analyze the schemes in large system limit and characterize the energy--delay tradeoff. We show that delay tolerance in data transmission helps us to exploit multiuser diversity and results in an energy efficient use of the system resources. We assume a general multiuser environment but the proposed scheduling schemes are specifically suitable for the wireless sensor network applications where saving of transmit energyat the cost of delay in transmission is extremely useful to increase the life of battery for the sensor node. In the first part of the thesis, we propose scheduling schemes withthe objective of minimizing transmit energy for a given fixed tolerable transmission delay. The fixed delay is termed as hard deadline. A group of users with channels better than a transmission threshold are scheduled for transmission simultaneously using superposition coding. The transmission thresholds depend onthe fading statistics of the underlying channel and hard deadline of the data to be scheduled. As deadline is approached, the thresholds decrease monotonically to reflect the scheduling priority for theuser. We analyze the proposed schedulers in the large system limit. We compute the optimized transmission thresholds for the proposed scheduling schemes. We analyze the proposed schemes for practically relevant scenarios when the randomly arriving packets have individual, non--identical deadlines. We analyze the case when loss tolerance of the application is exploited to further decrease the system energy. The transmitted energy is not a convex function oftransmission thresholds. Therefore, we propose heuristic optimization procedures to compute the transmission thresholds and evaluate the performance of the schemes. Finally, we study the effect of outer cell interference on the proposed scheduling schemes. The second part of the thesis investigates the problem of cooperative communication between the nodes which relay the data of other sources multiplex with their own data towards a common destination, i.e. a relay node performs as a relay and data source at the same time. This problem setting is very useful in case of some wireless sensor network (WSN) applications where all the nodes relay sensed data towards a common destination sink node. The capacity region of a relay region is still an open problem. We use deterministic network model to study the problem. We characterizethe capacity region for a cooperative deterministic network with single source, multiple relays and single destination. We also characterize the capacity region when communicating nodes have correlated information to be sent to the destination. / Cross Layer Optimization of Wireless Sensor Networks

We investigate in detail two multiuser opportunistic scheduling problems in centralized wireless systems: the scheduling of "delay-sensitive" flows with packet delay requirements of a few tens to few hundreds of milliseconds over the air interface, and the scheduling of "best-effort" flows with the objective of minimizing mean file transfer delay. Schedulers for delay-sensitive flows are characterized by a fundamental tradeoff between "maximizing total service rate by being opportunistic" and "balancing unequal queues (or delays) across users". In choosing how to realize this tradeoff in schedulers, our key premise is that "robustness" should be a primary design objective alongside performance. Different performance objectives -- mean packet delay, the tail of worst user's queue distribution, or that of the overall queue distribution -- result in remarkably different scheduling policies. Different design objectives and resulting schedulers are also not equally robust, which is important due to the uncertainty and variability in both the wireless environment and the traffic. The proposed class of schedulers offers low packet delays, less sensitivity to the scheduler parameters and channel characteristics, and a more graceful degradation of service in terms of the fraction of users meeting their delay requirements under transient overloads, when compared with other well-known schedulers. Schedulers for best-effort flows are characterized by a fundamental tradeoff between "maximizing the total service rate" and "prioritizing flows with short residual sizes". We characterize two regimes based on the "degree" of opportunistic gain present in the system. In the first regime -- where the opportunistic capacity of the system increases sharply with the number of users -- the use of residual flow-size information in scheduling will 'not' result in a significant reduction in flow-level delays. Whereas, in the second regime -- where the opportunistic capacity increases slowly with the number of users -- using flow-size information alongside channel state information 'may' result in a significant reduction. We then propose a class of schedulers which offers good performance in either regime, in terms of mean file transfer delays as well as probability of blocking for systems that enforce flow admission control. This thesis provides a comprehensive theoretical study of these fundamental tradeoffs for opportunistic schedulers, as well as an exploration of some of the practical ramifications to engineering wireless systems. / text

abstract: This dissertation builds a clear understanding of the role of information in wireless networks, and devises adaptive strategies to optimize the overall performance. The meaning of information ranges from channel/network states to the structure of the signal itself. Under the common thread of characterizing the role of information, this dissertation investigates opportunistic scheduling, relaying and multicast in wireless networks. To assess the role of channel state information, the problem of opportunistic distributed opportunistic scheduling (DOS) with incomplete information is considered for ad-hoc networks in which many links contend for the same channel using random access. The objective is to maximize the system throughput. In practice, link state information is noisy, and may result in throughput degradation. Therefore, refining the state information by additional probing can improve the throughput, but at the cost of further probing. Capitalizing on optimal stopping theory, the optimal scheduling policy is shown to be threshold-based and is characterized by either one or two thresholds, depending on network settings. To understand the benefits of side information in cooperative relaying scenarios, a basic model is explored for two-hop transmissions of two information flows which interfere with each other. While the first hop is a classical interference channel, the second hop can be treated as an interference channel with transmitter side information. Various cooperative relaying strategies are developed to enhance the achievable rate. In another context, a simple sensor network is considered, where a sensor node acts as a relay, and aids fusion center in detecting an event. Two relaying schemes are considered: analog relaying and digital relaying. Sufficient conditions are provided for the optimality of analog relaying over digital relaying in this network. To illustrate the role of information about the signal structure in joint source-channel coding, multicast of compressible signals over lossy channels is studied. The focus is on the network outage from the perspective of signal distortion across all receivers. Based on extreme value theory, the network outage is characterized in terms of key parameters. A new method using subblock network coding is devised, which prioritizes resource allocation based on the signal information structure. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

With the increasing demand for wireless communications, radio resource management (RRM) plays an important role in future wireless networks in order to provide higher data rates and better quality of services, given the limited amount of available radio resources. Although some specific features of wireless communication networks cause challenges to effective and efficient RRM, they bring opportunities that help improv- ing network performance and resource utilization. In this thesis, we focus on RRM issues related to the broadcast/multicast nature in wireless communication networks. The work is divided into two parts. In the first part, we exploit how to take advantage of the broadcast nature of wire- less transmissions in RRM by opportunistically applying two-way relaying (or network coding) and traditional one-way relaying. Different objectives are considered, includ- ing maximizing total packet transmission throughput (Chapter 2), minimizing costs related to transmission power and delay (Chapter 3), and minimizing packet transmis- sion delay subject to maximum and average transmission power limits (Chapter 4). While designing these scheduling schemes, the random traffic and channel conditions are also taken into consideration. Our results show that the proposed opportunis- tic scheduling schemes can indeed take good advantage of the broadcast feature at the relay nodes and achieve much higher throughput and, in some scenarios, provide close-to-optimum QoS performance. The second part (Chapter 5) of the thesis deals with the issue of efficient resource management in multicast communications, where we study channel sharing and power allocations for multicast device-to-divice (D2D) communication groups underlaying a cellular network. In such a scenario, D2D multicasting together with the mutual inter- ference between cellular and D2D communications, makes the interference conditions and power allocations a very complicated issue. Different approaches are proposed that allow each D2D group to share the cellular channels and allocate transmission power to each D2D and cellular transmitter, so that the sum throughput of D2D and cellular users is maximized. Our results indicate that it is possible to achieve close-to-optimum throughput performance in such a network. / Dissertation / Doctor of Philosophy (PhD)

As more and more wireless applications/services emerge in the market, the already heavily crowded radio spectrum becomes much scarcer. Meanwhile, however,as it is reported in the recent literature, there is a large amount of radio spectrum that is under-utilized. This motivates the concept of cognitive radio wireless networks that allow the unlicensed secondary-users (SUs) to dynamically use the vacant radio spectrum which is not being used by the licensed primary-users (PUs). In this dissertation, we investigate protocol design for both the synchronous and asynchronous cognitive radio networks with emphasis on the medium access control (MAC) layer. We propose various spectrum sharing schemes, opportunistic packet scheduling schemes, and spectrum sensing schemes in the MAC and physical (PHY) layers for different types of cognitive radio networks, allowing the SUs to opportunistically utilize the licensed spectrum while confining the level of interference to the range the PUs can tolerate. First, we propose the cross-layer based multi-channel MAC protocol, which integrates the cooperative spectrum sensing at PHY layer and the interweave-based spectrum access at MAC layer, for the synchronous cognitive radio networks. Second, we propose the channel-hopping based single-transceiver MAC protocol for the hardware-constrained synchronous cognitive radio networks, under which the SUs can identify and exploit the vacant channels by dynamically switching across the licensed channels with their distinct channel-hopping sequences. Third, we propose the opportunistic multi-channel MAC protocol with the two-threshold sequential spectrum sensing algorithm for asynchronous cognitive radio networks. Fourth, by combining the interweave and underlay spectrum sharing modes, we propose the adaptive spectrum sharing scheme for code division multiple access (CDMA) based cognitive MAC in the uplink communications over the asynchronous cognitive radio networks, where the PUs may have different types of channel usage patterns. Finally, we develop a packet scheduling scheme for the PU MAC protocol in the context of time division multiple access (TDMA)-based cognitive radio wireless networks, which is designed to operate friendly towards the SUs in terms of the vacant-channel probability. We also develop various analytical models, including the Markov chain models, M=GY =1 queuing models, cross-layer optimization models, etc., to rigorously analyze the performance of our proposed MAC protocols in terms of aggregate throughput, access delay, and packet drop rate for both the saturation network case and non-saturation network case. In addition, we conducted extensive simulations to validate our analytical models and evaluate our proposed MAC protocols/schemes. Both the numerical and simulation results show that our proposed MAC protocols/schemes can significantly improve the spectrum utilization efficiency of wireless networks.

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Auf dem Gebiet der drahtlosen Kommunikation und insbesondere auf den tieferen Netzwerkschichten sind gewaltige Fortschritte zu verzeichnen. Innovative Konzepte und Technologien auf der physikalischen Schicht (PHY) gehen dabei zeitnah in zelluläre Netze ein. Drahtlose Maschennetzwerke (WMNs) können mit diesem Innovationstempo nicht mithalten. Die Mehrnutzer-Kommunikation ist ein Grundpfeiler vieler angewandter PHY Technologien, die sich in WMNs nur ungenügend auf die etablierte Schichtenarchitektur abbilden lässt. Insbesondere ist das Problem des Scheduling in WMNs inhärent komplex. Erstaunlicherweise ist der Mehrfachzugriff mit Trägerprüfung (CSMA) in WMNs asymptotisch optimal obwohl das Verfahren eine geringe Durchführungskomplexität aufweist. Daher stellt sich die Frage, in welcher Weise das dem CSMA zugrunde liegende Konzept des konkurrierenden Wettbewerbs (engl. Contention) für die Integration innovativer PHY Technologien verwendet werden kann. Opportunistische Kommunikation ist eine Technik, die die inhärenten Besonderheiten des drahtlosen Kanals ausnutzt. In der vorliegenden Dissertation werden CSMA-basierte Protokolle für die opportunistische Kommunikation in WMNs entwickelt und evaluiert. Es werden dabei opportunistisches Routing (OR) im zustandslosen Kanal und opportunistisches Scheduling (OS) im zustandsbehafteten Kanal betrachtet. Ziel ist es, den Durchsatz von elastischen Paketflüssen gerecht zu maximieren. Es werden Modelle für Überlastkontrolle, Routing und konkurrenzbasierte opportunistische Kommunikation vorgestellt. Am Beispiel von IEEE 802.11 wird illustriert, wie der schichtübergreifende Entwurf in einem Netzwerksimulator prototypisch implementiert werden kann. Auf Grundlage der Evaluationsresultate kann der Schluss gezogen werden, dass die opportunistische Kommunikation konkurrenzbasiert realisierbar ist. Darüber hinaus steigern die vorgestellten Protokolle den Durchsatz im Vergleich zu etablierten Lösungen wie etwa DCF, DSR, ExOR, RBAR und ETT. / In the field of wireless communication, a tremendous progress can be observed especially at the lower layers. Innovative physical layer (PHY) concepts and technologies can be rapidly assimilated in cellular networks. Wireless mesh networks (WMNs), on the other hand, cannot keep up with the speed of innovation at the PHY due to their flat and decentralized architecture. Many innovative PHY technologies rely on multi-user communication, so that the established abstraction of the network stack does not work well for WMNs. The scheduling problem in WMNs is inherent complex. Surprisingly, carrier sense multiple access (CSMA) in WMNs is asymptotically utility-optimal even though it has a low computational complexity and does not involve message exchange. Hence, the question arises whether CSMA and the underlying concept of contention allows for the assimilation of advanced PHY technologies into WMNs. In this thesis, we design and evaluate contention protocols based on CSMA for opportunistic communication in WMNs. Opportunistic communication is a technique that relies on multi-user diversity in order to exploit the inherent characteristics of the wireless channel. In particular, we consider opportunistic routing (OR) and opportunistic scheduling (OS) in memoryless and slow fading channels, respectively. We present models for congestion control, routing and contention-based opportunistic communication in WMNs in order to maximize both throughput and fairness of elastic unicast traffic flows. At the instance of IEEE 802.11, we illustrate how the cross-layer algorithms can be implemented within a network simulator prototype. Our evaluation results lead to the conclusion that contention-based opportunistic communication is feasible. Furthermore, the proposed protocols increase both throughput and fairness in comparison to state-of-the-art approaches like DCF, DSR, ExOR, RBAR and ETT.