One of the fundamental assumptions made in the design of wireless networks is that the wireless devices have to be half-duplex, i.e., they cannot simultaneously transmit and receive in the same frequency band. The key deterrent in implementing a full-duplex wireless device, which can simultaneously transmit and receive in the same frequency band, is the large power differential between the self-interference from a device's own transmissions and the signal of interest coming from a distant source. In this thesis, we revisit this basic assumption and propose a full-duplex radio design. The design suppresses the self-interference signal by employing a combination of passive suppression, and active analog and digital cancellation mechanisms. The active cancellations are designed for wideband, multiple subcarrier (OFDM), and multiple antenna (MIMO) wireless communications systems. We then implement our design as a 20 MHz MIMO OFDM system with a 2.4 GHz center frequency, suitable for Wi-Fi systems. We perform extensive over-the-air tests to characterize our implementation. Our main contributions are the following: (a) the average amount of active cancellation increases as the received self-interference power increases and as a result, the rate of a full-duplex link increases as the transmit power of communicating devices increases, (b) applying digital cancellation after analog cancellation can sometimes increase the self-interference and the effectiveness of digital cancellation in a full-duplex system will depend on the performance of the cancellation stages that precede it, (c) our full-duplex device design achieves an average of 85 dB of self-interference cancellation over a 20 MHz bandwidth at 2.4 GHz, which is the best cancellation performance reported to date, (d) our full-duplex device design achieves 30-84% higher ergodic rates than its half-duplex counterpart for received powers in the range of [-75, -60] dBm. As a result, our design is the first one to achieve Wi-Fi ranges; in comparison, no implementation to date has achieved Wi-Fi ranges. Consequently, we have conclusively demonstrated that Wi-Fi full-duplex is practically feasible and hence shown that one of the commonly made assumptions in wireless networks is not fundamental.

In most wireless networks, nodes have only limited local information about the state of the network, which includes connectivity and channel state information. With limited local information about the network, each node’s knowledge is mismatched; therefore, they must make distributed decisions. In this thesis, we pose the following question - if every node has network state information only about a small neighborhood, how and when should nodes choose to transmit? While link scheduling answers the above question for point-to-point physical layers which are designed for an interference-avoidance paradigm, we look for answers in cases when interference can be embraced by advanced code design, as suggested by results in network information theory. To make progress on this challenging problem, we propose two constructive distributed algorithms, one conservative and one aggressive, which achieve rates higher than link scheduling based on interference avoidance, especially if each node knows more than one hop of network state information. Both algorithms schedule sub-networks such that each sub-network can employ advanced interference-embracing coding schemes to achieve higher rates. Our innovation is in the identification, selection and scheduling of sub-networks, especially when sub-networks are larger than a single link. Using normalized sum-rate as the metric of network performance, we prove that the proposed conservative sub-network scheduling algorithm is guaranteed to have performance greater than or equal to pure coloring-based link scheduling. In addition, the proposed aggressive sub-network scheduling algorithm is shown, through simulations, to achieve better normalized sum-rate than the conservative algorithm for several network classes. Our results highlight the advantages of extending the design space of possible scheduling strategies to include those that leverage local network information.

How to improve the radio resource utilization and provide better quality-of-service (QoS) is an everlasting challenge to the designers of wireless networks. As an indispensable element of the solution to the above task, medium access control (MAC) protocols coordinate the stations and resolve the channel access contentions so that the scarce radio resources are shared fairly and efficiently among the participating users. With a given physical layer, a properly designed MAC protocol is the key to desired system performance, and directly affects the perceived QoS of end users. Distributed random access protocols are widely used MAC protocols in both infrastructure-based and infrastructureless wireless networks. To understand the characteristics of these protocols, there have been enormous efforts on their performance study by means of analytical modeling in the literature. However, the existing approaches are inflexible to adapt to different protocol variants and traffic situations, due to either many unrealistic assumptions or high complexity. In this thesis, we propose a simple and scalable generic performance analysis framework for a family of carrier sense multiple access with collision avoidance (CSMA/CA) based distributed MAC protocols, regardless of the detailed backoff and channel access policies, with more realistic and fewer assumptions. It provides a systematic approach to the performance study and comparison of diverse MAC protocols in various situations. Developed from the viewpoint of a tagged station, the proposed framework focuses on modeling the backoff and channel access behavior of an individual station. A set of fixed point equations is obtained based on a novel three-level renewal process concept, which leads to the fundamental MAC performance metric, average frame service time. With this result, the important network saturation throughput is then obtained straightforwardly. The above distinctive approach makes the proposed analytical framework unified for both saturated and unsaturated stations. The proposed framework is successfully applied to study and compare the performance of three representative distributed MAC protocols: the legacy p-persistent CSMA/CA protocol, the IEEE 802.15.4 contention access period MAC protocol, and the IEEE 802.11 distributed coordination function, in a network with homogeneous service. It is also extended naturally to study the effects of three prevalent mechanisms for prioritized channel access in a network with service differentiation. In particular, the novel concepts of ``virtual backoff event'' and ``pre-backoff waiting periods'' greatly simplify the analysis of the arbitration interframe space mechanism, which is the most challenging one among the three, as shown in the previous works reported in the literature. The comparison with comprehensive simulations shows that the proposed analytical framework provides accurate performance predictions in a broad range of stations. The results obtained provide many helpful insights into how to improve the performance of current protocols and design better new ones.

The next-generation wireless communications have been envisioned to be supported by heterogeneous networks using various wireless access technologies. The popular cellular networks and wireless local area networks (WLANs) present perfectly complementary characteristics in terms of service capacity, mobility support, and quality-of-service (QoS) provisioning. The cellular/WLAN interworking is thus an effective way to promote the evolution of wireless networks. As an essential aspect of the interworking, resource allocation is vital for efficient utilization of the overall resources. Specially, multi-service provisioning can be enhanced with cellular/WLAN interworking by taking advantage of the complementary network strength and an overlay structure. Call assignment/reassignment strategies and admission control policies are effective resource allocation mechanisms for the cellular/WLAN integrated network. Initially, the incoming calls are distributed to the overlay cell or WLAN according to call assignment strategies, which are enhanced with admission control policies in the target network. Further, call reassignment can be enabled to dynamically transfer the traffic load between the overlay cell and WLAN via vertical handoff. By these means, the multi-service traffic load can be properly shared between the interworked systems. In this thesis, we investigate the load sharing problem for this heterogeneous wireless overlay network. Three load sharing schemes with different call assignment/reassignment strategies and admission control policies are proposed and analyzed. Effective analytical models are developed to evaluate the QoS performance and determine the call admission and assignment parameters. First, an admission control scheme with service-differentiated call assignment is studied to gain insights on the effects of load sharing on interworking effectiveness. Then, the admission scheme is extended by using randomized call assignment to enable distributed implementation. Also, we analyze the impact of user mobility and data traffic variability. Further, an enhanced call assignment strategy is developed to exploit the heavy-tailedness of data call size. Last, the study is extended to a multi-service scenario. The overall resource utilization and QoS satisfaction are improved substantially by taking into account the multi-service traffic characteristics, such as the delay-sensitivity of voice traffic, elasticity and heavy-tailedness of data traffic, and rate-adaptiveness of video streaming traffic.

This thesis explores the optimal node placement for linear Gaussian multiple relay networks of an arbitrary size and with one source-destination pair. Consider the the low attenuation regime (path loss exponent less than 3/2). Under the condition that the minimum achievable rate from source to destination is maintained, we derive upper bounds of node placement with the incoherent and coherent coding schemes, and examine the optimal power assignment related to the node placement with the coherent coding scheme. We prove that the farthest distance between two adjacent nodes is bounded even for an infinite total number of relay nodes, and closed-form formulas of the bounds are derived for both the coding schemes. Furthermore, the distance from the source to the destination is of the same order as the total number of nodes, given the path loss exponent greater than one half under the incoherent coding scheme and the path loss exponent greater than 1 with coherent relaying with interference subtraction coding scheme. Conditioned on a conjecture based on the simulation results, we also provide heuristic upper bounds, which are a little tighter than the strictly proved bounds. The bounds provided in this thesis can serve as a helpful guideline for the relay extension problem in practical network implementation.

The next-generation wireless networks are expected to integrate diverse network architectures and various wireless access technologies to provide a robust solution for ubiquitous broadband wireless access, such as wireless local area networks (WLANs), Ultra-Wideband (UWB), and millimeter-wave (mmWave) based wireless personal area networks (WPANs), etc. To enhance the spectral efficiency and link reliability, smart antenna systems have been proposed as a promising candidate for future broadband access networks. To effectively exploit the increased capabilities of the emerging wireless networks, the different network characteristics and the underlying physical layer features need to be considered in the medium access control (MAC) design, which plays a critical role in providing efficient and fair resource sharing among multiple users. In this thesis, we comprehensively investigate the MAC design in both single- and multi-hop broadband wireless networks, with and without infrastructure support. We first develop mathematical models to identify the performance bottlenecks and constraints in the design and operation of existing MAC. We then use a cross-layer approach to mitigate the identified bottleneck problems. Finally, by evaluating the performance of the proposed protocols with analytical models and extensive simulations, we determine the optimal protocol parameters to maximize the network performance. In specific, a generic analytical framework is developed for capacity study of an IEEE 802.11 WLAN in support of non-persistent asymmetric traffic flows. The analysis can be applied for effective admission control to guarantee the quality of service (QoS) performance of multimedia applications. As the access point (AP) becomes the bottleneck in an infrastructure based WLAN, we explore the multiple-input multiple-output (MIMO) capability in the future IEEE 802.11n WLANs and propose a MIMO-aware multi-user (MU) MAC. By exploiting the multi-user degree of freedom in a MIMO system to allow the AP to communicate with multiple users in the downlink simultaneously, the proposed MU MAC can minimize the AP-bottleneck effect and significantly improve the network capacity. Other enhanced MAC mechanisms, e.g., frame aggregation and bidirectional transmissions, are also studied. Furthermore, different from a narrowband system where simultaneous transmissions by nearby neighbors collide with each other, wideband system can support multiple concurrent transmissions if the multi-user interference can be properly managed. Taking advantage of the salient features of UWB and mmWave communications, we propose an exclusive region (ER) based MAC protocol to exploit the spatial multiplexing gain of centralized UWB and mmWave based wireless networks. Moreover, instead of studying the asymptotic capacity bounds of arbitrary networks which may be too loose to be useful in realistic networks, we derive the expected capacity or transport capacity of UWB and mmWave based networks with random topology. The analysis reveals the main factors affecting the network (transport) capacity, and how to determine the best protocol parameters to maximize the network capacity. In addition, due to limited transmission range, multi-hop relay is necessary to extend the communication coverage of UWB networks. A simple, scalable, and distributed UWB MAC protocol is crucial for efficiently utilizing the large bandwidth of UWB channels and enabling numerous new applications cost-effectively. To address this issue, we further design a distributed asynchronous ER based MAC for multi-hop UWB networks and derive the optimal ER size towards the maximum network throughput. The proposed MAC can significantly improve both network throughput and fairness performance, while the throughput and fairness are usually treated as a tradeoff in other MAC protocols.

The recent years have seen an enormous advance in wireless communication technology and co-existence of various types of wireless networks, which requires effective inter-networking among the heterogeneous wireless networks in order to support user roaming over the networks while maintaining the connectivity. One of main challenges to achieve the connectivity over heterogeneous wireless networks is potential intermittent connections caused by user roaming. The issue is how to maintain the connection as the user roams and how to ensure service quality in the presence of a long disconnection period. In this dissertation, we apply the delay tolerant network (DTN) framework to heterogeneous terrestrial wireless networks, and propose a system architecture to achieve the connectivity in the presence of excessive long delays and intermittent paths. We study several possible approaches, discuss the applicability of each of the approaches and propose the super node architecture. To demonstrate the effectiveness of the proposed super node architecture, we give a simulation study that compares the system performance under the super node architecture and under the epidemic based architecture. Within the proposed architecture that employs the idea of super nodes, we further study how to effectively route a message over access networks. We present a new routing technique for mobile ad-hoc networks (MANETs) based on the DTN system architecture. We introduce the concept of virtual network topology and redefine the dominating-set based routing for the challenged network environment under consideration. In addition, we propose a time based methodology to predict the probability of future contacts between node pairs to construct the virtual network topology. We present a simulation study that demonstrates the effectiveness of the proposed routing approach as compared with the epidemic routing, and that the time based technique for predicting the future contacts gives better performance compared with that using the number of previous contacts. We further extend the dominating set routing technique through analyzing the underlying node mobility model. We shed some light on how using node mobility model can improve contact probability estimation. Based on our findings we propose a new algorithm that improves the routing performance by minimizing the selected dominating set size. Information security challenges in the super node architecture are introduced. We further address two main security challenges: The first is how to prevent unauthorized nodes from using the network resources, and the second is how to achieve end-to-end secure message exchange over the network. Our proposed solutions are based on asymmetric key cryptography techniques. Moreover, we introduce a new idea of separating the problem of source authentication from the problem of message authorization. We propose a new technique that employs the one-way key chain to use symmetric key cryptographic techniques to address the problems under consideration.

Resource management in wireless networks is one of the key factors in maximizing the overall throughput. Contrary to popular belief, dividing the resources in a dense network does not yield the best results. A method that has been developed recently shares the spectrum amongst all the users in such a way that each node can potentially utilize about half of all the available resources. This new technique is often referred to as Interference Alignment and excels based on the fact that the amount of the network resources assigned to a user does not go to zero as the number of users in the network increases. Unfortunately it is still very difficult to implement the interference alignment concepts in practice. This thesis investigates some of the low-complexity solutions to integrate interference alignment ideas into the existing wireless networks. In the third and fourth chapters of this thesis, it is shown that introducing relays to a quasi-static wireless network can be very beneficial in terms of achieving higher degrees of freedom. The relays store the signals being communicated in the network and then send a linear combination of those signals. Using the proposed scheme, it is shown that although the relays cannot decode the original information, they can transform the equivalent channel in such a way that performing interference alignment becomes much easier. Investigating the required output power of the relays shows that it can scale either slower or faster than the output power of the main transmitters. This opens new doors for the applications that have constraints on the accessible output powers in the network nodes. The results are valid for both $X$ Channel and Interference Channel network topologies. In Chapter Five, the similarities between full-duplex transmitters and relays are examined. The results suggest that the transmitters can play the relay roles for offering easier interference alignment. Similar to the relay-based alignment, in the presented scheme full-duplex transmitters listen to the signals from other transmitters and use this information during the subsequent transmission periods. Studying the functionality of the full-duplex transmitters from the receivers' side shows the benefits of having a minimal cooperation between transmitters without even being able to decode the signals. It is also proved that the degrees of freedom for the $N$-user Interference Channel with full-duplex transmitters can be $\sqrt{\frac{N}{2}}$. The results offer an easy way to recover a portion of degrees of freedom with manageable complexity suited for practical systems.

Body Area Networks (BANs) are emerging as a convenient option for patient monitoring. They have shown potential in improving health care services through a network of external or implanted biosensors and actuators collecting real-time physiological data. Advancements in wireless networking and sensor development are expediting the adoption of BANs. However, real-time patient monitoring still remains a challenge due to network failures and congestion. In order to improve channel loss resilience and thus link availability, a multi-radio systems approach is adopted incorporating Bluetooth and Wi-Fi. In this work, we propose a multi-radio interface designed for a BAN to improve end-to-end communications. A multi-radio BAN controller is introduced to interface between the two wireless protocols (Wi-Fi and Bluetooth), control inter-radio handovers, manage a shared transmission buffer, and overall, route data accordingly through the protocol stacks. Simulations are conducted to study the performance of the system by adjusting handover timing and its effect on link availability. Advancing a handover has the benefit of a higher throughput at the cost of an increase in power consumption and timing overhead. Furthermore, various human mobility models, AP placement arrangements, and network densities are simulated to evaluate the performance of the BAN multi-radio interface. Sparse networks were found to have the most gain from the addition of the secondary Bluetooth radio system, as primary AP coverage was already very limited. Simulation results for various combinations of simulation parameters are presented to illustrate the improvement in BAN dependability through a multi-radio interface.

<p>In our homes and in the enterprise, in our leisure and in our professions, mobile computing is no longer merely "exciting;" it is becoming an essential, ubiquitous tool of the modern world. New and innovative mobile applications continue to inform, entertain, and surprise users. But, to make the daily use of mobile technologies more gratifying and worthwhile, we must move forward with new levels of sophistication. The Mobile App Stores of the future must be built on stronger foundations. </p><p>This dissertation considers a broad view of the challenges and intuitions behind a diverse selection of such new primitives. Some of these primitives will mitigate existing and fundamental challenges of mobile computing, especially relating to wireless communication. Others will take an application-driven approach, being designed to serve a novel purpose, and be adapted to the unique and varied challenges from their disparate domains. However, all are related through a unifying goal, to provide a seamless, enjoyable, and productive mobile experience. This dissertation takes view that by bringing together nontrivial enhancements across a selection of disparate-but-interrelated domains, the impact is synergistically stronger than the sum of each in isolation. Through their collective impact, these new "building blocks" can help lay a foundation to upgrade mobile technology beyond the expectations of early-adopters, and into seamless integration with all of our lives.</p> / Dissertation