Distributed Power Control and Medium Access Control Protocol Design for Multi-Channel Ad Hoc Wireless Networks

In the past decade, the development of wireless communication technologies has made the use of the Internet ubiquitous. With the increasing number of new inventions and applications using wireless communication, more interference is introduced among wireless devices that results in limiting the capacity of wireless networks. Many approaches have been proposed to improve the capacity. One approach is to exploit multiple channels by allowing concurrent transmissions, and therefore it can provide high capacity. Many available, license-exempt, and non-overlapping channels are the main advantages of using this approach. Another approach that increases the network capacity is to adjust the transmission power; hence, it reduces interference among devices and increases the spatial reuse. Integrating both approaches provides further capacity. However, without careful transmission power control (TPC) design, the network performance is limited. The first part of this thesis tackles the integration to efficiently use multiple channels with an effective TPC design in a distributed manner. We examine the deficiency of uncontrolled asymmetrical transmission power in multi-channel ad hoc wireless networks. To overcome this deficiency, we propose a novel distributed transmission power control protocol called the distributed power level (DPL) protocol for multi-channel ad hoc wireless networks. DPL allocates different maximum allowable power values to different channels so that the nodes that require higher transmission power are separated from interfering with the nodes that require lower transmission power. As a result, nodes select their channels based on their minimum required transmission power to reduce interference over the channels. We also introduce two TPC modes for the DPL protocol: symmetrical and asymmetrical. For the symmetrical mode, nodes transmit at the power that has been assigned to the selected channel, thereby creating symmetrical links over any channel. The asymmetrical mode, on the other hand, allows nodes to transmit at a power that can be lower than or equal to the power assigned to the selected channel. In the second part of this thesis, we propose the multi-channel MAC protocol with hopping reservation (MMAC-HR) for multi-hop ad hoc networks to overcome the multi-channel exposed terminal problem, which leads to poor channel utilization over multiple channels. The proposed protocol is distributed, does not require clock synchronization, and fully supports broadcasting information. In addition, MMAC-HR does not require nodes to monitor the control channel in order to determine whether or not data channels are idle; instead, MMAC-HR employs carrier sensing and independent slow channel hopping without exchanging information to reduce the overhead. In the last part of this thesis, a novel multi-channel MAC protocol is developed without requiring any change to the IEEE 802.11 standard known as the dynamic switching protocol (DSP) based on the parallel rendezvous approach. DSP utilizes the available channels by allowing multiple transmissions at the same time and avoids congestion because it does not need a dedicated control channel and enables nodes dynamically switch among channels. Specifically, DSP employs two half-duplex interfaces: One interface follows fast hopping and the other one follows slow hopping. The fast hopping interface is used primarily for transmission and the slow hopping interface is used generally for reception. Moreover, the slow hopping interface never deviates from its default hopping sequence to avoid the busy receiver problem. Under single-hop ad hoc environments, an analytical model is developed and validated. The maximum saturation throughput and theoretical throughput upper limit of the proposed protocol are also obtained

Random Access Transport Capacity

We develop a new metric for quantifying end-to-end throughput in multihop wireless networks, which we term random access transport capacity, since the interference model presumes uncoordinated transmissions. The metric quantifies the average maximum rate of successful end-to-end transmissions, multiplied by the communication distance, and normalized by the network area. We show that a simple upper bound on this quantity is computable in closed-form in terms of key network parameters when the number of retransmissions is not restricted and the hops are assumed to be equally spaced on a line between the source and destination. We also derive the optimum number of hops and optimal per hop success probability and show that our result follows the well-known square root scaling law while providing exact expressions for the preconstants as well. Numerical results demonstrate that the upper bound is accurate for the purpose of determining the optimal hop count and success (or outage) probability.Comment: Submitted to IEEE Trans. on Wireless Communications, Sept. 200

On the Power Efficiency of Sensory and Ad Hoc Wireless Networks

We consider the power efficiency of a communications channel, i.e., the maximum bit rate that can be achieved per unit power (energy rate). For additive white Gaussian noise (AWGN) channels, it is well known that power efficiency is attained in the low signal-to-noise ratio (SNR) regime where capacity is proportional to the transmit power. In this paper, we first show that for a random sensory wireless network with n users (nodes) placed in a domain of fixed area, with probability converging to one as n grows, the power efficiency scales at least by a factor of sqrt n. In other words, each user in a wireless channel with n nodes can support the same communication rate as a single-user system, but by expending only 1/(sqrt n) times the energy. Then we look at a random ad hoc network with n relay nodes and r simultaneous transmitter/receiver pairs located in a domain of fixed area. We show that as long as r ≤ sqrt n, we can achieve a power efficiency that scales by a factor of sqrt n. We also give a description of how to achieve these gains

Enhancing Secrecy with Multi-Antenna Transmission in Wireless Ad Hoc Networks

We study physical-layer security in wireless ad hoc networks and investigate two types of multi-antenna transmission schemes for providing secrecy enhancements. To establish secure transmission against malicious eavesdroppers, we consider the generation of artificial noise with either sectoring or beamforming. For both approaches, we provide a statistical characterization and tradeoff analysis of the outage performance of the legitimate communication and the eavesdropping links. We then investigate the networkwide secrecy throughput performance of both schemes in terms of the secrecy transmission capacity, and study the optimal power allocation between the information signal and the artificial noise. Our analysis indicates that, under transmit power optimization, the beamforming scheme outperforms the sectoring scheme, except for the case where the number of transmit antennas are sufficiently large. Our study also reveals some interesting differences between the optimal power allocation for the sectoring and beamforming schemes.Comment: to appear in IEEE Transactions on Information Forensics and Securit

Research on Wireless Multi-hop Networks: Current State and Challenges

Wireless multi-hop networks, in various forms and under various names, are being increasingly used in military and civilian applications. Studying connectivity and capacity of these networks is an important problem. The scaling behavior of connectivity and capacity when the network becomes sufficiently large is of particular interest. In this position paper, we briefly overview recent development and discuss research challenges and opportunities in the area, with a focus on the network connectivity.Comment: invited position paper to International Conference on Computing, Networking and Communications, Hawaii, USA, 201

Capacity of Cellular Wireless Network

Earlier definitions of capacity for wireless networks, e.g., transport or transmission capacity, for which exact theoretical results are known, are well suited for ad hoc networks but are not directly applicable for cellular wireless networks, where large-scale basestation (BS) coordination is not possible, and retransmissions/ARQ under the SINR model is a universal feature. In this paper, cellular wireless networks, where both BS locations and mobile user (MU) locations are distributed as independent Poisson point processes are considered, and each MU connects to its nearest BS. With ARQ, under the SINR model, the effective downlink rate of packet transmission is the reciprocal of the expected delay (number of retransmissions needed till success), which we use as our network capacity definition after scaling it with the BS density. Exact characterization of this natural capacity metric for cellular wireless networks is derived. The capacity is shown to first increase polynomially with the BS density in the low BS density regime and then scale inverse exponentially with the increasing BS density. Two distinct upper bounds are derived that are relevant for the low and the high BS density regimes. A single power control strategy is shown to achieve the upper bounds in both the regimes. This result is fundamentally different from the well known capacity results for ad hoc networks, such as transport and transmission capacity that scale as the square root of the (high) BS density. Our results show that the strong temporal correlations of SINRs with PPP distributed BS locations is limiting, and the realizable capacity in cellular wireless networks in high-BS density regime is much smaller than previously thought. A byproduct of our analysis shows that the capacity of the ALOHA strategy with retransmissions is zero.Comment: A shorter version to appear in WiOpt 201