Connectivity of ad hoc wireless networks: an alternative to graph-theoretic approaches

Connectivity in wireless ad hoc and sensor networks is typically analyzed using a graph-theoretic approach. In this paper, we investigate an alternative communication-theoretic approach for determining the minimum transmit power required for achieving connectivity. Our results show that, if there is significant multipath fading and/or multiple access interference in the network, then graph-theoretic approaches can substantially underestimate the minimum transmit power required for connectivity. This is due to the fact that graph-theoretic approaches do not take the route quality into consideration. Therefore, while in scenarios with line-of-sight (LOS) communications a graph-theoretic approach could be adequate for determining the minimum transmit power required for connectivity, in scenarios with strong multipath fading and/or multiple access interference a communication-theoretic approach could yield much more accurate results and, therefore, be preferable.     

Providing secondary access to licensed spectrum through coordination

Most wireless systems receive a license that gives them exclusive access to a block of spectrum. Exclusivity guarantees adequate quality of service, but it also leads to inefficient use of spectrum. Even when the license holder is idle, no other device can use the spectrum. This paper explores an alternative paradigm for secondary access to spectrum, where a secondary device can transmit when and only when the primary license holder grants permission. In this spectrum usage paradigm, each secondary device makes a request for temporary access to spectrum by providing a primary license holder with information such as its required bandwidth, its required signal to interference ratio, its transmit power, and its location, which is essential for a primary license holder in making an admission decision. This explicit coordination makes it possible to protect the quality of service of both primary and secondary, while gaining the efficiency of spectrum sharing. In this paper, we consider the case where the primary license holder is a GSM-based cellular carrier. We show that our proposed sharing scheme works well even with simple admission control and primitive frequency assignment algorithms. Moreover, imprecise location information does not significantly undermine the performance of our scheme. We also demonstrate that our scheme is attractive to a license holder by showing that a cellular carrier can profit from offering a secondary device access to spectrum at a price lower than it would normally charge a cellular call. Sooksan Panichpapiboon is currently a Ph.D. candidate in the Electrical and Computer Engineering (ECE) Department at Carnegie Mellon University (CMU), Pittsburgh, PA. He received both B.S. and M.S. degrees in electrical and computer engineering from CMU in 2000 and 2002, respectively. His research interests include wireless networks, sensor networks, and performance modeling. Home page: www.andrew.cmu.edu/~sooksan Jon M. Peha (M 87, SM 96) is Associate Director of the CMU Center for Wireless and Broadband Networking at Carnegie Mellon University, and a Full Professor in the Department of Engineering and Public Policy and the Department of Electrical and Computer Engineering. He has served as Chief Technical Officer of successful networking start-ups with products related to wireless networks, video and voice over IP, and secure network payment systems. He has been a member of Legislative Staff in the US Congress, and a member of technical staff at SRI International, AT&T Bell Laboratories, and Microsoft. Dr. Peha also launched a US Government program to assist developing countries with information infrastructure. He holds a Ph.D. in electrical engineering from Stanford. Home page: www.ece.cmu.edu/~peha     

Optimal Transmit Power in Wireless Sensor Networks

Power conservation is one of the most important issues in wireless ad hoc and sensor networks, where nodes are likely to rely on limited battery power. Transmitting at unnecessarily high power not only reduces the lifetime of the nodes and the network, but also introduces excessive interference. It is in the network designer's best interest to have each node transmit at the lowest possible power while preserving network connectivity. In this paper, we investigate the optimal common transmit power, defined as the minimum transmit power used by all nodes necessary to guarantee network connectivity. This is desirable in sensor networks where nodes are relatively simple and it is difficult to modify the transmit power after deployment. The optimal transmit power derived in this paper is subject to the specific routing and medium access control (MAC) protocols considered; however, the approach can be extended to other routing and MAC protocols as well. In deriving the optimal transmit power, we distinguish ourselves from a conventional graph-theoretic approach by taking realistic physical layer characteristics into consideration. In fact, connectivity in this paper is defined in terms of a quality of service (QoS) constraint given by the maximum tolerable bit error rate (BER) at the end of a multihop route with an average number of hops.     

Route Reservation in Ad Hoc Wireless Networks

This paper investigates whether and when route reservation-based (RB) communication can yield better delay performance than non-reservation-based (NRB) communication in ad hoc wireless networks. In addition to posing this fundamental question, the requirements (in terms of route discovery, medium access control (MAC) protocol, and pipelining, etc.) for making RB switching superior to NRB switching are also identified. A novel analytical framework is developed and the network performance under both RB and NRB schemes is quantified. It is shown that if the aforementioned requirements are met, then RB schemes can indeed yield better delay performance than NRB schemes. This advantage, however, comes at the expense of lower throughput and goodput compared to NRB schemes