Showing papers by "Gam D. Nguyen published in 2011"

Stable throughput tradeoffs in cognitive shared channels with cooperative relaying

Abstract: This paper addresses fundamental issues in a shared channel where the users have different priority levels. In particular, we characterize the stable-throughput region in a two user cognitive shared channel where the primary (higher priority) user transmits whenever it has packets to transmit while the secondary (cognitive) node transmits its packets with probability p. Therefore, in this system, the secondary link is allowed to share the channel along with the primary link, in contrast to the traditional notion of cognitive radio, in which the secondary user is required to relinquish the channel as soon as the primary is detected. The analysis also takes into account the compound effects of multi-packet reception as well as of the relaying capability on the stability region of the network. We start by analyzing the non-cooperation case where nodes transmit their own packets to their respective destinations. We then extend the analysis to a system where the secondary node cooperatively relays some of the primary's packets. Specifically, in the cooperation case, the secondary node relays those packets that it receives successfully from the primary, but are not decoded properly by the primary destination. In such cognitive shared channels, a tradeoff arises in terms of activating the secondary along with the primary so that both transmissions may be successful, but with a lower probability, compared to the case of the secondary node staying idle when the primary user transmits. Results show the benefits of relaying for both the primary as well as the secondary nodes in terms of the stable-throughput region.

Impact of relay placement on energy efficiency in Underwater Acoustic Networks

Abstract: In spite of the body of work in modeling the underwater channel, there is still considerable lack of fundamental understanding that is required to enable the widespread deployment of Underwater Acoustic Networks (UANs). In this paper we study one such aspect, namely the impact of relay placement on the overall transmit power required to maintain a certain quality of communication between two nodes, in a multi-hop underwater line network. We start by analyzing a simple case involving one relay and a common signal frequency over all hops. We then extend the analysis to include different signal frequencies over different hops. Specifically, in each case, we find through analysis and numerical evaluation the optimal relay placement that minimizes total transmit power, thereby improving energy efficiency. We observe that the optimal relay locations are not always equidistant, especially when each hop can choose from a set of signal frequencies for transmission.

Transmission strategies for single-destination wireless networks

Abstract: We consider the media-access control problem for nodes with heavy traffic in single-destination wireless networks We assume that each source transmits in each time slot according to a transmission probability, which is a continuous value between 0 and 1 Our goal is to determine the values of the transmission probabilities so that the network throughput is maximized In this paper, we show that the maximum throughput is achieved only if these values are either 0 or 1 We obtain closed-form results for optimal throughput for networks that operate under a homogenous situation in which the expected value of the received power at the destination is the same for each source We then extend our studies to more general networks, which rely on exhaustive search for the optimal set of transmissions The search has exponential complexity and is feasible only for networks with small or moderate sizes Thus, we also develop heuristic algorithms, which have polynomial-time complexity and are suitable for large and general networks

Parallel TDMA Scheduling for Multiple-Destination Wireless Networks

Abstract: We study transmission strategies in a multiple-source, multiple-destination wireless network. Each source transmits packets that are intended for a particular destination. However, a transmitted packet can cause interference at other destinations. Our primary performance measure is throughput, which we define to be the average number of packets that are successfully received per intended destination per time slot. The sources are first divided into groups, based on the intended destination of their packets. In our parallel method, each group operates according to its own local protocol (e.g., TDMA), concurrently with and independently of the other groups. Our results show the impact of transmission schedules, channel fading, receiver noise, and other-user interference on network performance. We then show that, for given channel statistics and topology configurations, the network performance can be significantly improved when the groups in the network coordinate their transmissions according to an optimal schedule. Further, in many cases, even the use of randomly generated parallel schedules can provide considerably higher performance than traditional TDMA.