Overlapped transmission in wireless networks

In wireless networks, interference is one of the major impairments that deteriorates the performance of a system. Conventional channel-sharing schemes such as TDMA, FDMA, CDMA, etc, orthogonalize the channel resources among users to minimize interference. However, information-theoretic results indicate that orthogonalization of the channel resources is not the most efficient way to transmit to multiple users. We use the term overlapped transmission to describe non-orthogonal transmission schemes because these schemes allocate the same channel resources to more than one user, thus overlapping their transmissions. We conducted an investigation into the potential benefits of overlapped transmissions as well as practical approaches to overlapped transmission in both cellular and ad hoc wireless networks.;We first analyzed the potential of overlapped transmissions to improve the performance of wireless cellular networks. We considered the use of cooperative broadcasting techniques in the downlink of a cellular network to support additional users compared to a system that orthogonalizes the channel resources among the users, such as TDMA, FDMA, CDMA, etc. We evaluated the performance gains of cooperative broadcasting techniques in terms of number of users that can be supported by the base station in a cellular network when the number of users in the system is finite. We also evaluated the user capacity of a cooperative broadcasting system, when the number of users in the system is large. We compared the performance of the optimal broadcasting to several optimal and suboptimal forward-link channel-sharing schemes.;Next, we studied the use of overlapped transmission in ad hoc networks to improve the spatial re-use and throughput of the network. We showed how multihop routing can result in mobile radios having knowledge of interfering signals during the reception of a transmission. We then demonstrated how this knowledge can be exploited to schedule additional transmissions by performing interference cancellation at the physical layer. We evaluated the performance limits of employing overlapped transmissions in wireless ad hoc networks with randomly distributed nodes. We developed a MAC protocol that takes advantage of the knowledge of the interfering signals to schedule additional transmissions, thereby increasing the spatial re-use and throughput of the network. We evaluated the performance of this MAC protocol in a variety of network scenarios and compared to that of IEEE 802.11 MAC protocol. We also analyzed the impact of overlapped transmissions on the performance of Transmission Control Protocol (TCP) in wireless ad hoc networks.