Cooperative Protocols for Relay and Interference Channels with Half-Duplex Constraint

Abstract

Enabling cooperation among nodes of a wireless network can significantly reduce the required transmit power as well as the induced intra-network interference. Due to the practical half-duplexity constraint of the cooperating nodes, they are prohibited to simultaneously transmit and receive data at the same time-frequency resource. The purpose of this dissertation is to illustrate the value of cooperation in such an environment. To understand how to cooperate efficiently, information theory is employed as a useful tool, which not only determines the fundamental limits of communication (i.e., capacity) over the considered network, but also provides insights into the design of a proper transmission scheme for that network. In this thesis, two simple but yet important types of wireless networks, namely Relay Channel, and Interference Channel are studied. In fact, these models constitute building blocks for larger networks. The first considered channel is a diamond-shaped relay channel consisting of a source, a destination, and two parallel relays. The second analyzed channel is an interference channel composed of two transmitter-receiver pairs with out-of-band transmitter cooperation, also referred to as conferencing encoders. While characterizing the capacity of these channels are difficult, a simpler and a more common approach is to find an achievable scheme for each channel that ensures a small gap from the capacity for all channel parameters. In chapter 2, the diamond relay channel is investigated in detail. Because of the half-duplex nature of the relays, each relay is either in transmit or receive mode, making four modes possible for the two-relay combination, specifically, 1) broadcast mode (both relays receive) 2,3) routing modes (one relay transmits, another receives) 4) multiple-access mode (both relays transmit). An appropriate scheduling ( i.e., timing over the modes) and transmission scheme based on the decode-and-forward strategy are proposed and shown to be able to achieve either the capacity for certain channel conditions or at most 3.6 bits below the capacity for general channel conditions. Particularly, by assuming each transmitter has a constant power constraint over all modes, a parameter Δ is defined, which captures some important features of the channel. It is proven that for Δ=0 the capacity of the channel can be attained by successive relaying, i.e., using modes 2 and 3 defined above in a successive manner. This strategy may have an infinite gap from the capacity of the channel when Δ≠0. To achieve rates as close as 0.71 bits to the capacity, it is shown that the cases of Δ>0 and Δ<0 should be treated differently. Using new upper bounds based on the dual problem of the linear program associated with the cut-set bounds, it is proven that the successive relaying strategy needs to be enhanced by an additional broadcast mode (mode 1), or multiple access mode (mode 4), for the cases of Δ<0 and Δ>0, respectively. Furthermore, it is established that under average power constraints the aforementioned strategies achieve rates as close as 3.6 bits to the capacity of the channel.

In chapter 3, a two-user Gaussian Interference Channel (GIC) is considered, in which encoders are connected through noiseless links with finite capacities. The setup can be motivated by downlink cellular systems, where base stations are connected via infrastructure backhaul networks. In this setting, prior to each transmission block the encoders communicate with each other over the cooperative links. The capacity region and the sum-capacity of the channel are characterized within some constant number of bits for some special classes of symmetric and Z interference channels. It is also established that properly sharing the total limited cooperation capacity between the cooperative links may enhance the achievable region, even when compared to the case of unidirectional transmitter cooperation with infinite cooperation capacity. To obtain the results, genie-aided upper bounds on the sum-capacity and cut-set bounds on the individual rates are compared with the achievable rate region. The achievable scheme enjoys a simple type of Han-Kobayashi signaling, together with the zero-forcing, and basic relaying techniques.