Cooperation in Wireless Networks

Spatial diversity, in the form of employing multiple antennas (i.e., MIMO), has proved to be very effective in increasing network capacity and reliability. However, equipping a wireless node with multiple antennas may not be practical, as the footprint of multiple antennas may not fit on a wireless node (particularly on handheld wireless devices). In order to achieve spatial diversity without requiring multiple antennas on the same node, the so-called cooperative communications (CC) has been introduced. Under CC, each node is equipped with only a single antenna and spatial diversity is achieved by exploiting the antennas on other nodes in the network through cooperative relaying.

The goal of this dissertation is to maximize throughput at network level through CC at the physical layer. A number of problems are explored in this investigation. The main contributions of this dissertation can be summarized as follows.

1. Optimal Relay Assignment. We first consider a simple CC model where each source-destination pair may employ only a single relay. For this three-node model, the choice of a relay node (among a set of available relay nodes) for a given session is critical in the overall network performance. We study the relay node assignment problem in a cooperative ad hoc network environment, where multiple source-destination pairs compete for the same pool of relay nodes in the network. Our objective is to assign the available relay nodes to different source-destination pairs so as to maximize the minimum data rate among all pairs. We present an optimal polynomial time algorithm, called ORA, that solves this problem. A novel idea in this algorithm is a "linear marking" mechanism, which maintains linear complexity at each iteration. We offer a formal proof of optimality for ORA and use numerical results to demonstrate its capability.

2. Incorporating Network Coding. It has been shown that network coding (NC) can reduce the time-slot overhead when multiple session share the same relay node in CC. Such an approach is called network-coded CC (or NC-CC). Most of the existing works have mainly focused on the benefits of this approach. The potential adverse effect under NC-CC remains unknown. We explore this important problem by introducing the concept of network coding noise (NC noise). We show that due to NC noise, NC may not be always beneficial to CC. We substantiate this important finding in two important scenarios: analog network coding (ANC) in amplify-and-forward (AF) CC, and digital network coding (DNC) in decode-and-forward (DF) CC. We analyze the origin of NC noise via a careful study of signal aggregation at a relay node and signal extraction at a destination node. We derive a closed-form expression for NC noise at each destination node and show that the existence of NC noise could diminish the advantage of NC in CC. Our results shed new light on how to use NC in CC effectively.

3. Session Grouping and Relay Node Selection. When there are multiple sessions in the network, it may be necessary to combine sessions into different groups, and then have each group select the most beneficial relay node for NC-CC. We study this joint grouping and relay node selection problem for NC-CC. By studying matching problems in hypergraphs, we show that this problem is NP-hard. We then propose a distributed and online algorithm to solve this problem. The key idea in our algorithm is to have each neighboring relay node of a newly joined session determine and offer the best group for this session from the groups that it is currently serving; and then to have the source node of this newly joined session select the best group among all received offers. We show that our distributed algorithm has polynomial complexity. Using extensive numerical results, we show that our distributed algorithm is near-optimal and adapts well to online network dynamics.

4. Grouping and Matching for Multi-Relay Cooperation. Existing models of NC-CC consider only single relay node for each session group. We investigate how NC-CC behaves when multiple relay nodes are employed. For a given session, we develop closed form formulas for the mutual information and achievable rate under multi-relay NC-CC. In multi-relay NC-CC, the achievable rate of a session depends on the other sessions in its group as well as the set of relay nodes used for NC-CC. Therefore, we study NC-CC via joint optimization of grouping and matching of session and relay groups in an ad hoc network. Although we show that the joint problem is NP-hard, we develop an efficient polynomial time algorithm for grouping and matching (called G²M). G²M first builds beneficial relay groups for individual sessions. This is followed by multiple iterations during which sessions are combined with other sessions to form larger and better session groups (while corresponding relay groups are merged and updated accordingly). Using extensive numerical results, we show the efficiency and near optimality of our G²M algorithm.