We consider the problem of energy-efficient transmission in multi-flow multihop cooperative wireless networks. Although the performance gains of cooperative approaches are well known, the combinatorial nature of these schemes makes it difficult to design efficient polynomial-time algorithms for joint routing, scheduling and power control. This becomes more so when there is more than one flow in the network. It has been conjectured by many authors, in the literature, that the multiflow problem in cooperative networks is an NP-hard problem. In this paper, we formulate the problem, as a combinatorial optimization problem, for a general setting of k-flows, and formally prove that the problem not only NP-hard but it is o(n1/7-ε) inapproxmiable. To our knowledge, the results in this paper provide the first such inapproxmiablity proof in the context of multiflow cooperative wireless networks. We further prove that for a special case of k = 1 the solution is a simple path, and offer a polynomial time algorithm for jointly optimizing routing, scheduling and power control.

We formulate the problem of delay constrained energy-efficient broadcast in cooperative multihop wireless networks. We show that this important problem is not only NP-complete, but also o(log(n)) inapproximable. We derive approximation results and an analytical lower-bound for this problem. We break this NP hard problem into three parts: ordering, scheduling and power control. We show that when the ordering is given, the joint scheduling and power-control problem can be solved in polynomial time by a novel algorithm that combines dynamic programming and linear programming to yield the minimum energy broadcast for a given delay constraint. We further show empirically that this algorithm used in conjunction with an ordering derived heuristically using the Dijkstra's shortest path algorithm yields near-optimal performance in typical settings. We use our algorithm to study numerically the trade-off between delay and power-efficiency in cooperative broadcast and compare the performance of our cooperative algorithm with a smart non-cooperative algorithm.

Physical layer cooperation can be a powerful tool for enhancing the performance of multi-hop wireless networks. In this paper, we analyze the time to complete a cooperative broadcast to flood some information from one node to all nodes in a wireless network. We show that with cooperation the total time to complete the broadcast grows only logarithmically with the network diameter (unlike in traditional systems where time to flood increases linearly with the diameter). Simulation results validate the analysis, and show that the improvements in flooding time are more pronounced for higher density networks. We further compare the energy costs of cooperative and traditional flooding, and show that the improvements in flooding time with cooperation do not come at the expense of higher energy costs. These results, albeit based on an idealized form of cooperation, provide a strong motivation to develop and test practical schemes for cooperative flooding in multi-hop wireless networks.

The use of multiple-input multiple-output (MIMO) configurations in wireless systems is becoming increasingly popular, due to the potential capacity enhancing properties. The use of such configurations, requires deployment of multiple RF chains and results in an increase in cost and complexity. This paper focuses on transmitter design, where cost is often dominated by the need to utilise linearized power amplifiers. Thus, the effect of reducing the number of active transmit antennae through selection can yield a significant cost advantage. Three criteria for transmit antenna selection are evaluated and characterised through the use of real channel data in the UHF band. The criteria are based only on the channel gains and have relatively low complexity; thus are suitable for practical purposes. This study examines the performance of the tested selection algorithms under a variety of operating conditions. The paper also considers the issue of delayed switch time due to reverse-link communication latency