Diversity and spectral efficiency in wireless relay networks
Authors: J. Nicholas Laneman
Venue:  (2008)

Motivated by the increasing demand for higher data rate, broader coverage, and lower infrastructure cost in wireless systems, a major effort is being made to study the use of relay stations in wireless networks. In this dissertation, we propose efficient relaying protocols for two types of wireless networks, i.e., networks with one destination, one relay and multiple sources, and networks with one destination, one source and multiple relays. We model the former as a multi-access relay channel (MARC) and refer the latter as a multihop network. For a MARC, we propose the multi-access relay amplify-forward (MAF) protocol. The proposed protocol allows for a low-complexity relay, allows the users to operate as if in a normal (non-cooperative) multi-access channel (MAC), and allows multiple users to achieve significant gains from sharing a single amplify-forward relay in slow fading environments. MAF achieves the optimal diversity-multiplexing trade-off at high multiplexing gains. Analysis of the protocol reveals that it outperforms the compress-forward strategy at low multiplexing gains and further outperforms the dynamic decode-forward protocol at high multiplexing gains. We also propose different routing protocols for wireless multi-hop networks. An information theoretic analysis suggests that there is an optimum number of hops in maximizing the end-to-end spectral efficiency for a multihop network with AWGN channels. Motivated by this observation, this dissertation takes the trade-off between power and bandwidth efficiency into account and proposes two routing schemes, namely, approximately-ideal-path routing (AIPR) and distributed spectrum-efficient routing (DSER). AIPR finds a path to approximate an optimum regular path and requires location information. DSER is more amenable to distributed implementations based on Bellman-Ford or Dijkstra&rsquo;s algorithms. We show by simulations that the spectral efficiencies of AIPR and DSER for random networks approach that of nearest-neighbor routing in the low signal-to-noise ratio (SNR) regime and that of single-hop routing in the high SNR regime. In the moderate SNR regime, DSER offers significant gains compared with nearest-neighbor or single-hop routing. We further demonstrate the trade-off between the number of hops and spectral efficiency in a broadband multipath fading system by considering resource allocation in an orthogonal frequency division multiplexing (OFDM) multihop network. We formulate a power and subcarrier allocation problem for a network with one destination and multiple sources and relays and discuss the properties of the optimum solution for a convex relaxation of the problem. We then focus on low-complexity, efficient algorithms for a multihop network with one destination, one source and multiple relays. We develop an algorithm that is optimum at high SNR for a two-hop network. For a general multihop network, we propose a greedy approach to subcarrier allocation. We show by simulations that judicious allocation of power and subcarriers offers significant gains compared to a fixed assignment of subcarriers in a multi-hop network. Furthermore, our results demonstrate that, even for a broadband system with frequency selective fading, there exits an optimum number of hops that maximizes the end-to-end spectral efficiency.