COOPERATIVE COMMUNICATION WITH WIRELESS NETWORK CODING
Liu, K. J. Ray
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Cooperative communication is a new communication paradigm that allows multiple transceivers to collaborate as a cluster for data transmission, and such clustering could greatly improve the transmission quality due to cooperative diversity. For conventional cooperation protocols, each cooperating device uses orthogonal channels to relay different messages for mitigating co-channel interference and avoiding transmission collision, but doing so would significantly reduce the bandwidth efficiency. One way to tackle this issue is to use wireless network coding, in which different messages are smartly combined at cooperating devices to save the channel use for data relaying. Network coding has been widely used in wireline networks, but only until very recently was grafted onto the wireless networks. In the research community, it has been unknown for a long time whether network-coded cooperation is able to achieve the same diversity gain as the conventional diversity technique. On the industry side, how to efficiently apply network coding in the current wireless systems has also been an open design problem in the past few years. This thesis work aims to address these important issues and challenges and provide some theoretical guidelines for real system design. In the first part of this work, we study the fundamental diversity performance of uncoded cooperation systems with wireless network coding. It is demonstrated that network-coded cooperation generally cannot achieve the same diversity gain as the conventional diversity schemes; however, the diversity loss is usually very limited and occurs only under particular channel conditions. For example, for digital network coding we show that the error propagation issue would cause half of the total available diversity gain to be lost, and we develop several link adaptive schemes to mitigate the diversity loss. For analog network coding, we demonstrate that the associated co-channel interference may reduce the diversity as well, but such loss gradually diminishes as the transmitted power goes up. Finally for non-coherent network coding, we show that when the receivers do not know the channel state information, using blind signal detection would not hurt the dominant diversity gain, and the diversity loss occurs only at modest signal-to-noise ratio. The second part of this work is focused on coded cooperation systems. The unique feature of coded systems is that the devices could somehow know the network dynamics such as the decoding status of a transmitted packet. We explore two transmission strategies that could efficiently exploit such information. For two-way relay channel, we propose a network-coded retransmission strategy, where wireless relaying is employed only when the direct link is in outage. To reduce the number of retransmissions, network coding is performed in a static or dynamic way to combine the to-be-retransmitted packets intended for different end terminals. We analyze the throughput and develop power allocation scheme to maximize the throughput. We also develop a hybrid network coding scheme that can fully exploit the network coding gain in the multi-relay environment. Next for wireless uplink channel, we come up with multi-user cooperation scheme based on node clustering. We develop inter-cluster cooperation strategy and intra-cluster transmit beamforming scheme to exploit the cooperative diversity gain. We demonstrate that there is a basic tradeoff between diversity gain and bandwidth efficiency, and different tradeoffs could be achieved by changing the formation of the clusters.