HARQ protocols for cooperative networks

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The objective of this research program is to increase our understanding of the fundamental principles, limitations, and trade-offs related to the deployment of the so-called HARQ protocols in the collaborative communications networks. The collaboration between the terminals in the network is seen as a way to increase the coverage, the rate, and the reliability of the communication in the wireless environment. It usually takes the form of relaying, where the terminals help each other to convey information to their respective destinations. In the wireless context, the transmission errors are unavoidable so it is necessary to consider retransmission via the so-called hybrid automatic repeat request (HARQ) protocol. While it is a well-known mechanism in point-to-point communications, deploying HARQ in the context of collaborative network is not trivial because the number of retransmissions possibilities is significantly increased due to the presence of many terminals. Another dimension to the problem is added since the relays may use the so-called network-coding and thus it is important to elucidate and analyze how to combine it efficiently with the relay-based HARQ. Our goal is to find the most convenient cooperative transmission schemes, which is possible only when we are able to optimize the rate and/or power of the HARQ retransmission rounds. We will also consider the HARQ parameters' adaptation strategies, which rely on the information being exchanged between the communication terminals. Since, in practice, this information may be subject to errors, we will analyze the effects caused by these errors and propose mechanisms to deal with these undesirable effects. Developing the methods to efficiently optimize and characterize the collaborative HARQ is crucial to use them and integrate these transmission strategies into high-level optimization of the network communications.