Physical Layer Network Coding for Mobile Communications

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The objective of this project is to investigate a novel approach to wireless multi-hop transmission, which is an extension of the network coding scheme as presented by Ahlswede et al. In the course of the project, a variety of equalization techniques that enable such a physical layer network coding (PNC) relaying system to operate in an environment where transmissions are subject to frequency-selective fading have been developed.We have designed three sequence estimation schemes for the relay nodes. Two of these algorithms operate on a full-state trellis and therefore their performance represents an upper bound for the performance of all subsequently investigated approaches. Since the computational complexity and memory consumption of full-state trellis-based schemes becomes prohibitively large very fast as the length of the channel impulse response increases, we also devised a complexity-reduced approach. However, the reduction in complexity comes with a considerable performance penalty. Furthermore, we devised optimal transmit filtering approaches that allow the relay node to either use linear equalization (LE) or decision-feedback equalization (DFE). By employing those filters at the source nodes, the transmissions from both nodes to the relay are influenced by the same overall channel impulse response, and the signal-to-noise ratio (SNR) with respect to the chosen equalization approach (LE or DFE) is maximized. An optimal Tomlinson-Harashima precoding (THP) based approach complements the above mentioned algorithms. Using a combination of suitable feedback (precoding) and feedforward filters at the source nodes, a transmission that is free of intersymbol interference (ISI) can be achieved for both communicating nodes.For the renewal of the project, the extension of PNC to single-carrier frequency-division multiple access (SC-FDMA) transmission will be one of the research topics. SC-FDMA has been already standardized for the uplink of Long Term Evolution (LTE) due to its appealing features. New frequency domain equalizers will be investigated for a PNC scheme with SC-FDMA. Furthermore, we will consider more realistic network topologies. To date, our focus was mainly on the two-way relay channel (TWRC). In the last year of the project, we will expand our investigations to the multiple access relay channel (MARC).Additionally, there are several open points for further research that came to our attention while working on the project. The THP based relaying system currently uses DFE at the destination nodes in the broadcast phase. We will develop a scheme that uses THP instead also in this phase in order to partly compensate for the ISI caused by both radio channels and removes the residual ISI at each destination node with LE. Finally, an optimal transmit filtering scheme that facilitates the use of low complexity trellis-based equalizers like reduced-state sequence estimation (RSSE) at the relay node will be developed.