Novel half-duplex relay strategy: An LTE implementation

Thomas, Robin Rajan; Cardone, Martina; Knopp, Raymond; Tuninetti, Daniela; Maharaj, Bodhaswar
Research Report RR-16-325

This report presents a practical implementation of a novel two-phase three-message strategy for half-duplex relaying, which consists of superposition coding and interference-aware cancellation decoding. As opposed to past works, the channel model has a direct link between the source and the destination, through which the source
continuously sends information to the destination at a rate close to the capacity of that link. At the same time the source leverages the relay to convey extra information to the destination. With the aim to bridge the gap between theory and practice, this study derives the block error rate with finite block-length and discrete constellation
signaling and compares it to the theoretical performance of Gaussian codes with asymptotically large block-lengths. The performance evaluation is carried out on an LTE physical layer compliant simulation test bench. The model assumes a single-antenna source and relay, and multi-antenna destination. During each phase of the transmission, the modulation and coding scheme is adapted to the channel link qualities and selected among those defined by the 3GPP LTE standard. First the nonfading/static Gaussian additive noise channel is considered. For the case of all single-antenna
nodes, the maximum spectral efficiency gap between theory and the proposed practical implementation is of 0.36 bits/dim when the strengths of the source-destination and relay-destination links are the same, and of 0.88 bits/dim when the relay-destination link is 5 dB stronger than the source-destination link. The performance of the proposed
strategy is also compared to a baseline scheme, where there is no physical cooperation between the source and the relay. When the source-destination and relay-destination links are of the same quality, the improvement over the baseline scheme is of 3.45 bits/dim for the single-antenna destination case, and of 3.39 bits/dim for the two-antenna destination case. The fading case is then considered and performance gaps computed as for the static case. These results confirm once again that physical-layer cooperation and the use of multiple antennas are of critical importance for performance enhancement in broadband wireless systems. More importantly, they show that
(i) a practical implementation of high-performing half-duplex relay techniques for future heterogeneous network deployments is possible with the modulation and coding formats already specified by the 3GPP LTE standard, and (ii) that the gap between theory and practice is small. Optimizing the physical-layer parameters and benchmark
its performance against second-order moderate block-length capacity results could potentially show the actual optimality of the proposed two-phase three-message relaying strategy.

Communication systems
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