To Code in Space and Time or Not in Multihop Relay Channels

Multihop relay channels use multiple relay stages, each with multiple relay nodes, to facilitate communication between a source and destination. Previously, distributed space-time coding was used to maximize diversity gain. Assuming a low-rate feedback link from the destination to each relay stage and the source, this paper proposes end-to-end antenna selection strategies as an alternative to distributed space-time coding. One-way (where only the source has data for destination) and two-way (where the destination also has data for the source) multihop relay channels are considered with both the full-duplex and half-duplex relay nodes. For the full-duplex case, end-to-end antenna selection strategies are designed and proven to achieve maximum diversity gain by using a single-antenna path (using single antenna of the source, each relay stage and the destination) with the maximum signal-to-noise ratio at the destination. For the half-duplex case, two paths with the two best signal-to-noise ratios in alternate time slots are used to overcome the rate loss with half-duplex nodes, with a small diversity gain penalty. A multiple stream end-to-end antenna selection strategy for full-duplex multihop relay channel is also proposed to obtain a lower bound on the diversity multiplexing tradeoff of multihop relay channels. Finally, to answer the question of whether to code in space and time or not in a multihop relay channel, end-to-end antenna selection strategy and distributed space-time coding is compared with respect to several important performance metrics such as encoding/decoding complexity, rate of transmission, latency, bit-error-rate performance, and resource requirements.