Abstract:

This paper proposes a minimum mean-square-error bi-directional amplify-and-forward (MMSE-BAF) relaying protocol for multi-hop wireless networks employing multi-antenna relays. MMSE-BAF is a two-phase relaying protocol which allows for two sources to exchange independent messages via a relay node equiped with multiple antennas. The latter performs a joint linear MMSE filtering of the received signal after the multiple access (MA) phase before amplifying and forwarding using a single transmit antenna, possibly through a specific antenna selection procedure, during the broadcast phase. The proposed MMSE-BAF protocol extends upon the so-called analog network coding schemes in the literature in that it inherently exploits the multiple antennas at the relay station in order to reduce the noise enhanced effects typical of an AF protocol. Owing to its joint linear MMSE filtering approach, it can also compensate for link imbalances between the relay and the sources and is agnostic to sources' modulation and coding schemes (MCS), which is especially relevant when these experience dissimilar channel conditions and wish to adapt their MCS accordingly. We derive the instantaneous signal-to-noise ratio expressions for the received signal by the source nodes in the downlink and provide extensive link-level simulation results for the MMSE-BAF protocol subject to both frequency flat and selective fading. Furthermore, we detail the modifications needed to the IEEE 802.16e orthogonal-frequency-division multiple access (OFDMA) cellular standard (mobile WiMax) to enable support of multiple-antenna bi-directional communications and show that MMSE-BAF is a viable solution within that framework.