Relay-based communication is being actively investigated to meet the demand of higher data rates and ubiquitous coverage in wireless systems. The half-duplex constraint imposed on a relay, for ease of implementation, prevents it from concurrently transmitting and receiving in the same time slot. This reduces the spectral efficiency of conventional relaying, as two time slots are required to transmit a single data unit. Two-way relaying (TWR) exploits bi-directional data flow between two nodes and eliminates this spectral loss by exchanging two data units in two time slots. In TWR, it is assumed that a user simultaneously sends and receives data from the base station via a relay. In cellular systems, data flow between the base station and a user is usually not simultaneous e.g., a user (say, TUE) may have uplink data to send in the first slot, but may not have downlink data to receive in the second slot. Such uni-directional data flow reduces TWR to spectrally-inefficient conventional relaying. We propose an asymmetric two-way relaying (ATWR) protocol that restores the bi-directional data flow via a relay and enables transmission of two data units in two time slots. In ATWR, the second slot following the first-slot transmission of a TUE is used to send downlink data to another user (say, RUE). However, the RUE cannot cancel the back-propagating interference. We design a structured precoder at the relay to cancel this interference. For this precoder, we show that the optimization of various metrics e.g., sum rate and relay transmit power, can be cast as convex geometric programs. We demonstrate that the proposed solution outperforms the conventional ones.
Rohit Budhiraja received his MS in electrical engineering from IIT Madras in 2004. From January 2004 to July 2011, he worked in two startups and designed, from scratch, both base stations and user equipments for WiMAX- and LTE-based cellular systems. For these systems, he developed both hardware and software algorithms for physical layer processing. He joined PhD at IIT Madras in August 2011 and defended his thesis in July 2015. He is currently a Post-doctoral researcher at the same institute. His paper was shortlisted as one of the finalists for the Best Student Paper Awards at IEEE SPCOM, Bangalore, 2014. His current research interests include cross-layer design between digital baseband and analog RF for wireless systems, energy-efficient MIMO wireless communications, and hardware design for communication systems. For more information,