Low-Complexity Energy-Efficient Joint Resource Allocation for Two-Hop MIMO-AF Systems

Energy efficiency (EE) is becoming an important system design criterion to ensure that the next generation of communication networks is sustainable. Equally, cooperative communication and resource allocation are well-known techniques for improving the performance of communication systems. In this paper, we propose a low-complexity energy-efficient joint resource allocation method for the two-hop multiple-input-multiple-output (MIMO) amplify-and-forward (AF) system. We derive explicit formulations of the near-optimal energy-per-bit consumption, subchannels' power and rate for the unconstrained, total transmit power and sum-rate constrained EE optimization problems as well as detail how to solve these problems in a low-complexity manner. We then use our novel method for comparing the performances of two-hop MIMO-AF and MIMO systems in terms of EE. Our results indicate that the usage of a relay is only energy efficient when the quality of the direct link is very poor. We also show that the extra fixed power consumption induced by transmitting over two hops can seriously disadvantage MIMO-AF in terms of EE, but on the other hand, the usage of relay can be useful for downsizing the donor cell, which in turn provides EE gain.