Adaptive hierarchical cooperative communication using energy harvesting relay

In this paper, we present an adaptive wireless communication system with energy harvesting relay where, two sources wish to send data to a destination via an energy harvesting relay that utilizes hierarchical constellation. In our proposed model, the relay based on the channel side information, adaptively decides between sending information bits of each source and saving energy, which leads to three modes of cooperation. In the first mode, after successfully decoding two sources, the relay puts the information bit of the source with lower channel amplitude in the first layer and the information bits of the other source in the second layer of the hierarchical constellation and in the second mode, the relay only transmits information bits from the source that was decodable and harvest the energy received by the other source and in the third mode that is, when the relay is not able to decode both sources, effectively, decides to stay silent and harvest the received energy from each source. We analyze the performance of these transmission modes in terms of average BER and outage probability. Our analytical results show that with non-zero energy efficiency factor, improvements in performance can be achieved in comparison with the case that the relay is not capable of harvesting energy and does not adaptively send data. In addition, we find the optimum value of the hierarchical modulation parameter in order to minimize the BER of each source.