New Efficient Transmission Technique for HetNets With Massive MIMO Wireless Backhaul

In order to cope with the rapid increase in power consumption of heterogeneous cellular networks, this article proposes a new efficient transmission technique for heterogeneous networks with massive MIMO wireless backhaul with the objective of minimizing power consumption cost. We assume that transmissions on the backhaul link and the access link occur simultaneously on the same frequency band (in-band) thanks to MIMO spatial multiplexing. On the other hand, we consider that uplink and downlink transmissions are separated in time. In order to prevent multi-user and inter-tier interference, block diagonalization beamforming is considered at the macro base station (MBS) and the signal from the small base station (SBS) to the MBS is transmitted orthogonally to the channel of the SBS's users. The problem of minimizing the transmit power of base stations under users' minimum-rate constraints is formulated. We first derive analytically the optimal time splitting parameter and the allocated transmit power considering that the inter-SBS interference is generated by fixed power. Next, we solve the power allocation problem when the generated inter-SBS interference is no longer considered fixed by proposing an efficient iterative power allocation algorithm. A heuristic user scheduling algorithm is devised in order to deal with the feasibility problem. Finally, simulations validate our analysis and show that the proposed transmission technique outperforms the conventional reverse time division duplex with bandwidth splitting (out-band) in terms of total power consumption.