Wireless Power Transfer in a Full-Duplex MIMO Communication System

Figure 1: MIMO two-hop relaying channel model

Project Overview:

With the rapid evolution of wireless networks, energy efficiency is now deemed as a figure of merit for the design of next generation communication systems. We can envision future communication networks to employ relays capable of providing cooperation in terms of both information and energy. Far-field, radio frequency (RF) energy harvesting has recently garnered significant interest for communication systems with the prospect of simultaneous information and power transfer. In this work, we consider a MIMO communication system assisted by a full-duplex relay, where the relay is capable of harvesting energy. To focus on the benefits of MIMO and full duplex features, we consider the scenario in which the relay is self-sustained, that is, it has no power source of its own and hence relies solely on energy harvesting for its operations.

Motivation:

Energy harvesting is a promising direction for upcoming technology era for example in mobile edge computing, 5G networks and wireless charging of wearable/IoT sensors. Most commercially available devices/standards are for near field power transfer, however, far-field power transfer for communicating nodes offers the inherent advantage of untethered mobility. RF energy harvesting is of significant interest due to its flexibility, portability and availability in a wide array of frequency bands for example cell phones, radio towers, Wi-Fi routers and TV signals. Utility of wireless power transfer in self-sustained relays, in particular, has emerged as an interesting research avenue, where the relays use harvested power for information forwarding, rather than depleting their own power resources.

Figure 2: Full-Duplex MIMO Relay Model with Self-Interference

Optimal Transmission using Self-Sustained Relay:

We formulate a novel optimization problem to maximize the throughput in a wirelessly powered two-hop full duplex MIMO relay channel, where all the nodes, source, relay and destination, are equipped with multiple antennas. We show how using a combination of harvesting energy from both the source signal and the self-interference signal in the RF domain, and using active self-interference cancellation in the baseband, allows us to exploit performance gains of MIMO and full-duplex communications. Based on the Lagrange duality theory, we design an efficient primal-dual algorithm for jointly optimizing the source, relay precoders and the relay power splitting ratios. The algorithm allows us to demonstrate the significant performance gain of our system over the traditionally used uniform power splitting scheme, half-duplex MIMO transmission, and also full-duplex MIMO transmission without self-interference energy harvesting.

Results:

Figure 3: Performance Comparison with half-duplex communication

Our results in Figure 3 suggest an interesting observation that a hybrid self-interference cancellation methodology could be implemented, where in the low SNR regime, passive cancellation only should suffice, whereas in the medium and high SNR regime, active cancellation helps in maximally utilizing the benefits of full-duplex communication over half-duplex communication.

Our proposed algorithm has significantly faster convergence than standard solvers as shown in Figure 4. Despite having the additional features of non-uniform power splitting and self-interference harvesting, it shows superior run-time performance in comparison to sequential algorithms in literature which solve for both hops separately and uses grid search to find the uniform power splitting ratio.

Figure 5: Comparison to Uniform Power Splitting

Figure 5 shows how the proposed non-uniform power splitting scheme offers significant rate gain over uniform power splitting especially at a moderate number of antennas. Here we fix the number of antennas at source and destination at two (Ns = Nd = 2), and increase the number of antennas at the relay. The rate gain of non-uniform over uniform power splitting is pronounced at a small to moderate number of relay antennas but reduces as the number of relay antennas increases. This is due to the channel hardening effect observed when the number of relay antennas is significantly larger than the number of antennas at the source and destination.

Publications:

“Optimal Transmission Using a Self-sustained Relay in a Full-Duplex MIMO System,”
R. Malik and Mai Vu, IEEE Journal on Selected Areas in Communications, vol. 37, no. 2, pp. 374-390, Feb. 2019.
“Link-State Optimized Decode-Forward Transmission for Two-Way Relaying,”“Optimizing Throughput in a MIMO System with a Self-sustained Relay and Non-uniform Power Splitting,”
R. Malik and Mai Vu, IEEE Wireless Communications Letters, vol. 8, no. 1, pp. 205-208, Feb. 2019.