[Paper Review] Simultaneous Wireless Information and Power Transfer for Two-hop OFDM Relay System

Xiaofei Di, Ke Xiong+1 more|arXiv (Cornell University)|Jul 1, 2014

Energy Harvesting in Wireless Networks16 references16 citations

TL;DR

This paper proposes a resource allocation scheme for a two-hop OFDM decode-and-forward relay system with simultaneous wireless information and power transfer (SWIPT), where the relay harvests energy from the source's RF signals using power splitting. The scheme jointly optimizes power allocation, subcarrier pairing, and power splitting ratios to maximize the total achievable rate, with simulation results showing significant performance gains and a key insight that EH relays should be placed closer to the source than conventional relays.

ABSTRACT

This paper investigates the simultaneous wireless information and power transfer (SWIPT) for two-hop orthogonal frequency division multiplexing (OFDM) decode-and-forward (DF) relay communication system, where a relay harvests energy from radio frequency signals transmitted by the source and then uses the harvested energy to assist the information transmission from the source to its destination. The power splitting receiver is considered at the relay, which splits the received signal into two power streams to perform information decoding (ID) and energy harvesting (EH) respectively. For better understanding the behavior and exploring the performance limit of such a system, resource allocation is studied to maximize the total achievable transmission rate. An optimization problem, which jointly takes into account the power allocation, the subcarrier pairing and the power splitting, is formulated. Due to its non-convexity, a resource allocation policy with low complexity based on separation principle is designed. Simulation results show that the system performance can be significantly improved by using our proposed policy. Moreover, the system performance behavior to the relay position is also discussed, and results show that in the two-hop OFDM system with EH relay, the relay should be deployed near the source, while in that with conventional non-EH relay, it should be deployed at the middle between the source and the destination.

Motivation & Objective

• To address the challenge of energy-constrained relay nodes in OFDM relay systems by enabling them to harvest energy from RF signals.
• To investigate the performance limits of a two-hop OFDM decode-and-forward relay system with simultaneous wireless information and power transfer (SWIPT).
• To design a low-complexity resource allocation policy that jointly optimizes power allocation, subcarrier pairing, and power splitting for maximum achievable rate.

Proposed method

• Models a two-hop OFDM relay system with a source, an energy-harvesting (EH) relay, and a destination, using decode-and-forward (DF) relaying.
• Applies power splitting (PS) at the relay to split the received RF signal into two streams for information decoding (ID) and energy harvesting (EH).
• Formulates a non-convex optimization problem to maximize the total achievable rate by jointly optimizing power allocation, subcarrier pairing, and power splitting ratios.
• Develops a low-complexity resource allocation policy based on the separation principle to handle the non-convexity of the optimization problem.
• Proves that optimal subcarrier pairing follows a sorted pairing rule based on channel gains, using a contradiction argument for the multi-subcarrier case.
• Derives closed-form expressions for power allocation in the two-subcarrier case and validates optimality across multiple cases using inequality analysis.

Experimental results

Research questions

• RQ1 How does the placement of an energy-harvesting relay affect system performance in a two-hop OFDM relay system with SWIPT?
• RQ2 What is the optimal joint resource allocation strategy for power allocation, subcarrier pairing, and power splitting in a SWIPT-enabled OFDM relay system?
• RQ3 Can a low-complexity resource allocation policy achieve near-optimal performance in a non-convex SWIPT-OFDM relay system?
• RQ4 Does the sorted pairing of subcarriers based on channel gains maximize the system's achievable rate in this setup?

Key findings

• The proposed low-complexity resource allocation policy significantly improves the system's total achievable rate compared to baseline schemes.
• Simulation results confirm that the optimal subcarrier pairing follows a sorted pairing rule, where subcarriers are paired based on descending channel gains.
• The relay should be deployed closer to the source in an EH-based system to maximize performance, unlike conventional non-EH relays that perform best at the midpoint.
• For the two-subcarrier case, closed-form solutions for optimal power allocation are derived and proven to be optimal across all feasible cases.
• The performance gain from joint optimization of power allocation, subcarrier pairing, and power splitting is substantial, demonstrating the value of coordinated design in SWIPT-OFDM relay systems.
• The analysis proves that any non-sorted subcarrier pairing leads to a lower achievable rate, confirming the optimality of the sorted pairing rule.