Wireless Information and Power Transfer in Multiuser OFDM Systems (1308.2462v2)

Abstract: In this paper, we study the optimal design for simultaneous wireless information and power transfer (SWIPT) in downlink multiuser orthogonal frequency division multiplexing (OFDM) systems. For information transmission, we consider two types of multiple access schemes, namely, time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA). At the receiver side, due to the practical limitation that circuits for harvesting energy from radio signals are not yet able to decode the carried information directly, each user applies either time switching (TS) or power splitting (PS) to coordinate the energy harvesting (EH) and information decoding (ID) processes. For the TDMA-based information transmission, we employ TS at the receivers; for the OFDMA-based information transmission, we employ PS at the receivers. Under the above two scenarios, we address the problem of maximizing the weighted sum-rate over all users by varying the time/frequency power allocation and either TS or PS ratio, subject to a minimum harvested energy constraint on each user as well as a peak and/or total transmission power constraint. For the TS scheme, by an appropriate variable transformation the problem is reformulated as a convex problem, for which the optimal power allocation and TS ratio are obtained by the Lagrange duality method. For the PS scheme, we propose an iterative algorithm to optimize the power allocation, subcarrier (SC) allocation and the PS ratio for each user. The performances of the two schemes are compared numerically as well as analytically for the special case of single-user setup. It is revealed that the peak power constraint imposed on each OFDM SC as well as the number of users in the system play a key role in the rate-energy performance comparison by the two proposed schemes.

• The paper investigates simultaneous wireless information and power transfer (SWIPT) in multiuser OFDM systems, analyzing time switching (TS) and power splitting (PS) strategies for receiver design.
• It proposes analytical solutions for weighted sum-rate maximization under minimum energy harvest and power constraints using Lagrange duality for TDMA/TS and an iterative algorithm for OFDMA/PS.
• Numerical results show that the performance superiority between TS and PS schemes depends significantly on system parameters like peak power constraints and minimum required harvested energy.

Overview of "Wireless Information and Power Transfer in Multiuser OFDM Systems"

The paper by Zhou, Zhang, and Ho investigates the problem of simultaneous wireless information and power transfer (SWIPT) in multiuser orthogonal frequency division multiplexing (OFDM) systems. The paper primarily explores the optimal design strategies for the integration of SWIPT technology in downlink multiuser scenarios, employing both time division multiple access (TDMA) and orthogonal frequency division multiple access (OFDMA) schemes. This work is motivated by the perpetuity potential of the SWIPT paradigm in providing a consistent source of energy to wireless networks, thereby enhancing both the energy efficiency and operational convenience of mobile systems.

The paper addresses the challenges of real-world implementation where circuits involved are not well-suited for simultaneous information decoding and energy harvesting. As a practical solution, two strategies—time switching (TS) and power splitting (PS)—are systematically analyzed. These strategies are used to coordinate the dual functions of energy harvesting (EH) and information decoding (ID) at the receiver side.

Methodology and Analytical Solutions

1. TDMA with Time Switching: In this scheme, a weighted sum-rate maximization problem is formulated, subject to constraints of minimum energy harvest, as well as peak and total transmission power. The problem is cast into a convex form by introducing appropriate variable transformations, enabling a solution via Lagrange duality.
2. OFDMA with Power Splitting: A different approach is adopted for OFDMA, where power splitting is applied at the user level to facilitate simultaneous information transmission and harvesting. An iterative optimization algorithm is proposed to handle this non-convex problem. Here, variables such as subcarrier allocation and power ratios are iteratively optimized until convergence.

The authors benchmark the effectiveness of both methods under different conditions, concluding that the strategies present distinct performance characteristics based on system specifics like the number of users and the constraints imposed by peak power.

Numerical Results and Key Findings

Numerical simulations complement analytical derivations, providing insights into the comparative performances of TS and PS schemes. Interestingly, the simulations indicate that the advantage of one scheme over the other can depend significantly on the system parameters, like the peak power constraint and required minimum harvested energy. For instance, with infinite peak power, the TDMA/TS scheme tends to outperform OFDMA/PS in rate efficiency, while under stringent peak power conditions, OFDMA/PS could be superior.

Implications and Future Directions

This paper contributes to the theoretical understanding of SWIPT in multiuser OFDM systems, offering insights that can guide the future development of energy-efficient wireless communication protocols. The results suggest that SWIPT applications could be tailored through strategic selection between TS and PS approaches based on specific application environments, leading to optimized design and implementation in practical networks.

Future research could expand on the incorporation of practical hardware constraints, including insertion losses in power splitting devices, and explore resilient designs against imperfect channel state information. Additionally, research could delve into integrating adaptive resource allocation strategies to dynamically adjust to varying transmission conditions in real-time, maximizing SWIPT deployment benefits in evolving communication networks.

In summary, this paper provides a foundational framework for realizing SWIPT capabilities in multiuser OFDM systems, aligning with broader goals of enhancing energy efficiency and sustainability in wireless communications.