[Paper Review] Multi-user Scheduling Schemes for Simultaneous Wireless Information and Power Transfer Over Fading Channels

Rania Morsi, Diomidis S. Michalopoulos+1 more|arXiv (Cornell University)|Jan 9, 2014

Energy Harvesting in Wireless Networks16 references4 citations

TL;DR

This paper proposes order-based SNR and normalized SNR (N-SNR) scheduling schemes for multi-user downlink systems with simultaneous wireless information and energy transfer (SWIET). By scheduling the user with the jth-best channel quality (rather than the best), the system controls the trade-off between ergodic capacity and harvested energy. Smaller j values increase energy harvesting at the cost of reduced throughput, enabling energy-efficient operation with potential cubic power reduction via dynamic voltage scaling.

ABSTRACT

In this paper, we study downlink multi-user scheduling for a time-slotted system with simultaneous wireless information and power transfer. In particular, in each time slot, a single user is scheduled to receive information, while the remaining users opportunistically harvest the ambient radio frequency energy. We devise novel online scheduling schemes in which the tradeoff between the users' ergodic rates and their average amount of harvested energy can be controlled. In particular, we modify the well-known maximum signal-to-noise ratio (SNR) and maximum normalized-SNR (N-SNR) schedulers by scheduling the user whose SNR/N-SNR has a certain ascending order (selection order) rather than the maximum one. We refer to these new schemes as order-based SNR/N-SNR scheduling and show that the lower the selection order, the higher the average amount of harvested energy in the system at the expense of a reduced ergodic sum rate. The order-based N-SNR scheduling scheme provides proportional fairness among the users in terms of both the ergodic achievable rate and the average harvested energy. Furthermore, we propose an order-based equal throughput (ET) fair scheduler, which schedules the user having the minimum moving average throughput out of the users whose N-SNR orders fall into a given set of allowed orders. We show that this scheme provides the users with proportionally fair average harvested energies. In this context, we also derive feasibility conditions for achieving ET with the order-based ET scheduler. Using the theory of order statistics, the average per-user harvested energy and ergodic achievable rate of all proposed scheduling schemes are analyzed and obtained in closed form for independent and non-identically distributed Rayleigh, Ricean, Nakagami-m, and Weibull fading channels. Our closed-form analytical results are corroborated by simulations.

Motivation & Objective

• To address the trade-off between system capacity and energy harvesting in multi-user SWIET systems.
• To extend conventional scheduling schemes (e.g., max-SNR, round-robin) to jointly optimize information and energy transfer.
• To enable proportional fairness and energy efficiency by introducing a tunable parameter j that controls the rate-energy trade-off.
• To analyze ergodic capacity and average harvested energy in i.n.d. Ricean and Rayleigh fading channels.

Proposed method

• Proposes order-based SNR scheduling by selecting the user with the jth ascendingly ordered SNR for information transmission.
• Introduces order-based N-SNR scheduling, where the user with the jth ordered normalized SNR (relative to its average) is scheduled.
• Applies order statistics theory to derive closed-form expressions for ergodic capacity and average harvested energy in i.n.d. Rayleigh fading.
• Analyzes baseline schemes: round-robin (RR) and equal-throughput (ET) scheduling under joint WIET.
• Uses system-level simulations to validate analytical results and evaluate performance trade-offs across different fading conditions.
• Evaluates the impact of user count N and scheduling order j on system capacity and total harvested energy.

Experimental results

Research questions

• RQ1 How does scheduling the jth-best user instead of the best user affect the ergodic capacity and average harvested energy in a SWIET system?
• RQ2 What is the impact of the number of users N on the system’s rate-energy performance under order-based scheduling?
• RQ3 How does the order-based N-SNR scheme compare to absolute SNR scheduling in terms of fairness and energy efficiency?
• RQ4 What is the performance gap between baseline schemes (RR and ET) and the proposed order-based schemes in terms of capacity and energy harvesting?

Key findings

• Smaller values of j in order-based SNR/N-SNR scheduling lead to higher average harvested energy but lower ergodic system capacity, enabling tunable rate-energy trade-off.
• The order-based N-SNR scheme achieves proportional fairness among users with diverse channel conditions, unlike max-SNR which favors strong channels.
• For j=1, the average harvested energy increases with the number of users N, while ergodic capacity decreases due to the MUD loss from selecting the weakest channel.
• For j=N, ergodic capacity increases with N due to the MUD gain from selecting the strongest channel, while total harvested energy also increases.
• For intermediate j (e.g., j=N/2), system capacity remains nearly constant for N≥8, indicating a balanced trade-off.
• The performance difference between absolute SNR and normalized SNR scheduling is minimal, but N-SNR scheduling offers superior fairness and is thus preferred in practice.