Optimal Save-Then-Transmit Protocol for Energy Harvesting Wireless Transmitters (1204.1240v3)

Abstract: In this paper, the design of a wireless communication device relying exclusively on energy harvesting is considered. Due to the inability of rechargeable energy sources to charge and discharge at the same time, a constraint we term the energy half-duplex constraint, two rechargeable energy storage devices (ESDs) are assumed so that at any given time, there is always one ESD being recharged. The energy harvesting rate is assumed to be a random variable that is constant over the time interval of interest. A save-then-transmit (ST) protocol is introduced, in which a fraction of time {\rho} (dubbed the save-ratio) is devoted exclusively to energy harvesting, with the remaining fraction 1 - {\rho} used for data transmission. The ratio of the energy obtainable from an ESD to the energy harvested is termed the energy storage efficiency, {\eta}. We address the practical case of the secondary ESD being a battery with {\eta} < 1, and the main ESD being a super-capacitor with {\eta} = 1. The optimal save-ratio that minimizes outage probability is derived, from which some useful design guidelines are drawn. In addition, we compare the outage performance of random power supply to that of constant power supply over the Rayleigh fading channel. The diversity order with random power is shown to be the same as that of constant power, but the performance gap can be large. Furthermore, we extend the proposed ST protocol to wireless networks with multiple transmitters. It is shown that the system-level outage performance is critically dependent on the relationship between the number of transmitters and the optimal save-ratio for single-channel outage minimization. Numerical results are provided to validate our proposed study.

• The paper’s main contribution is the development of an optimal protocol that minimizes outage probability by adjusting the save-ratio based on energy storage efficiency and circuit power.
• It employs a dual energy storage system using a super-capacitor and battery, demonstrating significant performance impacts under varied energy supply conditions.
• The study extends its findings to multi-transmitter scenarios, providing actionable insights for balancing energy harvest and data transmission in wireless networks.

Optimal Save-Then-Transmit Protocol for Energy Harvesting Wireless Transmitters

The paper "Optimal Save-Then-Transmit Protocol for Energy Harvesting Wireless Transmitters" by Shixin Luo, Rui Zhang, and Teng Joon Lim addresses the innovative concept of energy harvesting in wireless communication systems. This paper introduces an optimal communication strategy focused on the Save-Then-Transmit (ST) protocol tailored for energy harvesting transmitters. The authors concentrate on efficiently managing energy harvested from renewable sources to ensure reliable data transmission.

Summary of Methodology and Findings

The paper builds on existing research into energy-constrained wireless sensor networks, emphasizing the design constraints posed by the energy half-duplex constraint. This constraint prevents simultaneous charging and discharging of a single energy storage device (ESD). The authors develop a protocol using two distinct ESDs: a super-capacitor (MESD) with perfect efficiency and a battery (SESD) that possesses lower efficiency, governed by the efficiency parameter, $\eta$. The challenge addressed is how to balance energy harvesting and data transmission optimally.

The paper formulates the problem of minimizing the outage probability — the probability that the system fails to meet its minimum required data rate. For this, the authors define a 'save-ratio', $\rho$, which is the fraction of each time frame dedicated to energy harvesting.

Key Findings

1. Save-Ratio Optimization: The paper finds that the optimal save-ratio, $\rho^*$, minimizes the outage probability and varies with the energy storage efficiency $\eta$ and circuit power $P_c$. For low $\eta$ or significant $P_c$, the save-ratio diverges from zero, requiring more time allocated to energy harvesting rather than immediate data transmission.
2. Performance Metrics: Numerical results highlight the significant impact of $\eta$ and $P_c$ on the outage probability. For substantial circuit power and non-ideal $\eta$, optimizing the save-ratio is crucial to system reliability, while an ideal scenario (with $\eta = 1$ and $P_c = 0$) would almost always favor continuous data transmission ($\rho = 0$).
3. Impact of Random Power Supply: The paper demonstrates a notable performance gap between systems with random versus constant power supplies, using the Rayleigh fading model to show that diversity order for random power is the same as constant power, although the performance gap is significant due to the inefficiencies introduced by changing energy availability.
4. Multiple Transmitter Systems: They extend the ST protocol to multi-transmitter scenarios, analyzing both independent and common data cases. For independent data, transmitters can achieve minimal outage when their number remains below a critical threshold correlated with the single-channel optimal transmit-ratio. For common data, although diversity improvements can reduce outage initially, exceeding this threshold leads to increased outage probability due to suboptimal save-ratios.

Theoretical and Practical Implications

The theoretical framework facilitates further exploration into other energy harvesting communication models, laying groundwork for developing efficient protocols for systems with similar dual-ESD constraints. Practically, these insights are particularly pertinent as sensor networks expand in scope, necessitating efficient energy harvesting to minimize maintenance and ensure network longevity.

Future Developments

The insights gained from this research can be extended to investigate the impact of delaying feedback in adaptive systems or integrating advanced multiple-access schemes beyond TDMA for optimization in multi-transmitter environments. Additionally, there remains potential exploration into dynamic circuit configurations and the optimization of battery-supercapacitor combinations for enhanced performance.

Overall, this paper contributes valuable strategies for advancing energy harvesting communication protocols, framing an important cornerstone in the field's ongoing research.