Achievable rate-energy region in two-way decode-and-forward energy harvesting relay systems

Abstract

At an energy harvesting relay, securing residual harvested energy, net remaining energy after each receiving and forwarding cycle, is of importance for sustainable operation. However, there exists a tradeoff between achievable rate and residual harvested energy whereby understanding this tradeoff concretely is crucial for energy harvesting relay system design. This dissertation analyzes rate-energy (R-E) region for achievable rate and residual harvested energy in two-way decode-and-forward (DF) relay systems with various relaying protocols.

Firstly, we consider a wireless energy transfer (WET) based relaying protocol. In particular, we characterize R-E regions of WET-and-multiple access broadcast (WET-MABC) and WET-and-time division broadcast (WET-TDBC) protocols. The boundary of the R-E regions is identified by optimizing the time duration factor in each protocol. Moreover, the approximated R-E regions of all protocols for two asymptotic cases are derived to have better analytic comparisons and useful insights on performance. Based on the approximated R-E region, it is shown that the R-E region of the WET-TDBC is always included in that of the WET-MABC in low SNR case. On the other hand, in high SNR case, either the WET-TDBC protocol or the WET-MABC protocol is preferred according to a certain condition.

Secondly, we consider a power splitting (PS) based relaying protocol such as PS based MABC (PS-MABC) and PS based TDBC (PS-TDBC) protocols. In addition, to improve the R-E region, we propose a new energy harvesting relaying protocol, namely, PS based information and energy signals multiple access broadcast (PS-IEBC) protocol. We analyze the R-E regions of the PS-MABC, PS-TDBC, and PS-IEBC protocols. The boundary of the R-E regions is identified by optimizing the power splitting factor in each protocol. The proposed PS-IEBC is shown to considerably improve the achievable R-E region compared to the other protocols. Moreover, the approximated R-E regions of all protocols for two asymptotic cases are derived to have better analytic comparisons and useful insights on performance. Based on the approximated R-E region, it is shown that if the required residual energy is large, the PS-IEBC protocol outperforms the others, but if the required residual energy is small, either the PS-IEBC protocol or the PS-MABC protocol is preferred.

Thirdly, we consider a WET-and-PS (WPS) based relaying protocol. In particular, we describe the R-E regions of the WPS based MABC (WPS-MABC), WPS based TDBC (WPS-TDBC), and WPS based IEBC (WPS-IEBC) protocol. The boundary of the R-E regions is characterized by optimizing both time duration and power splitting factors in each protocol. To obtain the effect of each factor, we compare the WET, PS, and WPS based relaying protocols in each protocol. Then, we show that WET based protocol is not included in PS based protocol and vice versa. Numerical results show that the rate performance of the PS based protocol is the same as one of the WPS based protocol in high SNR case when the required residual energy is small. On the other hand, the WET based protocol outperforms PS based protocol when the required residual energy is large for both low and high SNR cases. Among all protocols, in high SNR case, the WPS-IEBC has the largest R-E region compared with other protocols, while the WPS-MABC outperforms the others in low SNR case.