In this paper, we introduce a full-duplex backscatter-assisted wireless powered communication network (FDBA-WPCN) with a full-duplex access point (FAP) and multiple energy harvesting wireless devices (WDs). The communication mode is a combination of backscatter communication (BC) and harvest-then-transmit (HTT). The entire time period of network is divided into energy harvesting/backscattering (EHB) period and information transmission (IT) period. In the EHB period, each WD either reflects information to the FAP by backscatter or harvests energy to prepare for the IT period. In the IT period, the WDs use their harvested energy to transmit information to FAP in time division multiple access (TDMA). However, under the setting, WDs with different distances from FAP will encounter unfairness in throughput due to the round-trip path loss in backscatter and the doubly near-far problem in HTT. To overcome the drawback, an optimization problem is considered to maximize the sum throughput under the condition of ensuring throughput fairness. By using convex optimization techniques, we obtain the optimal time allocation and the maximum same throughput of each WD. Comparing to the other two benchmark schemes, the simulation results prove the superiority of our proposed method.
With the rapid evolution of the Internet of Things (IoT) and the proposal of green communication, energy harvesting (EH) technology for power-constrained wireless devices (WDs) has aroused great concerns in academia and industry. EH replaces traditional battery-powered or wired power supply methods and enables WDs to achieve contact-less and sustainable power supply. It can not only can extend the life of the WDs but also reduce green gas emissions [
RF-EH is the WD harvest energy from the radio frequency (RF) signal radiated by the energy source (ES). As the main technology of wireless energy transfer (WET), it is usually used in conjunction with wireless information transmission (WIT). The most typical application is wireless powered communication networks (WPCNs). WPCNs were first studied in [
The backscatter communication system is different from the traditional wireless communication system. In this system, the backscatter transmitter does not actively generate the RF signal, but reflects the RF signal radiated by the RF source to transmit its own information. Specifically, the BC transmitter transmits bit data by adjusting the matching degree between the antenna impedance and the load impedance. When the antenna impedance matches the load impedance, the antenna will be in absorbing state to collect the incident signal, which means that the BC transmitter indicates the information bit “0” to its corresponding receiver. Otherwise, the antenna will be in the reflection state to reflect the incident signal, which means the transmission information bit is “1.” The whole process is called load modulation. Here, WDs are equivalent to BC transmitter, and HAP is equivalent to RF signal source and receiver.
Since the BC transmitter itself does not process the signal, the energy consumption generated by it is very low, and passive communication can be achieved almost without harvesting energy. Therefore, the dedicated time for harvesting energy can be ignored. It is a promising method to improve the system performance by introducing backscatter into the traditional WPCN. However, if ES and IR are both placed on the same device, WDs at different distances will be affected to different degrees by the round-trip path loss of BC [
According to [
Motivated by the advantages of the above research, this paper investigates an FDBA-WPCN with an FD access point (FAP) and multiple WDs, where each WD is equipped with HTT module and BC module. Due to the introduction of BC, this system is relatively suitable for low-power networks, such as tracking devices, medical telemetry, and low-cost sensor networks [ We first propose a new scheme that combines BC mode and HTT mode. In this scheme, each WD can allocate a portion of the original energy harvesting time in HTT to backscatter. By ingeniously integrating backscatter into the HTT mode, the energy harvesting period in HTT is modified as the energy harvesting/backscatter (EHB) period. In this period, WDs perform backscattering in a time division multiple access (TDMA) manner. When one of the WD backscatters, the other WDs harvest energy. And then the WDs use the harvested energy to transmit information in TDMA. Under the setting, the time can be utilized more efficiently, and thus the system throughput will be increased. In addition, the combination of BC mode and HTT mode will drive to more complex WDs. There are two independent circuit modules inside all WDs, namely, the BC module and the HTT module. Each WD can adaptively allocate time to call a certain module in real time according to its own channel state information (CSI). In the above scheme, we propose an optimal problem of maximizing throughput and ensuring fairness simultaneously. As far as we know, there is no work to study fairness enhancement in our proposed multi-WD case. By applying convex optimization techniques, we can get each WD’s optimal time allocations of backscattering, harvesting energy and information transmission to maximize the same throughput that the WDs can achieve Through simulation results, we compare system performance between the proposed scheme and the benchmark schemes. The results show that the throughput maximization scheme of backscatter-assisted transmission produces serious throughput unfairness, while the fairness enhancement scheme that only uses HTT makes the average throughput very small. In comparison, the proposed scheme can provide better performance, which can guarantee high fairness and improve average throughput
As shown in Figure
Multi-WD full-duplex backscatter-assisted wireless powered communication networks (FDBA-WPCN).
The time design of the FDBA-WPCN system is illuminated in Figure
Time design of the FDBA-WPCN system.
In the EHB period, it is assumed that the unmodulated baseband signal broadcast by FAP is defined as
Moreover, note that WDs can also harvest energy from the signal broadcast by FAP during
The throughput of
To sum up, the total throughput of each WD is given by
where
In the paper, we aim to solve the unfairness between WDs caused by round-trip path loss and the doubly near-far problem. Thus, a minimum throughput maximization problem was proposed to enhance the fairness of the system. The formula is as follows:
It can be observed that the above minimum throughput maximization problem is a nonconvex optimization problem. In this section, we first convert the problem to a convex optimization problem by introducing an extra variable
Since P1 is a nonconvex problem that is difficult to solve, it is transformed into the equivalent problem P2 as follows:
P2 is a convex optimization problem.
In P2, the objective function is a linear function, and the constraints from (
Carefully, it is found that each new constraint function consists of linear function
Thus, we need to prove that
and drive the Hessian of
Given an arbitrary nonzero real column vector
Therefore,
P2 is designed to gradually solve the unfairness in throughput of WDs caused by difference channel conditions. Assuming that the EHB period
In order to facilitate the solution, it is assumed that the value of
where
Here, the dual function yields higher bounds on the optimal value
P2 is infeasible if and only if there exists any
For simplicity, please refer to Appendix
This shows that
Because there is a boundary for the linear function
It is easy to see from (
For given
where
Please refer to Appendix
With Corollary
Initialize Initialize Initialize Update Update Update Compute
In this section, numerical results are presented to evaluate the superiority of the proposed scheme for FDBA-WPCN. The simulation environment is as follows unless otherwise specified. The transmission power of FAP is set as
We first consider the case that the number of WDs is
Energy harvesting and backscatter time allocation ratio of two WDs.
Figure
Comparison between the TM-BAT and the proposed scheme of two WDs.
Next, Figure
The average throughput comparisons of the MTM-HTT and the proposed scheme with respect to different parameters.
Finally, we promote the two-WD scenario to a multi-WD scenario by changing the number of WDs from 2 to 8. Other parameter settings are the same as before. Figure
Average throughput and fairness index versus the number of WDs.
This study considered fairness enhancement and throughput improvement in multi-WD FD WPCN assisted by backscatter. When the FAP emits a radio frequency signal, the WD either reflects the data to the FAP by backscatter or harvests energy to prepare for information transmission, which greatly improves time utilization. However, WD farther away from FAP will encounter the round-trip path loss of backscattering and the doubly near-far problem of HTT. This paper proposes the minimum throughput maximization problem, so that WDs at different distances have equal throughput. In order to solve this problem, the algorithm in the paper is used to reasonably arrange the time of backscattering, energy harvesting, and information transmission and make the throughput of each WD as high as possible. By comparing with TM-BAT, it is highlighted that the proposed scheme can guarantee good fairness when the throughput is not much different from that of TM-BAT. By comparing with MTM-HTT, it is highlighted that the proposed scheme can provide higher common throughput. Finally, the simulation numerical results reveal that the proposed scheme can not only ensure high fairness, but can also achieve high throughput compared to the two benchmark schemes.
Firstly we prove the sufficiency of proposition
Next, we prove the necessity of Proposition
Through the Lagrange multiplier method, the problem P2 is approximately transformed into an unconstrained optimization problem. Thus, we can directly calculate
where
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This work was supported by the National Key Research and Development Program of China under grant 2016YFE0200200, the National Science Foundation of China (61801240), the Natural Science Foundation of Jiangsu Province of China (BK20180753).