Research On Resource Allocation In Wireless Powered Body Area Networks With Cooperative Transmission

This paper studies a wireless powered body area network(WPBAN),where an access point(AP)provides sustainable energy supply by using wireless power transfer(WPT)technology.Sensor nodes with low-power consumption receive and harvest the radio frequency(RF)energy from the AP for transmitting the sensing data of human physiology and surrounding environment information.The WPBAN is of great significance to support the national development strategy of "facing the people’s life and health and advancing to the breadth and depth of science and technology".However,there are still some problems in WPBAN,such as the unfair RF energy collection capability of sensor nodes,low transmitting power of sensor nodes and low WPT efficiency of APs.These problems may lead to the unreliable wireless information transmission(WIT)of a sensor node.To address the above problems,this paper investigates the cooperation of multiple sensor nodes and the cooperation of multiple antennas of the AP for improving WIT reliability.In addition,the resource allocation optimization of WPBAN based on collaborative transmission is studied.The main contributions of this paper are as follows:(1)To address the problem of unreliable WIT due to the unfair RF energy collection capability of sensor nodes in WPBAN,we design two cooperative beamforming(CBF)schemes based on decode-and-forward(DF)and backscatter cooperation(BC),respectively.To enable the CBF,a group of sensor nodes act as a virtual antenna array for improving the signal-to-noise ratio(SNR)and throughput performance.Based on these two schemes,A joint time and harvested energy allocation(JTHEA)algorithm and a time allocation(TA)algorithm are designed,respectively with the consideration of WIT fairness.For the DF-based CDF scheme,a minimum throughput maximization problem is solved by using the JTHEA algorithm.For BC-based CBF scheme,the optimal TA solution for uplink and downlink transmissions of each sensor node is derived based on Karush-Kuhn-Tucker(KKT)conditions.Simulation results show that the proposed two schemes improve both SNR and throughput performance.(2)To address the problem of unreliable WIT due to the low transmit power of sensor nodes in WPBAN,we design a joint CBF and cooperative backscattering(CBS)scheme to maximize the sum throughput.A joint phase design and TA(JPDTA)algorithm is proposed based on the joint CBF and CBS scheme.Considering a WPBAN with multiple sensor types,we incorporate the CBF of sensor nodes of one type with the CBS of sensor nodes of other types to improve the received signal strength at the AP.Since the formulated sum-throughput maximization problem is difficult to be solved due to the tightly coupled optimizing variables,we decouple it the problem is decoupled into a TA subproblem and a phase shift optimization subproblem,and then solve them in two steps by using the proposed JPDTA algorithm.Firstly,the optimal solution for the TA subproblem is derived according to KKT conditions.Secondly,a centralized algorithm based on iterative optimization and onedimensional search and a decentralized algorithm with low complexities are designed for the phase shift optimization subproblem.Compared with traditional independent transmission,simulation results show that the proposed joint CBF and CBS scheme can improve 5.3 d B SNR and 16.6% throughput and save 29.4% WPT time length.(3)To address the unreliable WIT problem due to the low WPT efficiency of AP in WPBAN,we design a weighted beamforming scheme.A joint beamforming weights and time allocation(JBWTA)algorithm is proposed to adjust the beamforming weights for different sensor nodes and WPT time ratio.Considering the SNR and transmit power constraints,A throughput-energy ratio maximization problem is solved by using the proposed JBWTA algorithm.We transform the nonconvex optimization problem into a convex optimization problem and solved it by using convex optimization tools.Compared with the traditional equal-weight beamforming,simulation results show that the proposed weighted beamforming scheme can improve 10.2% energy efficiency and save 2.98% WPT time length.