| Wireless Powered Backscatter Communication(WP-BackCom)is one of the effective technical solutions to tackle the energy constraint problem of the large-scale Internet of Things,and the transmission performance of the large-scale WP-BackCom network is closely related to the network spatial location,which is characterized by the randomness and uncertainty.Although studies have been conducted to characterize the random spatial position of the large-scale WP-BackCom network and analyze the impact mechanism of network parameters on the transmission performance based on stochastic geometry,they have mostly ignored the assumptions such as the non-linear energy harvesting model and successive interference cancellation(SIC)that fit the realistic network properties,which affects the accuracy of the transmission performance expressions.Therefore,by considering the above factors in a large-scale WP-B ackCom network,this dissertation aims to establish a tractable research framework of transmission performance based on stochastic geometry,and evaluate the reliability and secrecy of the network in terms of the successful transmission probability and secrecy outage probability in turn,while exploring the impact of network parameters such as the reflection coefficient on the successful transmission probability and secrecy outage probability.The main research contents of this dissertation are as follows:(1)For a large-scale WP-BackCom network,the reliability of information transmission is evaluated using the successful transmission probability,and the trend of the successful transmission probability with network parameters such as the reflection coefficient is also analyzed.First,the successful transmission probability expression based on the random spectrum allocation(RSA)policy is derived in the general case,from which the relationship between different network parameters and the successful transmission probability is explored.Results point to the existence of an optimal reflection coefficient that maximizes the successful transmission probability within the range of the reflection coefficient.To achieve tractability of theoretical analysis,three special cases are considered and the closed-form expressions for successful transmission probability are obtained successively.Secondly,with the objective of improving the transmission reliability of the network,the study of successful transmission probability maximization is carried out by optimizing the reflection coefficient,and a low-complexity optimization scheme is adopted to design a sub-optimal reflection coefficient in closed form,which is proportional to the imperfect SIC coefficient.Compared to the optimal successful transmission probability achieved by the one-dimensional exhaustive search scheme,the proposed optimization scheme is able to obtain a sub-optimal successful transmission probability second only to the optimal one with lower computational complexity.Finally,Monte Carlo simulations verify the correctness of the theoretical results,and the variation of successful transmission probability with different parameters was revealed.(2)The secrecy performance study for physical layer security is not clear enough in large-scale WP-BackCom networks with random spatial locations.Therefore,for a large-scale WP-BackCom network with illegal eavesdropping users,the secrecy of information transmission is evaluated in terms of the secrecy outage probability,and the effect of network parameters such as the reflection coefficient on the secrecy outage probability is explored.First,an integral expression for the secrecy outage probability is derived under the RSA policy,which is applicable to general network parameters but loses tractability theoretically.Secondly,a closed-form expression for the secrecy outage probability is obtained using the special case of typical path loss exponent and no-noise.The secrecy outage probability shows a tendency to decrease first and then increase or only increase as the reflection coefficient increases,i.e.the secrecy outage probability is a concave or an increasing function with respect to the reflection coefficient,and the optimal reflection coefficient that minimizes the secrecy outage probability exists only in the case of the concave function.Finally,numerical calculations and Monte Carlo simulations verify the correctness of the theoretical results,while investigating the trend of the secrecy outage probability with different parameters.In particular,the optimal reflection coefficient is in direct proportion to the imperfect SIC coefficient at the gateway.The transmission performance study in terms of the successful transmission probability(reliability)and secrecy outage probability(secrecy)is helpful to provide theoretical guidance for the performance analysis,optimization design and parameter configuration of the large-scale WP-BackCom networks. |
