Secrecy Performance Analysis of Multi-Tag Bistatic Backscatter Communication Systems With Outdated CSI and Link Correlation
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摘要: 针对信道时变及链路相关条件下多标签双站反向散射通信系统的安全传输问题，本文提出一种基于反向散射链路信道增益排序的标签选择方案。充分考虑标签选择阶段与传输阶段之间的信道状态信息不一致，以及合法链路与窃听链路之间的相关性，在独立非同分布瑞利信道下，推导了系统保密中断概率的闭式表达式及高发射功率下的渐近表达式，并从合法链路与窃听链路增益比的角度分析了系统保密性能的变化规律。分析结果表明：信道状态信息过时会显著削弱系统保密性能；当合法链路与窃听链路增益比固定时，系统在高发射功率区域存在保密中断平层，而提高该增益比可有效缓解该性能瓶颈；此外，在本文考虑的模型下，合法链路与窃听链路的相关性能够降低窃听链路相对占优的概率，从而改善系统保密性能。最后，蒙特卡洛仿真验证了理论推导的准确性，并表明所提方案在信道状态信息过时条件下仍能有效提升系统保密性能。Abstract:
Objective Due to feedback delay, the channel state information (CSI) used in the tag selection phase may become outdated during the actual data transmission phase, leading to a mismatch between the selected tag and the actual optimal tag. Meanwhile, most existing studies on outdated CSI are based on the independent and identically distributed (i.i.d.) channel assumption, which may fail to accurately reflect the heterogeneous characteristics of practical links caused by different propagation distances. In addition, when the eavesdropping node is close to the destination, the legitimate and eavesdropping links may experience correlated fading. Ignoring these factors may lead to theoretical results that deviate from the actual system performance. Accordingly, under independent and non-identically distributed (i.n.i.d.) channel conditions, this paper studies the secrecy performance of a multi-tag BBC system subjected to the joint impact of outdated CSI and the correlation between the legitimate link and the eavesdropping link. Methods In this paper, candidate tags are ranked by their backscatter-link channel gains, and the tag with the largest gain is selected for transmission. An outdated CSI model is introduced to characterize the mismatch between the CSI used for tag selection and the actual CSI during data transmission. Since tag selection depends only on the backscatter-link CSI, outdated CSI is modeled only for this link. Meanwhile, a correlated Rayleigh fading model is adopted to capture the statistical dependence between the legitimate and eavesdropping links. Under i.n.i.d. Rayleigh fading, order statistics are used to derive the PDF of the selected tag’s outdated backscatter-link channel gain in (30)–(36) and the joint PDF of the legitimate and eavesdropping channel gains in (10)–(13). Based on these results, closed-form and high-transmit-power asymptotic expressions for the secrecy outage probability are obtained, and the secrecy performance is analyzed in terms of the gain ratio between the legitimate and eavesdropping links. Results and Discussions Monte Carlo simulations validate the analytical and asymptotic results. The results show that outdated CSI significantly degrades secrecy performance, as feedback delay may cause the CSI used for tag selection to differ from the actual CSI during transmission, so the selected tag may no longer provide the largest backscatter-link gain. For a fixed gain ratio between the legitimate and eavesdropping links, a secrecy outage floor emerges at high transmit power ( Fig. 2 ), indicating that increasing transmit power alone cannot eliminate the performance bottleneck. In contrast, increasing the gain ratio effectively mitigates this floor and enables a secrecy diversity order of 1 (Fig. 4 ). Meanwhile, under the considered system model, correlation between the legitimate and eavesdropping links also improves secrecy performance (Fig. 3 ) by reducing the likelihood that the legitimate link experiences deep fading while the eavesdropping link remains strong. Moreover, despite outdated CSI, the proposed gain-order-based tag selection scheme remains effective and consistently outperforms random tag selection (Fig. 3 ).Conclusions This paper develops a secrecy-performance analysis framework for multi-tag BBC systems with outdated CSI and correlated legitimate and eavesdropping links. Closed-form SOP and high-transmit-power asymptotic expressions characterize the effects of CSI outdated, link correlation, and the legitimate-to-eavesdropping gain ratio. The results identify outdated CSI as a major source of secrecy degradation, highlighting the need for low-latency feedback. Enhancing the legitimate-link gain advantage and employing backscatter-link gain-based tag selection effectively improve system secrecy performance. -
表 1 主要符号及其物理意义
符号 符号含义 $ K $ 候选标签总数 $ i $ 原始标签索引 $ r $ 升序排列后的阶次 $ {i}_{\left(r\right)} $ 第$ r $个升序排序位置对应的标签索引 $ {i}^{*}\triangleq {i}_{\left(K\right)} $ 反向散射链路增益最大标签的索引 $ {T}_{{{i}^{*}}} $ 本文选取的反向散射链路增益最大的标签 $ {g}_{{{i}^{*}}} $ 选择阶段选定标签的反向散射链路信道系数 $ g_{{i}^{*}}^{\tau } $ 传输阶段选定标签的反向散射链路信道系数 $ {h}_{{{i}^{*}}} $ 仅表示选定标签对应的前向链路信道系数 $ {g}_{0{{i}^{*}}} $ 仅表示选定标签对应的窃听链路信道系数 $ {\lambda }_{1{{i}^{*}}} $ $ S\rightarrow {T}_{{{i}^{*}}} $链路的方差 $ {\lambda }_{2{{i}^{*}}} $ $ {T}_{{{i}^{*}}}\rightarrow D $链路的方差 $ {\lambda }_{0{{i}^{*}}} $ $ {T}_{{{i}^{*}}}\rightarrow E $链路的方差 
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