The channel hardening effect is less pronounced in the cell-free massive multiple-input multiple-output (mMIMO) system compared to its cellular counterpart, making it necessary to estimate the downlink effective channel gains to ensure decent performance. However, the downlink training inadvertently creates an opportunity for adversarial nodes to launch pilot spoofing attacks (PSAs). First, we demonstrate that adversarial distributed access points (APs) can severely degrade the achievable downlink rate. They achieve this by estimating their channels to users in the uplink training phase and then precoding and sending the same pilot sequences as those used by legitimate APs during the downlink training phase. Then, the impact of the downlink PSA is investigated by rigorously deriving a closed-form expression of the per-user achievable downlink rate. By employing the min-max criterion to optimize the power allocation coefficients, the maximum per-user achievable rate of downlink transmission is minimized from the perspective of adversarial APs. As an alternative to the downlink PSA, adversarial APs may opt to precode random interference during the downlink data transmission phase in order to disrupt legitimate communications. In this scenario, the achievable downlink rate is derived, and then power optimization algorithms are also developed. We present numerical results to showcase the detrimental impact of the downlink PSA and compare the effects of these two types of attacks.

中文翻译：

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无蜂窝大规模 MIMO 下行链路的导频欺骗攻击：从对手的角度


与蜂窝系统相比，无蜂窝大规模多输入多输出（mMIMO）系统中的信道硬化效应不太明显，因此有必要估计下行链路有效信道增益以确保良好的性能。然而，下行链路训练无意中为敌方节点发起飞行员欺骗攻击（PSA）创造了机会。首先，我们证明对抗性分布式接入点（AP）会严重降低可实现的下行链路速率。他们通过在上行链路训练阶段估计到用户的信道，然后预编码并发送与合法接入点在下行链路训练阶段使用的导频序列相同的导频序列来实现这一点。然后，通过严格推导每用户可实现的下行链路速率的封闭式表达式来研究下行链路 PSA 的影响。通过采用最小-最大准则来优化功率分配系数，从敌对AP的角度来看，每个用户可实现的下行链路传输的最大速率被最小化。作为下行 PSA 的替代方案，敌对 AP 可能会选择在下行数据传输阶段对随机干扰进行预编码，以破坏合法通信。在这种情况下，导出可实现的下行链路速率，然后还开发功率优化算法。我们提供数值结果来展示下行链路 PSA 的有害影响并比较这两种类型的攻击的影响。