We consider that a transmitter covertly communicates with multiple receivers under the help of a friendly jammer. The messages intended for different receivers are transmitted in mutually orthogonal frequency bands. An adversary observes all these frequency bands aiming at detecting whether or not communication occurs, while the friendly jammer broadcasts jamming signals to degrade the detection performance of the adversary. We consider a block Rayleigh fading channel model and evaluate the performance of covert communication in two situations: 1) the wireless channels vary slowly such that the transmission ends within one channel coherent time block, and 2) the wireless channels vary fast such that the wireless channels have changed several times before the whole transmission is finished. In the former case, subject to a covertness constraint, we maximize the sum of the effective rates by optimizing the transmit power allocation and the transmission rate for each receiver. In the latter case, we take the channel training process into consideration, and subject to a covertness constraint, we maximize the sum of the ergodic rates by optimizing the power allocation and the pilot length. Though both of the two optimization problems are non-convex, we presented methods to find their global optimal solutions. Besides, we also present methods to find sub-optimal solutions with lower computational complexities. Numerical results are presented to evaluate the performance under the two situations.

中文翻译：

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干扰辅助与多个接收器的秘密通信


我们认为发射机在友方干扰机的帮助下秘密地与多个接收机进行通信。发送给不同接收者的消息在相互正交的频带中传输。对手观察所有这些频段，旨在检测是否发生通信，而友方干扰机则广播干扰信号，以降低对手的检测性能。我们考虑一种块瑞利衰落信道模型，并评估两种情况下隐蔽通信的性能：1）无线信道缓慢变化，使得传输在一个信道相干时间块内结束；2）无线信道快速变化，使得无线信道快速变化，使得传输在一个信道相干时间块内结束。在整个传输完成之前，频道已经改变了几次。在前一种情况下，受隐蔽性约束，我们通过优化每个接收器的发射功率分配和传输速率来最大化有效速率的总和。在后一种情况下，我们考虑信道训练过程，并在隐蔽性约束下，通过优化功率分配和导频长度来最大化遍历率之和。尽管这两个优化问题都是非凸的，但我们提出了找到其全局最优解的方法。此外，我们还提出了寻找计算复杂度较低的次优解决方案的方法。给出了数值结果来评估两种情况下的性能。