This paper considers a Gaussian multi-input multi-output (MIMO) wiretap channel with a legitimate transmitter, a legitimate receiver (Bob), an eavesdropper (Eve), and a cooperative jammer. All nodes may be equipped with multiple antennas. Traditionally, the jammer transmits Gaussian noise (GN) to enhance the security. However, using this approach, the jamming signal interferes not only with Eve but also with Bob. In this paper, besides the GN strategy, we assume that the jammer can also choose to use the encoded jammer (EJ) strategy, i.e., instead of GN, it transmits a codeword from an appropriate codebook. In certain conditions, the EJ scheme enables Bob to decode the jamming codeword and thus cancel the interference, while Eve remains unable to do so even if it knows all the codebooks. We first derive an inner bound on the system's secrecy rate under the strong secrecy metric, and then consider the maximization this bound through precoder design in a computationally efficient manner. In the single-input multi-output (SIMO) case, we prove that although non-convex, the power control problems can be optimally solved for both GN and EJ schemes. In the MIMO case, we propose to solve the problems using the matrix simultaneous diagonalization (SD) technique, which requires quite a low computational complexity. Simulation results show that by introducing a cooperative jammer with coding capability, and allowing it to switch between the GN and EJ schemes, a dramatic increase in the secrecy rate can be achieved. In addition, the proposed algorithms can significantly outperform the current state of the art benchmarks in terms of both secrecy rate and computation time.

翻译：本文考虑包含合法发射机、合法接收者（Bob）、窃听者（Eve）和协作干扰机的高斯多输入多输出（MIMO）窃听信道。所有节点可配备多根天线。传统上，干扰机通过发射高斯噪声（GN）来增强安全性，但该方法会导致干扰信号同时干扰Eve和Bob。除GN策略外，本文假设干扰机也可选择使用编码干扰机（EJ）策略，即从适当码本中传输码字而非高斯噪声。在特定条件下，EJ方案使Bob能够解码干扰码字并消除干扰，而Eve即使已知所有码本也无法实现。我们首先推导系统在强保密度量下的保密率内界，进而通过预编码器设计以计算高效方式最大化该内界。在单输入多输出（SIMO）情形下，我们证明尽管功率控制问题非凸，但针对GN和EJ方案均可求得最优解。针对MIMO情形，我们提出使用矩阵联合对角化（SD）技术求解问题，该方法计算复杂度极低。仿真结果表明，通过引入具有编码能力的协作干扰机并允许其在GN和EJ方案间切换，可显著提升保密率。此外，所提算法在保密率和计算时间两方面均显著优于现有最优基准方法。