WP1: Hierarchical multi-resolution codebook and IRS phase-shift matrix design.
In this work package, Dr. Xu designed coding schemes for multi-access wiretap channels under the strong secrecy metric. Different decoding schemes were considered at the legitimate receiver, i.e. joint decoding scheme and independent decoding scheme. Codebooks that involve secret, open, and redundant messages were designed for both cases. It was shown that by allowing users with zero secrecy rate to play different roles, the achievable region in the existing literature can be strictly enlarged. In addition, Dr. Xu also analyzed the outage probability of FAS-assisted systems. By proposing two schemes to approximate the channel model, the computational complexity of the analysis was significantly reduced.
WP2: Adaptive beam alignment methods design and IRS-assisted THz testbed setup.
In this work package, Dr. Xu proposed the coding-enhanced cooperative jamming scheme to enhance the communication secrecy and also developed a novel beamforming approach with extremely low computational complexity. Traditionally, the cooperative jammer transmits Gaussian noise (GN) to enhance the security. However, with this approach, the jamming signal interferes not only with Eve but also with Bob. Therefore, Dr. Xu proposed that besides the GN strategy, 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 so that Bob can decode the information and cancel the interference, but Eve cannot. The beamforming problem requires the solution of log-determinant optimizations in each iteration, yielding extremely high complexity. To address the issue, Dr. Xu proposed a novel approach based on matrix simultaneous diagonalization (SD). Using this technique, the beamforming problems associated with different jamming schemes can be efficiently solved with comparable performance (often better) and much less computational complexity than the MM-based method.
WP3: EKF-based beam tracking and ML-empowered dynamic beam switching methods design.
By using the compressed sensing methods, Dr. Xu developed a channel estimation scheme for a multi-user mmWave system, where users apply the fluid antenna technique to communicate with the BS. Taking advantage of the sparse characteristics of the channels, the proposed scheme requires each antenna to move and measure the channel over only a few estimating locations and can reconstruct full CSI with quite low hardware switching and pilot overhead.
WP4: Joint resource allocation for an IRS-assisted multi-UE multi-carrier THz network.
Dr. Xu investigated the capacity of a FAS-assisted MAC system. First, he derived upper bounds and approximations for the capacity, which not only provide valuable insights, but also serve as benchmarks for evaluating the performance of FAS. Then, he maximized the sum capacity by designing the transmit covariance matrices and antenna position vectors of the users. Simulation results have validated the effectiveness of FAS in increasing the system capacity, and also the proposed algorithms over the considered benchmarks.
During this project, Dr. Xu has published 7 papers in high-impact journals and flagship conferences in our research area. Two more papers are still under review.
