Joint Beamforming Optimization for IRS-Aided THz Communication with Time Delays

Intelligent reflecting surface (IRS)-aided terahertz (THz) communication has been regarded as a promising technology for future wireless networks. However, IRS generally suffers from the severe beam squint problem due to the frequency independent characteristic of IRS elements, especially for the ultra-wide THz frequency. To deal with this issue, we apply time delays to IRS, and formulate the weighted sum rate maximization problem via joint optimization of the base station beamforming as well as IRS reflection coefficients and time delays. To solve it, we propose an alternate optimization algorithm to divide the original optimization problem into three subproblems. The successive convex approximation, semidefinite relaxation, fractional programming and majorization minimization techniques are, respectively, applied to obtain the solution of each subproblem. Next, by alternatively solving these three subproblems until convergence, and solutions of the original problem are obtained. Simulation results illustrate that our proposed time delay-based IRS scheme can effectively mitigates the beam squint effect and achieve a better system performance compared to the conventional scheme.