  In this paper, we study how to jointly design the phase shift of cascaded
multi-IRSs and the precoding vector of the BS to improve the coverage in dense
urban areas. We aim to maximize the signal-to-noise ratio (SNR) of the user
equipment (UE) received signal by employing this method. However, it is a
constrained non-convex optimization problem and is NP-hard. In order to solve
this problem, we simplify it by utilizing the characteristic of the mmWave
wireless system to decompose the optimization problem into multiple
sub-optimization problems. By employing the asymptotic orthogonality of
wireless channel in mmWave system to solve the sub-optimization problems, we
finally yield a closed-form sub-optimal solution. The simulation results verify
that our solution can effectively improve the coverage of deep dense urban
areas.
