Distributed Beam Training for Intelligent Reflecting Surface Enabled Multi-Hop Routing

Intelligent reflecting surface (IRS) is an emerging technology to enhance the spectral and energy efficiency of wireless communications cost-effectively. This letter considers a new multi-IRS aided network where cascaded line-of-sight (LoS) link established between base station (BS) remote user by leveraging multi-hop signal reflection selected IRSs. As compared conventional single-/double-hop IRS system, system provides more pronounced path diversity cooperative passive beamforming gains, especially in environment with dense obstacles. However, challenging joint active/passive beam routing problem also arises for maximizing end-to-end channel gain. Furthermore, number IRS-associated coefficients increases drastically hops. To tackle above issues, this we propose efficient training based solution considering use practical codebook-based BS/IRS without need explicit estimation. Instead exhaustively or sequentially searching over all combinations active patterns each route, distributed scheme proposed reduce complexity, exploiting (nearly) time-invariant BS-IRS inter-IRS channels among BS IRSs’ controllers. Simulation results show that our design achieves gain close sequential search, but at much lower overhead complexity.