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Intelligent reflecting surface (IRS) is considered as a promising technology for enhancing the transmission rate in cellular networks. Such improvement is attributed to considering a large IRS with high number of passive reflecting elements, optimized to properly focus the incident beams towards the receiver. However, to achieve this beamforming gain, the channel state information (CSI) should be efficiently acquired at the base station (BS). Unfortunately, the traditional pilot estimation method is challenging, because the passive IRS does not have radio frequency (RF) chains and the number of channel coefficients is proportional to the number of IRS elements. In this paper, we propose a novel semi-blind channel estimation method where the reflected channels are estimated using not only pilot but also data symbols, reducing the channel estimation overhead. The performance of the system is analytically investigated in terms of the uplink achievable sum-rate. The proposed scheme achieves higher energy and spectrum efficiency while being robust to channel estimation errors. For instance, the proposed scheme achieves an 80&#x0025; increase in spectrum efficiency compared to pilot-only based schemes, for IRSs with <inline-formula> <tex-math notation="LaTeX">$N=32$ </tex-math></inline-formula> elements.

Semi-Blind Channel Estimation for Intelligent Reflecting Surfaces in Massive MIMO Systems