Table 1.

Overhead and complexity of existing CSI acquisition methods for acquiring H₂, h_{1, k}, and d_k, k = 1, …, K.

CSI acquisition method		Overhead	Complexity
Implicit CSI acquisition	Beam training
	Blind beamforming
Double link channel separation	ON/OFF RIS	K(N + 1)
	ON-only RIS	K(N + 1)
Cascaded channel estimation	Scaling law
	Channel sparsity
Individual channel estimation	IRS/active RIS (LoS)
	Selection type	2K + M
	Beamforming type