IRS Compensation of Hyper-Rayleigh Fading: How Many Elements are Needed?

The letter introduces and studies the problem of defining the minimum number of Intelligent Reflecting Surface (IRS) elements needed to compensate for heavy fading conditions in multipath fading channels. The fading severity is quantified in terms of Hyper-Rayleigh Regimes (HRRs) (i.e., full-HRR (worst-case conditions), strong-, weak-, and no-HRR), and the channel model used (Inverse Power Lomax (IPL)) was chosen since it can account for all HRRs. The research presents the derived closed-form channel coefficient envelope statistics for the single IRS-element channel with IPL statistics in both subchannels and total IRS-assisted channel, as well as tight approximations for the channel coefficient and instantaneous signal-to-noise ratio (SNR) statistics for the latter. The derived expressions helped estimate channel parameters corresponding to the specific HRRs of the total channel and demonstrate that while both single links (i.e., ''source-IRS'' and ''IRS-destination'') are in full-HRR, the minimum number of IRS elements needed to bring the total IRS-assisted link (''source-IRS-destination'') out of full-HRR is no less than $6$ (for the whole range on the IPL scale parameter corresponding full-HRR). Furthermore, the minimum number of IRS elements required to bring the total IRS-assisted link into no-HRR is $14$ (under the same conditions).