On-Demand RIS-Assisted Free Space Optical Access System for 6G Networks

In reconfigurable intelligent surface (RIS)-assisted systems where the line-of-sight (LoS) path is entirely obstructed, there are the same number of RIS elements as pilot signals. This makes RIS implementation challenging and incurs signaling costs. In addition, there is power consumption due to multiplicative fading resulting from the fact that each RIS element induces two sub-channels. Therefore, a permanent use of the RIS module when not needed can be seen as resource mismanagement. A solution to reduce this cost and power consumption is the use of the RIS technology on-demand (OnD). This offers resource management, cost, and energy efficiency advantages. Reflecting on this situation, in this paper, we analyze a free space optical (FSO) system aided by the OnD RIS technology. We consider that the LoS link can be severely obstructed and its channel significantly degraded. This degradation affects the signal-to-noise ratio (SNR) of the LoS path, $\gamma _{o}$. We set its threshold value, $\gamma _{th}$, which corresponds to the system's triggering point. The analysis considers three main scenarios, namely $\gamma _{o} > \gamma _{th}$, $\gamma _{o} \leq \gamma _{th}$ with total interruption of the LoS link, and $\gamma _{o} \leq \gamma _{th}$ with gradual degradation of the LoS link. For these three scenarios, we define the sub-channel SNRs, derive the end-to-end (e2e) channel statistics, including the probability density functions, cumulative distribution functions, and moment-generating functions, for the LoS link alone, the non-LoS path through the RIS module alone, and the combined LoS and non-LoS paths. Furthermore, we derive, analyze, and evaluate the e2e ergodic channel capacity and average bit error rate, and provide results. We demonstrate that when the LoS link experiences high disturbances, a RIS module can restore the LoS system's performance for a specific time slot. Finally, we show that the RIS can be solicited in an OnD manner to prevent FSO degradation when the weather condition worsens.