Impact of RIS Inter-Element Spacing on Array Gain and Energy Efficiency

Reconfigurable Intelligent Surfaces (RIS) have emerged as a promising technology for enabling seamless wire-less connectivity, particularly in environments characterized by non-line-of-sight (NLoS) propagation and signal blockages. To enhance performance in such scenarios, densely deployed RIS configurations is a promising solution. In this context, this paper investigates the impact of inter-element spacing on the gain and the energy efficiency (EE) of RIS-assisted communication system. Unlike previous works, we formulate the received signal model that incorporates the array directivity and show how the array gain varies with the change in the inter-element spacing. The performance of the standard (where the spacing is half of the wavelength) and dense RIS (where the spacing is less than half of the wavelength) configurations is compared in terms of, 1)beam pattern and array gain while considering quantized and unquantized phase shifts, 2) variation in the number of RIS elements, and 3) the trade-off between EE and array gain. Results reveal that while dense RIS offers slightly improved array gain compared to standard RIS, it incurs higher power consumption and reduced EE.