Intelligent Refelecting Surface (or Intelligent Reconfigurable Surface), IRS, consists of a large number of passive elements that are able to reflect the incident signal passively. The size of each element may depend on the wavelength of the signal to be reflected. My question here is how much should be the size of the IRS element in order to successufly reflect a signal with a wavelength \lambda.
The typical size of the elements being used in experiments is lambda/10 to lambda/5. The reason is that you want each element to scatter the signals without a directivity, so that the combined effect of having many elements will determine the directivity of the "reflected" signal.
I also explain this at 26:00 in the following video:
https://youtu.be/9cBn5piI9Ms?t=1560
You can also listen to the Episode 3 in my podcast "Wireless Future", we we discuss this. It is available on YouTube and in podcast apps.
Reflection angle is taken with the direction of wavefront, intensity of wave need for a free particles, then type of material and surface annealing will affect, then not less than one wave length to prevent diffraction and scattering and it may be more.
I don't have a criteria, it must be tested when don't have a standard or looking for there where are.
If the typical size of the elements is lambda/10 to lambda/5, what is the element spacing between adjcent elements?
As I Know, lambda/2 is the typical array spacing in MIMO systems.
From a designer's perspective, a reflective element has to be resonant to the specific frequency of operation. Lambda is the most optimum size. The width and length needed for each unit element calculated can use the tool here,
https://www.pasternack.com/t-calculator-microstrip-ant.aspx
Although the above tool is for a feedline microstrip patch, the same applies for a reflective patch as well. Shrinking a unit element to lambda/2 or lambda by 10 is however possible using a loading plane, but this comes at the cost of bandwidth and some gain.
Conference Paper Size reduction of Microstrip Patch Antennas via Artificial D...
I used this miniaturization concept to built my reconfigurable surface, the design can be seen in the paper:
Preprint Injecting Reliable Radio Frequency Fingerprints Using Metasu...
Spacing is typically lambda/2 for maintaining the independence of the unit elements.
It depends on the speci c applications and practical implementation, typically from dozens of refecting elements to thousands of refecting elements . For example, a person wearing the t-shirt is coated with a recon gurable intelligent meta-surface or the IRS is attached to the street lamp, only several or dozens of re
ecting elements is needed. In addition, for the thousands of refecting elements and more, it is difficult to obtain the perfect channel state information, which makes it difficult to implement in practice.