## 2 IRS Technology Fundamentals
### IRS Hardware Architecture and Its Working Principle  
Based on the meta-surface, the implementation of IRS hardware is digitally controllable, and is made of two-dimensional meta-material [27]. In order to operate in the subwavelength frequency of interest, the meta-surface is a planar array that consists of a large number of meta-atoms with electrical thickness [28]. The individual signal response, such as a reflection, amplitude, and phase shift of the IRS meta surface atom element, can be changed by properly designing the elements, including the geometrical shape (e.g., square or split-ring), size/dimension, orientation, arrangement, and so on. The reflection coefficient of each element should be tunable to cater to dynamic wireless channels arising from the user (UE) mobility in the wireless communication system. Three main approaches are introduced in the literature for controlling IRS reflection mechanical attenuation (e.g., mechanical translation and rotation), functional materials (e.g., liquid crystal and graphene), and electronic devices (e.g., positive-intrinsic-negative (PIN) diodes, field-effect transistors (FETs), micro-electromechanical system (MEMS) switches, etc.). Among them, electronic devices are widely adopted because of their fast response time, low reflection loss, and low energy cost. Finally, dynamically adjustable reflection coefficients are needed to create the IRS elements, and it is essential to link them to the network to acquire knowledge of the exterior communication environment in order to qualify its real-time adaptive reflection [29]. One typical architecture of an IRS device is illustrated in Figure 1, where the IRS consists of a smart controller and three layers. The first layer is made up of a large number of reconfigurable metallic patches that are printed on the dielectric substrate to directly manipulate the incident signals. Behind this layer, a copper plate is usually used to avoid the signal energy leakage during IRS's reflection. A control circuit board excites the reflecting element and is responsible for adjusting the reflection amplitude/phase shift of each element. In addition, the reflection adaption is triggered by a smart controller attached to the IRS. Moreover, the controller between BS and UE is accomplished by a field-programmable gate array (FPGA), which also works as an entrance to communicate and coordinate with other network parameters (e.g., base stations (BSs), access points (APs), and UE terminals) by different wireless links for low-rate information interchange between them. Figure 1 shows the equivalent circuit as a sample of a separate element structure where each element is embedded with a PIN diode in the middle point of the element. The PIN diode can be changed to either the \"ON\" or \"OFF\" state by regulating various biasing voltages to the PIN diode through a direct-current feeding line, which allows the element to result in a phase-shift difference in \(\pi\) in the coming signal [30]. Setting up the corresponding biasing voltages by the smart controller, different phase shifts of IRS elements can be realized separately. According to [30], the changing frequency can be up to 5 megahertz (MHz) in the PIN diode element, which is equal to a changing time of 0.2 microseconds (\(\upmu\)s). This is much smaller than the typical channel coherence time, which well matches with the application of a mobile with time-varying channels. Besides tuning the phase shift, extra control of the reflection amplitude of every element of IRS gives much greater flexibility in reshaping the reflected signal to obtain different communication objectives successfully. Additionally, this provides an effective way to trade  
off the relationship between the cost of hardware and the performance of the reflection. In practice, the amplitude control is basically more cost-effective than phase control and there are many ways to obtain amplitude adjustment for the IRS network. One of the common processes is to adjust the load resistance in every IRS element [31].