# Reviews Based on the Reconfigurable Intelligent Surface Technical Issues
## 2 Technical Considerations for RIS
### Differences between Relay and RIS [101]  
Currently, relays are deployed in mobile communications to identify alternative propagation paths in real time that can receive signals carrying the same information as a way to avoid the channel instability [102]. Relays can be used to effectively turn a single non-line-of-sight (NLOS) link into multiple LOS links. This approach requires that each relay be equipped with a dedicated power source and the necessary front-end circuitry to receive, process, and retransmit. For this reason, using relays has the disadvantage of increasing network power consumption and cost. The network spectral efficiency provided by a relay-enabled system depends on the duplexing protocol used for transmission. When using a HD relay protocol, the transmitter and repeater cannot transmit simultaneously from the same physical resource. FD relay protocols can overcome this problem, but suffer from high loopback self-interference at the relay due to the simultaneous transmission and reception of signals, co-channel interference at the destination because the relay and transmitter emit different information from the same physical resource, and increased signal processing complexity and power consumption at the relay. Relays are typically active devices that require power and consist of active electronic components such as DACs and ADCs, mixers, power amplifiers, and low-noise amplifiers for receiving. The implementation of decode-and-forward (DF) and amplify-and-forward (AF) relays, especially in millimeter-wave and submillimeter-wave frequency bands, typically requires multiple electronic components, resulting in high cost and power consumption [103]. In addition, as previously discussed, the use of FD relays further increases complexity because they require the use of custom antennas and analog/digital signal processing methods to eliminate loopback self-interference. Furthermore, the active electronics used in relays increase noise, which negatively impacts the performance of existing relay protocols. For example, in AF relays, noise is amplified at the relay. With DF relays, the effects of additive noise can be mitigated at the expense of increased signal processing complexity and power consumption because the signal is decoded and re-encoded (regenerated) at the relay. However, in FD relays, system performance is degraded by the effects of residual loopback self-interference. The spectral efficiency of a relay-assisted system depends on the duplexing protocol adopted. In a HD relay, the achievable rate is typically doubled because different physical resources are used for the data emitted by the transmitter and the repeater. On the other hand, the end-to-end signal-to-noise ratio can be increased by taking advantage of more favorable propagation conditions for the relayed signal and by optimally combining the direct and relayed signals. In an FD relay, the achievable speed is not doubled, but the relay is affected by residual loopback self-interference and the receiver is degraded by interference generated by the simultaneous transmission of the transmitter and relay. Figure 26: Cost effective RIS design. (**a**) RIS unit cell with FET switches. Adapted with permission from [99]. Copyright 2023 IEEE. (**b**) RIS unit cell without Vias. Adapted with permission from [100]. Copyright 2023 IEEE. The RIS is a technology that uses metasurfaces to reflect signals from the source node to the destination node with low energy consumption, as described earlier, and is not constrained by power supply. Considering the low-cost nature of RISs [104], implementing an RIS in practical applications is more favorable than relays. Therefore, it is meaningful to compare RISs and relays. In [105], it was shown that a multiple-input single-output (MISO) system using an RIS can achieve higher energy efficiency compared to an AF relay; in [101], an RIS-enabled single-input single-output (SISO) system was compared to a conventional DF relay.