RIS vs. Relay: Key Differences in Wireless Communication

This article compares Reconfigurable Intelligent Surface (RIS) and Relay technologies, highlighting their key differences in the context of wireless communication.

Introduction

In wireless communication, the signal travels from a transmitter (like a Base Station or eNB) to a receiver (UE or mobile subscriber) through a channel. Obstacles in this channel can significantly impede signal propagation, making it difficult to serve subscribers located behind these blockages. Two primary solutions address this challenge: employing intelligent reflecting surfaces (or metasurfaces) and utilizing classical relays.

Reconfigurable Intelligent Surface (RIS)

RIS utilizes two-dimensional passive reflecting elements, often referred to as metasurfaces. It operates based on a modified Snell’s law principle, converting plane waves into scattered waves. A RIS controller manages the elements, enabling control over the delay, phase, and polarization of the scattered waves. Figure 1 illustrates RIS deployment in a wireless network scenario. Metasurfaces are particularly useful for serving subscribers blocked by obstacles.

Relay

Figure 2 depicts a relay used in an LTE network. As shown, it receives the signal from the LTE Base Station (eNB), processes it, and transmits it to the UE. Common functional blocks within a relay include:

• RF Down conversion
• Demodulation and decoding
• Modulation and Encoding
• RF Up conversion
• Signal power amplification

RIS vs. Relay: A Detailed Comparison

Figures 3 and 4 illustrate wireless communication systems employing IRS and Relay, respectively. The table below details the key differences between these technologies.

Parameters	RIS (Reconfigurable Intelligent Surface)	Relay
Definition	The reflecting surface which scatters incoming EM waves with controllable delay/phase and polarization.	The device which decodes incoming EM signal, amplifies and retransmits it back.
Operating mechanism	Passive/active reflection	Active reception and transmission
Duplex	Full	Half/Full
Number of transmit RF chains	Zero	N ADC/DAC and amplifier
Hardware cost	Low	High
Energy consumption	Low	High
Prelog Penalty	No	Yes ( ~ 1/2)
Spectral efficiency formula