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Difference between Reconfigurable Intelligent Surface and Relay | RIS vs Relay

This page compares Reconfigurable Intelligent Surface (RIS) Vs Relay and mentions difference between Reconfigurable Intelligent Surface and Relay.

Introduction: Wireless communication consists of transmitter (i.e. Base Station or eNB), receiver (UE or mobile subcriber) and channel. When there is blockage between transmitter and receiver, it will be difficult to serve subscribers behind this blockage. There are two main ways, these subscribers can be served viz. by using intelligent reflecting surfaces or metasurfaces or using classical relay.

Reconfigurable Intelligent Surface (RIS)

Reconfigurable Intelligent Surface RIS in wireless network

The RIS uses two dimensional passive reflecting elements. These surfaces are also known as metasurfaces. It works based on modified snell's law principle. RIS converts plane waves into scattered waves. The elements of RIS are controlled by RIS controller which helps in controlling delay/phase and polarization of scattered waves. The figure-1 depicts RIS used in one such wireless network scenario. Metasurfaces are mainly used to serve subscribers which are blocked by any obstacles.

Relay

LTE relay type1

The figure-2 depicts relay used in LTE network. As shown it receives the signal from LTE Base Station i.e. eNB. It processes the received signal and transmits back to the UE. The common functional blocks which relay uses include following.
• RF Down conversion
• Demodulation and decoding
• Modulation and Encoding
• RF Up conversion
• Signal power amplification

Difference between RIS and Relay

The figure-3 and figure-4 depict wireless communication systems using IRS and Relay respectively. The spectral efficiencies of both the cases are mentioned below in the table taking into considering path losses, transmit power and 'N' elements in RIS.

Wireless Communication using IRS Wireless Communication using Relay

Following table mentions difference between RIS and Relay.

Parameters RIS (Reconfigurable Intelligent Surface) Relay
Definition The reflecting surface which scatters incoming EM waves with controllable delay/phase and polarization is known as RIS. The device which decodes incoming EM signal, amplifies and retransmit it back is known as relay.
Operating mechanism Passive/active reflection Active reception and transmission
Duplex Full Half/Full
Number of transmit RF chains required Zero N
ADC/DAC and amplifier Not required Required
Hardware cost Low High
Energy consumption Low High
Prelog Penalty No Yes ( ~ 1/2)
Sprectral efficiency formula spectral efficiency formula of IRS Spectral efficiency formula of Relay

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