Ratio of Self-Interference

Description

The Ratio of Self-Interference (RSI) is a dimensionless metric defined as the power ratio of the residual self-interference after cancellation to the power of the desired received signal. In mathematical terms, RSI = P_residual_SI / P_desired, often expressed in decibels (dB). It is a central figure of merit in systems employing in-band full-duplex (IBFD) radio operation, where a device transmits and receives simultaneously on the same center frequency. The primary challenge in IBFD is the overwhelming power of the device's own transmitted signal that leaks into its own receiver chain, potentially drowning out the weak desired signal from a remote transmitter. This leakage is termed self-interference (SI).

The process of mitigating SI involves sophisticated Self-Interference Cancellation (SIC) techniques that operate across multiple domains: the antenna domain (using antenna isolation and directional separation), the analog/RF domain (using balancing circuits, tunable filters, and RF cancellation), and the digital domain (using adaptive digital signal processing to model and subtract the estimated interference). The RSI metric captures the net effectiveness of this entire SIC chain. After all cancellation stages, the remaining interference power (P_residual_SI) is what corrupts the desired signal. A lower RSI value indicates more successful cancellation, meaning the residual SI is much weaker than the desired signal, allowing for successful demodulation and decoding.

In 3GPP specifications, particularly in the context of NR and studies on advanced transceivers (e.g., for Integrated Access and Backhaul - IAB), RSI is used to characterize transceiver performance requirements and to model system-level performance. The achievable RSI dictates the practical feasibility of IBFD. For instance, if the transmitted power is +20 dBm and the receiver noise floor is -90 dBm, the SIC system must achieve roughly 110 dB of cancellation to bring the residual SI down to the noise floor. The RSI, in this case, would be the ratio of this noise-floor-level residual power to the power of the incoming desired signal. System designers use RSI to calculate the effective signal-to-interference-plus-noise ratio (SINR), which directly determines the achievable data rates and reliability of a full-duplex link.

Purpose & Motivation

The RSI metric was introduced to address the fundamental challenge of in-band full-duplex communication, a technology that promises to double spectral efficiency by allowing simultaneous transmission and reception on the same frequency resource. Traditional wireless systems operate in half-duplex modes (either time-division or frequency-division) specifically to avoid the severe self-interference problem. The pursuit of IBFD as a means to dramatically increase capacity for 5G and beyond necessitated a standardized way to quantify the core technical hurdle: how well a transceiver can suppress its own transmission.

Prior to the formal definition of RSI, research into full-duplex systems used various ad-hoc metrics like 'cancellation depth' or simply quoted the absolute residual power. The RSI provides a more system-relevant metric because it relates the interference to the strength of the desired signal. This is crucial because the impact of residual interference is not absolute; it is relative to what the receiver is trying to decode. A certain level of residual power might be negligible when receiving a strong signal from a nearby base station but catastrophic when receiving a weak, cell-edge signal. The RSI directly feeds into the SINR calculation, making it an essential parameter for link budget analysis and system-level simulation.

Its creation was motivated by 3GPP's work on advanced antenna systems and new deployment scenarios in Releases 15 and beyond, such as IAB. In IAB, a node relays backhaul and access traffic, and full-duplex operation could significantly enhance its efficiency. Defining RSI allowed 3GPP to specify performance requirements for transceivers capable of supporting such features. It provides a common language for equipment vendors and network operators to specify, test, and verify the self-interference cancellation capabilities of hardware, ensuring interoperability and predictable performance gains from full-duplex technologies when they are deployed.

Key Features

• Quantifies the effectiveness of self-interference cancellation (SIC) in full-duplex systems
• Defined as the power ratio of residual self-interference to desired signal power
• A critical input for calculating the effective SINR in full-duplex links
• Used to specify transceiver performance requirements in 3GPP standards
• Applicable across multiple cancellation domains: antenna, analog/RF, and digital
• Enables system-level modeling and capacity analysis for in-band full-duplex operation

Evolution Across Releases

Defining Specifications