Self Interference Cancellation (SIC) in 5G: Analog vs Digital

This article explains Self Interference Cancellation (SIC) for full duplex radio systems in 5G networks. We will explore the differences between Analog SIC and Digital SIC techniques, along with their advantages and benefits.

Introduction to Interference

Anything that disrupts the normal operation of a system can be considered interference. This applies to various domains, including communication, personal life, and entertainment. In communication, interference refers to an electromagnetic (EM) or radio frequency (RF) signal that disrupts another EM or RF signal. RF interference can manifest as poor video quality on TVs, distorted sound on radios or mobile phones, and so on.

Interference can be categorized into the following types:

• Co-channel Interference
• Adjacent channel Interference
• Electromagnetic Interference
• Sound Interference
• Light Interference
• Inter carrier Interference
• Inter Symbol Interference

Now, let’s focus on Self Interference.

Self interference occurs in the receiver part of a full duplex radio due to the leakage of the transmitter signal that is adjacent to it. This interference originates within the RF system itself, unlike interference from nearby RF systems. The problem is generally not observed in half duplex radios.

A transmitter emits a high-magnitude energy signal to its intended channel. Some of this signal leaks into the receiver, acting as a blocker. Furthermore, the transmitter leaks noise into the receiver, known as the “noise skirt” of the transmitter. Both these sources contribute to self interference.

Full duplex radios utilize both transmitter and receiver components, enabling simultaneous communication with other radios. They can employ both Time Division Duplexing (TDD) and Frequency Division Duplexing (FDD) topologies. In TDD, devices communicate on the same channel but at different time intervals. In FDD, devices communicate simultaneously on two different channels.

What is Self Interference Cancellation?

Self Interference Cancellation (SIC) refers to the process of canceling or suppressing the interference caused by a transmitter to its co-located receiver within the same full duplex radio system.

5G New Radio (NR) utilizes both TDD and FDD frequency allocations for User Equipments (UEs).

• 5G TDD: In TDD, the same antenna array element is connected to an SPDT switch and RF filters. The power to be transmitted passes through a Power Amplifier (PA), while the received RF signal passes through a Low Noise Amplifier (LNA).

• 5G FDD: In FDD, the same antenna array element is connected to diplexers, which separate the transmit/receive frequencies for the PA and LNA, respectively. FDD uses two unique frequencies for the uplink and downlink directions. TDD, on the other hand, uses a single RF carrier frequency for both uplink/downlink at different time instants.

Analog SIC vs Digital SIC

Image courtesy: https://www.orca-project.eu/

Here’s a breakdown of the two main types of SIC:

• Digital SIC: This technique utilizes the baseband or digital portion of the transmitter. As shown in the image, the baseband Tx signal is extracted before the Digital-to-Analog Converter (DAC). This feedback is used to cancel the received baseband signal, which is derived after the Analog-to-Digital Converter (ADC). Digital SIC can achieve approximately 30 to 40 dB of suppression.

• Analog SIC: This technique utilizes the analog portion of the transmitter. It extracts the analog signal to be transmitted after the Power Amplifier (PA). This feedback is used to cancel the received analog signal before the Low Noise Amplifier (LNA). Analog SIC can achieve approximately 50 to 70 dB of suppression.

To maximize the advantages of both Analog SIC and Digital SIC, they are often employed together in full duplex communication systems.

Benefits of Self Interference Cancellation

Here are the key benefits of Self Interference Cancellation over conventional RF filters:

• Does not degrade receiver sensitivity: Unlike RF filters, SIC does not negatively impact the noise figure of the receiver.

• Reduces size and weight: RF filters with sharper frequency responses tend to be larger and heavier, increasing the overall size and weight of the full duplex radio. SIC helps overcome this issue.

• Suitable for mobile applications: The size and weight problem associated with filters is a significant concern for mobile platform-based products. Therefore, SIC is a better choice for mobile applications.

• Tunability: Filters are typically used when the transmitter and receiver operate at frequencies far apart with sufficient guard bands. However, they are static, passive, and their frequency response cannot be dynamically changed. SIC offers a solution to this limitation, as the Digital SIC technique can be software-configurable (or tunable) with minimal effort and without changing the hardware of the radio system.

• Large dynamic range: SIC techniques enable a large dynamic range in wireless receivers or full duplex radio systems.

• Minimal insertion loss: Unlike RF filters which are placed directly in the RF path, SIC circuitry is connected to the RF path using RF couplers or directional couplers, introducing negligible insertion loss.