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What is Self Interference cancellation in 5G | Analog SIC, Digital SIC

This article describes Self interference cancellation (SIC) for full duplex radio system in 5G. The difference between Analog SIC and Digital SIC (self interference cancellation) techniques along with advantages or benefits are described.

Introduction Anything which interferes in the normal functioning of system is referred as interference. It can be applied to many domains including communication, personal life, entertainment and so on.

In communication, it refers to EM (Electromagnetic) signal or RF signal which interferes with other EM signal or RF signal. RF interference results into poor video signal on TV, poor sound on radio or mobile phones and so on.

Interference can be categorized into following types.
• Co-channel Interference
• Adjacent channel Interference
• Electromagnetic Interference
• Sound Interference
• Light Interference
• Inter carrier Interference
• Inter Symbol Interference
➨Refer article on Interference basics and types >>.

Let us understand Self interference. The interference produced in the receiver part of full duplex radio due to leakage of transmitter adjacent to it is known as self interference. This interference is caused within the RF system itself and it is not caused by the nearby RF system, hence the term "Self". The problem is not observed in half duplex radio.

Transmitter emits large magnitude of energy to its desired channel. This leaks into receiver which is a blocker signal for receiver. Moreover transmitter leaks noise also into receiver which is known as "noise skirt" of transmitter. Both of these sources of interference contribute to the self interference.

The full duplex radio uses transmitter and receiver part which communicates with other radio simultaneously. It can use both TDD and FDD topologies. In TDD, handsets communicate at same channel but at difference time instants. In FDD, handsets communicate simultaneously at two difference channels and also at the same time instants.

What is Self Interference cancellation ?

The cancellation or suppression of the interference caused by transmitter to its co-located receiver on the same full duplex radio is known as self interference cancellation.

Self interference cancellation

5G NR (New Radio) uses both TDD and FDD frequency allocations for UEs (User Equipments). In 5G TDD, same antenna array element is connected with SPDT switch and RF filters. The power to be transmitted is passed through PA (Power Amplifier) where as received RF signal is passed through LNA (Low Noise Amplifier). In 5G FDD, same antenna array element is connected with diplexers which separates transmit/receive frequencies for PA and LNA respectively. FDD uses two unique frequencies for uplink and downlink directions. TDD uses single RF carrier frequency for both uplink/downlink at different instants.

Analog SIC vs Digital SIC

Analog SIC vs Digital SIC, Difference between Analog SIC and Digital SIC
Image courtesy : https://www.orca-project.eu/

Digital SIC: This type utilizes baseband or digital part of transmitter. As shown baseband Tx part is extracted before DAC. This feedback is utilized for cancellation of received baseband signal derived after ADC. Digital SIC technique helps in achieving about 30 to 40 dB of suppression.

Analog SIC: This type utilizes analog part of transmitter. As shown it extracts analog signal to be transmitted after PA (Power Amplifier). This feedback is utilized for cancellation of received analog signal before LNA. Analog SIC technique helps in achieving about 50 to 70 dB of suppression.

In order to leverage benefits or advantages of both analog SIC and digital SIC, they are employed together in full duplex communication systems.

Benefits or advantages of Self Interference cancellation

Following are the benefits or advantages of Self Interference cancellation over conventional RF filters. Hence SIC does not decrease the receiver sensitivity.
➨RF filters affect the noise figure of the receiver where as self interference cancellation (SIC) does not.
➨Sharper frequency response of the RF filters make them larger in size and heavier in weight. This increases overall size and weight of the full duplex radio. This problem is overcome by introduction of SIC in the full duplex radio.
➨The problem of weight and size is big concern for mobile platform based products. Hence for mobile applications, SIC is a better choice.
➨Filters are applicable in the scenarios where transmitter and receiver operate at frequencies far apart with sufficient guard band between them. The major drawback is they are static and passive and their frequency response can not be changed dynamically as per requirements. Self interference cancellation offers solution to above problem of static RF filters which are not tunable. Digital SIC technique can be software configurable (or tunable) with minimum efforts without changing the hardware of the radio system.
➨Large dynamic range can be achieved with the use of SIC techniques in the wireless receiver or full duplex radio system.
➨The self interference cancellation is not placed in the RF path unlike RF filters. SIC circuit is connected to RF path using RF couplers or directional couplers which introduces insertion loss negligibly.



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