Full Duplex in 6G: Features, Benefits, and Challenges

There are two topologies in which current wireless networks operate TDD and FDD.

Time Division Duplex (TDD): Devices separate their uplink (transmit) and downlink (receive) signals by assigning them different time slots. It’s like a conversation on a walkie talkie where you have to say “over” before the other person can speak. You can’t talk and listen at the same time.

Frequency Division Duplex (FDD): Devices separate their signals by assigning them different frequency bands one for transmitting and one for receiving. This is like having a phone with two separate channels.

Full duplex breaks this limitation. It aims to use the exact same time slot and the exact same frequency band for both transmitting and receiving, much like a natural human conversation where we can listen and speak simultaneously.

Fundamnetal & immense challenge in full duplex is “self-interference”. full-duplex hinges on the ability to perfectly cancel out this self interference. There are various methods used for this purpose which include antenna cancellation, analog rf cancellation and machine learning based digital cancellation.

Advantages of Full Duplex Communication

Following are some of the benefits of full duplex communication.

1. By using the same frequency resource for both uplink and downlink simultaneously, you can theoretically push twice the amount of data through the network, which is a massive benefit in a world of limited spectrum.
2. By eliminating the “waiting time” inherent in TDD (where a device must wait for its time slot to transmit), full duplex can significantly reduce the end to end communication delay. This is crucial for achieving the ultra-low latency goals of 6G required for applications like tactile internet and remote robotics.
3. Full duplex removes the need for paired spectrum bands required by FDD. This gives network operators more flexibility in how they deploy their spectrum assets.
4. It can improve the efficiency of network protocols that require rapid back and forth handshaking. It can also enhance physical layer security by allowing a device to sense for eavesdroppers while it is transmitting.

Disadvantages of Full Duplex Communication

Following are some of the challenges or limitations of Full Duplex Communication.

1. Extreme Self Interference : While SIC ( Self Interference Cancellation ) techniques have improved, achieving the near perfect cancellation required for commercial use is incredibly difficult.
2. The multi-stage SIC circuitry is complex and adds significant cost, size and power consumption to the radio transceivers.
3. Limited Dynamic range : Even with good cancellation, the massive power difference between the transmitted and received signals can still overwhelm the receiver’s electronics, limiting the effective communication range and performance.
4. Increased Inter user Interference: While full duplex solves self interference, it can create new and more complex forms of interference between users. In a network with multiple full duplex devices, a signal from one user’s transmitter can now interfere with the reception of another nearby user, requiring more sophisticated network-level interference management.

Conclusion: Full duplex communication stands as a groundbreaking advancement that could significantly boost 6G network capacity and flexibility. Yet, deploying FD at scale brings formidable challenges: self interference mitigation, cross link coordination, hardware isolation and multi-cell interference remain open issues. Only by overcoming these hurdles can full duplex realise its potential and become a mainstream pillar of 6G architecture.