5G Full Duplex

- preliminary details and information the 5G full duplex schemes for simultaneous transmission and reception on a single channel being developed for 5G mobile communications

One of the schemes being proposed for use with 5G mobile communications systems is that of single channel full duplex.

While existing mobile cellular communications systems currently use forms of full duplex, these use different frequencies for transmit and receive, FDD - frequency division duplex, or different times for transmission and reception, TDD - time division duplex.The 5G full duplex scheme being developed allows simultaneous transmission and reception on the same channel at the same time.

Existing duplex schemes

For mobile communications it is important to be able to communicate in both directions simultaneously, or virtually simultaneously.

In previous mobile phone systems, this has been accomplished using either FDD or TDD systems.

  • FDD - Frequency Division Duplex:   As the name implies, this form of full duplex, sometimes now referred to as half duplex requires the use of two channels - one for the uplink and the other for the downlink. This system requires the full channel bandwidth in two areas of the spectrum. In addition to this, complex filtering schemes are required to ensure that the transmitter signal does not enter the receiver and desensitize the receiver. Using filters this becomes increasingly difficult if multiple bands are used because multiple filters are required adding size, weight and cost. Additionally the more complicated filters introduce additional loss. Propagation characteristics between the transmit and receive bands will also be slightly different, especially with regard to fading.
  • TDD - Time Division Duplex:   This form of full duplex, sometimes now referred to as half duplex allocates time slots to the transmission and reception functions. The slots are relatively short and in this way transmission in both directions is virtually instantaneous, although some latency is obviously introduced, It also requires additional management and time is lost as switching between transmit and receive occurs.

What is 5G full duplex?

The concept being developed for 5G full duplex is to be able to transmit and receive simultaneous on the same channel.

This 5G full duplex offers several advantages:

  • Effectively doubles spectrum efficiency:   By employing a 5G full duplex scheme using a single channel, only one channel is needed to transmit data to and from the base station rather than two for an FDD scheme, or when using a TDD scheme the full transmission time can be utilised in both direction rather than half - the scheme effectively makes TDD schemes redundant. This effectively doubles the spectrum efficiency.
  • Fading characteristics:   As the same channel is used in both direction the fading / propagation characteristics will be the same. Difficulties can arise using an FDD scheme when one channel is affected by fading and the other less so.
  • Filtering:   FDD requires filters to be used to ensure that the transmitted signal did not enter the receiver and desensitize it. As more bands were added, more filters were required with a resulting increase in loss and drop in performance. By using single channel 5G full duplex, this issue can be overcome as techniques used have been shown to be capable of use over a wide bandwidth.
  • Novel relay solutions:   The techniques used for 5G full duplex on a single channel enable the simultaneous re-sue of spectrum in backhaul as well as the main user access can allow for almost instantaneous retransmission and high throughput mesh operation for heterogeneous networks.
  • Enhanced interference coordination:   The simultaneous reception of feedback information while transmitting data, possible using 5G full duplex reduce the air interface delays and provide much tighter time / phase synchronisation for techniques like Coordinated Multipoint, CoMP (which is also part of the 4G LTE standard).

Techniques for 5G duplex

The concept of full duplex using a single channel has been thought impossible for many years. The level of interference cause to the receiver by the transmitter has been thought to be far too great to make the concept of single frequency full duplex possible.

However many researchers have spent years investigating and developing the technology and now it seems likely that full duplex can be used as part of the 5G communications system.

There are two main technologies involved in 5G full duplex:

  • Electrical balance isolation:   The isolation technique employed effectively uses the same technology as used in landline telephones to provide isolation between the incoming and outgoing signals and this is obviously modified for RF. It can provide around 20dB or so of isolation.

    The scheme uses a hybrid that has four ports, between three of them the phase shift is 0°, but the fourth has 180°. Balance is achieved when a balancing impedance matches that of the antenna.
  • RF self-interference cancellation:   The main amount of reduction of the transmitted signal is provided by using RF cancellation techniques - often referred to as self-interference cancelation, SIC. Much investigation work has been ongoing to improve the performance and enable 5G full duplex in a single channel to be a realistic option.

Using a combination of both the electrical balance isolation and the SIC it is possible to operate a single channel full duplex system for 5G.

Single channel full duplex is an exciting technology that will bring many benefits in terms of operation, cost and efficiency to 5G mobile communications systems. Once fully developed, the implementation may be able to provide cost reductions when compared to the filters currently needed for FDD systems that are the most widespread for of duplex used.

By Ian Poole

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