Band on the Run

US Army Tactical Radio
The introduction of in-band full duplex techniques could help to reduce the spectrum congestion caused by tactical radios on the battlefield, while potentially enabling transceivers to perform addition, simultaneous missions such as electronic attack and electronic support.

In-band full duplex radio communications is a potentially game-changing technology enabling simultaneous voice and data transmission and reception, and electronic warfare applications, using the same frequency and transceiver.

PLATH’s annual intelligence workshop is always a useful opportunity to explore interesting innovations in the military communications and Electronic Warfare (EW) domains. This year’s event was held on 13th May in the town of Lillestrøm, southeast Norway. The workshop included a presentation by Professor Taneli Riihonen from Tampere University, southwest Finland. Prof. Riihonen’s presentation explained In-Band Full Duplex (IBFD) radio to delegates; an area of expertise for him and his colleagues.

IBFD Defined

Most tactical radios are half-duplex meaning that one person can transmit (Tx) but occupies a specific frequency, waveband or channel while they do so which means that the traffic can only move in one direction at a time. The receiver (Rx) must wait to obtain the traffic, be that voice or data, before they can transmit a response. Full duplex communications can move traffic in both directions simultaneously. Full duplex communications use one channel or frequency for Radio A to transmit to Radio B, and a different channel or frequency for Radio B to transmit to Radio A, for example. This approach means that a certain amount of radio bandwidth is needed to facilitate full duplex communications. Spectrum is an increasingly precious and finite commodity on and off the battlefield. Anything that can be done to reduce radio spectrum occupancy can only be beneficial.

Prof. Riihonen says that IBFD takes a different approach: The same frequency or channel can be used for the simultaneous transmission and reception of traffic. How is this achieved? The main challenge, Prof. Riihonen notes, is avoiding self-interference within the radio, primarily by preventing the Tx signal from leaking into the receiver. The problem is that the comparatively high power of the outgoing Tx signal risks ‘washing out’ the lower power incoming Rx signal. This phenomenon is akin to “trying to listen to someone whispering while someone is shouting at you.” Prof. Riihonen said that self-interference levels can reach up to 150 decibels in signal strength. The approach taken by Prof. Riihonen and his colleagues is hardware-specific, chiefly to isolate the Tx and Rx chains from one another using analogue and digital cancellers as one part of this approach. Resonators on the radio’s antennas also help reduce self-interference.

As well as offering benefits from a communications perspective, Prof. Riihonen highlighted that IBFD could be employed for EW. An Rx channel could receive traffic while a Tx channel transmits a jamming signal. Likewise, the Rx function could assist Signals Intelligence (SIGINT) gathering while the Tx function is sending traffic. This approach could put a combined EW and communications system using a single architecture and hardware into the hands of the operator. Several other capabilities were touted by Prof. Riihonen exploiting the IBFD approach including “two-way tactical radio links moving simultaneously between two or more networks, self-organising radio networks, real-time spectrum sensing, network jamming detection and uninhabited aerial vehicle swarm detection.”

Demonstrations

Prof. Riihonen said the IBFD technology has already been demonstrated to the North Atlantic Treaty Organisation’s Science and Technology Organisation. The demonstration saw IBFD radio architectures being used for simultaneous electronic support and electronic attack. The radios used frequencies of 225 megahertz/MHz to 400MHz and transmitted 100 watts of spot jamming power. Although the hardware in this demonstration was used to demonstrate EW acumen, Prof. Riihonen said that the same architectures can be employed for two-way communications.

Into the hands of the operator

Prof. Riihonen asserts that IBFD is now mature enough for civilian and commercial applications and expects the adoption of the technology from the early 2030s onwards. He anticipates that IBFD will be supporting the 3rd Generation Partnership Project (3GPP). 3GPP initiative brings together several standards organisations involved in cellular communications protocol development. Other applications include IBFD incorporation into the long-term evolution of fifth-generation (5G) cellular protocols, and IBFD’s inclusion as standard in future 6G cellular communications.

The technology could be made available to the military in a similar 2030 timeframe but will require the creation of a new generation of software-defined radios. Prof. Riihonen added that IBFD protocols can work comfortably with frequency hopping techniques. Nonetheless, tactical radios are increasingly adopting MIMO (Multiple-In/Multiple-Out) techniques. MIMO transmits traffic simultaneously across several different frequencies and wavebands. The goal of MIMO is to avoid signal disruption through phenomenon like multipath interference. Multipath interference can corrupt signals and the traffic they carry as signals collide with solid surfaces; a notable problem in built-up urban environments. Prof. Riihonen said that, while it can be difficult to get IBFD to work with MIMO protocols, it is not impossible. He also expects in-band full duplex techniques to be used as standard in future cognitive radio architectures. IBFD looks destined to equip both the civilian and military worlds in the coming years. “The academic research has been completed,” Prof. Riihonen says, “it is now time to transition the technology to a commercial environment in partnership with industry.”

by Dr. Thomas Withington