Why RFoF means wired communication still has a place in military applications

This article originally appeared in the Sept'23 magazine issue of Electronic Specifier Design – see ES's Magazine Archives for more featured publications.

Communication is the backbone of the military. Yet often the vehicles of communication are subject to incredibly difficult environments that put it at risk. Military communication is often governed by protocol and procedure, like the US Department of Defence’s Common Data Link – a secure communications protocol for transporting signals intelligence and imagery – but even in this, the method of transmission is less strictly governed. Therefore, antenna and wireless wave technology are still widely used. Yet large issues exist within these modes, which can drastically affect operational efficiency. But can anything be done to increase the integrity of military communications?

“As electronic warfare becomes a bigger concern in the military, fibre will be used more frequently beyond traditional antenna use cases because of its immunity to electromagnetic interference, low attenuation, and ease of deployment,” says Meir Bartur, Co-Founder and CEO of Optical Zonu.

The fibre Bartur is referring to is Radio Frequency over Fibre (RFoF). RFoF is an analog optoelectronic transmission technology where radio frequency is converted into light waves for transport and converted back again to original native radio frequency format at its destination. It is an ideal technology to transport high frequencies, which are becoming necessary for all data-intensive industries that require more bandwidth and low-latency.

This is preferential for the military in circumstances where continuous communication is key as traditional antennas can inadvertently create electromagnetic interference due to the way they transmit and receive signals. Malicious actors could exploit this interference by intentionally designing or configuring antennas to emit signals that disrupt or jam the communications of nearby devices. This could be used to interrupt wireless networks, interfere with GPS signals, or disrupt radio communications. Yet RFoF technology travels through cables and is not a wireless technology.

Currently, many militaries across the world use a combination of communication methods. The US Navy largely uses waveguides for its USV to transport Ku-Band CDL communications payloads between the radomes mounted high above the decks and equipment rooms situated within the hulls. Waveguides are metal tubes functioning as conduits for carrying electromagnetic waves. Yet, due to this wireless method of carrying signals, they are practical only for signals of extremely high frequency, where the signal wavelength approaches the cross-sectional dimensions of the waveguide.

But, despite the mixture of modes, Bartur ponders if the wrong technology is being applied in the wrong situation. “The disadvantage of waveguides is they are rigid and radio frequency performance is highly susceptible to any mechanical stress like shock or vibration. The waveguide joints are fragile and must be kept extremely clean and dry. The slightest amount of moisture or the smallest contamination of dirt or salt degrades the transmission of waveforms,” explains Bartur. "In order to keep moisture out, waveguides are usually filled with dried air or purified nitrogen, but any corrosion or oxidation of the waveguide inner metal surface will degrade RF performance. However, even with this precaution in place, waveguides are too unreliable for the critical communication requirements of the modern military.”

Fibre optics on the other hand is so thin that it's flexible. Fibre cables are also constructed with protection layers, stiffening and stretch elimination measures. Fibre is also more resilient to environmental factors than coaxial cables, another instrument of communication transfer used by the military for things like radar systems and command and control centres, due to their inherent physical and material properties. They are immune to EMI and RFI, and the light signals in fibre experience significantly less attenuation (loss of signal strength), less than 0.5dB per km, which is orders of magnitude better then coaxial cables. Fibre can also support much higher bandwidth and data rates since light frequency is so high that it can carry more information than electrical signals, allowing for greater data capacity.

Yet, despite its promises and improvements, RFoF still suffers from some problems. “As a replacement for cable/waveguide, RFoF is power-intensive as it often requires power sources for both the RF and optical components, which can in some cases be problematic,” says Bartur. “The inherently poor noise figure of the RFoF physical media transport, the conversion of RF signal to light (via direct modulation laser or external modulator) and back to RF must be compensated by mission-specific electronics that condition the signal via additional electronic stages. Additionally, troubleshooting and maintenance of RFoF systems can be challenging due to the technology's hybrid nature.”

Yet despite the drawbacks, the applications show how cabled communication still has a place in the military alongside, and even over, wireless solutions, and Bartur believes its place will grow. “The military is gravitating towards higher frequency ranges for communication, like mmWave, that are characterised by high throughput and low latency but are also easily obstructed by man-made and natural elements. This makes traditional signal transmission, such as coaxial cables and waveguides, increasingly cumbersome and less efficient.”