The critical role of spectrum superiority in the success of battlefield campaigns is evidenced by the enormous investments being made in electronic warfare (EW) capabilities by governments worldwide. Communication technologies, such as 5G, are quickly being adopted by militaries in an attempt to satisfy the demand for exponentially larger amounts of data transmission in a shorter period of time. As quickly as secure communication strategies are being developed to encrypt mission critical data, so too are the technologies used to detect, decode, and disrupt such communications. The security and integrity of critical communications is of the utmost importance as the world progresses towards an increasingly networked theater of operations.
The militaries of the world appear to be in widespread agreement that the critical communication infrastructure of tomorrow's battlefields need to be:
Rapidly deployable and reconfigurable for mission readiness.
Designed for minimal spectral footprint to minimize risk of detection.
Secure against spectral manipulation tactics and immune to remote disruption.
Ruggedized to survive harsh environment deployment, but small enough in form factor to enable maximum mobility.
Open-source and future-proof to enable the seamless integration of next-generation systems and technologies.
To compromise and sacrifice any one of these criteria in favor of utilizing legacy communications technology and infrastructure places the security of personnel, operations, and sensitive data at risk.
The Paradox of Modern Secure Communications
There are three major initiatives that are signaling a significant increase in spectrum congestion within next-generation operations:
Demand for real-time, 360-degree audio and visual situational awareness in the multi-domain networked battlefield.
The need for more autonomous combat vehicles, which is driving the integration of higher density EW payloads onboard next-generation airborne, naval, and armored vehicle platforms.
The proliferation of the use of remote-piloted and AI-enabled unmanned systems in Command, Control, Communications, Computers, Cyber-Defense and Combat Systems and Intelligence, Surveillance, and Reconnaissance (C6ISR) campaigns.
The combination of these three initiatives creates the following paradox: how can critical communications infrastructure achieve the volume, bandwidth, and range demands of the modern battlefield, while protecting transmit and receive elements from detection, manipulation, and destruction?
Legacy communications infrastructure typically consists of long runs of coaxial cable used to relay transmit/receive signals between antenna elements and data processing equipment. In addition to inducing a significant amount of signal loss, coaxial lines used in such infrastructure are not electrically isolated from the ambient environment. This creates an opportunity for enemy SIGINT/ELINT equipment to detect low-level radio signal leakage from the coaxial line and trace these signals to the source. This ultimately provides the location of the antenna and the command post where data processing is taking place. Once the antenna has been located, jamming, spoofing, or hard-kill measures can be taken by an enemy force to attack the antenna feed. Once a command post has been located, there is a clear and dire threat to personnel safety and operational capability.
Signal Processing at the Antenna
Several solutions exist today for reducing the spectral footprint and signal losses of remote antenna installations, when compared to traditional passive coaxial cable infrastructure. The most prominent of these solutions entails placing a signal processing capability at the antenna. Up-/down-conversion and digitization at the antenna enables range extension and signal security by placing transmit/receive signals onto ethernet connections between the antenna and the data processing center.
However, this strategy requires a great deal of expensive equipment, including thermal control infrastructure, to be placed at the antenna to ensure signal integrity is maintained while the analog signal is manipulated under dynamic operating conditions. For communication network installations linking multiple remote transmit and receive elements to a central data processing center, the cost of utilizing this signal processing strategy can become quite prohibitive. In addition, as the frequencies and bandwidths of future secure communications change, analog signal processing equipment and connectivity must keep pace.
The Case for RF-Over-Fiber
There exists a viable alternative solution for extending and securing remote communication links, while eliminating the need for expensive decentralized signal processing infrastructure. Radio Frequency-over-Fiber (RFoF) technology - the practice of converting radio signals into optical signals using laser diodes - is not a new concept. Since its inception over 30 years ago, RFoF technology has found widespread adoption in the commercial wireless communications industry. Seen as a solution for centralizing wireless network infrastructure and hedging against costly infrastructure obsolescence, due to a rapidly advancing communications industry, RFoF networks offer a range of benefits. The networks reduce system losses, provide immunity to electromagnetic interference (EMI), reduce system complexity and integration costs, and drastically extend the bandwidth-distance limit of a communication link.
Although adoption of RFoF by militaries has been slow so far, the time is right for systems engineers to reevaluate the potential for optical data links in next-generation secure communications infrastructure. Over the last several decades, the industry has seen significant advancements in the fabrication methodologies of glass fibers, optical connector design, and defense industry standardization of the manufacture, assembly, and verification of fiber optic componentry. This has led to an increased advocacy for optical network architecture within modern secure communications infrastructure deployed in extreme operating environments.
Denser, Lighter, Stronger Critical Communications
Further channel density is possible as optical fibers enable wavelength division multiplexing (WDM) applications. Numerous RF communication channels can be placed on a single optical fiber by converting each of the signals into different wavelengths of visible light, isolating the signals from one another as they are carried along the fiber (similar to how a prism works).
Fiber lines are extremely well-aligned with SWaP-C (Size, Weight, Power and Cost) initiatives as they are smaller, lighter, and enable greater transmission efficiency when compared to coaxial cable - without sacrificing harsh environment robustness. Expanded beam optical connectors enable more resilient optical connections, reducing the sensitivity of a link to contamination by dirt, dust, or damage in the field. Extraordinary cost savings are possible since optical fibers in a communication link do not need to be modified or replaced when communication parameters change, and RF-over-Fiber conversion eliminates the need for costly data processing at the antenna.
Traditional coaxial lines must be optimized for phase stability and signal attenuation at the transmit/receive frequency. Therefore, they must be replaced when operating frequencies shift or waveform integrity becomes too sensitive to losses in the data link. However, optical fibers are frequency agnostic, enabling rapid reconfigurability of an existing communications infrastructure to optimize the data link for new operating parameters.
Optical transmission offers a 99% reduction in signal attenuation per unit length when compared to large coaxial lines. This allows for the antenna to be placed further away from areas of spectral congestion or mission criticality. RF-to-optical signal conversion utilizes direct modulation, seamlessly capturing the properties of the analog signal when converting to the optical domain. Once in the optical domain, the signal can be transported miles away with near-lossless efficiency and complete EMI immunity. Furthermore, opting for a provider that can implement direct beam steering all-optical switching technology provides near-zero latency connections that eliminate amplitude distortion of the RFoF signals. This enables a nearly invisible switching process in the data link which is not possible with MEMS based switching technologies.
The Future of Secure Communications
Rapid advancements in the state of the art of laser diode design and photoelectric conversion technology drive the continued refinement of the RF-to-optical conversion process. This enables even the most phase-critical of RF communications systems to be reliably converted to the optical domain for range extension, data security, a future-ready system, and rapid reconfigurability of antenna elements for mission readiness.
The overall performance and cost potential of RFoF data links in secure communications topologies implores RF system engineers to re-evaluate how next-generation network infrastructure will be prepared for the future of the connected battlefield.
This article was written by Mike Teri, Business Development Manager, A+D Solutions, Huber+Suhner (Herisau, Switzerland). For more information, visit here .