Whether in the field of battle, search-and-rescue or humanitarian aid efforts, the ability to share real-time, networked information between ground, sea and airborne forces is rapidly becoming the defining factor in a mission’s success. Russia and china have developed sophiticated Electronic warfare (EW) syatems through which they can jam satellite and other communications in higher frequecies. Advancements in HF technologies over the past two decades, especially automatic link establishment (ALE) and wideband HF (WBHF), improve the ease of use and capacity of HF communications.
High frequency communications have been used since the 1930s as a means to communicate beyond line of sight. HF operates in the ~1.8 MHz to 30 MHz frequency band by either reflecting off the ionosphere (called Skywave) or refracting off the surface of the earth (called Surface Wave). HF’s single greatest value is its ability to provide reliable short AND longrange Beyond Line Of Sight (BLOS) communications. It can support Point-Point and P-Multipoint data rates up to ~10kbps w/o relays . HF is generally available, rapidly and readily deployable – requires very little infrastructure and can be made extremely reliable.
The US has a large installed base of HF radios for ground, maritime, and airborne operations. For example, there are more than 4000 fixed and rotary wing airborne platforms with HF radios installed. The US uses HF to communicate directly from platform to platform, such as air-to-ship communications. The US also has fixed and deployable HF entry station infrastructure for connectivity from deployed platforms to the Global Grid.
Recent advances in HF radio and Digital Signal Processing (DSP) technology, along with new U.S. and international regulatory flexibility in spectrum allocation policies, have ushered in a new era for terrestrial-based, long-range communications capabilities. As a result, HF radio is now no longer limited to agonizingly slow 9,600 bps data transfer rates – slower than dial-up modems of the early 1990s. Today, modernized Wideband HF (WBHF) can deliver rates up to 240 kpbs on a 48 kHz wide channel.
Rockwell Collins demonstrated that its wideband high frequency (WBHF) communications capability can stream video and digital audio, conduct real-time chat, and transfer files across a 5,000-mile distance from the east coast of the United States to Hawaii. The demonstration, which was done in partnership with the Air Force Research Lab and the Air Combat Command (ACC), took place in June 2017 aboard a C-17 transport aircraft. The goal is to provide the capability to aircraft that have long-haul missions and where there are gaps in those missions.
Ron Broden, account manager for high frequency systems at Rockwell Collins, said it is mostly seen as a back-up system in today’s aircraft as most pilots use satellite links. Legacy systems use three kilohertz wide channels, while the wideband system goes up to 48. Wideband high frequency channels offer users four to seven times more capacity than the legacy high frequency channels,
“Whether it’s a real time conversation, streamed live video or the rapid transfer of large data files between an aviation platform, and support or command and control elements for a broad range of missions, this technology has the ability to deliver a true sovereign beyond line of sight communications capability for defence, and one that complements and hardens existing networks.”
Rockwell Collins’ modernized HF capabilities, coupled with the inherent anti-jam nature of the widely dispersed nodes in Automatic Link Establishment (ALE) based HF networks, create an ideal alternative to narrow-band SATCOM in Anti-Access/Area Denial (A2/AD) battlefield environments. “Being able to transfer secure data via the WBHF radio could provide greater operational resilience to the Australian Defence Force in the future, especially in satellite denied environments,” said Air Force’s Director of Plan Jericho Group Captain Carl Newman.
Rockwell Collins Asia-Pacific vice-president and managing director Jim Walker said: “WBHF technology is the only modernised HF solution that will deliver net-centric, high-speed communications at costs that are in line with today’s tighter military budgets.”
Since the ionosphere changes conditions, much like Earth’s weather, there are good days and bad days for using it as a means of communications. The experiment took place on average days — not the worst that a radio operator could encounter but not the best, either, he said.
HF has had its downside too, Dave Schreck, vice-president and general manager airborne solutions government systems for Rockwell Collins, acknowledged. For example, it could take time to establish a link. But today, that problem has been solved with Automatic Link Establishment (ALE) technology.
The brand new ALE capability, known as 4G ALE, makes HF linking easier, faster and more reliable than ever before, while taking full advantage of wider channels. Its 4G ALE adds the spectral sensing parameter, as well as other signal enhancing characteristics, to create a new protocol that not only automatically determines the optimal bandwidth but also links much faster than legacy 2G or 3G ALE. Spectral sensing ensures that the established link not only has the best signal, but also the maximum available bandwidth. And it does it all without any operator intervention.
WBHF technology applications
Eric E. Johnson Klipsch School of Electrical and Computer Engineering New Mexico State University discusses a few example applications in which the higher data rate of the wider-bandwidth waveforms could offer a qualitative improvement in mission performance.
Currently, most UAV video is sent via satellite or lineof-sight radio channels. WBHF offers the intriguing possibility of beyond line-of-sight communications to and from a UAV via HF radio. Of course, powering an HF transmitter and mounting an HF antenna on a UAV pose challenges. A link is established as soon as the aircraft comes within extended line of sight. At this extreme range, the path loss may be as much as 100 dB greater than when the aircraft passes near the ground transmitter, so the data rate must adapt to the varying SNR during the link.
A somewhat less challenging opportunity for delivering video over WBHF arises with ground forces in mountainous or dense urban terrain, where near-vertical incidence skywave (NVIS) is used to overcome the lack of line-of-sight between a video source and its users.
The authors consider their third example application, the one-to many communication involved in maintaining a common operating picture (COP) among vessels in a naval battle group. Propagation is via the (relatively benign) surface wave channel, but nodes in this HF LAN must share the channel. Often, a token passing channel access protocol is used.
High Frequency Communication
HF operates BLOS in the 2 to 30 MHz frequency band by either reflecting off the ionosphere (called Skywave) or refracting off the surface of the earth (called Surface Wave). Different frequencies will reflect off the ionized layers depending upon their height and ionization density which varies depending upon time of day and solar activity.
Thus, multiple HF frequency assignments are required to ensure 24/7/365 reliable communications. It is possible to predict which frequencies will be propagating based upon time of day and solar conditions as well as physical measurement of the ionosphere layers via sounding stations.
However, actual propagation performance for frequency channels will often be better or worse than expected. Thus, automatic protocols are required to determine which frequencies are propagating and adapt the waveform modulation and coding to actual channel conditions, write James A. Stevens and others from Rockwell Collins.
Automatic Link Establishment (ALE) protocols probe (sound) HF frequencies to identify good propagating channels
to setup a link. In the past, successful HF communications required skilled operators to know when to manually switch
frequencies to obtain or maintain good links to exchange voice or data. ALE enables radios to automatically determine which
links are propagating and use those links.
Single sideband (up to 3 kHz bandwidths) channel modulations support data rates up to 9.6 kbps and independent sideband (6 kHz bandwidth) channel modulations support data rates up to 19.2 kbps. The serial tone data waveform modulations feature an “autobaud” capability that enables the receiver to automatically adapt the transmitter’s data rate and interleaver configuration without operator intervention.
HF link layer protocols dynamically adapt data rate to channel conditions and improve reliability through acknowledgement and retransmissions. These link layer protocols support higher layer applications including email, chat, and file transfer.
New HF Wideband Technology for A2AD enironment
The lowest data rate modulation for each WBHF channel bandwidth operates with a Signal to Noise Ratio (SNR) of
about -9 dB for mid-latitude links. This provides data rates from 75 bps (3 kHz channel bandwidth) to 600 bps (24 kHz
channel bandwidth). The latter is sufficient for the lowest data rate Mixed Excitation Linear Prediction – Enhanced (MELPe)
vocoder and thus provides voice at significantly lower SNR than possible today, write James A. Stevens and others from Rockwell Collins.
The lowest data rate WBHF modulation is based upon Walsh codes that spread the transmitter power out in a wider
channel than the information actually requires. This means that a transmitter has additional transmit power reserve.
A WBHF solution leverages existing DoD worldwide HF infrastructure and platforms with installed HF. Only the HF
radios/modems would have to be upgraded to run the latest military standards, including ALE and WBHF. This means that airborne platforms with existing HF only have to upgrade their radios and could reuse their existing HF antennas without
having to make costly modifications to the outside of the aircraft , write James A. Stevens and others from Rockwell Collins.
WBHF, with its low SNR requirement, operates below the noise floor and can be used with lower transmit powers for lower detectability or with higher transmit powers for additional anti-jam margin.
Anti-Access and Area Denial (A2AD) occurs when an enemy gains the ability to disrupt command and control to such an extent that friendly forces cannot control their assets nor risk sending assets to that region. Example A2AD communications threats include: loss of communications through satellite denial, loss of communications through jamming, platform detection and location from communications transmissions, and loss of communications due to nuclear effects. Nuclear events disturb the ionosphere by increasing its electron density which increases the absorption of lower HF frequencies and the reflection of higher HF and VHF frequencies .
WBHF mitigates many of the A2AD environment threats, including jamming, detection, and satellite denial . The HFGCS system can be used in an A2AD scenario to provide reachback connectivity if satellite communications are denied. HFGCS can also be used to heal jammed LOS links in the A2AD area by allowing forward deployed nodes to communicate via relay through the HFGCS.
WBHF’s voice service can heal A2AD jammed voice nets like UHF SATCOM voice; WBHF’s IP data service can heal A2AD jammed networks like SHF SATCOM; and WBHF, combined with JREAP, can heal A2AD jammed Link-16, Link-11, and VMF tactical data links.
References and resources also include:
Wideband High Frequency (WBHF) for Anti-Access Area-Denial (A2AD) Environments; James A. Stevens PhD, Lizy Paul, Timothy E. Snodgrass, Randy W. Nelson Rockwell Collins Government Systems