22 December 2021
For those unfamiliar with the applications of RF-over-Fibre in satellite communications, it may seem a contradiction that satcom would depend on fibre, the very terrestrial technology that satellite communications is often purported to replace.
However, in the past few years, something has happened in the world of satcom. With the advent of High Throughput Satellites (HTS), satellite bandwidth capacity has grown exponentially. Satellite frequencies have moved from C-band to higher frequencies such as Ku and KA-band. These higher frequencies usually require the implementation of diversity gateway sites to address rain fade. In some countries, regulatory authorities require commercial satcom operators to build backup gateway sites to improve network resiliency in case of natural disasters.
All of these satcom trends have arrived in the SE Asian region, and have put a strain on the ground interconnectivity infrastructure of satellite gateways. Traditionally, coaxial cable has been the mainstay for connection of baseband satcom equipment to the RF components. However, as RF capacity, RF frequency, and distances between RF and baseband equipment increases, the signal loss inherent in coax cable becomes problematic.
Enter RF over Fibre. The Transmission of RF signals over Fibre (RoF) is an established technology, and the continued fall in the price of electro-optical converters over the last ten years has led to RoF becoming a competitive transmission platform.
As a market leader in RoF solutions for satcom, broadcast and other areas, ViaLite Communications has seen continued growth in the Southeast Asian region for the reasons mentioned above. However, the design and implementation of these solutions has also evolved, and this has benefited our – ViaLite’s - customers in the region.
A decade ago, RF over Fibre was used to connect gateway baseband equipment (operating at L-band or 70/140 MHF IF frequencies) to the BUC/LNB on large gateway “hub” antennas that were located more than 100 meters from the equipment room. Coax cable is typically limited to a 100 meter distance in this application.
Today, there is also a need for Long Distance links that connect the baseband equipment to both a local gateway Radio Frequency Transmitter (RFT) and a diversity or backup RFT site. These remote gateway RFTs are typically 30-100 km away, but depending on how the fibre is physically run, the actual distance may be more. During rain fade or hardware failure at the main gateway, the RF signal may need to be seamlessly switched between the local RFT and the remote RFT with no disruption to user traffic.
ViaLite uses DWDM (Dense Wavelength Division Multiplexing) and CWDM (Coarse Wavelength Division Multiplexing) methods to transport L-band RF signals between baseband equipment and remote gateways at distances up to 600 km. DWDM and CWDM allows many RF frequencies (satellite carriers) to be multiplexed and transported over a single fibre or fibre pair.
Proper link design is critical when implementing a long distance link, as amplifiers and boosters are typically employed. If a design is incorrect, the link may not close. ViaLite has created a tool to address this concern.
When satcom engineers design a satellite link, they utilise link budget tools to take into account link losses, gain, power, noise, availability and other factors to properly design a reliable satellite link.
In the same way, ViaLite has made available to its customers a link budget tool for designing Long Distance RoF links. Called the System Designer tool, it is a cloud-based link budget tool used by ViaLite engineers to confirm the performance of Long Distance Link designs. It is also available for our customers to use and “tweak” the inputs, as performance details of the actual fibre become available or change over time.
This is a great time saver for our customers, as they can confirm changes to the design as well as the link reliability by themselves. A proper link budget is especially critical when DWDM or CWDM technology is utilised for Long Distance links.
We have noted varying degrees of the quality of dark fibre in some markets in the region. A poorly performing 30 km fibre run may have the same end-to-end losses (dB/km) as a well performing 100 km fibre run. This makes the need for a link budget tool to confirm a long distance DWDM/CWDM link design even more critical. If a Long Distance link is designed improperly, the link may not close, resulting in additional labour and equipment costs to replace the supplied optical amplifiers and other equipment with higher power equipment.
Remote access and support
Over the past two years, the Covid-19 pandemic has impacted the installation and support capabilities of many communications systems in the SE Asian region. Not only have ViaLite engineers been unable to travel to other countries to install equipment, but similarly, the in-country engineers of our customers often had difficulty accessing the satcom gateway site to install and commission the ViaLite equipment. ViaLite addressed this shortcoming with our remote access capability. Using PC-based remote access tools such as Teamviewer and the M&C capability of the ViaLite equipment, ViaLite engineers in the UK have been able to remotely configure, test and finalize customer acceptance of RoF equipment in countries such as Indonesia and Hong Kong.
When troubleshooting was required, our engineers were able to quickly access the M&C system of the RoF equipment to assist our customers. Of course, following training on these remote access tools, many of our customers use this report access capability by themselves as well.
Several LEO (Low Earth Orbit) satellite networks are planned to be on-station and covering portions of SE Asia within the next two years. From the RF-over-Fibre perspective, this represents increased activity for companies like ViaLite. Unlike a typical GEO earth station, which may have one or two RFTs, a LEO gateway may have a dozen or more antennas that need to be connected together to allow the tracking and handoff between the multiple LEO spacecraft as they move across the sky.
In addition, a country may need several gateways (Starlink says it will need thousands of gateway antennas for its LEO constellation). ViaLite has already provided this type of gateway connectivity in North America and we foresee similar requirements across the APAC region.
The Southeast Asian region will always be an active region for satcom and satellite broadcast, due to the prevalence of islands and dispersed rural areas across a large geographic area. Even landlocked countries such as Thailand and Vietnam, which have a significant installed base of terrestrial fibre will always have some requirements for satellite communications.
Broadcasters continue to have requirements for connecting multiple Television Receive Only (TVRO) antennas to broadcast studios – along with satcom, this has always been a sweet spot for ViaLite.
ViaLite is addressing the RF-over-Fibre aspect of these requirements, as well as other areas including broadcast, GPS/GNSS extension and military RoF solutions. We see an exciting future in the SE Asian region.