Increasing numbers of low earth orbit (LEO) satellites are being developed and deployed to address the challenge of providing ubiquitous broadband data coverage around the globe. These ‘mega constellations’ of new satellites will help to deliver the full potential of 5G and provide high-speed connectivity to remote parts of the world, including moving platforms such as aircraft and ships, as well as providing low latency connectivity for business users.

New opportunities and limitations

For the developers of satellite technologies and components, the proliferation of LEO satellite constellations presents new challenges, as well as exciting opportunities, not previously encountered with traditional Geostationary Equatorial Orbit (GEO) and Medium Earth Orbit (MEO) satellites.

Conventional geostationary satellites remain in their fixed orbits for 25 years or more. To achieve such lengthy operational lifetimes in space, all components must meet stringent requirements for reliability and radiation tolerance. The testing procedures are arduous and, where component reliability is in doubt, redundancy is achieved at the component level, by installing two or more identical sub-systems as a back-up.

Finding the right standard

For new LEO satellites used in mega constellations, redundancy is achieved at the satellite level, meaning operators launch replacement satellites to repair the constellation. These satellites have much shorter operational lives than their traditional geostationary counterparts, remaining operational for five to ten years. Nevertheless, there remains a need for the components installed on these satellites to offer high reliability, quality and performance in the high-radiation environment of space.

Using the stringent standards set for GEO and MEO satellite components would result in costly over-engineering for LEO satellite applications. So, to achieve the high quality and reliability levels required, LEO operators have tended to look to other demanding, high-performance applications – such as automotive – to set the standard for their satellite sub-systems. However, these general standards are not always fit for purpose, and there is a lack of suppliers with the necessary expertise at higher frequencies. These factors, combined with a lack of space heritage among suppliers, mean the LEO market requires a fresh approach if it is to meet growing demand for space-qualified RF components.

Meeting demand for more capacity

The volume of data being consumed worldwide is increasing apace, so there is an urgent need to increase satellite capabilities and network capacity. One example is through phased array antennas that make it possible to steer beams and target areas where extra capacity is needed, creating multiple beams at the same frequency. Care is required to avoid congestion and interference caused by overlapping beams, but in general it is possible to increase the capacity of satellites by reusing frequencies for different geographical areas.

Deploying digital processing capabilities within the satellite payload is another way to expand capacity. This enables data being brought to the satellite from users to be repackaged and consolidated on the satellite, creating extra capacity and allowing more efficient use of the frequency spectrum.

Exploring higher frequencies

However, as demand for data increases worldwide, the ultimate constraint on expansion will be RF capacity. Delivering the extra bandwidth needed will require expansion into higher frequencies, not currently widely used for earth-to-satellite communications. Ku and Ka bands offer around 2GHz of available bandwidth each, although there is some protected bandwidth within these bands. This creates additional obstacles when it comes to developing frequency plans and implementing hardware. What’s more, communications channels are becoming very congested at Ka and Ku bands, which are also shared with geostationary satellites. The orbits of any new LEO constellations therefore must be very carefully plotted to avoid interference with existing geostationary satellite transmissions.

Future satellite systems will move to Q and V bands, and indeed the International Telecommunication Union (ITU) has already approved these bands for use in forthcoming constellations. These higher frequency mmWave bands are currently little used for satellite communications and provide an important way to increase the capacity of the feeder links between satellites and the terrestrial network. Q and V bands each provide up to 5GHz of additional bandwidth, with a few excluded sub-bands. Looking further into the future, even higher frequencies will offer greater scope for expansion, with E band providing two wide-open 5GHz segments of contiguous bandwidth.

Challenges posed by mmWave

As frequency increases, so does atmospheric absorption, which, along with the difficulties in generating power in these bands, makes the links more sensitive to environmental conditions. However, the wider bandwidth available means that modulation levels can be reduced and output power increased to maintain the link, retaining data rates comparable with lower frequencies. The capacity gains thus make it an attractive option that’s worth investing in to secure long-term broadband connectivity via LEO satellites.

Currently, there are very few RF mmWave payload systems available with space heritage. That’s a limiting factor in the market, but also a huge opportunity for RF component manufacturers with the necessary expertise and track record in critical terrestrial applications. With new and well-resourced satellite operators joining the burgeoning market for LEO satellites, the demand for high-quality, space-compliant mmWave components is set to grow significantly.

UK expertise in mmWave components

High-reliability mmWave transceivers and SSPAs for communications and defence applications are already being designed and volume manufactured in the UK – and these are precisely the components needed for next-generation satellite products. Proven UK expertise in producing high reliability u-wave and mmWave modules for defence and communications systems can be applied directly to satellite applications. And because these devices have been rigorously tested and successfully deployed in terrestrial networks, satellite operators can be confident in their capabilities for non-terrestrial applications. The semiconductor processes already used in mmWave devices for terrestrial applications are inherently tolerant to radiation, so do not need further testing for space compatibility. The microprocessors and transistors used to provide power and control can be sensitive to radiation, but cost- effective radiation-tolerant alternatives are readily available.

Recently, Filtronic has designed high-capacity transceiver modules for use in high altitude pseudo-satellite (HAPS) systems, providing communications links of up to 40Gbps at E-Band. HAPS stations are unmanned aerial vehicles that provide moving 5G base stations, operating in the stratosphere at an altitude of around 20km to provide a ‘base station in the sky’. They play an important role in expanding connectivity around the globe and directly to LEO satellites at much higher altitudes where atmospheric absorption is very low. Filtronic RF technology provides solutions for both HAPS and LEO satellites, and the functionality of its terrestrial communication systems has been expanded to provide the environmental suitability for satellite applications. Filtronic is actively involved in supplying LEO demonstration hardware for both ground and payload applications.

Bold steps into new bandwidths

The solution to meeting the rapidly increasing demand for global broadband via LEO satellite constellations lies both in developing and manufacturing more commercially viable, high-performance RF components that can withstand the rigours of space, and in transitioning the bandwidths used for satellite communications into higher frequencies.

Since the u-Wave spectrum is limited and subject to many conflicting and overlapping demands, the development of feeder links operating in mmWave will be an important factor in the success of new satellite constellations.

While exploiting the possibilities of mmWave bands presents technological challenges, the experience we have in the UK of developing long-range terrestrial mmWave transceiver solutions with high data rates provides a solid foundation for further development to meet the demands of new LEO satellite systems.

Further reading:

Ground Segment Architectures for Large LEO Constellations with Feeder Links in EHF-bands, Iñigo del Portillo, Bruce Cameron, Edward Crawley. Massachusetts Institute of Technology, 2018 IEEE Aerospace Conference, March 2018.

Using E-band for Wideband SATCOM – Opportunities and Challenges. Sam Morrar Hughes Network Systems. Microwave Journal, August 2021.

Application of mmWave technology in High Altitude Pseudo Satellites (HAPS). Mike Geen, Filtronic. Microwave Journal, Feb 12th 2021.

The post Time to step up to mmWave to unlock potential of LEO satellites for global data connectivity  appeared first on Filtronic.

New TTA specification delivers greater consistency, audio quality and reliability for first-responder radio communications in North America

In 2018, one of the leading OEMs of critical communication equipment in North America changed its specification for the Tower Top Amplifiers (TTAs) installed in critical communications network base stations. This has now become their only specification of TTA available for new or replacement installations throughout the land mobile radio (LMR) network in North America. The rationale for the switch to this new specification was not widely publicised, and there remains limited understanding in the market about the benefits of the change.

Filtronic was instrumental in developing a TTA product to meet the new specification. Working closely with the OEM gave us a clear understanding of the thinking behind the new specification, the advantages it offers to radio system installers, and the improvements in LMR communications it brings to first responders.

In this paper, we outline the background to the specification change, the problems associated with the old system, the changes brought about by the new specification, and its benefits for public-safety communications in North America.

Background: Public-safety communications in North America

The current public-safety communications network in North America was established in the 1980s and early 1990s. The network of base stations aims to give 100% area coverage across North America, providing guaranteed communications connections for police and other emergency responders using land mobile radios in any location.

When the network was initially designed and implemented, it was based predominantly on analogue radio technologies, which were state-of-the-art at the time. This system proved invaluable, however, it was insecure, enabling new, easily obtainable radio scanners to pick up signals and listen in to police radio and other first-responder communications. Over the next decade or so, the radios were gradually updated to use digital modulation schemes and eventually the data was encrypted, making them secure and preventing casual eavesdropping. At the same time, channel bandwidths were reduced to enable more channels to be accommodated. More recently, the individual communication channel bandwidth is being halved again in order to double the number of channels – so the evolution continues.

However, while the radios themselves have been constantly upgraded, the radio frequency (RF) components and base stations that support the communications network have not really altered since the 1990s. This means that some components installed in base stations, such as power amplifiers, power combiners, filters, antennas and other RF conditioning products, have remained unchanged since their installation almost 30 years ago.

Technology: The role of Tower Top Amplifiers

One crucial component used in the majority of network base stations is the Tower Top Amplifier, which is used to improve receiver performance. TTAs incorporate a low noise amplifier installed at the top of the mast and a control/distribution unit installed at the base. The tower-top element incorporates a low-loss, bandpass filter to protect the receiver from out-of-band interference and a low noise amplifier to boost the received signal. The control/distribution unit at the base amplifies and splits the output signal to feed multiple different radio receivers. The tower top and base units are connected by a coaxial feeder cable running down the mast.

Over the years, many different companies have made the components for tower-top and base units, and there was some level of interoperability between them. But, over time, manufacturers opted to produce both the tower top and base elements together, meaning they had to be purchased as a pair which helped improve reliability and system performance.

An important feature of the original TTA specification was that the gain of the receive signal could be set to achieve the desired performance. That meant using attenuators to adjust the level of amplification and prevent the receiver from being overloaded. So, for example, in a dense urban environment where there is a lot of signal traffic, the gain can be reduced to prevent the receiver from picking up too many signals. This higher level of attenuation can also reduce system performance as well as restrict the range of the receiver. Conversely, for base stations in sparsely populated areas, the receiver needs to be far more sensitive so it can pick up signals from far away – so the attenuation would be reduced to achieve higher gain. At each base station, the gain would be set at the point of installation, according to the location of the site and its application.

Challenges: Complications caused by dual attenuators

The problem with the original specification for TTAs was that they featured two separate attenuators, providing two locations at which gain could be set. Both were housed in the control/distribution unit. The first ‘Reserve Gain Attenuator’ was located before the amplifier, and the second ‘Distribution Attenuator’ was located after the amplifier. As there are two locations for setting the gain, there are almost infinite possibilities for altering the ratio between them to achieve the same required level of overall gain. So, while the overall gain achieved might be the same, you would get different system performance depending on the balance between the settings of the two attenuators.  

The Reserve Gain Attenuator influences the sensitivity and noise figure of the system and its performance in the presence of high-power interfering signals. The Distribution Attenuator effects the system linearity. So, setting the first attenuator high and the second attenuator low, produces poor noise figure, poor range, but high immunity to interference. The other extreme is to have little or no attenuation at the input and all the attenuation after the distribution amplifier. In this case, you would achieve the same overall gain as the above scenario, but with very good noise figure and increased range, but greater susceptibility to interference.

Because there were multiple ways to set the two attenuators to achieve the same level of gain, different installers could set up sites differently. Each manufacturer provided guidance on how to set attenuation levels for different locations, with little constancy from one product to another, meaning it was up to individual installers to achieve the required gain levels for each site by setting the attenuators in whatever way they chose. This meant that once the base stations were operational, it was sometimes difficult to diagnose the cause of any underperformance, since the attenuators could have been set in many different ways. Any problems, such as audio drop-outs or poor call quality, were difficult to rectify without knowing how the two attenuators at each base station had been configured.

Solution: Delivering a new TTA specification

The potential problems caused by this uncertainty were one of the reasons why the leading OEM requested changes to the TTA specifications in 2018. The two significant changes made to the specification were:

1. To remove the second attenuator completely, and instead have a single adjustable attenuator located before the amplifier in the control/distribution unit. That gave installers a single way to adjust gain for the whole system.

2. To improve the low noise amplifier in the tower top by making it more linear, with better noise figure than previously required.

The new specification for Tower Top Amplifiers proved particularly challenging to achieve, and several RF suppliers attempted the task before a successful product was developed. Previously, TTAs had incorporated stand-alone filters, amplifiers and other connectorized components, which were cabled together in a waterproof housing. In the new product, all components are fully integrated into a single cast housing. As well as meeting all the performance standards stipulated by the OEM, this new product provides a lighter-weight solution in a smaller footprint, enabling better utilisation of space at the communication tower.

Benefits: Improved performance, control and reliability 

There are significant benefits of the new TTA specification for installers, end users and – ultimately – the general public. Fundamentally, each base station can now be set up very simply via a single control to optimise gain levels for the site, according to its location and the density of base stations in the region.

Having a single attenuator means there is only one point of adjustment to set the overall gain for the site. That immediately removes any ambiguity about how gain levels should be achieved. It makes setting gain levels far simpler and more consistent across the entire network. It means that if you’re in a remote rural area, you can simply set the attenuator to achieve a high system gain of 15dB, while in an urban area you set a low gain of 5dB. That eliminates idiosyncrasies in the way different installers set up gain levels at different base stations.

To compensate for the loss of an attenuator, the specification for the amplifier at the tower top has been significantly improved. This means there is no loss of performance, despite only having a single attenuator in the base unit. It gives you the best of both worlds, having the benefit of a tower-mounted amplifier with the gain effectively set to maximum, while providing the control simplicity of having a single attenuator to set the overall gain level for the site.

Outcome: Better communications for public safety

The new TTA specification supports better LMR performance for all emergency service providers. It gives radio system operators the confidence that their mission-critical networks will operate reliably with resilient connections and higher quality audio, especially in congested urban environments. Ultimately, that means greater peace of mind and security for the citizens who rely on these vital rapid-response services.

The post How upgrading tower top amplifier specification has improved first responder radio communications appeared first on Filtronic.