Satellite communication of the future

Compared to its predecessors, the DVB-S2X standard is much more powerful, and offers improved flexibility and spectral efficiency. This means that each and every bit transmitted in this way costs less than it did before. The aim is an ambitious one: the new standard is intended to be 20 to 30 percent more efficient on average than its predecessor, and this should rise to as much as 50 percent for some applications. The new standard also makes communication less prone to error, and so paves the way for new applications. “With DVB-S2X, we can improve the flexibility and utilization of the satellites. This has an impact on reception at sea and in the air, and on reception via small satellite antennas,” says Rainer Wansch, head of RF and SatCom Systems department.

Numerous optimizations make this possible. One example concerns the receivers and transmitters on the ground. At the moment, they use many small transponders that can each transmit up to 36 megahertz (MHz). The problem is that not every signal uses the full 36 MHz. For example, if one user only needs 15 MHz, it will still block the entire transponder. Another user will not be able to get a signal through in case it would exceed the 36 MHz. “That’s why the new standard uses a wide transponder with a bandwidth of roughly 400 to 500 megahertz. This makes it possible to divide the data streams more efficiently,” says Wansch. In other words, users receive the exact amount of transmission capacity that they need in each case. As a result, the full bandwidth of a satellite channel can be used efficiently and signal distortions decrease.

The new standard offers numerous applications and configurations, and this makes it hard to implement. With that in mind, we have developed a special receiver and a testbed that allows us to simulate the complete wideband transmission chain for DVB-S2X, and to test devices thoroughly. The testbed covers every aspect – from wideband transmission, to signal modulation, to receiver signal processing. “Our testbed offers customers enormous added value. It allows us to simulate real transmission situations and conditions. That means we can test and optimize the entire transmission chain from transmitter to receiver under actual operating conditions,” says Wansch.

A new processor for the satellites

Aside from the transmission standard, the satellites themselves also have room for improvement when it comes to efficiency. The current models are highly configured, which means that they can perform the tasks for which they were built – for 15 years if possible. Still, over such a long period of time, there is a good chance that something will change, or even be revolutionized.

“WE’VE MADE SATELLITE COMMUNICATION FIT FOR MOBILE USE.”

Today’s satellites, however, can only respond to changes within certain limits; they are relatively “stupid.” They receive the signals, amplify them, and send them back. This is perfectly adequate for many applications, such as radio broadcasting. With other applications, satellite operators want more flexibility. After all, it is almost impossible to know today what demands a satellite will have to fulfil in 15 years’ time. And with new services, we can only roughly estimate what the utilization situation will be. In short, only a few satellites continue doing the job that they were designed for. At the moment, repurposing satellites is an extremely complex task – if it is even possible at all.

We now want to make satellites significantly more flexible, and are aiming to achieve this with the Fraunhofer On-Board Processor (FOBP), which we are developing in our laboratories. “You can usually only program the satellite-integrated processors once, and then they’re fixed for a single application. Our on-board processor, however, can be reprogrammed as many times as you like. It’s also very straightforward – you can do it from down on the ground,” says Wansch. In future, therefore, it will be easy to tailor satellites equipped with this type of flexible on-board processer (OBP) to other applications. The main achievement here lay in developing the necessary programs and the firmware and software architecture, and in making them usable in the harsh conditions encountered in space.

The signal that a satellite transmits using the OBP is much clearer than current signals. This is because conventional satellites also amplify the noise that occurs during signal transmission. The OBP, by contrast, does not simply amplify the signals it receives, but instead interprets them and then produces them again. The noise is therefore not amplified, and the satellite transmits only useful information. In Low Earth Orbit (LEO), for instance, the OBP is a key element in mega-constellations, as it can act as intermediary between the satellites. This is useful because although two-thirds of low-flying satellites are located above the sea, as the Earth is, after all, roughly two-thirds ocean. But these satellites do not have all that much to do because few signals are sent into space from the sea. Therefore, the processors in these satellites have the majority of their capacity free. Using the OBP and inter-satellite connections, the satellites could send “work” between each other and share the processing load. The users would not notice anything; there are no disadvantages to doing this. For satellite operators, however, this kind of collaboration offers enormous advantages. They would need fewer ground stations, which in turn would save them money. The FOBP is scheduled for completion at the end of 2017, when we will deliver it to the satellite manufacturer. The plan is for it to take off with the DLR Space Administration’s Heinrich Hertz mission in 2021.

Satellite communication even on a bumpy ride

Having an Internet connection on a plane or on board a ship is undoubtedly useful. A more fundamental role for satellite communication, however, concerns extreme events like tsunamis and earthquakes. Telephone lines and cellular networks are often no use to rescue teams after these kinds of natural disasters, as the lines are generally dead. Communication via satellite is therefore the only option left. However, this is also not without problems. If the links are overloaded, the connection fails – and this can happen even during a simple storm. It also takes time for the rescue teams to set up the necessary small satellite stations in among all the destruction. Another shortcoming is that the receiver must not make any fast movements. Instead, the satellite antenna has to be pointing directly at the satellite (as is the case with TVs). While the steady movement of aircraft and cruise ships means the links work well in those contexts, someone driving a car down a dirt road has almost no chance. Drivers have to stop if they need a connection.

Rescue teams should be able to communicate more easily in future thanks to the KASYMOSA project (Ka-band systems for mobile satellite communications) in which we together with a number of other partners developed an innovative communications system. Among those involved were Technische Universität Ilmenau, Industrieanlagen-Betriebsgesellschaft mbH Ottobrunn IABG, and the German Aerospace Center (DLR). “We’ve basically made satellite communication fit for mobile use,” says project manager Florian Raschke. “Our development eradicates the main disadvantage of satellite communication. The connection is also much more reliable: the bandwidth during transmission is so big that the connection won’t even cut out if it’s overloaded. We’ve also removed the inconvenient and time-consuming task of setting up a transmitting station.”

Let’s start by taking a look at the moving systems, and at how we overcame this shortcoming in satellite communication: When a car drives over potholes and around bends in the road, the antenna corrects its position in a fraction of a second. This happens with a high level of precision, as the antenna only ever moves 0.2 degrees out of the satellite’s focus. To put this in perspective, the car’s antenna would move more than that if a person got into the car. We achieved this using a special mechanical system and, above all, with algorithms that allow us to adjust the antenna precisely and quickly. The antenna itself is also the product of much development work. With diameters of 60 centimeters, conventional satellite dishes are too big to install on the roof of a car. This is why we use a flat satellite antenna, known as a panel antenna. Although panel antennas are already available on the market (they are installed in aircraft, for example), these models cannot withstand being driven over bumpy roads. “So our partners at Technische Universität Ilmenau developed their own flat antenna that meets the requirements very well,” says Raschke.

Another improvement lies in the reliability of the connections. We have modified the data processing so that the data rate changes according to the situation. So if the line is overloaded, the conversation doesn’t cut out entirely, as was previously the case. All that happens is the transmission quality decreases – in the same way as happens with Internet connections. We achieved this with a special modem developed as part of the project that transmits the data. “It enables us to reach very high data rates of several megabits per second,” says Raschke. “Of course, this is nowhere near the gigabit streams of a fixed data line, but it is a major step forward for satellite communications.” This means rescue teams will be able to send maps, and videos of the situation on the ground, secure in the knowledge that the connection will not cut out. They will also be able to make clear satellite calls without any interruptions and without being disconnected.

Rescue teams will also no longer have to set up transmitting stations. “With our system, every terminal – which comprises an antenna and a modem installed in the car – can transmit directly to the satellite. This means, for instance, that the systems in two separate cars can communicate directly with one another without having to first send their data to a central hub.” Not only does this make it easier to communicate, but it also increases data security. Consequently, as well as being suitable for use on water and in the air, satellite communication will soon also be available while driving down bumpy roads.