Satellite communications are in trouble. How can we meet the increasing demand for bandwidth?

The need for high-bandwidth data access for commercial aircraft has also grown significantly for business jets and large passenger aircraft. New satellites supporting higher frequencies are being launched to enable this bandwidth growth. This article will examine these technology trends and the solutions that can achieve the required performance and reduce time to market through customizable architectures available in the market.

The need for ever-increasing data rates is driving many new developments in the SATCOM space. The data rate of SATCOM links will increase from kbps to Mbps, which will enable more efficient data and video transmission. The significant increase in drones has created a new arena for SATCOM links. Moreover, the growing demand for data and internet access in the commercial aerospace market is driving the development of Ku _- band and Ka _- band to support data rates up to 1000 Mbps. At the same time, supporting legacy data links, minimizing size, weight and power (SWaP) and reducing system development efforts are also driving the need to develop flexible architectures and maximize system reuse.

SATCOM systems typically utilize satellites in geostationary orbit (GEO)—satellites that are stationary relative to the Earth's surface. To achieve geostationary orbit, the satellite must be at a very high altitude—more than 30 km from the Earth's surface. The benefit of such a high orbit is that only a few satellites are needed to cover a large area of ​​ground, and it is simple to transmit data to the satellite because its fixed coordinates are known. Because these systems are expensive to launch, they are designed for a long service life and are very stable, but can also be a bit outdated at times.

Due to the high altitude and the presence of radiation, additional equipment shielding or satellite shielding is often required. Also, because the satellite is so far away, users on the ground may experience significant signal loss, which also affects signal chain design and component selection. The long distance from the ground to the satellite also causes high latency between the user and the satellite, which can affect some data and communication links.

Recently, many alternative or complementary systems to GEO satellites have been proposed, with UAVs and low Earth orbit (LEO) satellites also being considered. These systems reduce the challenges of GEO-based systems by using low orbits, but will compromise coverage, requiring more satellites or UAVs to achieve similar global coverage.

Airplane and commercial jet passengers need to be connected to the Internet as they travel around the world. Airlines are seeking to increase data links in the cockpit, and enabling IoT system monitoring and reporting requires high data rate SATCOM platforms with hundreds or even thousands of Mbps data links.

Until now, such high-bandwidth data links have been provided primarily when the aircraft is on the ground, using a ground-mounted system to connect to the aircraft. If transcontinental coverage is to be achieved, SATCOM is the only effective method to achieve connectivity, such as Inmarsat’s L-band coverage. In the future, to achieve the required bandwidth, operating frequencies must move to the Ku-band or Ka-band. These higher frequencies provide the required bandwidth, but there are still many design challenges, and the system must support legacy data links.

Inmarsat is addressing some of the challenges mentioned above by offering users the ability to use GEO satellites with Ka-band data links. From an architectural perspective, this provides a solution to the bandwidth shortage problem, but it also introduces some new challenges for design engineers. Figure 1 depicts a typical superheterodyne receive and transmit signal chain operating in the Ka _and Ku _bands . These systems often require two, sometimes even three, analog up- and down-conversion stages, each requiring a synthesizer, amplification, and filtering systems that increase the system SWaP. However, it is not possible to fit within the existing aircraft architecture and power distribution system that includes such a signal chain for all possible data links.

While this is obviously a simplified schematic, by assuming that each function is implemented using discrete components, the SWaP implications are clear. The high component count, high power consumption, and isolation challenges mean that the printed circuit board (PCB) will be very large. And due to the high frequency routing, more RF-appropriate PCB material may be required, which will significantly affect the cost. In addition to the need to continue to support operating frequencies in the L-band, the SWaP and design effort challenges are also complicated.

LEO satellites may offer some relief. These satellites operate at much lower altitudes—about 1 km above the Earth’s surface—but at these altitudes, they are not stationary but rather quickly pass over the Earth’s surface, with one orbital period of about 30 minutes. Low altitudes reduce launch costs and require less shielding and protection because the environment is less harsh. Crucially, low altitudes also mean less propagation delays. But the main difficulty with LEO systems is that the satellites are within range of users for relatively short periods of time, necessitating the use of a transmission system.

Drones may also be a solution to this problem, and some platforms may be considered as a means to extend Internet coverage. Drones can provide low latency, high bandwidth links, similar to LEO, but now with the advantage of being relatively stationary. However, the cost and coverage of this approach are challenging for global adoption.

While the SATCOM challenges outlined above may seem daunting, there are many new and advanced solutions that address these challenges, reduce SWaP, or provide signal architectures that can be partially reused or used between systems.

For high-bandwidth UHF SATCOMs such as MUOS, new continuous-time Σ-Δ (CTSD) bandpass analog-to-digital converters (ADCs) provide RF sampling solutions. For example, the AD6676 is an IF receiver subsystem that integrates an ADC, analog gain control (AGC), and digital down-conversion. The CTSD ADC can trade bandwidth for noise floor, providing system flexibility and an inherent bandpass filter response, thereby reducing external filtering requirements. Because the AD6676 can directly sample the MUOS downlink, the front-end mixing stage and synthesizer are eliminated, and the signal chain is reduced to a low noise amplifier and a simple passive filter.

However, because MUOS uses full-duplex mode, the power consumption of the power amplifier (PA) also becomes critical. Handheld SATCOM radios need to transmit at power levels between 1 W and 10 W. New gallium nitride (GaN) amplifier devices, such as the HMC1099, can provide higher power efficiency, and when combined with other linearization techniques such as digital pre-distortion (DPD), they can provide an attractive SWaP solution for these systems.

For Ku _- band and Ka _- band systems, new, more integrated architectures provide SWaP and signal chain simplification, as well as capabilities that enable important system reuse between L-band and Ka-band. Figure 3 illustrates the power savings that the AD9361 RF transceiver can achieve when used as an IF converter, eliminating two up- and down-conversion stages, amplifiers and filters, as well as the ADC and DAC.

The RF transceiver is typically used as a flexible direct conversion radio, which enables it to be used as part of an L-band solution. When used in this manner, it provides significant commonality across these platforms and maximizes software and firmware reuse. Overall SWaP is also reduced, consuming only 1.1 W of power in most applications and being able to be packaged in a 10 mm × 10 mm space.

In addition, new PLL and VCO devices, such as the ADF5355, provide ultra-wideband, high-performance, low SWaP frequency sources. The ADF5355, in a 5 mm × 5 mm package, provides low power, high-performance LO sources that can sweep from VHF all the way up to 13.6 GHz—an ideal solution for common platform designs.

Finally, for future LEO systems, beam steering architectures are critical to ensuring the efficiency of the link. While analog beamforming solutions using digital phase shifters such as the HMC247 provide solutions today, digital beamforming becomes a very attractive architecture as converter technology becomes more integrated and enhanced signal processing becomes more accessible in low-power devices. In this approach, the RF signal chain remains the same across the array, and the beams are formed in the digital domain. The main difficulty with digital beam steering is managing the size, timing, and power of multiple ADC or DAC devices. Any timing or processing skew between devices will affect the quality of the beam. New devices such as the AD9681 can greatly simplify digital beam steering design. Using a common voltage reference and clock source for all eight ADCs improves beam quality, while integrated devices reduce package size and power consumption.

SATCOM has played an increasingly important role in commercial communications and data systems in recent decades. However, the world’s growing demand for bandwidth creates new challenges for future aerospace SATCOM designs, requiring new architectures and system designs. Whether the goal is to fit into a smaller drone payload or provide internet on the next flight, the SWaP of SATCOM radios will become increasingly important. New high linearity IF subsystems, multi-channel high resolution ADCs, integrated RF transceivers, and VCO and PLL combinations will deliver low SWaP solutions to next generation SATCOM radios