The Bandwidth Demand of Satellite Communication Brings New Pressure to the Design

Over the past two decades, the field of commercial aviation has been relying on satellite communication to coordinate the travel of civil aviation passengers. With the growth of data traffic and Internet of things (lot) applications, the demand for satellite communication systems has reached its peak.

For commercial jets and large passenger aircraft, the demand for high bandwidth data access of commercial aircraft has also increased significantly. We have launched new satellites supporting higher frequencies to achieve this bandwidth growth. This article examines these technology trends and solutions that can achieve the required performance and reduce time to market through customizable architectures available in the market.

Satcom introduction and history

The demand for increasing data rate is driving many new developments in the field of Satcom. The data rate of Satcom link will be increased from Kbps to Mbps, which will realize more efficient data and video transmission. The significant increase of UAVs has created a new stage for Satcom link. Moreover, the growing demand for data and Internet access in the commercial aerospace market is driving the development of Ku and Ka bands to support data rates up to 1000 Mbps. At the same time, supporting traditional data links, minimizing size, weight and power consumption (SWAP) and reducing system development investment are also driving the demand for developing flexible architecture and maximizing system reuse.

Satcom systems usually use geostationary orbit (GEO) satellites - satellites that are stationary relative to the earth's surface. To achieve geostationary orbit, satellites must have a very high altitude - more than 30 km from the earth's surface. The advantage of such a high orbit is that only a few satellites are required to cover a large area of the ground, and because its fixed coordinates are known, it is easier to transmit data to satellites. Due to the high launch cost of these systems, they are designed for long service life, very stable, but sometimes a little outdated.

Due to high altitude and radiation, additional equipment shielding or satellite shielding measures are often required. Moreover, because the satellite is too far away, users on the ground may have significant signal loss, which will also affect the signal chain design and component selection. The long distance from the ground to the satellite will also cause high delay between the user and the satellite, which will affect some data and communication links.

Recently, many alternatives or supplementary systems for Geo satellites have been proposed, and unmanned aerial vehicles and low earth orbit (LEO) satellites are also under consideration. With the help of low orbit, these systems can reduce the challenges of geo based systems, but will affect the coverage, and more satellites or unmanned aerial vehicles are required to achieve similar global coverage.

Commercial aviation

Aircraft and commercial jet passengers need to be connected to the Internet when traveling around the world. Airlines are striving to increase the data link in the cockpit, while the realization of lot system monitoring and reporting requires a high data rate Satcom platform with hundreds or even thousands of Mbps data links.

So far, this high bandwidth data link is mainly provided when the aircraft lands, and a system installed on the ground is used to connect with the aircraft. Satcom is the only effective way to realize cross continental coverage, such as L-band coverage of international maritime communication satellite. In the future, in order to achieve the required bandwidth, the operating frequency must be moved to Ku band or Ka band. These high frequencies can provide the required bandwidth, but there are still many design challenges, and the system must support traditional data links.

Ku Band / Ka band and Leo system

INMARSAT is providing users with the ability to use geo satellites with Ka band data links to meet some of the challenges mentioned above. From the perspective of architecture, this provides a solution to the problem of insufficient bandwidth, but it also introduces some new challenges to design engineers. Fig. 1 depicts a typical superheterodyne receive and transmit signal chain operating in Ka band and Ku band. These systems often need two analog up conversion and down conversion stages, sometimes even three. Each stage requires a synthesizer, an amplification system and a filter system to increase the system swap. However, it is unlikely to achieve matching within existing aircraft architectures and power distribution systems that contain such signal links for all possible data links.

Figure 1. Traditional Ka Band / Ku band superheterodyne receiving and transmitting signal chain

Although this is obviously a simplified schematic diagram, the meaning of swap is clear by assuming that each function is implemented using separate elements. The large number of components, high power consumption and many isolation problems mean that the printed circuit board (PCB) will be very large. Moreover, due to high-frequency wiring, more RF appropriate PCB materials may be required, which will significantly affect the cost. In addition to the need to continue to support the working frequency of L-band, the problems of swap and design are also very complex.

LEO satellite may relieve some pressure. Such satellites operate at much lower altitudes - about 1 km from the earth's surface - but at this altitude, they are not stationary, but quickly sweep over the earth's surface, with an orbital period of about 30 minutes. Low altitude can reduce the launch cost, and because the environment is not so bad, it needs less shielding and protection. Most importantly, low altitude also means less propagation delay. However, the main difficulty of Leo system is that the time of satellite in the user range is quite short, and the transmission system must be used.

UAV may also be a solution to this problem, and some platforms can also be regarded as a means to expand Internet coverage. UAV can provide low latency and high bandwidth links, similar to Leo, but now it also has the advantage of relatively static. However, the cost and coverage of this scheme are challenging for global applications.

Solve Satcom dilemma

Although the satcom challenges described above seem very difficult, there are many new advanced solutions to address these challenges, reduce swap, or provide a signal architecture that can be partially reused or used between systems.

For high bandwidth UHF Satcom such as MUOS, new continuous time Σ-Δ CTSD bandpass analog-to-digital converter (ADC) can provide RF sampling solution. For example, ad6676 is an IF receiver subsystem integrating ADC, analog gain control (AGC) and digital down conversion. The CTSD ADC can use the noise floor switching bandwidth to provide system flexibility and inherent band-pass filtering response, so as to reduce the external filtering requirements. Since the ad6676 can directly collect MUOS downlink lines, 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.

Figure 2. Ad6676 receiver subsystem architecture

However, because MUOS adopts full duplex mode, the power consumption of power amplifier (PA) also becomes very important. Handheld Satcom radios require power level transmission between 1 W and 10 W. new gallium nitride (GAN) amplifier devices, such as hmc1099, can provide higher power efficiency. Combined with other linearization technologies such as digital predistortion (DPD), they can provide attractive swap solutions for these systems.

For Ku band and Ka band systems, the new and more integrated architecture provides the functions of swap and signal chain simplification, and supports the reuse of important systems between L band and Ka band. Fig. 3 depicts the power savings of the ad9361 RF transceiver when used as an if converter, eliminating two up and down conversion stages, amplifiers and filters, as well as ADC and DAC.

Figure 3. Ka Band / Ku band receiving and transmitting signal chain based on integrated if receiver

RF transceivers are often used as a flexible direct frequency conversion radio, which enables them to be used as part of an L-band solution. When used in this way, it can provide obvious commonalities in these platforms and maximize the reuse rate of software and firmware. The total swap is also reduced, consuming only 1.1 w in most applications and can be encapsulated in 10 mm × In a space of 10 mm.

In addition, new PLL and VCO devices, such as adf5355, can provide ultra wideband, high performance and low swap frequency sources. 5 mm for adf5355 × The 5 mm package provides low-power, high-performance Lo sources that can scan from VHF to 13.6 GHz - an ideal solution for public platform design.

Finally, for the future Leo system, the beam control architecture is very important to ensure the efficiency of the link. Although analog beamforming solutions using digital phase shifters such as hmc247 can provide today's solutions, digital beamforming has become a very attractive architecture as converter technology becomes more and more integrated and enhanced signal processing becomes easier to use in low-power devices. In this method, the RF signal chain remains the same throughout the array, and the beam is formed in the digital domain. The main difficulty of digital beam control is to manage the size, timing and power of multiple ADC or DAC devices. Any time or processing deviation between devices will affect the beam quality. New equipment such as ad9681 can greatly simplify the design of digital beam control. Making all eight ADCs use the same voltage reference and clock source can improve beam quality, while integrated devices can reduce package size and power consumption.

summary

In recent decades, Satcom has played a more and more important role in commercial and military communication and data systems. However, the increasing global demand for bandwidth has created new challenges for the future aerospace and defense Satcom design, as well as new architecture and system design. Whether the goal is to extend the battery life of soldiers, match with smaller UAV loads, or provide the Internet in the next flight, the swap of Satcom radio will become more and more important. The new high linearity if subsystem, multi-channel high-resolution ADC, integrated RF transceiver and VCO and PLL combination will provide a low swap solution for the next generation Satcom radio.

The Bandwidth Demand of Satellite Communication Brings New Pressure to the Design 1

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