Addressing Low Earth Orbit (LEO) Satellite Communications System Design Challenges
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Addressing Low Earth Orbit (LEO) Satellite Communications System Design Challenges

Posted Date: 2024-01-20

Author: Mike McLernon, Chief Technical Marketing Engineer, MathWorks

Interest and investment in commercial space satellite systems are growing. Private investors have poured more than $23.5 billion in private sector funding into space-related companies since 2021, with tech giants such as SpaceX and Amazon (Kuiper) launching space programs to increase global broadband access. Satellite communications have long been used for voice communications, defense and space exploration; however, the introduction and popularity of low-Earth orbit (LEO) satellites has lowered the financial threshold for launching satellites and provided opportunities for new use cases. This economic benefit is due to two factors: 1. The size of the satellites - SpaceX's latest Starlink LEO satellite is only the size of a dining table; 2. Multiple LEO satellites can be launched simultaneously. While LEOs make satellite communications systems more economically viable, they also introduce complexities that require engineers to deal with higher Doppler shifts, interference, and network complexity.

The introduction and popularity of low-Earth orbit (LEO) satellites has lowered the financial threshold for launching satellites and provided opportunities for new use cases.

Trends Driving Adoption of Satellite Communications Systems

Ubiquitous connectivity—an environment where devices can create, share, and process data virtually anywhere—is one of the key trends driving LEO adoption. Although the world has made significant progress in building terrestrial wireless communications infrastructure, a considerable number of areas, such as remote rural and maritime areas, still lack cellular connectivity due to cost or geography. Satellite is a key enabling technology for the wireless industry to close the urban-rural connectivity gap.

LEO not only provides accessibility to cellular connectivity, but also increases the capacity of cellular connectivity. Please refer to the following market data from Statista: There are currently 4.6 billion smartphone users worldwide. It is estimated that by 2030, the number of connected devices worldwide will reach over 29 billion. More and more people are using the Internet, which is increasing the demand for cellular systems around the world. Wireless companies are still investing in ground infrastructure because using commercial satellites is not always economical; however, the cost of LEO satellites has been decreasing, making them a viable option to address increasingly limited bandwidth issues, especially in more remote areas. in this way.

Finally, as extreme weather events become increasingly severe and frequent, disaster recovery communications have become the main trend driving the application of satellite communications. Cellular infrastructure is often disrupted during these events, prompting satellites to be activated to ensure first responders, government officials and residents can broadcast and receive critical safety information. A strong demonstration of this type of use case is that Starlink positioned 120 satellites to cover Southwest Florida and other affected areas after terrestrial cellular infrastructure was devastated by Hurricane Ian.

Signal delay and power amplification

Before the advent of LEO satellites, satellite communications systems primarily used geostationary orbit (GEO) satellites. If three GEO satellites are properly spaced in longitude and rotate at the speed of the Earth's rotation, they can provide nearly global coverage. Three GEO satellites can cover the Earth with just a few cross-links, but unfortunately, their construction and launch costs are much higher than LEO satellites. Additionally, the distance of GEO satellites from the ground and from each other causes signal delays. While GEO satellites are great for email and other non-real-time communications, voice and video calls suffer from significant delays, preventing natural communication.

LEO satellites are closer to the Earth's surface and therefore have much shorter signal delays. However, transmitters require higher power to communicate with LEO satellites compared to terrestrial networks. This is because the transmission distance of terrestrial network signals is 5-10 kilometers, while the transmission distance of LEO signals is as long as 2,000 kilometers, and the signal loss is also greater.

The small size of LEO satellites is both an advantage and a design challenge. The power amplifier (PA) of the LEO satellite must be small in size and have sufficient power to transmit signals to the intended target. Ideally, satellite engineers want the PA to have linear characteristics, even when driven by high power inputs. However, excessive PA drive power will cause severe signal distortion, as shown in the figure below. Digital predistortion (DPD) subsystems in the transmitter can counteract these distortions.

DPD applies an "inverse PA" characteristic to the signal, making the PA's output signal more obviously linear. DPD tools such as Communications Toolbox® Tools in , are increasingly using AI to improve results.

Power amplifier characteristics showing nonlinearity (compression) and memory effects. The digital predistortion (DPD) feature shown compensates for nonlinearity.

RF links, optical links and phased arrays

Interference also presents a challenge when using LEO satellites for satellite communications systems. The primary reason is that there are currently nearly 6,000 LEO satellites in orbit.

Traditional radio frequency links have long been used in satellite communications systems, but engineers are opting for optical links as often as possible. The beam pattern is much narrower than traditional RF links, whose wide beams can spill into other receivers and cause interference. Because signal propagation is limited, interference in the optical system is significantly reduced.

Finally, satellite engineers can use phased arrays, which are groups of computer-controlled antennas that produce beams that can be electronically steered in different directions. Phased arrays can spatially cancel interference and direct energy to a specific point on the ground. Phased array systems maximize the signal-to-interference-plus-noise ratio (SINR) by maximizing beam energy in the direction of the target signal and inserting beam nulls in the direction of interference.

Doppler effect and frequency shift

Unlike GEO satellites, LEO satellites orbit the Earth at a speed different from the Earth's rotation rate. This means they are constantly moving closer or further away from the receiver. This movement creates the Doppler effect, which satellite engineers must control.

In engineering terms, the Doppler effect refers to the frequency difference between the transmitted and received waves due to the motion of the transmitter or receiver. The challenge posed by the Doppler effect requires satellite engineers to acquire and track the changing center frequencies of LEO satellites.

The frequency and phase of the transmitter and receiver must be perfectly locked to ensure that the waveform is successfully demodulated. However, if the Doppler shift is large, this can lead to frequency, phase, and timing desynchronization. Therefore, multiple closed loops must be implemented in these receivers to eliminate frequency shifts caused by the Doppler effect. Synchronization must be performed at the frame, symbol timing, carrier frequency, and carrier phase levels.

Conclusion

MATLAB is used by many satellite engineers reference receiver designs for other products, so they don’t have to “reinvent the wheel.” By making minor customizations to the reference design, satellite engineers can design robust receivers that operate in challenging RF environments.

LEO has received widespread attention due to its compelling short- and long-term use cases. Companies like Apple are already using satellite communications networks, and this is just the beginning. As satellite communications continue to impact the wireless industry, engineers should familiarize themselves with its uses, challenges, and enabling technologies.


About MathWorks

MathWorks is the world's leading developer of mathematical computing software. MATLAB from the company is called "the language of scientists and engineers" and is a programming environment that integrates algorithm development, data analysis, visualization and numerical calculations. Simulink is a modular modeling environment for simulation and model-based design of multi-domain and embedded engineering systems. These products serve engineers and scientists around the world, helping them accelerate invention, innovation and development in automotive, aerospace, communications, electronics, industrial automation and other industries. MATLAB and Simulink products are essential teaching and research tools for many of the world's top universities and academic institutions. MathWorks was founded in 1984 and is headquartered in Natick, Massachusetts. It has 34 branches around the world and a total of more than 6,000 employees. For more information, visit cn.mathworks.com.


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