Phased Array and LEO Technologies: Evolving Mobile NTN

Consumer Mobile Satellite Services (MSS) have historically been a niche segment in telecommunications, with specially designed portable user terminals equipped with a large antenna for satellite uplink and downlink capabilities (i.e., satphones) dominating the space. In recent times, however, the mobile user terminal has begun taking on different sizes and shapes entirely. Chiefly, with the launch of new satellite-to-phone communications services birthed from partnerships between smartphone manufacturers and Satcom operators like Apple and Hughes Network or Huawei and Global BeiDou, MSS technologies are entering the mainstream by shifting away from bulky ruggedized devices and integrating into everyday smartphones and devices. With new MSS players like SpaceX, AST SpaceMobile, and Lynk deploying satellite constellations in Low Earth Orbit (LEO) to allow direct-to-mobile phone connectivity, the future of MSS capabilities for wireless-enabled smart devices looks bright. Despite these recent developments, however, the current capabilities of MSS in commercial off-the-shelf smartphones are limited, with most only able to send or receive texts and others offering limited latency and bandwidth.

Transmitting electromagnetic waves from a mobile device to roughly 550 Kilometers (km) or farther outside the Earth’s atmosphere to a fast-moving satellite comes with a host of technical challenges. While some of these challenges have been overcome by integrating antennas into a portable form factor, the satellite network’s operational orbit will often impact the mobile device's build. Networks operating in Geostationary Orbit (GEO) for instance, often require handsets to be equipped with large antennas protruding from the device to transmit signals to and from a satellite roughly 36,000 km from Earth’s atmosphere. On the other hand, networks operating in LEO require handset signals to travel roughly 1/65 the distance and, therefore, can transmit with lower latency and higher bandwidth using a smaller user antenna. The drawback to this is that LEO satellites move at a very high velocity (roughly 27,350 Kilometers per Hour (km/h)) and either require a user to physically direct their device to a precise location in the sky for uplink and downlink (like with the iPhone 14) or the device is equipped with an array of antennas that can steer the beam to a passing satellite (i.e., phased array beam steering) like with Starlink terminals.

While many players are approaching the challenges presented with MSS connectivity differently, common constraints remain with miniaturizing hardware and optimizing software in the modern smartphone form factor for latency-sensitive satellite-enabled communications. For MSS to become a truly viable end-to-end communications channel for modern consumers, smartphone and satellite technologies need to symbiotically evolve. In this respect, the solution to NTN lies not only in adapting smartphone hardware and software to match space-based signals, but also adapting satellites to better pick up the signals emitted from smartphones. Integrating more phased array technologies for downlink beamforming in satellites operating in LEO may be the solution.

• Phased Array Antennas: Beamforming is one of the critical advantages of using a phased array of antennas. Instead of relying on a single large antenna to send and receive electromagnetic signals to a satellite 550 km away, an array of smaller antennas’ oscillating field vectors combine at all points in space to create a focused beam of constructive interference. The resulting zone of constructive interference or beam front/wave front is a laser-like beam with so much intensity and directionality that it can reach outer space. While this can reduce antenna size and allow signals to be sent over long distances, the beam front still needs to be directed accurately to the fast-moving satellites in LEO via phased array beam steering. Through phase shifting, the wavelength of one antenna is shifted to the left or right with respect to another antenna, allowing for the location of the constructive interference to angle in a different direction. By stacking the antennas together in a Two-Dimensional (2D) array, the beam can effectively be steered in any direction within a 100° field of view. To find the exact angle and location the wave front needs to be steered, the device embedded with phased array antennas uses its Global Positioning System (GPS) coordinates, alongside the orbital coordinates of the satellite to communicate via software. This software then computes the exact set of Three-Dimensional (3D) angles and required phase shift for each antenna in the array over and over within microseconds, so that the beam points perfectly at the satellite. By deploying this phased array antenna hardware layer underneath the back panel of a smartphone, alongside specialized software to manage and translate orbital and GPS positioning for the array, mobile devices could theoretically maintain a connection with a fast-moving satellite (like in LEO). This would likely not resolve all the challenges presented with this problem, as leveraging phased array antennas would require smartphone manufacturers to specially design antennas, power supply, and software for these operations in a mobile phone form factor that also meet the specifications required by satcom operators for reliable signals.
• LEO: Operating in LEO has many distinct advantages for wireless communications over other operational orbits in space. For one, the closer distance to the Earth’s atmosphere (generally 200 km to 1,600 km away) enables them to communicate with ground equipment with minimal path loss. As a result, this allows for a reliable link to be established with less power and antenna size. These advantages make LEO a strong operational orbit option for providing higher bandwidth, data rates, and latency for wireless applications. Despite these advantages, there are connectivity hurdles presented with operating in LEO. These issues primarily arise from the speed at which LEO satellites travel and the transmission power requirements to sustain a reliable link. In this respect, more sensitive and advanced microwave receivers can be used to work with lower field strengths and reduce the power needs of a transmission. Furthermore, the advancements in chip design and architecture have helped reduce the size of electronics and subsequent power consumption of operations. To establish a more reliable broadband connection with ground equipment, modern LEO satellites are being equipped with more phased array antennas, such as the Starlink or BlueWalker 3 satellites. In this respect, the higher concentration of phased antennas, and therefore focused beams, enables a stronger link with ground terminals. As with Massive Multiple Input, Multiple Output (mMIMO), both the network and mobile devices need to have tight coordination with each other to work. While these are some of the key aspects to consider in making LEO MSS work for modern mobile devices, there are still other aspects to consider, such as orbital decay and the high replacement rate of LEO (and therefore, higher upgrade frequency), and elevation needs to maximize Line of Sight (LoS), which will impact the feasibility of deployments in LEO.

Phased array technologies are seeing increasing integration in next-generation networks like 5G radio units, 5G mMIMO, and satellite communications, where antennas no longer transmit to a sector, but focus the signal for enhanced data rates and use in different frequency ranges. Alongside this, the advancements in LEO technologies have opened a feasible option for lower-latency communications from Non-Terrestrial Networks (NTNs). The marriage of both technologies could be the answer to establishing a reliable network connection between terrestrial mobile devices and satellites in orbit. Despite this potential, there are challenges to overcome that will require Communication Service Providers (CSPs), satcom operators, and smartphone manufacturers to build systems that are specially designed to communicate with one another. Below are some of the recommended areas that these co-development initiatives could focus on:

• Developing compact, energy-efficient, and powerful antennas for smartphones/devices.
• Deploying purpose-built satellites for LEO equipped with many powerful phased array antennas.
• Co-development between satellite operators, CSPs, and phone manufacturers to focus not only on interoperability, but on integration via chipsets, components, and standards.
• Data compression and bandwidth allocation, as limitations focused on data can impact service performance and the user experience.
• Uplink and downlink timing distribution (helps reduce latency by distributing timeslots throughout each second).