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Satcom could once be considered separately to terrestrial connectivity, but today we are moving swiftly towards full integration between satellite and terrestrial solutions.
Consumers, enterprises and governments increasingly expect uninterrupted ubiquitous access to data and communication, regardless of location or situation.

Two emerging trends, namely the convergence of terrestrial and non-terrestrial networks (NTN) and direct-to-device (D2D) services, are set to redefine global wireless coverage, ushering in an age where integrated seamless solutions connect everything from everyday mobile users to mission-critical communications.

The thought of having cellular coverage while hiking in the mountains or sailing out at sea may seem like heaven to some and hell to others but regardless, this is where we are undoubtedly heading.

Significant investments and partnerships are driving these developments to imminent reality. Examples include Apple’s $1.5bn investment in Globalstar’s direct-to-device satellite constellation.

T-Mobile and SpaceX subsidiary Starlink have announced public beta trials to access connectivity for compatible phones. Vodafone has recently achieved a successful video call from a remote mountain in Ceredigion, west Wales, using a standard smartphone connecting to AST SpaceMobile’s enormous Bluebird satellites, which measures just over 64m2, making it one of the brightest objects in the night sky.

AST SpaceMobile and Vodafone have also announced a new joint venture, SatCo, which aims to bring space-based D2D mobile broadband with 100% geographic coverage to Europe in 2026.

Comparing NTN and D2D

It is worth explaining the difference between NTN and D2D. NTNs complement terrestrial networks, utilising non-terrestrial platforms such as satellites in different orbits – low, medium and geostationary orbits (LEO, MEO, GEO) – high-altitude platform systems (HAPS) or airborne platforms such as aeroplanes, balloons and airships.

These technologies extend coverage to areas where traditional infrastructure is impractical or unavailable, supporting broadband, IoT applications and terrestrial network backhaul.

D2D is also known as direct-to-cellular (D2C). It is a specific technology within the broader NTN offering. It allows ordinary consumer devices, such as mobile phones, to connect directly to satellites without the need for additional hardware. Key features of D2D include compatibility with unmodified mobile phones, the ability to provide basic services such as texting, calling and data in areas without traditional cellular coverage and use of existing mobile frequency spectrum bands for communication with satellites.

Network architectures

In order to achieve this, there are a number of possible network architectures that can be employed, depending on where the 5G base station (gNB) is located.

Examples of basic cases are the transparent (or bent-pipe) architecture and the regenerative architecture. (There are other split architectures but this article will not focus on those.)

The transparent payload architecture, often referred to as ‘bent-pipe’, represents the simpler of the two approaches to satellite-based communication in NTNs. In this configuration, satellites function primarily as signal relays with minimal onboard processing capabilities.

The satellite performs basic operations such as RF filtering, frequency conversion and amplification without altering the underlying waveform structure of the transmitted signals.

In contrast to the transparent approach, regenerative architecture integrates significant processing capabilities directly onboard the satellite. This configuration effectively moves some or all the gNB functionality onto the satellite rather than on the ground.

Beyond the basic RF operations performed in transparent systems, regenerative satellites include sophisticated signal processing including demodulation/decoding, switching, routing and coding/modulation. This advanced processing transforms the satellite from a simple relay into an active network node with intelligence and decision-making capabilities.

Each architecture presents its own advantages and challenges. The transparent architecture benefits from its relative simplicity and lower deployment costs.

This simplicity, however, comes with drawbacks, including higher latency. Since all signal processing occurs on the ground, retransmissions or corrections must traverse the complete path from user equipment to satellite to ground station and back again. This extended signal path introduces substantial propagation delays, which is problematic for applications requiring real-time responsiveness.

Latency issues

Regenerative architecture offers numerous compelling advantages, including addressing the latency issue. By integrating gNB functionality directly on the satellite, retransmissions and corrections can be handled without the long propagation delays associated with ground-based processing.

This lower latency empowers the satellite-based gNB to make scheduling, adaptive modulation and power control decisions based on near real-time link conditions, resulting in superior quality of service and improved bandwidth utilisation.

Despite its numerous advantages, regenerative architecture faces significant implementation challenges. The most prominent is the substantially increased complexity and cost associated with placing sophisticated processing equipment in space.

Keeping grounded

While much of the focus is on the satellite and user equipment, the ground station serves as the link between the satellite and the ground network infrastructure, making this all possible.

Historically the ground segment has been almost exclusively analogue, moving and switching data between the antenna and modem as RF signals. A key enabler for NTN is virtualisation and cloud integration. They aim to move as much of the hardware functionality onto standard computing platforms as possible and to host them on the cloud. This provides ultimate flexibility and aligns the satcom ground architecture with that of the mobile network architecture, where network functions are already virtualised.

A key enabler for virtualisation is a device called a digitiser, which bridges the gap between the traditional RF technology and the evolving cloud virtualised ground segment by converting RF analogue signals into IP packets that can be transmitted over an Ethernet connection. This process is commonly referred to as RF over IP. This not only streamlines signal transport but also facilitates the integration of traditional RF systems into modern digital infrastructures, marking a significant advance in satcom technology.

Challenges

While this may sound positive, there are still many challenges to overcome to get to a sustainable, technically sound and commercially-viable operating model.

The first is closing the link budget between a satellite and a mobile phone designed to connect to a local terrestrial basestation. Mobile phones are limited in the permissible RF radiated power levels they can emit due to the potential negative effects on the human body. This limited transmit power inherently limits the potential upload speeds attainable and it is difficult to see an easy solution to achieving true broadband speeds. This is, however, far less relevant for most IoT or other low data rate use cases.

Another challenge is the significant expense required in building, launching and operating satellite constellations and ground infrastructure capable of providing uninterrupted coverage – LEO satellite lifespans are typically five to 10 years and require regular replacement. Connecting areas not currently connected by terrestrial infrastructure may prove a difficult prospect for a positive return on investment.

As the satcom industry continues to evolve, the convergence of terrestrial and non-terrestrial networks and the push towards 6G, enhanced digitisation, standardised interfaces and protocols are not merely incremental improvements, they are the building blocks of a fundamentally new connectivity paradigm.