The satellite communications industry is entering a decisive transition: Direct-to-Device (D2D) connectivity, often described as Satellite-to-Phone, Tower-to-Device (T2D), and Network-to-Device (N2D), is moving from early demonstrations to scaled, operational deployments. As operators pursue 3GPP-aligned 5G Non-Terrestrial Networks (NTN) and D2D service models, the limiting factor is increasingly not the spacecraft but the ground segment RF architecture that must carry wideband signals reliably, securely, and at scale.

For defense and dual-use stakeholders, this evolution matters more than commercial coverage. D2D and NTN concepts are accelerating demand for resilient, distributed, and rapidly deployable SATCOM gateway infrastructure, including architectures that support EMCON-minded operations, contested RF environments, and mission continuity when terrestrial pathways are degraded or denied.

Optical Zonu Corporation (OZC) supports this shift by providing analog RF-over-Fiber (RFoF) transport solutions that connect antenna-to-core with low loss, high dynamic range, and phase-stable performance, establishing a foundational layer for modern gateway, test, and tactical edge architectures.RFoF

What D2D, T2D, and N2D Require
Direct-to-Device satellite technology allows satellites to communicate directly with standard mobile devices and IoT endpoints without specialized handsets. The implications are substantial:

• Resilient communications when terrestrial infrastructure is disrupted
• Coverage extension into rural, remote, maritime, and austere locations
• Redundancy and continuity for critical services and mission operations
• Scalable capacity growth as constellations and coverage areas expand

Under the hood, these outcomes depend on ground infrastructure that can support multi-band RF, geographically distributed antennas, and centralized processing without disrupting signal integrity.

5G NTN also require high instantaneous bandwidth to support massive data rates needed for demanding applications like 4K/8K streaming, real-time AR/VR, massive IoT, and autonomous vehicles. 4G is incapable of providing the speed, low latency, and capacity to handle vast amounts of data simultaneously.In terms of transport, digital communication standards like DIFI are less ideal compared to analog since peak modulation bandwidth is locked and limited.

 Why RFoF Is Critical for 5G NTN/D2D Ground Segments
Traditional RF distribution approaches likecoax and waveguide can become problematic as D2D/NTN gateways 

Scale RFoF:

• Coax losses grow quickly with frequency and distance
• Waveguide adds weight, installation burden, alignment sensitivity, and maintenance complexity
• Electrical pathways invite EMI/EMC complications, especially in dense RF sites
• Centralized RF equipment architecture
• Distributed antenna layouts across facilities create long RF runs and inconsistent performance

Analog RFoF directly addresses these constraints by moving RF as light:

• Low-loss transport over long distances (without RF coax penalties)
• EMI immunity and improved survivability in high-RF/industrial environments
• Flexible antenna placement and simplified site design
• Centralized processing (modems/RAN/SDR resources can be consolidated)
• Phase-stable, multi-channel transport suited for calibration, beamforming support, and high-fidelity test

This matters for both commercial and defense-aligned deployments because gateway sites increasingly need to be modular, disaggregated, and rapidly scalable.

Bridging Antenna-to-Core: What OZC Brings to NTN/D2D

OZC’s RFoF approach is designed as a transport layer, transparent to waveform and modulation, so operators and integrators can evolve modems, RAN elements, and SDR processing without rebuilding the RF distribution backbone.

Key RFoF capabilities relevant to NTN/D2D ground segmentsRFoF

• Multi-orbit / multi-band support: L/S/C/X/Ku/Ka (architecture-dependent)
• Antenna/sensor remoting: from antenna sites into gateway/processing environments
• High dynamic range to meet telecom EVM (error vector magnitude) requirements
• DWDM-enabled scaling: multi-channel RF transport over single-mode fiber
• Rugged form factors: rack, modular, and outdoor-capable configurations (as required)
• Managed system monitoring: GUI, SNMP v2/v3, and NMS-aligned management options

In practice, this enables NTN gateway designs reimagined: more channels, fewer RF distribution constraints, and less sensitivity to facility layout.

Our largest deployment in NTN involves supporting a large-scale Direct-to-Device satellite project for space-based cellular broadband leader. This deployment highlights how RFoF can enable distributed antenna systems and gateway architectures essential for next-generation satellite-to-phone networks. 

Single-Aperture and Multi-Band Architectures

As NTN and D2D concepts converge, ground terminals increasingly need multi-band flexibility whether implemented via a single aperture with front-end splitting/combining, or multiple antennas with centralized transport.

RFoF supports both approaches by enabling:

• Simultaneous transport of multiple RF paths over fiber (CWDM/DWDM-based architectures)
• Modular scaling without redesigning the entire gateway layout
• Consistent performance across distributed antenna placements

This is particularly helpful when upgrades introduce new bands or when operators need to add capability without major topside rework RFoF.

EMCON and Contested Environment Considerations

In defense and dual-use scenarios, gateway survivability and signature management matter. RFoF supports passive, low-signature antenna remoting concepts by relocating processing and minimizing the need for active RF electronics at the antenna location (architecture-dependent). This can reduce:

• RF leakage and unintended emissions near the antenna site
• Thermal and maintenance burden at exposed edge locations
• Physical vulnerability of high-value electronics in harsh placements

While EMCON is ultimately a mission and system-level design choice, RFoF can be a strong enabler for distributed, resilient gateway postures.

Cost and Complexity: Why “Building Blocks” Win

D2D and 5G NTN ground segments scale fast, and cost and time-to-field become decisive. OZC’s approach is based on configurable commercially shipping building blocks (modules, boards, chassis systems, and managed elements). That structure helps reduce complexity by enabling:

• Faster integration cycles (proven modules assembled to the required architecture)
• Lower NRE exposure than fully bespoke RF distribution designs
• Simplified upgrades (new bands/channels via modular expansion)
• Reduced installation and sustainment burden compared to heavy RF distribution runs

For many gateway designs, the transport layer becomes a reusable foundation that support both today’s deployments and future upgrades.

Conclusion

Direct-to-Device and 5G NTN deployments will be judged on real-world availability, scalability, and resilience—not just link budgets on paper. The common denominator is a ground segment RF architecture that can carry multi-band signals reliably, with low loss and strong fidelity, while simplifying site design and enabling rapid growth.

RFoF is the quiet enabler behind many of the most practical, scalable NTN/D2D gateway designs, especially where performance, survivability, and lifecycle cost RFoF.