Blog Series Post 2 - Inside the Hybrid: How Castanet and our Co-Innovation Partners Built the World’s First ATSC 3.0 + 5G Broadcast Internet
- alexanderrenz
- 7 days ago
- 4 min read
In our previous post we explained why Castanet chose a hybrid ATSC 3.0 / 5G Broadcast architecture: it is fully compliant with existing FCC Broadcast Internet rules, aligned with 3GPP standards, and ready for the coming wave of 5G Broadcast-capable handsets.
This follow-up goes deeper into how that architecture actually works — and highlights the specialized co-innovation partners whose technology made the world’s first live hybrid deployment possible.
The Core Technical Challenge
ATSC 3.0 and 5G Broadcast (FeMBMS LTE-based 5G Broadcast under 3GPP) are designed to allow for the discontinuous transmission of their frames. To exploit that capability, we employ precise time-division multiplexing. Two independent mechanisms create the necessary gaps that allow us to combine the two signals.
According to the A/327 Guidelines for the Physical Layer Protocol (Recommended Practice: TDM of ATSC With Other Broadcasting Systems), the standard supports coexistence with non-ATSC 3.0 signals on the same channel. This is achieved by implementing the min_time_to_next parameter in the bootstrap signal (A/321), which informs every receiver how long it must wait before the next ATSC physical-layer frame. By deliberately setting this value, controlled silent intervals can be created within the ATSC 3.0 waveform.
On the 5G Broadcast side — 3GPP Release 19 CAS (Cell Acquisition Subframe) muting allows the broadcast cell to go completely quiet during those same intervals. The 5G Broadcast modulator simply stops transmitting control and data symbols for the programmed duration.
When these two mechanisms are synchronized, ATSC 3.0 frames and 5G Broadcast frames interleave cleanly on the same RF carrier. Receivers on both sides see only standards-compliant signals. No destructive interference occurs, and no capital equipment on either side is stranded.
The Co-Innovation Partners Who Made It Real
Turning the theory into a working, field-proven system required deep specialization across the signal chain. Three partners contributed critical building blocks:
GreyFox – Scheduler and ATSC-side tooling
GreyFox developed the intelligent scheduler that decides the exact duration and placement of ATSC 3.0 frames and the corresponding silent gaps. Their tooling configures the min_time_to_next values, manages frame length alignment (in 5 ms quanta), and ensures the ATSC 3.0 transport stream remains fully compliant while leaving precisely sized windows for the 5G Broadcast waveform. Without this precise ATSC-side control, the hybrid would remain theoretical.
Neutral Wireless – 5G Broadcast Core and Modulator and CAS-muting implementation
Neutral Wireless supplied the end-to-end 5G Broadcast transmission chain, which included the 5G Broadcast Core and the 5G Broadcast modulator. The 5G Broadcast Core manages content ingestion, orchestration, scheduling, and the centralized configuration of channel parameters, including the critical Release 19 CAS-muting capability. The 5G Broadcast modulator generates the FeMBMS LTE-based 5G Broadcast waveform, including the CAS-muting functionality. Their implementation creates the complementary quiet periods that match the ATSC gaps generated by GreyFox’s scheduler.
In the Las Vegas pilot, this produced clean periodic multiplexing (for example, 1280 ms of 5G Broadcast followed by an 80 ms ATSC 3.0 insertion window). Neutral Wireless also provided the SDR-based Receive-Only Mode (ROM) receivers used to validate the 5G Broadcast side of the hybrid signal.
Televes / TRedess – ATSC 3.0 + 5G Broadcast Multiplexer
Televes and its TRedess division delivered the physical-layer multiplexer that combines the two independent waveforms into a single RF output. Their solution takes the scheduled ATSC 3.0 frames and the CAS-muted 5G Broadcast frames and produces a continuous, transmission-ready signal that can be fed directly into an LPTV transmitter. The multiplexer maintains tight timing synchronization and handles the necessary guard intervals so that neither waveform is corrupted.
Together these three contributions helped Castanet create the world’s first commercial-ready implementation of a hybrid ATSC 3.0 / 5G Broadcast standards-based pipeline:
Content & IP streams → GreyFox scheduler (ATSC timing & gaps) → Neutral Wireless 5G Broadcast modulator (CAS muting) → Televes/TRedess hybrid multiplexer → LPTV transmitter → air
What This Enables in Practice
Because the hybrid signal remains fully compliant with both ATSC 3.0 and 3GPP specifications:
Existing and future ATSC 3.0 Standard-compliant receivers continue to decode the ATSC portion without modification.
Future 5G Broadcast-capable handsets (expected to reach handset devices in early 2028) will decode the 5G Broadcast portion natively — no SIM card required, no data-cap impact, Receive-Only Mode.
The same LPTV transmitter can deliver local and traditional television services and high-capacity one-to-all delivery of live streams, video-on-demand, and AI-generated content across fixed and mobile.
This is not a laboratory curiosity. The architecture has already been demonstrated, deployed and operated as commercial-ready pilot networks in Las Vegas and Silicon Valley during NAB 2026.
Looking Ahead
The hybrid ATSC 3.0 / 5G Broadcast Internet is now a proven, deployable overlay to the unicast internet. By combining the regulatory path already available under the FCC’s Broadcast Internet framework with the global scale of the 3GPP device ecosystem, and thanks to our trusted co-innovation partners, Castanet has removed the false choice that previously divided the industry.
We will continue to share technical progress, field results, and additional partner contributions as the network expands. The future of media delivery is no longer ATSC 3.0 or 5G Broadcast. It is both — operating together on the same spectrum, from the same transmitters, to every class of device.
The Castanet 5G Broadcast Internet is no longer a concept. It is on the air, ready to scale.
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