Blog Series Post 1: Building the Broadcast Internet - Why Castanet Chose Hybrid ATSC 3.0 + 5G Broadcast
- alexanderrenz
- Jul 25
- 3 min read
Updated: Jul 30
For years the industry framed a false choice: ATSC 3.0 or 5G Broadcast. One path protected existing broadcast investments and local service. The other aligned with the global mobile ecosystem and promised ubiquitous receivers in future smartphones. Castanet rejected the dichotomy. We built the hybrid that turns “or” into “and.”
Our approach time-multiplexes ATSC 3.0 and 5G Broadcast onto the same RF channel—typically a 6 MHz Low-Power TV (LPTV) allocation. The result is a single overlay network that carries both traditional and local content and high-capacity, direct-to-device delivery of live streams, video-on-demand, and AI-generated media. It is fully compliant with existing FCC rules, aligned with 3GPP standards, and designed for the moment 5G Broadcast becomes a native capability in mainstream handsets.
Why This Architecture?
Two constraints shaped the design from day one.
1. FCC compliance without waiting for new rulemakings
The FCC’s Broadcast Internet framework (clarified in the 2020 Report and Order and subsequent guidance) authorizes the inclusion of ancillary services within the required ATSC 3.0 transport stream. Castanet incorporates the 5G Broadcast Internet as those ancillary services. Critically, LPTV stations may transition directly to ATSC 3.0 without the same simulcast obligations that apply to full-power stations. This gives Castanet a clear, lawful path to operate a hybrid service today using licensed LPTV spectrum as the foundation. We did not need to wait for a separate 5G Broadcast authorization or a rewrite of the ATSC 3.0 transition rules. The architecture is deliberately built on what the rules already permit.
2. Alignment with the global telecom ecosystem
5G Broadcast is defined in 3GPP (LTE-based 5G Broadcast, with key enhancements in Release 19 including CAS muting and Receive-Only Mode). Once 5G Broadcast chipsets are integrated into mobile handsets—Qualcomm has indicated commercial availability beginning in early 2028, starting with premium devices—hundreds of millions of phones become potential receivers without requiring a separate tuner, a SIM card, or any contribution to a user’s data cap. Broadcast traffic is inherently zero-rated. By riding the 3GPP standard rather than inventing a proprietary waveform, Castanet inherits the entire global device ecosystem and the economies of scale that come with it.
The hybrid therefore satisfies both regulators and the mobile industry at the same time.
The Technical Foundation: Hybrid Time Multiplexing
The core idea is time-division multiplexing on a single RF channel. ATSC 3.0’s physical layer already contains a forward-compatibility mechanism: the min_time_to_next field in the bootstrap signal (A/321). It tells receivers how long they must wait before the next ATSC frame arrives, deliberately creating controlled gaps. On the 5G Broadcast side, 3GPP Release 19 CAS muting allows the broadcast cell to go quiet during those same intervals.
By coordinating the two—setting ATSC frame duration to fit the muted window and aligning the CAS cycle—the two waveforms interleave cleanly. Receivers on both sides see predictable, standards-compliant signals. No destructive interference. No stranded capital on either side of the fence.
Qualcomm originally conceived the hybrid time-multiplexing concept that makes this coexistence practical. Castanet took that foundation, turned it into a working end-to-end system, and implemented it with a consortium of co-innovation partners including GreyFox (scheduler and ATSC-side tooling), Neutral Wireless (5G Broadcast modulator and CAS-muting implementation), Televes/TRedess (ATSC 3.0+5GBroadcast Multiplexer), and others. The result is not a paper architecture; it is field-proven.
From Prototype to Industry Consensus
In October 2025 at NAB Show New York we demonstrated the world’s first working hybrid ATSC 3.0 / 5G Broadcast prototype. The reaction was immediate. Industry leaders who had spent years arguing “ATSC 3.0 or 5G Broadcast” began describing the future as “ATSC 3.0 and 5G Broadcast.” The false choice dissolved.
By NAB 2026 in Las Vegas we had moved from prototype to commercial-ready pilot networks in Las Vegas and Silicon Valley, transmitting live streams over the hybrid waveform from operational LPTV transmitters. The architecture had graduated from concept to deployable infrastructure.
An Overlay Network Built for Scale and Local Relevance
Castanet’s hybrid layer functions as an overlay on existing LPTV spectrum. It preserves and enhances the ability to deliver local content and traditional television services via the ATSC 3.0 portion of the multiplex. Simultaneously, the 5G Broadcast portion excels at one-to-all, direct-to-device delivery of:
Live linear and event streams
Video-on-demand
AI-generated and AI-augmented media
Software, data, and public-safety messaging
Because reception is a native 3GPP feature operating in Receive-Only Mode, no SIM is required and the traffic does not count against any cellular data allowance. The economics invert the unicast problem: audience growth multiplies value rather than cost.
This is the practical realization of the Broadcast Internet vision—compliant with today’s rules, ready for tomorrow’s devices, and designed so that scale creates opportunity instead of expense.
The hybrid is not a compromise. It is the architecture that lets both ecosystems win. And it is already on air.
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