Summary
At last month’s SCTE Tech Expo Event in Atlanta, two separate 3 GHz developments were announced, as cable operators and their equipment vendors look to define a future that continues to include HFC and DOCSIS in some capacity. First, CableLabs published Annex A to the DOCSIS 4.0 PHY spec, a year after announcing that an effort would begin to push existing HFC capacity to 25 Gbps. Second, AOI showed a live concept demonstration of a 3 GHz amplifier and published a corresponding whitepaper detailing some of the architectural options and challenges cable operators face when considering whether to evolve their outside plant to 3 GHz.
Although the updated CableLabs spec and AOI’s whitepaper tackle two different, but completely complementary, parts of the network, there are some interesting areas of alignment that emerge as well as some potential architectural differences, which is to be expected at these early stages of theorizing and development. It’s important to keep in mind that these technology cycles generally take many years before products are even introduced. The work on 1.8 GHz amplifiers and housings began in 2019, after 3 years of standards development, with final designs largely settled in 2025. If that holds true to form, then we are currently estimating 2033-2035 as the rough timeline for 3 GHz upgrades in current HFC networks.
In these early stages of conceptualization, the engineering questions often overshadow the business rationale. But in the case of 3 GHz, whether it makes sense from a financial and competitive standpoint is a very relevant question to ask, given the dynamic competitive landscape and given the fact that so few operators are even moving forward with DOCSIS 4.0 in the first place.
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- Will the effort required to bring 3 GHz to the market be worth it, considering that the number of operators deploying DOCSIS 4.0 remains limited to the largest North American tier 1 operators and a smattering of European operators?
- By the time 3 GHz technologies are fully baked, delivering 25 Gbps of capacity, will the world have already passed those capacity levels by, effectively stranding the industry-wide R&D investment?
These are critical questions that absolutely have to be answered in order for 3 GHz to reach maturity and find a set of operators large enough to create the required component ecosystem. Even before the industry tries to answer those difficult questions, the technology has to work. Right now, vendors and operators alike are coming up with early technology options that will ultimately evolve with more testing and with the availability of silicon.
Outside Plant Remains the Overarching Challenge
The Annex A update to the DOCSIS 4.0 PHY spec lays out the requirements to support 3 GHz at the node and the modem, which is the scope of the PHY working group. The specification keeps the door wide open on architectural approaches in the outside plant, but notes that the target power amplifier technology at the node is 75 dBmV of total composite power (TCP) across 2304 MHz of available spectrum. That compares to current TCP levels of 66-70 dBmV at 1794 MHz. The specification states that the amplifier technology for 3 GHz at the node is still under development (and is “highly demanding”) and that a future iteration of the spec will include more detailed interface D requirements.
SCTE provides the standards for amplifiers, taps, and passives. While the organization has developed standards (ANSI.SCTE 264 and 265) that call for taps with 3 GHz-capable housings with 1.8 GHz faceplates, the amplifiers remain an open question thus far. That is where AOI and others are seeking to outline options for evolving the plant to 3000 MHz.
AOI’s whitepaper makes the clear point that attenuation rises with frequency. Pushing today’s amplifiers to 3 GHz raises TCP by more than 10 dB, or roughly 10x the RF power. This means that current network powering options will likely not be able to handle this additional power load. Also, whether current amplifier housings can dissipate the additional heat each amplifier creates is an open question.
The whitepaper proposes several alternatives and organizes them into distinct categories, each with its own set of trade-offs:
- High-TCP wideband
- Low-TCP wideband
- Split-band
- Channelized
- Hybrid analog/digital
Each category provides a different combination of answers to two fundamental questions:
- Does the architecture concentrate gain in each amplifier using today’s spacing, or does it spread the gain across a larger number of smaller actives with shorter spacing?
- Does the architecture rely on amplifiers that are built to support a wideband path, a split-band pair, several channelized amps, or amps that support both analog RF and digital using a DSP?
Based on how each approach answers these two questions, we can boil things down to two architectural approaches. Let’s look at both.
Architecture One: Maintain Current Spacing and Make Each Amplifier Do More
In addition to AOI’s whitepaper, Richard Prodan, an Engineering Fellow at Comcast, published a technical paper for SCTE Tech Expo called “Evolving the DOCSIS 4.0 PHY to 3 GHz: Evaluating the Possibilities and Optimizing the Benefits.” In his paper, Prodan offers an option for keeping today’s amplifier spacing.
In one of his examples, Prodan starts with a node-plus-6 architecture with a 100-foot drop from the last tap after the amplifier furthest from the node, supporting up to 12 192 MHz OFDM channels anywhere from 108-258 MHz up to 3 GHz. The legacy power tilt is then extended to 3 GHz, which pushes the node and amplifier TCP to 84 dBmV, well above the 75 dBmV target identified in the CableLabs Annex A spec.
To bring the TCP down, Prodan suggests using step-downs in the power profile at higher frequencies. A single 9.5 dB step near 1 GHz reduces TCP to the 75 dBmV target while continuing to support 25 Gbps of channel rate using 2048-QAM.
From the perspective of maintaining RF signal quality and fidelity, the paper does demonstrate that cable operators could theoretically achieve their goal of 25 Gbps of capacity without having to put any additional active electronics in the field. The challenge that the paper doesn’t address and which AOI says is a deal-breaker for this high-TCP wideband option, is that maintaining 1.8 GHz amplifier spacing at 3 GHz would require “an incredible 50 times more power and dissipate more than a kilowatt of heat.”
Architecture Two: Add Gain Between Amplifiers
If a 50x increase in power is an excessive trade-off for not having to touch the installed amplifier base, then the next architectural option is to increase the number of actives in the plant and further distribute power amplification beyond what exists today. In other words, leave the legacy amplifiers in place but then add smaller booster amplifiers in the spans between them, using 2-3 booster amps per span. Additionally, booster amps can be located closer to taps or inside them as active hybrid taps. In the context of AOI’s whitepaper, this is the split-band option.
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- Technical trade-off: Overall loss is reduced, though at the cost of an increased number of actives in the plant, something most operators would desperately like to avoid;
- Commercial trade-off: New hybrid amplifier and tap platform doesn’t exist yet, and there is no guarantee that the R&D effort required to build it would generate enough ROI, especially with so many operators looking at 50G PON as their possible fiber entry point.
Also, every split-band design sacrifices some amount of spectrum between the original amplifiers and the new booster amplifiers. According to the various papers presented, the range of loss is roughly 4-7%. This is precious capacity that, as of yet, can’t be reclaimed by any of the approaches suggested.
Why 3 GHz-Capable Tap Housings Matter Today
Though the papers disagree on the ideal approach for operators to upgrade their amplifier networks to support 3 GHz, there is alignment on the current need to deploy taps, nodes, and amplifier housings that can all be upgradeable to 3 GHz. For taps, start with deploying 3 GHz-capable housings, even with 1.8 GHz electronics inside.
Over time, and assuming alignment on the most cost-effective approach among operators, as well as the availability of silicon, deploy 3 GHz smart amplifiers and tap faceplates in those systems where the taps were already deployed within 3 GHz-capable housings.
In reality, however, operators upgrading to 1.8 GHz amplifiers aren’t universally pairing these upgrades with 3 GHz-capable tap housings.
Instead, operators appear to be installing 3 GHz-capable tap housings in select systems, rather than across the new 1.8 GHz amplifier footprint. This could be due simply to the original taps having ingress noise issues, or it could be part of a long-term strategy in those systems to help offload MVNO traffic.
But the fact that operators that are in the process of upgrading to 1.8 GHz DOCSIS 4.0 aren’t universally deploying 3 GHz tap housings at the same time could say something about their belief in 3 GHz as a realistic and cost-effective option to remain competitive in the 2030s and beyond.
What we believe will determine the viability of 3 GHz in the minds of operators is the success of DOCSIS 4.0 in slowing subscriber losses. Over the next 18 months, we should start seeing whether the systems that get upgraded to full DOCSIS 4.0 deliver better subscriber gains, more reliability, and higher NPS scores.
If they do, then 3 GHz remains on the table. If they don’t, then we believe there will be an effort to rethink the mix of fiber and HFC going forward.
Our summary. The original keeps its own byline.