Worth The Squeeze - Wringing More Bits/Hz Out of Your Upstream With OFDMA

Advanced Upstream Blog Series Introduction

In the Advanced Upstream blog series we will explore how cable operators have traditionally managed upstream bandwidth capacity planning, how market dynamics and technical innovation are driving changes, and where things are likely headed in the coming years. In this entry we will discuss the “More Bits/Hz” approach – or how to get the maximum throughput out of the upstream spectrum available today by using OFDMA.

What is OFDMA?

Orthogonal frequency division multiple access (OFDMA) was introduced as part of the DOCSIS 3.1 specification, but its adoption has lagged its downstream companion OFDM considerably. At the time of the 3.1 release, all eyes were on the downstream as operators struggled to keep up with the rise on OTT video services like Netflix. Enter the pandemic and the resulting videoconference-driven upstream demand spike and suddenly OFDMA deployment plans were accelerated, especially for operators with sizeable DOCSIS 3.1 CPE deployments. Operators were eager to take advantage of the increased spectral efficiency offered by OFDMA as well as the ability to use noisy spectrum previously unusable with SC-QAM.

How To Get a Free 6.4MHz 64QAM Carrier

It is worth noting that while OFDMA can work in dirty spectrum, clean plants will pass considerably more packets. Just a 4dB increase in SNR for a 30MHz OFDMA carrier can net ~50Mbps increase – it’s like getting more than a 6.4MHz SC-QAM carriers worth of capacity out of the same spectrum for free! Plant integrity matters more than ever with OFDMA, tight plants can turn dB’s into dollar Bills!

More Bits/Hz Introduction

Of the three main options that operators have to increase upstream capacity, the More Bits/Hz option seems like the easiest route. Just make a few tweaks at the HFC network edges to enable OFDMA and instantly gain upstream throughput without ever touching the outside plant – right? This is at least partially true – all DOCSIS CPE should be capable of supporting OFDMA with maybe just a FW push to enable. CCAP’s may require a new card or two and/or licenses but still child’s play compared to heavy outside plant work required for the other options. But the real challenges come in when you look at what it takes to maintain and troubleshoot networks running OFDMA.

OFDMA Challenges

CCAP/CPE compatibility – CCAP’s must have appropriate HW/FW to support OFDMA, and only DOCSIS 3.1 CPE with OFDMA-ready FW can benefit from this method. The good news is that you don’t have to be 100% deployed with 3.1 CPE to benefit, your DOCSIS 3.0 CPE will continue to work fine with SC-QAM’s even when they frequency share with OFDMA due to TDMA nature of upstream transmissions. Some operators will selectively deploy 3.1 CPE to their heaviest users to help maximize spectrum utilization.
Reverse Sweep – Traditional sweep systems have placed sweep pulses every 6.4MHz between upstream carriers but with up-to-96 MHz wide OFDMA carriers this is no longer possible. How this challenge is addressed depend on the sweep use case.
For troubleshooting critical outages, operators will sweep right through OFDMA carriers using traditional sweep processes. There is potential for minor service disruptions doing this, but the benefit of clearing an outage quickly outweighs this risk.
For use cases where service disruptions are not permissible like routine amplifier alignment and balance, an improved option is now available which uses shorter sweep pulses to minimize chances of service disruption while retaining the responsiveness of traditional sweep.
Sweepless return sweep is 100% non-intrusive and can be used on field meters to provide the highest resolution view of upstream response, and has the added bonus of not requiring any hub-based gear or management of sweep plans. The primary downsides to return sweepless sweep are that it is not as responsive as traditional sweep variants, only sweeps occupied spectrum, and requires DOCSIS services to be operational.

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Ingress Detection and Troubleshooting – the Low Density Parity Check (LDPC) error correction included in OFDMA is quite robust against ingress, but operators still need tools to detect when ingress is occurring and find and fix it in the field. Wide OFDMA carriers complicate this process by negating the use of guard bands between carriers to see ingress. Heatmap analysis in both return path monitoring applications and field meters has proven effective by early adopters at providing visibility of ingress including bursty impulse noise under these wide, seemingly always-on carriers.

What’s Next For OFDMA?

Despite these challenges, turning on OFDMA is likely a no-brainer for many operators. It is the least intrusive option on its own and is highly synergistic with the other options

Required to leverage spectrum above 85MHz for high split
Increased SNR from DAA’s digital optical link enables higher OFDMA modulation orders

For regions with significant DOCSIS 3.1 CPE deployments OFDMA is already becoming ubiquitous, for lagging regions it is yet another driver for CPE modernization. Operational benefits delivered by OFDMA will continue to improve as 3.1-specfic PNM capabilities gain adoption, and spectral efficiency will continue to improve as artificial intelligence (AI) and machine learning (ML) concepts evolve to further optimize profile management. OFDMA is already helping us get More Bits/Hz today, but we’ve only scratched the surface of what it can deliver in the future.

Want to learn more? Listen to the Broadband Lounge Podcast Series or visit the VIAVI Advanced Upstream Page for more information.

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