What’s trending in cable? It’s OFDMA or orthogonal frequency division multiple access. OFDMA is the upstream counterpart to OFDM, which stands for orthogonal frequency division multiplexing. Both are part of the DOCSIS 3.1 specification and are intended to increase speed over the DOCSIS network far beyond what can be achieved using legacy SC-QAM channels. Most cable operators deployed OFDM first because the downstream had already reached data capacity limits. OFDM enabled expanded data capacity without node splits. Now that OFDM has been successfully deployed, it is time to deploy OFDMA to achieve the same capacity benefits in the upstream.

Why deploy OFDMA?

Should you or shouldn’t you deploy OFDMA in your plant? This is an important question that should be asked inside any organization before rolling out any new technology. Sure, OFDMA is trending now, and it seems like everyone is trialing OFDMA, but does that mean you should be doing it? There are some factors to consider.

First, have you successfully deployed OFDM in the downstream? Prior to considering OFDMA in the upstream, it is a sound decision to deploy OFDM in the downstream first. The downstream typically suffers from fewer impairments than the upstream. This means deploying OFDM has a higher chance of success than deploying OFDMA. During the deployment process, cable operators will have an opportunity to learn a lot about the differences between SC-QAM channels and OFDM channels, which will translate to upstream OFDMA later.

Next, does your market require OFDMA technology? OFDMA is an enabling technology to increase data speeds in the upstream. If the market you are in does not require upstream speed expansion, then OFDMA is likely not required at this time. The bottom line is that every market and perhaps every fiber node should be assessed on a need basis. This is a not just a technical decision, but also a business decision. Deploying OFDMA requires new tools, software, and licensing in the CMTS. Choose wisely.

What are the differences between OFDM and OFDMA?

If you have already deployed OFDM in the downstream, the good news is that OFDMA has a lot of similarities with OFDM.

Much of the terminology is identical, such as subcarrier spacing, error correction, and getting away from standard 6 MHz channel spacing. The same DOCSIS 3.1 CMTS and cable modems that work with OFDM channels will also support OFDMA channels. Similarly, test equipment and software that works with OFDM channels should also support OFDMA channels — perhaps with some software and/or firmware updates. Again, this is a good reason to deploy OFDM before attempting to deploy OFDMA.

The differences between OFDM and OFDMA are where some of the challenges may arise. OFDM is a one-to-many technology. This is very similar to SC-QAM channels in the downstream; in both cases, the downstream channel is transmitting data to many cable modems. OFDMA, on the other hand, is a many-to-one technology. In this case, OFDMA channels can have multiple modems transmitting in the channel at the same time. The concept of many modems transmitting at the same time in the same OFDMA channel is where the “MA” part or “multiple access” comes into play in OFDMA. Why does this make a difference? In the downstream, the OFDM channel is always on and always visible because it is always transmitting data. In the upstream, the OFDMA channel is only on and visible when a modem or modems are transmitting upstream data. If only one modem is transmitting it is likely that a small portion of the OFDMA channel will be visible on a spectrum analyzer. The only time that the entire  . For those who are familiar with upstream SC-QAM, this partially occupied spectrum will be familiar, but partially occupied spectrum is much more pronounced in an OFDMA channel. So do not be concerned if the channel is often not visible, especially during non-peak hours or in a fiber node with few modems or no modems online.

OFMDA channels are also missing the PLC that is found on the downstream OFDM channel. The smallest channel bandwidth for an OFDMA channel is 6.4 MHz with the largest channel size being 96 MHz (95 MHz encompassed spectrum). But it is possible to use just about any bandwidth between 6.4 MHz and 96 MHz with some caveats. This makes for a very flexible upstream now that one is not limited to 3.2 or 6.4 MHz upstream channels.

OFDMA — the future

While I have just touched on a few key points, the takeaway I want to leave is that OFDMA did have some rocky starts, which I covered in a previous article “Why are the DOCSIS 3.1 Cable Modems Crashing with OFDMA?” (https://broadbandlibrary.com/ofdma-2/). However, vendors have made tremendous improvements in OFDMA performance on the hardware side. As an industry, we will eventually move towards a goal of two upstream 96 MHz OFDMA channels. This end objective will enable up to 1800 Mbps on the upstream. This means the ability to provide true 1 Gbps symmetrical service to subscribers over coax.