OFDM

DOCSIS 3.1 in ’13

By now, hopefully, you’ve heard that there’s a new chapter coming in cable modems. It’s the latest iteration in the specification known by technologists as “DOCSIS 3.1” for “Data Over Cable Service Interface Specification.”

DOCSIS 3.1 is a doozy — both in terms of what it will do for broadband capacity, and the sheer density of the tech talk that surrounds it.

Hey! Let’s face it. “QAM” is a little long in the tooth, as impressively nerdy industrial tech-talk goes. Not to worry. With 3.1, you too can impress your friends and colleagues by blurting out 3.1-speak like “Orthogonal Frequency Division Multiplexing with Low Density Parity Check.”

Feature-wise, DOCSIS 3.1 is so crammed with improvements that some of us wondered why it didn’t qualify as “DOCSIS 4.0.” (Answer: To thwart any misperceptions from the investor community about “forklift upgrades.”)

First off: DOCSIS 3.1 matters and was devised because of the billowing consumer demand for broadband usage — 50% and higher compound growth, since about 2009. Again: In the history of consumable goods, nothing has grown at a sustained rate of 50%, year over year.

DOCSIS 3.1 basics: When complete (2013) and in market (2014?), it will expand the industry’s downstream and upstream carrying capacity for digital, IP traffic by 50%.

“Half as much again” is always a big deal, especially for that spectrally anemic upstream signal path.

Also impressive about DOCSIS 3.1: It could enable connection speeds of 10 Gigabits per second (Gbps.) Note: Don’t inhale too deeply on this one. It’s 10 Gbps if and only if all other channels on a system are empty. No analog, no SD or HD video, no broadband, no voice.

Let’s get back to the tech-talk of 3.1. What makes for these enormous gains in IP capacity and speed is a new (to cable) form of modulation called “OFDM” (see above.) OFDM, when coupled with a new (to cable) form of forward error correction (LDPC), brings the 50% efficiency gains.

OFDM is widely used by mobile carriers, because they’re already pretty bandwidth-challenged (ship any video from your phone lately?). It works by chopping the typical 6 MHz digital cable channel into smaller “subcarriers,” in the lingo. That’s good for both transmission and dealing with impairments.

That’s the basics of DOCSIS 3.1 — why it matters, and how to talk about it with aplomb. Watch for it to be a major undercurrent of the 2013 cable-tech scene.

This column originally appeared in the Platforms section of Multichannel News.

How to Talk to People Who Use the Word “Orthogonal”

by Leslie Ellis // September 04 2012

Here’s one that’s sauntering back into tech-speak: Orthogonal.

Orthogonal is an oldie-but-goodie tech term, sure to imbue the person who utters it with an unmistakable whiff of tech intelligence. (It has nothing to do with your feet or shoes.)

As an everyday term, though, “orthogonal” is still largely inscrutable: A (math) term, meaning “at right angles.” Generally speaking, when people say “orthogonal” and they’re not talking about OFDM (yes there are people who say “orthogonal” conversationally), they mostly mean “irrelevant.” One thing doesn’t affect or disaffect the other thing.

In cable tech-talk, listen for “orthogonal” in tech-talk from broadband-side technologists working on what comes next with the DOCSIS cable modem specification. One of the potential expansions: OFDM, or, Orthogonal Frequency Division Multiplexing.

Know going in that OFDM is an upstream modulation thing, just as is QAM (quadrature amplitude modulation), QPSK (quaterny phase shift key), and S-CDMA (synchronous code division multiple access.)

Refresher: Modulation is the process of imprinting information onto a communications carrier, itself an electro-magnetic wave, so as to move that information from one physical location to another.

So, people talk about OFDM in the context of getting more stuff upstream, or responding to faster upstream speeds. (OFDM could be used for downstream modulation, too, but that’s not the initial application.)

OFDM, with improved error correction, could kick up some serious capacity – half as much again as what’s already down there. Which is good, because the upstream path is a slender five percent of total available capacity on any cable system.

How to earn style points when talking to engineers about OFDM: Ask the person who utters “OFDM” what they think Hedy Lamarr would say, if asked how it compares to her invention of CDMA. (A variation of which – Synchronous CDMA, or S-CDMA — is still used in cable modems.)

Lamarr, a celebrated MGM actress in the ‘30s and ‘40s, was romantically involved with a ranking military official in Europe, and endured the tedium of being treated as a bubblehead long enough to collect the facts she needed to develop what is now CDMA. (It also goes by “spread spectrum.”)

CDMA works by compartmentalizing a signal into a series of packets, which are smeared across a chunk of spectrum for transmission. It was initially used by the Dept. of Defense to transmit coded information. It’s like OFDM in that it codes data to move over smaller spectral slices.

But in every other sense, OFDM is essentially orthogonal to your everyday life.

This column originally appeared in the Platforms section of Multichannel News.