Back to School: ‘Orthogonal’


It seems that school starts earlier
every year, but where I came from,
the yellow buses started rolling
the day after Labor Day. In that
spirit, this week’s column tackles
a term you’ll likely hear more and
more as tech time marches on:

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

As an everyday term, though, “orthogonal” is still largely
inscrutable: It’s a math term, meaning “at right angles.” Generally
speaking, when people say “orthogonal” they mostly
mean “irrelevant” — one doesn’t affect or disaffect the other.

In cable-tech talk, listen for “orthogonal” from broadband-
side technologists working on what comes next with
the DOCSIS cable modem specifi cation. One of the potential
expansions: OFDM, or orthogonal frequency-division

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

Refresher: Modulation is the process of imprinting information
onto a communications carrier 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 downstream, too, if needed.)

OFDM, with improved error correction, could kick up
some serious capacity — half again as much as what’s already
down there. That’s good, because the upstream path
is a slender 5% of total available capacity on any cable system.

How does one 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, CDMA, a variation of which (synchronous
CDMA) is already used in today’s cable modems.

Lamarr, a celebrated MGM actress in the ’30s and ’40s,
was romantically involved with a ranking military official
in Europe; she endured being treated like a bubblehead
long enough to collect the facts she needed to develop
what is now CDMA.

CDMA works by compartmentalizing a signal into a series
of packets, which are smeared across a chunk of spectrum
for transmission. It’s like OFDM in that it moves
data over smaller spectral slices.

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

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