Subject: [NEXTEL-1] How Would Shannon View Broadband over Powerl Line?

From: [email-masked]
Date: Fri, 28 May 2004 12:26:20 -0400

How Would Shannon View Broadband Over Power Line?

F.H. "Rick" Smith
 ChevronTexaco, Inc.

Back in 1948, Claude Shannon, discovered the mathematical
and theoretical limits of a communications channel to carry
information. Today, we are pondering how Shannon's law may
impact proposals that would bring "Broadband Over Power Line"
into every home across America.

We see a classic example of Shannon's theories played out
in our own lives every day as we continue to wonder why our
dialup computer modem seldom runs at 56K! Because we are
talking about Broadband Over Power Line (BPL), not dialup
modems, we need to go back to Shannon's law, which in
simple terms states that the capacity of a communications
channel is both a function of the channel's bandwidth, and
the channel's signal-to-noise ratio. In the case of a dialup
modem, the channel bandwidth is fixed, and it is the signal-to-
noise ratio that exists when the signal reaches your house that
ends up establishing your speed.

The tradeoffs between usable bandwidth, signal-to-noise ratio
and information throughput are all big realities as we dig deeper
into the challenges of BPL. Keep in mind that BPL, like cable
modems, is a shared resource where a lot of users will be sharing
the same power wires. This fact, when viewed with the Moore's law
mentality (which says more is better and the marketplace will drive
performance to infinity) will likely cause the bar for BPL to be set
unrealistically high. This market may reluctantly accept data rates
starting out at 20 to 50 megabits per second, but the true Moore's
law believers will be turning up their noses at BPL technology if
100+ megabit per second speeds don't happen soon.

Let's look at the challenge of BPL within the context of the realities
Mr. Shannon discovered. The channel bandwidth that BPL has to
work with is limited. With frequencies below 1 MHz -- although
leakage and losses are low -- the electrical background noise
escalates rapidly. On the high side, in frequencies above 20 MHz,
electrical noise becomes less of an issue, but signal leakage and
losses escalate rapidly as the power wires start looking more like
an antenna/transmission line than a pure transmission line. There
has been some talk about BPL carriers going up to frequencies
around 80 MHz. At 80 MHz, a power line carries BPL about as well
as a soaker hose carries water. Clearly, there is a range of frequencies,
sort of a sweet zone, within which it makes sense for BPL to operate.

I don't know exactly what that sweet zone is, but it is important to note
that its width is much less than 100 megahertz-wide, maybe about 20
megahertz-wide. Mr. Shannon wouldn't tell you that you cannot get
100 megabits of information down a 20-megahertz wide channel, but
he would remind you that you better have a pretty good signal-to-noise
ratio to make it happen.

There are two ways to get the required signal-to-noise ratio. 1. Make
the signal bigger, or 2. Make the noise smaller. There is only so much
you can do to make the noise smaller, forcing BPL proponents to push
the limits on the BPL signal level. BPL proponents have suggested they
can get the job done and still comply with current Part 15 radiation limits
designed to protect through-the-air radio communication services from
interference. While I do not share their confidence that this can be done
within the context of moving 50 or 100 megabits per second of data, the
real challenge is more difficult than this.

Part 15 limits were never intended to have every nook and cranny of
spectrum filled to the limit with signal. Keep in mind that one can
transmit
a signal for some distance under the current Part 15 limits. Current limits
have generally served their intended purpose. The percentage of
frequencies that are affected at any one time (today) is typically small.
Therefore, the probability that a particular frequency on which a "through-
the-air" listener is using will be impaired is also small. BPL will change
the
probability of interference (at least as defined by through-the-air users
experience, not Part 15) from small to large.

Don't forget, it was not too long ago that we thought interleaved channels
at 800 MHz was a good idea. At that time, it was assumed that towers
would always be away from users, up on a mountain, or on a tall building
somewhere, not down in the neighborhoods. Nextel expanded into the
neighborhoods of America to meet the demand for service, and they did
it in compliance with the rules, yet problems with Public Safety
communications
arose. Why? The rules, at least with respect to the channel interleaving,
were made inappropriate by this new application.

The issue and implication that the current Part 15 rules may not be
appropriate for BPL in terms of their original purpose, which is to
protect through-the-air listeners, cannot be over stated! The real proof
of performance the Commission needs to push for is to shift the burden
of proof onto the BPL proponents to demonstrate that BPL, on the scale
that the marketplace demands, will not, as a practical matter, demolish
"through-the-air" services.

Source: http://www.ita-relay.com/magazine/PW2004/May2004/Chair.pdf

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