[LINK] RFC: Shannon's Law and RFID
rchirgwin at ozemail.com.au
rchirgwin at ozemail.com.au
Mon Aug 23 12:39:41 EST 2004
After last week's long RFID thread, I decided to write the article
pasted below the signature.
Before I publish it, I'm soliciting Linkers' comments on the technical
accuracy of the piece. I am trying to explain Shannon's Law to a lay
audience, which will be a trick, and I want to make sure I haven't
strayed into the sort of technical inaccuracies which I hate in others!
Replies probably offlist so as not to trigger more theology!
RFID Capacity and Reach
Why Shannon’s Law Matters
A couple of weeks ago, partly because of the misconceptions I observed
even in technically-savvy people, I decided I should write a series of
features in which I looked in-depth, and independently, at RFID
technology. That way, I might walk a fine line between the Philip K
Dick-like fears of its opponents, and the tech-utopian promotions of its
However, the more I discussed, debated, and analysed peoples’
understanding of RFID, the clearer it became that there are other things
which have to be explained along the way. RFID has a great many
interactions with very fundamental communications concepts - and one of
them is Shannon’s Law.
I suspect one reason that RFID is so feared on one side, and so revered
on the other, is that a great many people believe in a technology
development curve that will go upwards forever - and, because they’re
not mathematicians or technologists, they compare the unknown (like
RFID) to the familiar (like Moore’s Law). Moore’s Law (which predicted
the way the growth in processing power would stay in step with falling
prices) hasn’t run out of steam in processors, RFID is just another
application of processors, therefore RFID’s capabilities will be as
limitless as the development of the microprocessor, right?
Well, yes - if we’re just talking about the processor itself. I’m quite
happy to believe that Moore’s Law will work just as well in the silicon
we build for RFID chips as it works for Intel CPUs. It is, in fact, one
of the reasons that processors are now small enough to fit on
But the microprocessor isn’t the only constraint an RFID tag has to
respect. The tags communicate over radio channels - and in radio
communications, an older principle applies.
My guess is that people with radio communications training don’t need to
have the maths explained to them; and people without that background
don’t want the maths. So as far as possible, I’m going to follow
Hawking’s principle and avoid setting down formulae for people to memorise.
Fortunately Shannon’s Law is easy to put into words. Stated by one of
the founders of the discipline known as information theory, Claude
Shannon, it predicts the useful capacity of any communications channel.
In a perfect world, the “bandwidth” of a communications channel and its
throughput would be identical.
If you count /everything /occupying the channel, that’s true. The
problem for the user is that their data traffic isn’t the only thing
using a communication channel.
There’s also noise. Whatever space the noise takes up in your channel,
there’s less room for “payload”. The more noise, the more the “carrying
capacity” of a channel approaches zero.
The equation itself can be found here (a Wikipedia link).
So it’s simple: channel capacity falls as signal-to-noise (SNR) ratio
falls. In a perfect world, where SNR is 1, the channel capacity and the
bandwidth are identical.
One of the aims, for all radio communications designs, is to find better
ways to separate the signal from the noise - to preserve as much
bandwidth as possible for the payload.
In fact, Shannon’s Law doesn’t just work for radio communications - it’s
applicable for wireless, wired, even fibre-optic communications. The
system with the least noise will always preserve the greatest amount of
the “raw” channel capacity for user communications.
If you have a “raw” capacity of 10M bits/s, the wireless medium might
get a throughput of 1M bits/s. Copper wire, however, experiences less
noise, so it might preserve 4M bits/s of that channel for the end user.
Fibre - even starting just with that 10M bits/s channel - gets the best
throughput (maybe approaching 10M bps), because it suffers the least
noise. An optical fibre doesn’t couple with external signals - outside
light doesn’t get in.
And this example completely ignores the difference between “raw”
capacity - in signal-to-noise terms alone, the hierarchy puts light as
the best, copper in the middle, and wireless at the bottom.
So what can we do to “work around” Shannon? Only two things: improve the
“raw” capacity of the channel, or improve its signal-to-noise ratio.
Capacity, Power, Compression
Working at higher frequencies is one way to improve a channel’s raw
capacity - if you start with a carrier frequency in the Gigahertz range,
you’re starting well ahead of someone whose carrier frequency is in the
The second is to use a “wider” channel.
This needs a little explanation. Most people are accustomed to thinking
of radio signals as occupying a single frequency - for example, ABC 702
in Sydney just uses the 702kHz frequency, right?
Wrong. The “702kHz” designation refers only to its carrier wave
frequency. The carrier is a pure sinewave, but as soon as you add the
voice of Angela Catterns or Richard Glover, you end up with a complex
mix of many frequencies starting at 702kHz - X, and ending at 702kHz + X.
For someone talking on AM radio, “X” is only around 12kHz or so - and
one of the reasons we have spectrum management is so that the next radio
transmitter doesn’t send a signal which your radio confuses with 702.
If you are sending (for example) 10MHz of data, you will occupy a
“wider” channel (the emerging next-generation Wireless Ethernet
proposals need a 40MHz channel to achieve predicted throughput of 100M
bps using a 500M bps-plus “raw” speed).
“Raw” speed isn’t the only way to improve throughput: you could also
work to improve the signal-to-noise ratio, and the easiest way to do
this is to pump up the power. Apart from human-generated interference,
the amount of noise appearing on a radio channel is nice and predictable
- so a 1,000-watt transmitter gives you a lot better signal-to-noise
ratio than a 1-watt transmitter, and lets you keep more of your “raw”
Of course, if you endlesslly “pump up the volume”, you’re going to
eventually run afoul of the regulator; not to mention you’ll eventually
start to notice that your skin burns whenever you’re too near the
transmitter. Boosting the power is only viable up to a point.
The third option is to reduce your demands - compress the data so that
the user experiences throughput of 10M bits/s but is only actually
sending 1M bits/s.
But Aren’t Radios Smarter?
But they don’t somehow magically “break” Shannon’s Law. Techniques like
spread spectrum - and in fact all of the computationally-expensive new
modulation techniques - give you better throughput by combining the
above workarounds: they start with a “wider” communication channel (so
regardless of SNR, the starting point is more “raw” capacity); or they
use their “smarts” to tell the difference between “signal” and “noise”
(thus improving the SNR); or they compress the data traversing the radio
channel, so as to give the user more apparent throughput than the
They might even adjust the power of the transmitter to compensate for a
Smarter radios don’t break Shannon’s Law: they adjust capacity and
signal-to-It noise ratio to get the best possible throughput.
By the way, this also applies to copper-based communication. The history
of broadband on copper networks is one of using more intelligent, more
powerful, and faster devices to adjust the Shannon’s Law parameters of
Let’s apply all this in, for example, WiFi, which uses spread spectrum
radio technology to get really fast communications.
A full explanation of spread spectrum is another article entirely, but
if you combine a radio that’s able to transmit on multiple carriers at
once with a really smart processor at each end, you get a wider radio
channel than if you just used one carrier frequency. So the “raw”
Moreover, because the communication is using multiple frequencies,
negotiated between the devices at each end, the processor has a better
chance of distinguishing between signal and noise - so the SNR improves.
Up to a point, WiFi devices can tell whether a device in the same
“footprint” is part of their network or not - my access point can decide
that someone else’s access point is “noise” without losing performance.
It all comes unstuck, however, if there are too many overlapping devices
on different networks - which is why there have already been reports of
hotspot “wars” in which people just keep ramping up the output power of
their access point.
There is, of course, one last way to separate signal from noise: use a
highly directional antenna. That way, nearby transmitters don’t
contribute their own transmissions to “your” channel as “noise” - and
you get to keep more of the raw capacity. Once again, this doesn’t
change Shannon’s Law, it merely changes the value of one of the
Shannon’s Law variables.
Back in he world of RFID, what we’re talking about is really small
devices, with limited transmitter power, limited processing power, with
regulatory limits on the radio systems.
Active RFID devices - those which have their own battery to provide
power - are far less constrained than the passive tags, but since it’s
the passive devices that get all the attention in retail settings, I’m
going to stick with them. Active devices have one very important
characteristic for this debate: they’re expensive, so retailers don’t
like giving active tags away with products. Of course, there are
intrusive applications of active tags - for example, the possible
data-matching provided by tollway tags is a handy example - but it’s
consumer goods that worry consumers.
And it’s also, frankly, the retail application that’s getting hyped to
the skies. Just about everywhere you turn, there’s a vendor promising
things like completely automated supermarket checkouts, really cool CRM
applications based on the tag in the shoe, and so on.
I’ll give this more detail in a future article, but I’d at least like to
set down a general principle: Shannon’s Law, not Moore’s Law, is the one
that’s going to set the long-term constraints on just how effective RFID
can become in retail settings.
Your power budget in the device is extremely limited: you start by
getting power “over the air” (the devices are powered up by what they
get from the radio signal sent by the tag reader). They start with
microwatts or nanowatts; some of that is needed to power-up the
processor, some to run the radio transmitter, and the rest is wasted by
There’s been talk in the local media that the RFID business associations
would like the Australian Communications Authority to lift the
transponder power limit from the current 1W to 4W. I understand why -
but even that’s not going to make a huge difference to capability. At
best, it’s going to mean systems that are only marginally reliable might
become nearly-feasible, but it’s not going to give the world a pervasive
RFID scheme that actually works.
Because the tags have so little power, there’s none to spare for fancy
radio modulation schemes. Instead of something like spread spectrum,
which least improves the signal-to-noise ratio, passive tags use an AM
modulation scheme, and they all use the same frequency.
The tag reader has a moderately directional antenna - but the tag
itself, which is transmitting on tiny power levels, has to spray its
communications in all directions. It doesn’t know where the transponder
is located, and you can’t guarantee that someone is handling the tag to
make sure it’s “pointed” the right way. So a smarter antenna scheme
doesn’t help much.
All of these, as I said, fall under Shannon’s Law. There is one place
where Moore’s Law is going to help the tags: their electronics will get
smarter and use less power. Perhaps one day the microprocessor on the
RFID tag will have enough grunt to add compression to its capabilities,
and increase throughput that way - but that’s about all.
Retailers looking at RFID should be asking the vendors about the real,
practical constraints on the technology. And consumer advocates
preaching excessive fear would do well to learn just a little bit of
maths, get a handle on Shannon’s Law, and focus instead on the back-end
data applications that already represent dangerous privacy invasions.
If you’re not interested in RFID at all, try thinking about Shannon’s
Law and wonder whether wireless will truly destroy Telstra.
The second article in this series will look in more detail at passive
RFID tag technology.
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