Wireless Systems Design

# Breakthrough Modulation Method Minimizes Bandwidth At Any Speed

Those of you who work in wireless know the basics, such as the higher the data rate, the wider the bandwidth you are going to use. With spectrum being in such short supply, or perhaps just way too expensive, wireless engineers have resorted to all sorts of tricky methods to squeeze more bits per Hertz of bandwidth. Spectral efficiency is the name of the game, and researchers seek the holy grail of wireless that improves bits/s/Hz beyond current technology. Well, here at last is a technique that makes that problem go away—or at least be greatly mitigated. This new modulation method is so obvious, that no one has stumbled upon it. Yet here it is, proving that Shannon-Hartley is not the only law to follow. With this new method, you can achieve nearly any desired data rate in a bandwidth not a whole lot greater than zero.

Do You Believe in Magic?

We all know that Shannon and Hartley defined a basic law that all communications professionals learned early on and have been fighting since day one. In a nutshell it says:

C = B log2 (1 + S/N)

C is the channel capacity in bits per second (bits/s), B is the bandwidth in Hz and S/N is the power signal to noise ratio. We have all probably played around with this expression not only in trying to glean what the devil it really means, but also to figure out a way to beat it. I always did struggle with it in trying to figure out how a dial-up modem was able to get 50 kbits/s+ through a 4 kHz bandwidth telephone line. Or how to squeeze HDTV video into a narrow 6 MHz bandwidth in cable TV. Compressed 64QAM comes to mind. Anyway, you get the picture. But the bottom line remains: if you want higher data rates you have to have more bandwidth.

Some wireless techniques take a different approach. Take Orthogonal Frequency Division Multiplexing (OFDM) as an example. What new wireless method is not OFDM these days? Take the fast data, divide it up into slower parallel data streams, and modulate them on to hundreds or even thousands of orthogonal carriers spread over a wide bandwidth. Cool technique. It is a broadband method and not a narrow band method of getting more bit per Hz. Direct sequence spread spectrum (DSSS) is another widely-used technique.

Now you can have beat the normal Shannon-Hartley law and get near zero bandwidth for almost any data rate. I am sure you are saying that’s nuts. But just hear me out.

The Ultimate Near Zero Bandwidth Method

You may have heard of a technique called Ultra Narrowband Modulation. It has been around for years and is controversial to say the least. Everyone who studies it says, "You must be kidding me.” And many have tried to duplicate the method that some claim works like a dream. Well, here is another method that does really work to create a signal with near zero bandwidth and is independent of data rate.

The technique is best explained with the block diagram shown in the Figure. If you analyze the diagram closely, you will see that it uses standard circuits and techniques that do indeed produce a zero bandwidth for any data input. Do the math if you will and to prove this to yourself. Not bad, huh? The secret to getting zero bandwidth lies in careful match/balance in the balanced modulator/mixers and the precision of the 90-degree phase shifter.

Lots of you are probably thinking, “Duh....why didn't I think of that? Especially since (good news!) it’s IP free. No patents or any restrictions because it uses basic circuits and methods that have been around for years and are easily realized in ICs. Why isn't anyone using this? Beats me, but with this revelation, I suspect we will see multiple suppliers and new wireless standards developed to take advantage of a technique that is as basic as Ohm's law. And it doesn’t violate the Shannon-Hartley law.

Remember, you heard it here first in the Wireless Systems Design UPDATE.

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