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Some tricks for radio communication

German translation by Alica Slaba: http://www.piecesauto.fr/science/?p=3488
Georgian translation by Irakli Nishnianidze: http://theautoz.com/blog/some-tricks-for-radio-communication/ courtesy of theautoz.com





I thought of following systems in order to build devices I needed. Some of them are classical solutions.



1. The sign detector

A receiving antenna is a device that produces a weak electric signal. That signal is intended to be fed to the receiving electronics.

The trick is to first input the antenna signal into a sign detector. That is a device that outputs a binary signal: 0 if the antenna signal is negative, 1 if it is positive. A comparator, an operational amplifier with a high gain, a c-mos TTL gate or a saturating receiving antenna amplifier may be used for that purpose.

The fact the sophisticated antenna signal becomes a simple binary signal allows the rest of the receiving electronics to be mainly a rudimentary 1-bit digital circuit.

Take for example a superheterodyne FM receiver where the multiplicator is a precisely adjusted MOS-FET transistor used in his resistive functioning area. Thanks to the sign detector MOS-FET and passive components can be replaced by a simple X-OR gate. One input of the gate receives the binary antenna signal, the other gate receives a digital clock signal with a frequency slightly different from the signal frequency. The X-OR gate output may be directly fed into a rudimentary low frequency filter.

Imagine the use of a microcontroler to be an AM receiver. Thanks to the signal detector only one digital microcontroler pin will be necessary to input the signal. And only simple 1-bit instructions will be used to increment and decrement the two receiving accumulators, with no fear for saturation or counter loop.

This system may make you fear it implies a loss of information. There will indeed be a loss of information for receptions with a high signal/noise ratio. But there will be no loss for receptions with a weak signal/noise ratio, and that's what matters.

The sign detector offers three main advantages:

And four drawbacks:


2. The modulators and demodulators chain

A common radio emitter contains one modulator. That is a device that produces a high frequency signal whose amplitude (or frequency, or phase shift) is influenced by the signal you want to transmit. For example: full amplitude to transmit a binary 1 signal, zero amplitude to transmit a binary 0 signal.

A common radio receiver contains one demodulator. That is a device that inputs a high frequency signal and outputs a usable signal. For example: a binary 1 is output if a high frequency signal is detected, a binary 0 is output if no high frequency is detected

Suppose we send 1,000 binary bits per second. And we use a high frequency of 1 Mhz. Then we need clocks with a precision better than 1/1,000 th.

Suppose we decide to use a higher frequency; 100 MHz. That frequency being 100 times higher, we are supposed to be able to send the signal 10 times further. (This is not true in fact: because there is less environmental noise at higher frequencies we will have more than a 10 times increase. Unless there are obstacles like a hill or the earth curvature, then low frequencies, by diffracting around obstacles and bouncing back on the ionosphere, can travel a lot further than high frequency signals. But here we neglect these phenomenons.)

Yet this requires our clocks to have a precision of 1/100,000 th.

Suppose we do not own clocks with such a precision. We only have clocks with a precision of 1/1,000 th. Then, although we transmit at 100 MHz, we will have no increase in transmission distance.

A solution is to you use intermediate modulators and demodulators. (This is commonly done for signal encryption or transmission of several signals on a unique radio frequency. An example: the stereo difference signal, for standard FM receivers, is encoded on a subfrequency of 30 kHz that is added to the common mono signal. Another example: RC systems for modelists use several modulators in parallel, called "ways", using frequencies in the order of 10 kHz, to transmit commands to different actuators inside the model plane, ship or car. The modulators signals are added before being fed into the radio modulator. Inside the model the received signal is given to all demodulators and each one of them, having his own frequency, extracts the subfrequency intended for him. But here we do it to increase the transmission distance.)

We use our binary data signal at 1 kHz to modulate a binary frequency at 1 MHz, then we use that modulated 1 MHz signal to modulate a radio signal at 100 MHz.

The receiver will contain a demodulator with an output at 1 MHz that will be fed into a second demodulator that will render the 1 kHz digital binary data signal.

That way, we will able to transmit further while using low precision clocks!

The distance increase will not be of 10, it will be less. There is a factor two difference. Yet the result is still important.

A chain of more than two modulators and demodulators can be used. They may be three, four, five... That way it is theoretically possible to transmit a signal up to any distance, whatever the clock precision.

It is possible, for example, to transmit one bit per hour with a 1 GHz radio signal thanks to two modulators using little quartz crystals with a precision of 1/1,000,000 th (available in common electronic components stores for $2). That's very interesting for special purposes like space probes or measurement and activation systems in very noisy environments. Would such a transmission system have been programmed into the processor of the Pioneer 10 probe, we would still be in contact with it while using a much cheaper antenna system. Maybe the transmission rate would be of a few bytes per year, yet that's enough to receive some key information.

Most funny with this system is the fact at big distances the first demodulator only feeds a noise to the second demodulator. Yet the second demodulator manages to detect the signal out of that noise!

As mentioned above, this system is not strictly equivalent to using a single modulator and demodulator stage with high precision clocks. There is a loss (comparable to the particle/wave packet detection in quantum mechanics). Therefore the number of stages should be kept as little as possible and best possible clocks should be used.

Another problem is the fact the radio signal will use a bandwidth a lot broader than strictly necessary. This may be partially compensated by the fact several transmissions may work at the same time, using the same main radio frequency, but using different subfrequencies. If one emitter emits at 100 MHz a signal modulated by a signal at 1 MHz that is modulated by a signal at 1 kHz, it will not be heard by a receiver that receives also at 100 MHz but then demodulates at 2 MHz to produce also a signal at 1 kHz. That way the bandwidth can be used effectively, without too much wasting. It is simply another way of sharing the bandwidth.

Other advantages are:


3. The fractal signal

The system described above implies information is transmitted at a given rate; 1 kHz in the examples.

Suppose now you just want to transmit a "ON" signal. To call for help, for example.

The problem is it is difficult to choose at what speed you want that signal to be transmitted. Suppose you choose 1 second. Well that's quick. Okay, but what if you are very far from the receiver? Your signal will not be heard. Unless you transmit slower, say on one hour. That way you can transmit 60 times further. But that's stupid if you were close to the receiver. Why wait an hour before the rescuers are warned?

When using only one modulation and demodulation stage, the solution is simple: just let the receivers' accumulators accumulate information on a one hour basis, but look at their content every second. If a strong signal arrives it will make the accumulators increment very fast. That will be noticeable and will allow the system to react instantly.

But what if a demodulators chain is used ?

The solution is to allow each demodulator stage to have also the property to detect the signal at his frequency, instead of only feeding it to the next demodulator. That way, if the signal is very strong, the first demodulator will instantly react. "I hear it!". If it is weaker, the first demodulator will only hear a noise. That noise is fed to the second demodulator stage that will detect the signal and react, yet his reaction will be slower. If the signal is too weak for that second demodulator too, the noise fed to the third demodulator will perhaps make him react, yet still again a lot slower than the second. And so on, you may use a guirland of demodulators/detectors, allowing the signal to be detected in a few milliseconds or in a year...

At each stage there must always be a signal at any time. For example there must always be a frequency of x Hz or 2 x Hz. The way the signal changes between the two frequencies is used to feed the next demodulator. But the demodulator will react if any of the two frequencies is detected.

When you plot such a modulated signal, you get a one dimensional fractal pattern. Previous system also had a fractal look, yet here the full identity of a fractal appears: the fractality can be developed up to a virtually infinitely low frequency, and every stage has the same purpose: make a demodulator react.


4. The noise link

In general the basic signal is a sinusoid. That sinusoid is modulated in order to transmit information.

You may use anything else instead of a sinusoid. You may even use a noise; a totally non-repetitive random signal.

The only condition is that the receiver must know that noise. It must know any detail of it and know when it is emitted. Precise clocks and time shift adjustment techniques must be used.

For secret transmissions, the advantage of using a noise is obvious. It is totally unhearable for receivers who do not know what noise exactly is emitted. There are just three drawbacks:

Best way to modulate the noise signal is to invert it. For example it is emitted non inverted for digital 1 and inverted for digital 0. The receiver just has to multiply the antenna signal with the noise and sum the results to get big numbers that will indicate digital 1 if they are positive and digital 0 if they are negative. That way the emitter emits constantly with the same power. (This method of multiplicating the signals and summing the results is called a correlation.)

Again, the time shift is a serious problem. If the receiver is not locked on the emitter, it must try out all possible time shifts until the signal is found. Then the signal must be followed carefully in order not to be lost.

One way to emit a noise-like signal while being closer to usual techniques is to emit the sum of a wide range of ultraweak sinusoids. The receiver will also contain a wide range of demodulators, one for each frequency, yet all demodulators use the same shared accumulators to sum their sine periods. Those single accumulators, containing the sum of all sums of all demodulators, will then be used to tell if there was an emission or not. (Such a set of demodulators can also be used to determine if for example an emitter is emitting a continuous series of Dirac wave patterns.)


5. Modulate several bits together

At first hand the method to transmit bits along a very noisy channel is to modulate the status of each bit and send each bit the one after the other. Yet there is a drawback. If say a hundred or a thousand modulation periods are used for each bit then the transmission yield will be quite far away from the maximum baudrate allowed by the Shannon theorem. A better yield is achieved when a lot more modulation periods can be used for each bit. Say ten times up to hundred times more. Yet this would imply a slower baudrate. the solution is to transmit several bits together, each with its own modulation. If ten bits are transmitted together, then the number of signal periods available for each bit will be multiplicated by about ten.




Eric Brasseur  -  1 January 1997
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