Electronic – modem free high base AM data transfer possible

dataModulationtransmitterwireless

not a very experienced EE, but I was thinking about a system for high speed data transfer,
and it is fairly simple conceptually, so I imagine that there must be serious flaws with the method,
because to my knowledge it is not implemented, or at least not widespread. I apologize in advance for
my ignorance of the subject matter (I'm just starting my learning).

The way I understand it, binary signals enclosed in a carrier wave are modulated/demodulated through a modem.
I understand why this was done in the past – processors weren't that hardcore, and the hardware can only
understand binary. I guess I just don't understand why it is still done this way.

If we modulate the amplitude of a wave (I think by providing the oscillator different levels of current),
can we not sample this wave with some sort of analog to digital converter and process it on the CPU?

If this is possible, why stick to base 2? If we can have a unique value for each measurable amplitude,
data transfer rates would skyrocket. Imagine transferring data with base 1024, or even higher. If we
could accurately sample the wave (each oscillation), I don't see why the rate of transfer could be equal
to the frequency of the wave times base divided by 2 bits per second (this is probably not correct mathing).

If we have a processor running in the gigahertz, and a signal in the low megahertz, it seems feasible that
the processor would be able to sample and translate the data to base 2 (possibly sending to another core
for translation). This way, the data rate would be limited by the processor (faster processors would lead
to the utilization of higher frequencies for transfer).

Limiting factors that I can think of are how fast the current to the oscillator can be changed (for TX),
how fast the analog to digital conversion can be done (read that accurate sampling is possible into the
hundreds of megahertz), and the range of measurable amplitudes.

I'm aware that this question probably contains an unusal amount of stupidity, but I want to build this
system and I'm wondering why I shouldn't. There has to be something major that I am missing here. What could it be? Thanks.

Best Answer

You've just described two separate and entirely valid technologies used in communication theory today: software-defined radio and (for lack of a good general term that I can remember) multi-symbol/level communication.

If we modulate the amplitude of a wave (I think by providing the oscillator different levels of current), can we not sample this wave with some sort of analog to digital converter and process it on the CPU?

Yes - to a degree. You've just described software-defined radio. The basic idea is what you said: dispense with the majority of the radio frequency equipment and create the modulated sine wave directly from the output of a D/A converter and for the return path use a similarly fast A/D and plenty of DSP processing for both sides. The current problem is that although processor speeds are measured in gigahertz nowadays, the interface with the analog world hasn't yet reached those speeds. This means that direct waveform creation is limited to low frequencies (which, for communications, is still fearfully high compared to frequencies 'normal' analog designers worry about). However, if I read my articles correctly this as still allow removal of some of the intermediate-frequency hardware present in most radios. In the future it may be possible to dispense with more of the hardware.

If this is possible, why stick to base 2? If we can have a unique value for each measurable amplitude, data transfer rates would skyrocket. Imagine transferring data with base 1024, or even higher. If we could accurately sample the wave (each oscillation), I don't see why the rate of transfer could be equal to the frequency of the wave times base divided by 2 bits per second (this is probably not correct mathing).

You're right that it's not perfect but you definitely have the basic idea down. To give an example we'll stick with Amplitude Modulation. When you're trying to transmit 0 or 1 using AM it's called On-Off-Keying (link goes to a site with nice pictures and a description). This works by modulating a pure digital signal - 5v is '1', 0v is '0'. You're right that if you have a number of voltage levels you can send more data at once - this is called Amplitude Shift Keying (another nice description with picture). As you can see, there's multiple levels of voltage for various combinations of bits - 2 bits gives four different voltage levels, 3 gives 8, etc.

The problem with this and other similar schemes is not theoretical but practical - in a communication channel with noise it's very likely you'll have trouble figuring out what exactly was sent. It's just like with analog signals: if my only valid voltage levels are 0 and 5V then if I get 4.3V out I can be reasonably sure it should be 5V. If I have 1024 valid voltage levels then it gets a lot harder to determine.

Also note that you're not limited to Amplitude Modulation - the same techniques can be applied to Phase Modulated signals (similar to FM) or you can step into the realm of Frequency Shift Keying where distinct frequencies represent bits (ie, if you want to transmit '3' in binary that might mean sending a 3KHz sine wave and a 6KHz sine wave, then separating them at the receiving end where sending '1' might just be the 3KHz sine wave).

And these techniques are already in wide use - GSM cell phones use a form of Frequency Shift Keying called Gaussian Minimum Shift Keying. Although I do want to correct one incorrect idea you may have: modulation is still used in all of these schemes. The opposite of a modulated signal is a baseband signal (like a bitstream from a serial port). To communicate at any distance over the air you need modulation, period. It's not going away, but how we generate the modulated waveform will change.

I suggest you take a class in Communication Theory if you can - it sounds like you've got the knack for it.