Why is it harder to modulate/demodulate the PSK signals on high baseband frequencies(1 Ghz for instance) with high sample rates than modulate a multicarrier OFDM signal – do IFFT, FFT to receive/transmit.
Electronic – why use OFDM, not PSK/QAM for high speed data transfer
wireless
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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.
- X10 devices are the same. However than only defines the power-line interface. It doesn't cover wireless accessories.
- Consider USB to serial adapters. Serial to X-10 devices exist.
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Best Answer
Direct conversion of a 1Gbps baseband signal using say PSK/QAM is prone to wideband fading, group delay distortion, ISI, TV ghosting etc. Even if you reduced the symbol rate by N amplitude and phase levels which you can also do in ODFM, the wide bandwidth of a single carrier suffers more distortion and loss of SNR than multiple sub-carriers.
OFDM reduces the bandwidth of each channel by the # of mux'd sub-carriers plus a guard band, so the channel interference is significantly less. The symbol rate is also much lower with multiple channels, making low cost DSP realization possible.
ODFM is double sideband and some HDTV offers better spectral efficiency and lower error rates for same CNR using 8VSB (vestigial SB is nearly single sideband) preferred in America for over the air broadcast, while Europe seems to prefer COFDM.
But since your question is wide-band related (>0.5Gbps) depending on medium and your application, you may want to pursue http://en.wikipedia.org/wiki/Ultra-wideband.