Your perfectly single-sideband suppressed-carrier modulated sinusoid certainly has a phase which can be measured. However, what you cannot tell is what the contributions of that measured phase from the audio input and the RF oscillator were.

There is another form of single-sideband modulation, in which not only one sideband but also the carrier component is transmitted. This provides a reference which can be used to synchronize the receive LO to the transmit one - normally done to insure exact tuning, but it would also give you the ability to recover the original audio phase.

It is also quite possible, especially with modern DSP gear, to transmit two separate audio channels, one on each side band. This is commonly called independent sideband modulation (ISB).

Many spread spectrum implementations are DSP based and capable of receiving multiple channels at once - GPS being a good example.

RF bandwidth and data rate are related by the modulation format. Different modulation formats will require different bandwidths for the same data rate. For FM modulation, the bandwidth is approximately 2*(df + fm) where df is the maximum frequency deviation and fm is the frequency of the message. FSK is basically FM where the message signal is a square wave. The highest frequency component of a binary bit sequence transmitted serially occurs when the sequence is 01010101. This component is one half of the bit rate. So for FSK, the bandwidth is approximately Δf + r where Δf is the separation between the two frequencies and r is the bit rate. The reason this is bigger than Δf is because whenever the frequency is changed, extra frequency components are generated. Switching between frequencies more often (higher data rate) results in more power in these extra frequency components. Now, these can be filtered out to some extent, but if you filter more of them than Δf + r, the result will be too distorted to reliably extract the original bitstream.

Think about it this way: a pure sinewave consumes zero bandwidth, but it also contains zero information. As soon as you start changing a characteristic of a pure sinewave (frequency, phase, amplitude, etc.) its bandwidth must increase accordingly. In the case of amplitude modulation, modulating the amplitidue of a sinewave of frequency fc at frequency fm will result in a signal with components at fc, fc+fm, and fc-fm. If the message contains components all the way down to DC, then the resulting modulated signal will have twice the bandwidth of the message signal. FSK is basically transmitting two AM signals at the same time on different frequencies, so the bandwidth will naturally be increased by the separation of these two carrier frequencies.

For FSK, the bit rate and the symbol rate are the same. But for higher order modulations like QPSK and QAM, each transmitted symbol can code for more than one bit so the bit rate can be significantly higher than the symbol rate. This means that the required transmit bandwidth is less than what would be required for AM or FSK. QPSK and QAM have higher spectral efficiency. However, QPSK and QAM are more susceptible to noise and distortion and therefore require a relatively higher SNR.

Also, for FSK, you want the two frequencies to be integer multiples of the data rate. This will result in an integer number of cycles in each bit period so that the carrier always ends up at the same level on data bit transitions. This probably won't be done at RF, though. Generally the FSK signal would be generated at an intermediate frequency which would then be mixed up to the actual RF carrier frequency.

## Best Answer

I THINK this addresses your main question

However, the material quotes below certainly should if you spend the required time going through it.

If the above comment and the material below does not cover your question would you please explain the question in more detail.

For starters the following provides an excellent feel for a multi amplitude multi phase modulation scheme.

The two diagrams below, when watched together, give you a superb idea of how basic QAM functions.This is 4-QAM which is the "entry level".

Diagrams from this superb national instruments QAM tutorial

There is a good attempt to graphically display what is happening on this page but not as good as above.

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Wikipedia provides this excellent overview with "constellations" for square and circular QAM at various N values. (If those terms do not make sense, read the article).

Related, Wikipedia QAM TV

Very good Comparison of 8-QAM, 16-QAM, 32-QAM, 64-QAM 128-QAM, 256-QAM, etc Useful definition o=f sorts from here

QAM (quadrature amplitude modulation) is a method of combining two amplitude-modulated (AM) signals into a single channel, thereby doubling the effective bandwidth. QAM is used with pulse amplitude modulation (PAM) in digital systems, especially in wireless applications.

In a QAM signal, there are two carriers, each having the same frequency but differing in phase by 90 degrees (one quarter of a cycle, from which the term quadrature arises). One signal is called the I signal, and the other is called the Q signal. Mathematically, one of the signals can be represented by a sine wave, and the other by a cosine wave. The two modulated carriers are combined at the source for transmission. At the destination, the carriers are separated, the data is extracted from each, and then the data is combined into the original modulating information.