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Modulation is all about mapping digital data to analog mediums. We have (and use) different modulation techniques because they have different benefits and weaknesses. … The "ideal" modulation for a given application depends on the specifics of your application. …

Longer preambles make it easier for the demodulator to lock onto the signal well. This is due to getting more preamble signal energy and because longer preambles make it easier to detect stuff like c …

There is no inherent symbol rate for any modulation type. 64 QAM symbols transmit 6 bits per symbol, and 256 QAM symbols transmit 8 bits per symbol. … The general equation for a given modulation type and its symbol rate is: $symbols/s = \frac{bits/s}{bits/symbol}$ Determining the bits per symbol is usually pretty easy. …

For many years the state of the art was to use a convolutional "inner code" and a block "outer code". The "inner" and "outer" terminology come from the following block diagram: $$\boxed{\scriptstyle …

Yes and no. The mapping is arbitrary as long as the receiver correctly determines which constellation point a symbol is. If the receiver makes a mistake, though, it is most likely going to pick a "n …

While I do not have proof, it seems pretty clear, given the "Characteristics of Compass signals reported as of May 2008 compared to GPS-L1CA" table at the Wikipedia page that you offered as an example …

There are independent sub-carriers in both the positive and negative frequencies. In other words, yes, they have completely different data.

suppose, Carrier signal frequency = 2800KHz message signal frequency = 3KHz Then you will get a signal that looks like this in the frequency plane. Obviously this is not to scale, but you get the …

Ideal symbol pulses are sequences of rectangular pulses. Rectangular pulses, unfortunately, have horrible frequency profiles, as shown below. The main lobe is where most of the pulse power is, and …

If you do have feedback I would do the same but also dynamically change the modulation type from QPSK up to 16-QAM or something, depending on how well the signal is coming through. …

1) A typical approach would be to have a frame alignment word (and by "word" I mean bit sequence) at the beginning of every frame. You could time synchronize by looking for that FAW through cross-cor …

I would use a frame alignment word to synchronize your data. The basic idea is to insert your FAW at the beginning of every $N$ bytes. After you demodulate the data you look for the FAW to synchroni …

4-PAM transfers 2 bits ($log_2(4) = 2$) per symbol, while 2-PAM transfers 1 bit per symbol ($log_2(2) = 1$), so 4-PAM definitely transfers data faster. The catch is that you need higher SNR to get th …

I have created a C++ library that I haven't officially released yet that could be used for modulation. It is called NimbleDSP. … It doesn't have a function or method called "modulation", but it does have interpolation and supports complex numbers, so it could easily be used to do modulation. …

Yes, you are correct, though your formula puts the constellation points at 0, 90, 180, and 270 degrees, where usually the constellation points are represented as being at 45, 135, 225, and 315 degrees …