QPSK, from Square Root Raised Cosine to Rectangular Impulse Shaping

Question

I have implemented a QPSK modulator and demodulator using classic text-book examples: Square Root Raised Cosine Impulse shaping and Matched Filtering.

The problem is that I've been told that I could only used square waves for the modulation instead of sine waves on the emitter side (only +/-Vmod) and this really degrades the performance of the communication.

After further investigation it seems that the root cause of the lack of performance is not really the use of square wave but the change of impulse shaping/matched filtering implied by it.

Basically my question is the following: Using only +/-Vmod (and 0V if necessary) on the emitter side, am I wrong in assuming that the best case scenario is using a Rectangular Impulse Shaping with the according Matched Filter or is there some other combination that could provide better results?

If you have any ideas or links I could learn from don't hesitate! I'm struggling because I don't know if my reasoning is wrong or if its my implementation.

Answer 1

You can use rectangular pulses, but it is not necessary to do so. Rectangular pulses have advantages and disadvantages. The advantage is that they are very simple and easy to use, both for the modulator and the demodulator. It makes life easier for the demodulator because it makes symbol timing very forgiving- you can be off on where you sample the symbol and still get a good measurement.

There are two disadvantages to using rectangular pulses. The first is that it uses a lot of bandwidth. The Fourier transform of a rectangular pulse is a sinc function, which drops off very slowly. The second disadvantage is related to the first- very few real world solutions use rectangular pulses because they use a lot of bandwidth. If you want something realistic, in terms of what most designers would do professionally, you should not use rectangular pulses.

You could, perhaps, get it working with rectangular pulses, and then when it is solid reintroduce the root raised cosine pulses.