Pulse shaping and digital modulation

[magnetra]

digital modulation technique pulse

Digital modulation techniques like BPSK, QPSK, MPSK are described in books with inputs taken as sequence of rectangular pulse representing input binary sequence.
But then again, there is also this issue of pulse shaping to minimize ISI, which certainly reshapes the rectangular pulse. Can we use pulse shaped input with PSK/FSK modulators? How does these two issue match up? Could anyone please clarify it to me. Thanks

M

 

[nitu]

Hi..
As I understood your problem. You are saying that in texts authors always mention rectanglar pulse for analysis but in real world we would never use rectangular pulses.

You are right after modulation we have pulse shaping block which shapes the pulse to make it compliant to spectrum. This pulse shaping block might be missing in modulation section in texts as there the author is emphasising the modulation instead of pulse shaping.

Regarding PSK and FSK you can do pulse shaping.

 

[magnetra]

nitu, thanks for the reply. But I'm still confused. Isn't it that digital moduation techniques require their inputs to be digital data i.e sequence of rectangular pulses?
Is the pulse shaping done before or after modulation process?

Can you refer me to any books, urls or papers which present both modulation and pulse shaping together.
Thanks.

M

 

[hamdard]

pULSE sHAPING means we have to regive the proper amplitude to all the waveform.
If any of the pulse is not having suffucient amplitude, it has to go through EQUALIZER, which reshapes the waveform.
Equalizer works on proper training initially with known waveform, later the real transmission is done.

 

[buark]

Hy;
In the modulation block you encode the bit streams in to coefficients of possible transmitted signals. This coefficient is giving the charakteristiks of the signal which is going to be transmitted over the air. For example if you use MPAM (Pulse Amplitude Modulation) coefficients represents the amplitude of the transmitted signal. Ofcourse the shape of the transmitted signal can be any shape you like (rectengular, or a picture of donkey) but for example rectangular pulse is good for ISI but has very high spectral sidelobes, so for the ISI and effective baseband usage Raised Cosine Pulse shapes are used. So you first find the coefficients in modulation block and then multiplying this with signal shape (correlation).
you can see; Chapter 5 of WIRELESS COMMUNICATIONS Andrea Goldsmith Stanford University

 

[carlosftm]

Rectangular pulses are short in time domain, but in frequency domain are infinit. That means, you have to ad many sinusoid signals to form a square pulse. In the other way, Raised cosine signal, is infinit in time domain but limited in frequency domain. Raised cosine is a common pulse shape because is bandwith efficient.

Regards,

Carlos