# calculating the spectrum of digital modulation methods

I have just started studying the techniques of digital modulation. The book i am reading only has mentioned the psd of bpsk, bfsk , qpsk and msk. I would like to know, how the psd is actually calculated and I would also like to know, why the angles used in qpsk are (2m+1)pi/4 why cannot be one with a 89 degree and another with 91 degree instead of 90 degree and 90 degree. By these degree I indicate the angle between to signal vectors.

• Can you link to where "instead of 90 degree and 90 degree" is coming from? Have you grasped the concept that seperating binary codes with a phase "gap" that is detectable in a reasonable time span is what is needed? – Andy aka May 23 '13 at 22:01
• no i didnot learn it. but i would like to know. and the 90 degree is in the signal space representation, the orthonormal co-ordinate thing – shortCircuit May 23 '13 at 22:08
• i have learned aliasing , intersymbol interference but i think its not what you are saying.. – shortCircuit May 23 '13 at 22:11
• I'm sure there will be better answers but to "detect a change of something that is down to modulation you need something bigger than a 1 or 2 degree phase change in a carrier especially on high speed data links. – Andy aka May 23 '13 at 23:23
• 89 degree >> 1 or 2 degree . Unfortunately Google is bad at electronics. perhaps if i could post an image then i would be more clear. – shortCircuit May 23 '13 at 23:26