hi suha, u can read the book"electronic communication systems" by kennedy or modern digital communication system by lathi. these contain detail information about pwm and ppm.
another thing, pwm and ppm are not belong to frequency modulation. in frequency modulation , frequency of carrier wave is varied with the amplitude of message signal. in pwm and ppm the frequency of carrier are kept constant.
and for effective modulation technique, sampling rate f(s)>= 2 fmax