PCM and other keying Modulation

[varindernew]

What is the difference between PCM and other modulation techniques like PSK, FSK etc... I understand how they are done, but my question is where do they fit in communication system. lets say we have binary sequence we can use PCM and transmit it..then why do we need other keying techniques. do we something have to do with baseband abd bandpass modulation.

 

[KostasN]

It's been a long time since this has posted, but since it has not been answered yet, I decided to answer it.

Well, PCM it is a baseband modulation scheme (also known as pulse modulation), that is, simply stated, the transmitted signals are pulses, whereas PSK, FSK etc. are examples of bandpass modulation schemes (a.k.a. carrier modulation), that is, the information is imprinted on the changes of some characteristic (amplitude, phase, or/and frequency) of a high-frequency sinusoidal carrier.
In wireless channel you cannot have pulse modulation. Besides that, there are differences between these modulation schemes also in terms of bandwidth, transmitted power, tolerance in noise and interference, implementation complexity and cost, etc.