Help me understand the BPSK simulation process

BPSK

hi all
we were given simulation of BPSK as an assignment. what it did was to generate BPSK symbols(-1 1) and after upsampling and filtering(pulse shaping) transmitted to receiver.

i want to know where is the actual modulation happening in it. where is the carrier frequecy modulated by modulating signal.
also if a carrier(sinusoid) is modulated later by these BPSK symbols(which i dont know where), then why do we call it digital modulation because in analog modulation the same thing happens as well; an analog carrier wave is modulated . so where lies the difference?

one morething....if all carrier signals are analog signals then where actually is digital communication carried out since we read in text books that communication is now mostly digital.

please clearify my confusions as m kinda lost in it...have read quite a many articles on it but nothing helped

thnx

BPSK

hi zeerum

what we do in simulation a bit differ from what we should actually do in practice. all passband representation of signals have an equivalent baseband representation. if we have prefect synchronization in receiver both of these representation go to the same point. it's common in simulation when we want to explore phenomena like channel response, pulse shaping, ... but not prefect synchronization, (for simplicity) we can simulate it in baseband.
if you refer to Matlab (esp. 7.xx) all digital modulation perform in baseband and then if it's necessary in your simulation you can upconvert it to your interested frequency.

about your second question I gonna go a little in deep. modulation have two important types, Linear and Nonlinear. Linear modulation in baseband representation are simply a mapping (refer to Matlab). as the input give it symbol index and take modulate sample. but about the difference with analog modulation i should emphasis that in digital modulation we have two critical subsystem match filter, and hard decision. it's because of two different point of view, fidelity in analog communication and accuracy in digital modulation.

hope this help you.
regards.