I am sorry if my question is not in the right forum.
Why does the baseband signals are usually modulated onto a higher frequency carrier before transmission?
Thanks you very much for your consideration you may give me.
I googled this and some of your answer can be found here :
A baseband signal is a signal that can includes frequencies that are very near zero, by comparison with its highest frequency (for example, a sound waveform is a baseband signal, whereas a radio signal is not).
In the wireless SATCOM world I lived in, we always multiplexed the baseband signals that we remodulated to the higher frequencies for transmission some almost 22,300 miles up and 22,300 miles back down.
In general, signals can be described as including a whole range of different frequencies added together. In telecommunications in particular, it is often the case that those parts of the signal which are at low frequencies are 'copied' up to higher frequencies for transmission purposes, since there are few communications media that will pass low frequencies without distortion. Then, the original, low frequency components, are referred to as the baseband signal. Typically, the new, high-frequency copy is referred to as the 'RF' (radio-frequency) signal.
hope this helps