Digital modulation and RF bandwidth

[dnyberg2]

Why isn't it possible to invent a custom digital modulation scheme that would use much less RF bandwidth than all the schemes available? What are the limiting factors that do not allow for high data rates to be coded so that the occupied bandwidth is much less than all the others like ASK, PSK, MSK OOK etc...

 

[the_emi_guy]

Look up "channel capacity" on the Internet.
Roughly summarizing you are trading between three things:
1 - information rate
2 - channel bandwidth
3 - noise

You can reduce channel bandwidth if you reduce noise, accept lower information rate, or both.
Note that "information rate" is not the same as bit rate.
There are also practical considerations. It is rarely possible to actually achieve reliable operation right at the channel capacity because it would require too much coding complexity.

 

[marcusl]

Shannon showed that the capacity of a band-limited communication channel with AWGN is
$$C=Wlog_2(1+\frac{P}{N_0W})$$
where W is the channel bandwidth, P is the signal power and N0 is noise power spectral density. Reducing the bandwidth requires increasing transmit power to keep capacity the same, but the logarithmic function makes it very painful to do this.