Quick tutorial on information theory and nyquist and the difference.
The Nyquist rate deals with perfect reconstruction of a sampled signal.
Detection and demodulation do not require perfect reconstruction of the
signal.
Channel capacity is defined as the maximum rate for reliable
communication through a channel. This is a function of many things,
channel bandwidth, noise spectrum, SNR and a whole bunch of other
things. How to achieve channel capacity in the general case is a
challenging problem. It has been solved in some cases.
A clear example of this is the 56k telephone modem. The channel is 3-4
kHz wide. Perfect reconstruction of the received signal can be
accomplished with an 8kHz sampling rate. However we are getting almost
7 times that in data trasmission. (Yes a 56k modem does closer to 53k
in the telephone system.)
For the person looking for the SNR plots for the different bands such a
beast does not exist. The best that you can do is find a chart that
describes the noise in each of the bands. Everybody transmits with
different power to receivers at different range. This changes the
received signal power and therefore the SNR at the receiver.
Demodulations can be computationally intensive. 56k modems usually have
an ASIC or a dedicated processor to deal with the load. My question
is: Where are the computational bottlenecks and can anything be done
about them?
Charles Suprin
Not for work, not works opinions.
