The practical interpretation of the orthogonality definition is that
if a symbol ai(t) is mixed with a carrier reference equal to the
frequency and phase of a second symbol aj(t), and the mixer output is
then averaged over a symbol period using a matched filter or integrator, the output will
be zero. This means that with orthogonal signalling it is possible to increase the number of symbol states used without affecting the
output from an individual coherent detector, and hence
without increasing the probability of symbol error for each detector.
As we increase the number of orthogonal symbols used for transmission, we can increase the
duration of each symbol for a given data information rate, (see
section 2.3). The longer the symbol duration, the greater the time for
averaging each symbol in the receiver and the better the S/N ratio at the
detector output, improving the probability of correct symbol detection. Orthogonal FSK in
theory can have any number of orthogonal symbol states, but does so at the expense of
ever-increasing occupied bandwidth.
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