On Nov 10, 6:13 am, "alex65111" <alex65...@list.ru> wrote:
> How in OFDM the size and type of a cyclic prefix gets out? What
> characteristics of a liaison channel and a signal influence a prefix?

The OFDM cyclic prefix duration provides a guard interval between
consecutive OFDM symbols. Its purpose is to prevent energy from the
previous OFDM being smeared into the current one. It depends how
dispersive the communications channel is. The more dispersive the
channel, the longer the CP needs to be.

The CP also has another purpose. It helps to shape the frequency
spectrum of the subcarriers, such that their main lobes are widened
and more importantly, the side lobe heights are reduced. This means
that any loss of sub-carrier orthogonality at the receiver(will always
happen to some extent due to imperfect synchronisation, etc ), has
less of an effect of inter-subcarrier interference. I think of it as
the classic windowing scenario. Without the CP, the OFDM symbol has
been multiplied by a rectangular window in the time domain. This, of
course, is a convolution with sinc kernel in the frequency domain. By
choosing a CP that rolls off to zero(?) it has the effect of applying
a "nicer" window to the OFDM symbol.

What is a liaison channel?