guard period ofdm
Cyclic Prefix / Guard Interval
The Cyclic Prefix is a periodic extension of the last part of an OFDM symbol
that is added to the front of the symbol in the transmitter, and is removed
at the receiver before demodulation.
The Cyclic Prefix has two important benefits:
² The Cyclic Prefix acts as a guard space between successive OFDM
symbols and therefore prevents Inter-symbol Interference (ISI), as long
as the length of the CP is longer than the impulse response of the
channel.
² The Cyclic Prefix ensures orthogonality between the sub-carriers by
keeping the OFDM symbol periodic over the extended symbol duration,
and therefore avoiding Inter-carrier Interference (ICI).
Mathematically, the Cyclic Prefix / Guard Interval converts the linear convolution
with the channel impulse response into a cyclic convolution. This
results in a diagonalised channel, which is free of ISI and ICI interference.
(see part ??)
The disadvantage of the Cyclic Prefix is that there is a reduction in the
Signal to Noise Ratio due to a lower efficiency by duplicating the symbol.
This SNR loss is given by:
where Tcp is the length of the Cyclic Prefix and T = Tcp +Ts is the length of
the transmitted symbol.
To minimise the loss of SNR, the CP should not be made longer than necessary
to avoid ISI and ICI.