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What is cyclic prefix (CP)

cyclic prefix to avoid ISI

In the OFDM symbol transmission,ISI occurs at the receiver due to delay spread of the channel. ISI stands for Inter Symbol Interference. In order to avoid ISI, guard interval is inserted between two OFDM symbols. This guard internal is referred as cyclic prefix (CP). This is kept greater than delay spread of the channel to avoid ISI.

Following are basics of cyclic prefix(CP):
•  Zeros used in the guard time can alleviate interference between OFDM symbols.
•  Orthogonality of carriers is lost when multipath channels are involved, which can be restored by cyclic prefix.
•  It converts a linear convolution channel into a circular convolution channel.This restores the orthogonality at the receiver.
•   The disadvantage is wastage of energy in the cyclic prefix samples.

cyclic prefix

Figure depicts S1 and S2 as OFDM symbols.Tg is the guard time interval and Ts is the data symbol period.
OFDM symbol time = Tg+Ts

There are four options to append samples in the guard time. In other words there are 4 options of cyclic prefix(CP) viz. 1/4,1/8,1/16 and 1/32 of symbol time(no.of fft samples). If we take FFT as 1024 samples and CP is 1/8 then guard samples will be 128.

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