Hi,

I am trying to design an OFDM trasmitter.

My current structure is as follows:

IFFT -> Cyclic Prefix insertion on I and Q -> x4 interpolation filter on I
and Q -> I/Q quadrature modulator -> output to DAC.

My question is related to the interpolation filters.

The convolution between the input to the filter and the filter impulse
response smudges the symbol. For example, if the FFT size is 256 and the
cyclic prefix is 1/4, the input to the interpolation filter would be a 320
point sequence. Assuming the filter has 64 taps, the output of the filter
is a 384 point sequence. The filter has basically the same effect as a
fading channel (which is also modelled as FIR system). To combat the
effect of the channel, the cyclic prefix of length > than the channel
impulse response is added to the symbol. My question is, what is to be
done about the distortion introduced by the filter? I have never heard of
cyclic prefix inserted in order to combat the effects of interpolation
filtering.

I am aware that I am making a mistake somewhere (either in the structure
of the transmitter, or in the analysis above), but I just can't see it.

Thanks.