On Nov 11, 9:21 am, Andrew FPGA <andrew.newsgr...@gmail.com> wrote:
> >>And about shape the
> >>frequency spectrum of the subcarriers - I see that dependence is, but I
> >>can not mathematicly describe it.
>
> Well if there is no cyclic prefix, then it is a rectangular window =
> rect(t). The fourier transform of this is sinc(f). So a single sub-
> carrier of frequency f1 in the time domain multiplied by a rectangular
> function, is a pair of impulses at +/- f1 in the frequency domain,
> convolved with a sinc function.
>
> > What length of a prefix should be if the interval between subcarriers is
> > 100Hz?.
> > And what length of a prefix should be if the interval between subcarriers
> > is 60Hz?
>
> I don't know the answer, but thinking out loud: If the time duration
> (including cyclic prefix) of the OFDM symbol is T, and assuming a
> rectangular window, then the main-lobe width of a single sub-carrier
> in the frequency domain is 1/T. So I suppose the OFDM subcarriers
> cannot be spaced any closer than 1/T Hz. So presumably it follows that
> a longer cyclic prefix, allows the sub-carriers to be spaced closer
> together.

No. T is the length of the OFDM symbol before the cyclic prefix has
been added.

In general, the cyclic prefix should be at least as long as the delay
spread that you expect from your channel. In practice, it must be
longer than that to allow for symbol timing uncertainty, etc.


--
Oli