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I want to implement an OFDM transceiver chip and my question is how to choose the clock frequency of the IFFT block? The number of output bits of the IFFT block? Also, how has the clock frequency and the number of output bits of the digital-to-analog converter (DAC) to be chosen?

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closed as unclear what you're asking by hotpaw2, Jason R, Matt L., Phonon May 13 '14 at 4:14

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ What exactly do you mean by number of output bits of DAC? The vertical resolution? $\endgroup$ – Deve Jan 28 '14 at 9:47
  • $\begingroup$ The clock frequency that you need is a straightforward function of the symbol rate, any cyclic prefix length, and the number of subcarriers that you're using, as Deve pointed out in his answer. DAC resolution is completely unrelated to that, and is typically a higher-level system design parameter. $\endgroup$ – Jason R Jan 28 '14 at 13:01
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The clock frequency $f_\mathrm{s}$ of the digital-to-analog converter (DAC) ultimately depends on the desired bitrate $R$ and some other parameters like guard interval length and number of bits per subcarrier. You can find the detailed formulas in a related question.

The speed of the IFFT block depends on $f_\mathrm{s}$ and the number of subcarriers $N_\mathrm{SC}$. It has to transfrom $f_\mathrm{S}/N_\mathrm{SC}$ sets of values each set consisting of $N_\mathrm{SC}$ complex numbers. What clock frequencies are involved in such an IFFT block depends on implementation details (mainly degree of parallelization) and cannot be answered in general.

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  • $\begingroup$ Can you further explain about IFFT block? Does it mean that the IFFT block has to generate a complete transform in NSC/fs time period? $\endgroup$ – Sara Feb 18 '14 at 9:24
  • $\begingroup$ How to determine the bandwidth of an OFDM signal? $\endgroup$ – Sara Feb 18 '14 at 9:25
  • $\begingroup$ Also, If around half of the Sub-carriers only are used to send data, then do we need to perform oversampling to the signal? Is it necessary to have LPF in OFDM transceiver? $\endgroup$ – Sara Feb 18 '14 at 9:27
  • $\begingroup$ @Sara Actually, the IFFT block has to calculate one complete transform every $(N_\mathrm{SC}+N_\mathrm{G})/f_\mathrm{S}$ seconds, where $N_\mathrm{G}$ is the guard interval length. Your other comments are new questions, you should ask them as such. $\endgroup$ – Deve Feb 18 '14 at 9:38

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