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Why the following commands gives the same result for the LTE downlink signals with 75 resource blocks and 100 resource?

       enb = struct('NDLRB',75,'CyclicPrefix','Normal');
       lteOFDMInfo(enb)

answer:

       SamplingRate: 30720000
               Nfft: 2048
          Windowing: 8
   CyclicPrefixLengths: [160 144 144 144 144 144 144 160 144 144 144 144 144 144]

And the same command using 100 resource blocks:

      enb = struct('NDLRB',100,'CyclicPrefix','Normal');
      lteOFDMInfo(enb)

answer:

       SamplingRate: 30720000
               Nfft: 2048
          Windowing: 8
CyclicPrefixLengths: [160 144 144 144 144 144 144 160 144 144 144 144 144 144]

I belive that the size of the FFT and the sampling rate were supposed to be different. Am I forgetting something?

function link: https://www.mathworks.com/help/lte/ref/lteofdminfo.html

Luis Mendo
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Flávio Mendes
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  • I am using the Matlab LTE toolbox. – Flávio Mendes Jan 30 '18 at 19:06
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    A sampling rate 30.72 Msamples/s is valid for a 15-MHz baseband signal (although a smaller value would do), and it gives an FFT size that is a power of 2. From the [book by Dahlman et al.](https://www.amazon.es/4G-LTE-LTE-Advanced-Mobile-Broadband/dp/012385489X): _FFT-based implementations of OFDM are common practice and an FFT size of 2048, with a corresponding sampling rate of 30.72 MHz, is suitable for the wider LTE carrier bandwidths (such as bandwidths of the order of 15 MHz and above)._ – Luis Mendo Jan 30 '18 at 20:07
  • Thanks. Just a curiosity: Is there a way to specify a smaller sampling rate for the signals with 75 resource blocks? – Flávio Mendes Feb 01 '18 at 17:40
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    Not that I know of. The sampling frequency is automatically derived from the `enb` struct – Luis Mendo Feb 01 '18 at 18:02
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    Glad I could help! – Luis Mendo Feb 01 '18 at 22:17

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