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OFDM has advantage over frequency selective channel with addition of cyclic prefix after IFFT of MPSK or MQAM symbol. but I wonder that the spectral efficiency of with OFDM vs without OFDM.

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The spectral efficiency of OFDM is strictly worse than that of a pulse-shaped QAM signal with the same rate.

  • OFDM requires a guard interval, on which no useful information is transmitted.

  • It is common to dedicate several subcarriers to pilot signals.

  • It is often not pulse-shaped (or, more accurately, pulse-shaped with a rectangular pulse).

Since each non-pilot subcarrier is essentially a narrowband QAM signal, you can see that its spectral efficiency cannot be better. However, the point of OFDM is to allow for low-complexity frequency-domain equalization, with spectral efficiency given a less important role.

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    $\begingroup$ To add to your good list: the DC bin (bin 0) is typically not used due to inevitably competing with DC offsets in implementation. $\endgroup$
    – Dan Boschen
    Commented Mar 23, 2021 at 2:46
  • $\begingroup$ Good point, Dan! $\endgroup$
    – MBaz
    Commented Mar 23, 2021 at 13:11

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