  For DFT-spread-OFDM or OFDM, if the delay spread varies in a wide range and
the symbol duration is relatively short, adapting the cyclic prefix (CP)
duration rather than using a fixed one may significantly improve the spectral
efficiency while preventing inter-symbol interference (ISI). In practice, it
may be beneficial to have a constant overall DFT-spread-OFDM/OFDM symbol time,
which is the sum of the duration of a CP and the duration of a data portion. We
propose to adapt the CP duration to the delay spread without changing the
overall symbol time for DFT-spread-OFDM or OFDM, and address implementation
challenges. In particular, we propose changing the clocking rate of ADC and DAC
or using a Farrow filter to reduce the computational complexity of
arbitrary-size DFT/IDFT resulting from the adaptation.
