  This paper computes the distortion power at the receiver side of an
FBMC/OQAM-based OFDMA uplink channel under strong frequency selectivity and/or
user timing errors. More precisely, it provides a distortion expression that is
valid for a wide class of prototype pulses (not necessarily
perfect-reconstruction ones) when the number of subcarriers is sufficiently
large. This result is a valuable instrument for analyzing how users interfere
to one another and to justify, formally, the common choice of placing an empty
guard band between adjacent users. Interestingly, the number of out-band
subcarriers contaminated by each user only depends on the prototype pulses and
not on the channel nor on the equalizer. To conclude, the distortion analysis
presented in this paper, together with some simulation results for a realistic
scenario, also provide convincing evidence that FBMC/OQAM-based OFDMA is
superior to classic circular-prefix OFDMA in the case of asynchronous users.
