Principle of Mapper in frequency diversity in OFDM
In transmitter the N symbols from the series to parallel convertor are repeated M times at the mapper giving you M sets of N symbols. This is a guess, but I  assume the mapper is spreading the contents of each of these M sets out across the whole transmit frequency band so the repeated symbols are not concentrated in any one region. This way if any there is a null in any particular region of the band only a few of the repeated symbols would be affected. This would give you frequency diversity. 

AT the receiver the contents of each of the M sets are weighted and combined in a kind of majority rule detection scheme to determine the estimated N symbols.