Principle of Mapper in frequency diversity in OFDM I am trying to understand frequency diversity in OFDM. An extra block 'MAPPER' is introduced in conventional OFDM transmitter to apply frequency diversity. Can anyone explain the principle of this MAPPER with its working and mathematical formulation. The transmitter and Reciever block diagrams are as shown below:
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.