Multiantenna wireless systems have shown great promise in achieving high rates. We consider an OFDM multiple transmit/receive antenna system. We propose a linear non-redundant block pre-coding scheme that is applied at the input of the OFDM system. The coding spreads the symbols of each user over all subcarriers, thus increasing multipath diversity, and at the same time introduces some structure to the transmitted symbols, which can be exploited at the receiver to estimate the channel within a complex scalar ambiguity. The precoding does not increase power.