This paper introduces linear constellation precoding

(LCP) for OFDM transmissions over frequency-selective

fading channels. Exploiting the correlation structure

of OFDM subchannels, and choosing system parameters

properly, we first perform optimal subcarrier grouping

to divide the set of correlated subchannels into subsets.

Within each subset, a non-redundant constellation-specijic

unitary precoder is then designed to achieve maximum

multipath diversity and coding gains. While greatly reducing

the decoding complexity and simplifiing the LCP

design, subcarrier grouping offers optimal perjormance by

maximizing multipath diversity and coding gains. In addition,

the proposed system does not sacrifice the transmission

rate. Analytic evaluation and corroborating simulations

reveal its pe$ormance merits.