The increasing need of high data rate transmissions over the time and frequency selective fading channels has drawn attention to modulation schemes with high spectral efficiency, such as the multidimensional rotated QAM constellations, which is also referred to as the modulation diversity. An improved JCMD (Joint-Coding-Modulation-Diversity)-ID (iterative demodulation/decoding) scheme in OFDM (orthogonal frequency division multiplexing) system is proposed. For the rotated QAM (quadrature amplitude modulation), it unveils the optimum rotation matrix depends on the modulation order, code rate and iteration number. This new scheme can take full advantage of the modulation diversity of rotated QAM (quadrature amplitude modulation), the frequency diversity of OFDM system and the coding-gain of Turbo codes all together. Actually, it extends the original uncoded modulation diversity over an independent Rayleigh fading channel to a joint coding and modulation diversity over time/frequency selective fading channels. Simulation results have turned out this new scheme can significantly outperform the BICM (bit-interleaved coded modulation) scheme, which is up to 2 dB SNR gain.