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Active Constellation Extension (ACE) is one of techniques introduced for Peakto Average Power Ratio (PAPR) reduction for OFDM systems. In this technique,the constellation points are extended such that the PAPR is minimized but theminimum distance of the constellation points does not decrease. In this paper,an iterative ACE method is extended to spatially encoded OFDM systems. Theproposed methods are such that the PAPR is reduced simultaneously at allantennas, while the spatial encoding relationships still hold. It will be shownthat the original ACE method can be employed before Space Time Block Coding(STBC). But in case of Space Frequency Block Coding (SFBC), two modifiedtechniques have been proposed. In the first method, the OFDM frame is separatedby several subframes and the ACE method is applied to these subframesindependently to reduce their corresponding PAPRs. Then the low PAPR subframesare recombined based on SFBC relationships to yield the transmitted signalsfrom different antennas. In the second method, for each iteration, the ACE isapplied to the antenna with the maximum PAPR, and the signals of the otherantennas are generated from that of this antenna. Simulation results show thatboth algorithms converge, but the second method outperforms the first one whenthe number of antennas is increased.

PAPR reduction of space-time and space-frequency coded OFDM systems using active constellation extension