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