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This paper compares the performance of on-off keying (OOK) modulation and subcarrier phase shift keying (PSK) modulation through an optical wireless channel. The bit error rate (BER) for a turbulent channel is derived for optical wireless communication systems employing OOK or subcarrier BPSK. A lower bound is derived for BER performance when fixed-threshold OOK is used. It is also shown that the BER performance of the optical wireless communication system, employing subcarrier PSK modulation, is more than 3 dB better than that of a compatible system employing fixed-threshold OOK modulation at BER=10-6 and with turbulence parameter σ=0.1. When σ is 0.2 and fixed-threshold OOK is employed, the BER performance never falls down to 10-4. The BER performance degradation of optical wireless communication systems employing subcarrier BPSK between σ=0.1 and σ=0.2 is only 2.5 dB at BER=10-6.