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In a previous work by Wu et ah, it is shown that the performance of the pre-transformed space-time block coded orthogonal frequency division multiplexing (PT-STBC-OFDM) system has much superior performance compared to the normal STBC-OFDM system. In this paper, we present an analytical study on the bit error rate (BER) of the PT-STBC-OFDM system. We derive the noise distribution of PT-STBC-OFDM system and hence, obtain the BER numerically. We also derived two closed- form BER approximations for the PT-STBC-OFDM system at two different SNR regions. We show that by adding the PT operation, the system has better diversity compared to STBC-OFDM system at the medium SNR region. Using linear detection, the diversity of both systems at the high SNR regions are the same. Moreover, we demonstrate that at high SNR, a PT-STBC- OFDM system of transform size N requires 5log10(N) dB less SNR to achieve the same BER as without PT.