This paper investigates the bit error probability of a wavelet-time diversity transmission system, which exploits the orthogonality properties between wavelet matrix rows to spread the information of QPSK symbols before transmission. Numerical results of performance comparisons show that the proposed theoretical analysis very accurately matches with the Monte Carlo simulation results. Besides, analytical and simulation results show much higher performance under flat-fading conditions, while remains unaffected the spectral efficiency of the original modulation scheme, which offers significant advantages for next-generation broadband wireless communication systems.