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Ultra wide-band (UWB) indoor channel is highly time scattered with large number of multi-path and a transmit power of UWB systems is limited to low level. One method to overcome these weak points of UWB communication is diversity combining technique. In this paper, we simulate and analyze performance of pulse position modulation-time hopping (PPM-TH) UWB systems adopting a space time block coding (STBC) scheme in indoor wireless channels. Indoor wireless channel is modeled as a modified Saleh and Valenzuela (SV) model proposed as a UWB indoor channel model by the IEEE 802.15.SG3a in July 2003. In the STBC encoding process, an Alamouti algorithm for real-valued signals is employed because UWB signals have the type of real signal constellation. It is assumed that the transmitter has knowledge about channel state information. From simulation results, it is shown that the performance of PPM-TH UWB systems with the STBC scheme is improved due to diversity and array gains. And the system performance can increase although there is multi-user interference.