A three-dimensional implementation of the finite-difference timedomain method is used to estimate the down-link outage probability of a direct-sequence code division multiple access system operating in a multi-storey office building in the presence of co-channel interference. The numerical analysis is supported by experimental measurements and good agreement is found for the outage probability. Both simulation and measured results indicate that vertically aligned cochannel base stations have lower outage than a vertically staggered configuration, which is explained by examining the correlation between the desired and interfering signals.Introduction: Achieving reliable performance within highly confined areas, such as multi-storey buildings, remains a challenge for contemporary wireless systems. Co-channel interference from neighbouring systems is significant, as transmit powers are relatively high, frequency reuse separations are generally shorter, and unlike outdoor deployments, three-dimensional (3D) reuse strategies are possible [1]. Furthermore, predicting the system performance for indoor environments is complicated by the large variability in building layout, architectural styles, and building materials. Currently, empirical models based on experimental measurements are widely used, e.g. [2,3]. However, many of the parameters in these models lack an electromagnetic basis and can vary considerably between buildings, and thus often require further tuning measurements (which can be time-consuming and expensive) [2]. To investigate the factors limiting system performance, a 3D model of a multi-storey office building is analysed using the finitedifference time-domain (FDTD) method. The FDTD method has been shown to accurately model radiowave propagation within and around buildings, e.g. [4][5][6][7]. However, the previous research has largely focused on characterising propagation mechanisms, and not on the system performance. This Letter focuses on how the FDTD method can be applied to directly estimate the outage probabilities of a directsequence code division multiple access (DS-CDMA) system in the presence of co-channel interference.