A three-dimensional implementation of the finite-difference time-domain 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 co-channel base stations have lower outage than a vertically staggered configuration, which is explained by examining the correlation between the desired and interfering signals.