In a time division duplex (TDD) based adaptive antenna array (AAA) and space division multiple access (SDMA) systems, array weights determined from the up-link signals can be applied to the down-link signal transmission because of the channel reciprocity. However, under time-varying fading conditions, for example that created by a rapidly moving mobile station (MS), the channel change between the array weight determination time in the up-link and the down-link signal transmission degrades the accuracy of the array weight. Thus, the spectral efficiency of the system degrades in the down-link. In this paper, we introduce a prototype of an MS with a 2-element antenna array and present the down-link mobility performance results of computer simulations and field experiments in a field trial system of i-Burst, which is a TDD based wireless broadband system employing the AAA/SDMA techniques at the base station. Both results show that adaptive antenna processing at an MS is effective in improving the down-link mobility performance of TDD-AAA/SDMA systems.