UWB has a very large bandwidth in a WPAN network, which is best used for HD-video applications. Meanwhile, MBWA is a WMAN option optimized for wireless-IP in a fast moving vehicle. In this paper, we propose a practical engineering scenario that harnesses simultaneously the distinctive feature of both UWB and MBWA. However, this in-proximity operation of the technologies will inevitably cause mutual interference to both systems. In light of this, as a preliminary phase to coexistence, we have derived, under various circumstances, the maximum interference power limit that needs to be respected in order to ensure an acceptable system performance as requested by the new IEEE 802.20 standard.