Mobile station location in a DS-CDMA system

The FCC has mandated that by year 2001, cellular carriers must supply Public Safety Answering Points (PSAPs) with information that locates a wireless 911 caller within a radius of 125 meters, 67 percent of the time. In order to comply with FCC requirements, one can use trilateration techniques to locate a mobile station (MS), which requires that a MS be heard at 3 or more base stations (BTS). The ability to use trilateration techniques in a DS-CDMA system is difficult since a MS close to a cell is powered down to avoid the near-far problem, which typically results in a MS not being visible at multiple BTSs. One of the ways to increase the visibility of the MS to multiple BTSs is to increase the MS power level for a brief period at a predefined instance using the Power Up Function (PUF) command from the infrastructure equipment. This feature has already been accepted by the standards body for inclusion in IS-95B[1],[2]. This paper analyzes the impact of using the PUF on BTS receiver performance as well as on the whole DS-CDMA system using the following custom simulation tools; a) CDMA Link Simulator, b) CDMA Static System Simulator (CSSS) and CDMA Motion Simulator (CMS). Based on our simulation results it is concluded that in an infrastructure based MS location method a) PUF is essential to increase MS visibility, b) if the number of PUF events are managed the impact on DS-CDMA system performance is not catastrophic and c) in case the number of PUF events within the same cell occurs frequently a simple interference canceller at the BTS can mitigate the effect of PUF.