E911 overlay solution for gsm, for use in a wireless location system

Abstract

In a network overlay Location Services solution for a GSM communications network, the LMU monitors the uplink and/or downlink control channels and determines which call originations are for E911 emergencies and which call setup (Stand-alone Dedicated Control Channel, or SDCCH), assigned traffic (TCH) and associated (Slow Associated Control Channel, or SACCH) channels are subsequently assigned. Capturing the energy on the call setup channel, SDCCH, and its associated SACCH will typically provide a sub-100 meter level of accuracy. If not, the SMLC can redirect the LMU to the assigned traffic channel, TCH, to capture additional RF energy and improve the location accuracy as required. This approach takes advantage of the fact that Channel Request messages on the RACH include an information element that indicates the purpose for the access request. One of the values of this information element identifies that the Channel Request is for an emergency call. By monitoring the Access Grant channels (downlink), e.g., the AGCH, the primary (serving) LMU (LMUA in FIG.) can capture the Immediate Assignment Message (IMM ASS) for emergency calls. The Immediate Assignment Message contains the Channel Request message (with the emergency call indication) and the SDCCH channel to be used by the MS for subsequent call setup messaging. The primary LMU will forward the Immediate Assignment Message to the SMLC, which will task the cooperating LMUs to collect the RF energy from the associated SDCCH, SACCH and TCH channels and provide TDOA, AoA, and FDOA determinations.