Time Of Arrival Window Startpoint

Description

The Time Of Arrival Window Startpoint (TOAWS) is a network-controlled timing parameter essential for the operation of Uplink Time Difference of Arrival (UTDOA) positioning. It specifies the earliest time at which a Location Measurement Unit (LMU) should begin searching for the uplink transmission of a target User Equipment (UE). Together with the Time Of Arrival Window Endpoint (TOAWE), TOAWS creates a confined temporal 'gate' that focuses the LMU's signal processing efforts. The positioning node calculates an expected Time Of Arrival based on the approximate known locations of the UE and the LMU. The TOAWS is set to a time slightly before this expected value to account for negative error in the initial location estimate and propagation uncertainties.

In practice, when the positioning node (E-SMLC in LTE, LMF in 5G) requests a UTDOA measurement, it sends a command to the relevant LMUs. This command includes parameters identifying the target UE (e.g., its Cell Radio Network Temporary Identifier - C-RNTI, and scheduling information) and the critical window parameters: TOAWS and TOAWE. Upon receipt, the LMU configures its digital signal processor to start correlating the received wideband radio signal with the expected signature of the UE's uplink signal (such as the Sounding Reference Signal or the Physical Uplink Shared Channel demodulation reference signals) precisely at the time indicated by TOAWS. The correlation process continues until the TOAWE time is reached.

This mechanism is vital for performance. By starting the search at a calculated TOAWS, the LMU avoids processing irrelevant earlier time segments, saving computational resources and reducing the overall time to obtain a valid TOA measurement. The accuracy of the TOAWS setting directly impacts the probability of detection; if set too late, the actual signal arrival might be missed. Therefore, the positioning node uses quality of service parameters and uncertainty estimates to dynamically adjust the window's start point for each measurement request, ensuring robust operation across varying network geometries and UE velocities.

Purpose & Motivation

TOAWS was developed to enable practical and efficient uplink signal hunting in a multi-user, wideband cellular environment. The core problem it solves is the unbounded search problem. An LMU receiving signals from an entire cell sector must isolate one specific UE's transmission. Without a guided start point, the receiver would need to perform continuous, full-time correlation, which is prohibitively complex and slow for real-time location services, especially when serving multiple simultaneous location requests.

Its creation was motivated by the need to make network-based, UE-independent positioning feasible for meeting stringent emergency call location accuracy mandates. Downlink methods require UE cooperation and capability, but UTDOA (using TOAWS/TOAWE) allows the network to locate any device that transmits, including low-end IoT sensors and legacy phones. The TOAWS parameter provides the necessary 'seed' information to initiate a targeted search, transforming an intractable continuous search into a manageable, bounded task.

Furthermore, TOAWS supports network resource optimization. By precisely controlling when an LMU begins its processing for a specific task, the network can better schedule and multiplex measurement requests across a pool of LMUs, improving the overall capacity of the positioning system. This allows a single LMU to serve many cells and numerous location requests without being overwhelmed by processing load.