Enhanced time of arrival estimation and quantization for positioning in LTE networks

Observed Time Difference of Arrival (OTDOA) is a downlink positioning method in Long Term Evolution (LTE) networks, in which a User Equipment (UE) measures the time of arrival (TOA) of reference signals received from multiple base stations. The TOA measurements are reported to the network in terms of reference signal time difference (RSTD) measurements, which are used to determine the position of the UE. The accuracy of the estimated position relies on the accuracy of the measured and reported RSTDs (equivalently TOAs). A UE can determine TOA by measuring the time delay of the first channel tap in the estimated channel impulse response. This is usually a nontrivial task in indoor environments where the line-of-site (LOS) signal can be weak or even non-existent. This paper proposes an iterative method for the detection of the first channel tap in an estimated channel impulse response. The performance of the proposed method is evaluated using an ITU Macro 3D channel model. The proposed iterative method is shown to always outperform a commonly used fixed threshold/non-iterative method. Furthermore, the paper analyzes the impact of RSTD quantization resolution in scenarios where a good quality of TOA estimation is already available. By introducing higher resolutions, it has been shown that the current RSTD reporting in LTE specifications is limiting, and the positioning accuracy can be further improved by a finer RSTD reporting resolution.