Support of wireless transmit/receive unit (WTRU)-based and WTRU-assisted observed time difference of arrival (OTDOA) methods has been identified as one technique to improve upon achievable accuracy and speed for positioning in a long term evolution (LTE) system.
For OTDOA methods, a WTRU observes signals of several neighbor cells, measures the observed time differences of arrival (for example, as compared to its own serving cell), and reports these to the network. In one method, the network then derives a positioning estimate based on factors such as known deployment locations of the reported neighbor signals, known transmission time differences among various cells in the network, and the like. This is referred to as WTRU-assisted positioning. Alternatively, the network provides the factors to the WTRU so that the WTRU computes its own position from the measurements using these factors and the observed time differences of arrival measurements. This is referred to as WTRU-based positioning.
WTRU time difference measurements may be defined based on one or more reference signals (RS) from the cells being measured. The RS may be either Release 8 LTE Cell-Specific RS (CRS) and/or a newly designed Positioning RS (PRS). Either may be used individually or in combination by the WTRU to derive the measured metrics. As a result, the WTRU may need to know which RS to use for the time difference measurements.
The time difference measurement that may be used for OTDOA calculations may be the Reference Signal Time Difference (RSTD). The RSTD may be defined as the relative timing difference between a cell j and cell i, which may be defined as TSubframeRxj−TSubframeRxi, where TSubframeRxj is the time when the WTRU receives the start of one subframe from cell j and TSubframeRxi is the time when the WTRU receives the corresponding start of one subframe from cell i that is closest in time to the subframe received from cell j. The reference point for the observed subframe time difference may be the antenna connector of the WTRU. The RSTD measurement may be applicable to intra-frequency and inter-frequency cells. The WTRU, therefore, needs to support measurements made on neighbor cell carrier frequencies that are different than the serving cell carrier frequency.