Indoor positioning enhancements in LTE standardization
The positioning of terminals in indoor scenarios with cellular networks has attracted significant interest in recent years. A 3GPP Release 13 study item disclosed the extent to which LTE can meet specific requirements for indoor positioning and identified possible enhancements. A set of the most important enhancements have now been standardized in recently finalized 3GPP Release 13 and 14 work items.
The American Federal Communications Commission (FCC) launched new emergency call positioning requirements in February 2015 to address accuracy:
- Vertical and horizontal accuracy requirements
- Requirements on devices and networks to support reporting identifiers of Bluetooth and WiFi access points and uncompensated barometric pressure
When it comes to horizontal accuracy requirement, the objective of 50 meters has to be met for an increasing percentage of up to 80 percent of emergency calls within six years. The study item concluded that already Release 9 LTE can support such high percentages in studied scenarios (Read more about Indoor Positioning in LTE)
3GPP release 13 enhancements
The Release 13 work item introduced the device reporting of Bluetooth/WiFi identifiers and uncompensated barometric pressure. These reports are used for positioning via correlation with data in a National Emergency Address Database (NEAD), only supporting emergency call positioning. In addition, the work item introduced support for a dedicated positioning layer with Terrestrial Beacon Systems (TBS) based on the Metropolitan Beacon System. It can be seen as a terrestrial bound Global Navigation Satellite System (GNSS).
3GPP release 14 enhancements
The Release 14 work item was mainly dedicated to enhancements for Observed Time Difference of Arrival (OTDOA). The following enhancements were specified:
- Solutions to enable separation of positioning signals from different transmission points configured as a combined cell
- Improved measurement report resolution
- Reporting of received additional paths of positioning reference signals
- Terrestrial Beacon Systems based on LTE positioning reference signals
- The time of arrival estimation based on the combination of different reference signals
Separation of positioning signals of different transmission points
Originally in the 3GPP specifications, the positioning reference signals (PRS) used in OTDOA are generated based on the physical cell identifier (PCI) of the associated cell. This means that the PCI planning of a network also affects the PRS plan. One issue identified in the study item was the case of configuring a network with combined cells where multiple transmission points (TPs) share the same PCI. Thereby, it is not possible to configure PRSs that can be separable among the TPs sharing the same PCI. Figure 1 illustrates such a deployment, with 5 different PCIs assigned to 15 cells, where each group of three cells sharing the same PCI, and the time of arrival (TOA) estimation of those three cells cannot be distinguished separately, which is a positioning limitation.
Figure 1. Example of a combined cell deployment with groups of three combined cells sharing the same PCI.
Two different ways to separate TPs sharing the same PCI were introduced in 3GPP Release 14. One possibility is to configure mutually orthogonal muting patterns to different TPs, each configured with a TP ID that is unique within the combined cell. This means that the TPs still will transmit the same PRS but at mutually exclusive occasions. The other possibility is based on PRSs separated from the PCI and instead defined based on a PRS ID with a range of 4096 identifiers compared to the previous 504 identifiers available based on PCI. Thereby, TPs within the same combined cell can be configured with different PRS IDs and hence different PRSs. The support for these two possibilities is indicated via device capabilities. The enhancement is a fundamental enabler for OTDOA in scenarios with combined cells.
Improved measurement report resolution
The Release 13 study item also identified limitations due to the OTDOA report resolution, where the Reference Signal Time Difference (RSTD) resolution of 1 Ts (the basic LTE time unit, which is approximately equal to 32 ns) is concluded to be more limiting than the measurement accuracy. Figure 2 illustrates how the combined RSTD error (measurement error and quantization error) vary with the report resolution in comparison to an ideal non-quantized report. Clearly, there is a significant error contribution from the 1 Ts report resolution, and therefore the report resolution was changed to 0.5 Ts in Release 14. Further details about the time of arrival estimation and impact from the report resolution can be found in our PIMRC 2016 paper. For example, in a scenario with an indoor deployment of small cells, the horizontal 80-percentile positioning accuracy improves from about 4m to 2.5m by the improved RSTD resolution.
Figure 2. Impact from different RSTD report resolutions on the combined RSTD error.
Reporting of additional paths
The most significant positioning error component in macro deployments is due to the multipath propagation, where the device does not select a line of sight (LoS) path among the received paths. The reason for not selecting the LoS path is when a different path is erroneously believed to be the most reliable one. Release 14 introduced more rich reporting of measurements, where capable devices will include timing of up to two additional paths per PRS. This increases the probability that the LoS path will be included. In the scenario depicted in Figure 3, the device reports an additional path for the reference cell that corresponds to the LoS path, which enables the location server to estimate the true position. In the studied 3GPP scenario based on an outdoor macro deployment, the horizontal 80-percentile positioning accuracy improves from 40m to 20m when introducing reporting of additional paths.
Figure 3. Scenario where the device observes and reports one additional path τ2,2 (in this case the LoS path) for the reference cell.
Release 14 also introduced the possibility to configure TPs dedicated to transmitting PRSs and nothing else in a frequency band. This means that the PRSs can be transmitted very frequently – even contiguous if considered relevant. Such configurations can support very short response times, and very good coverage due to the possibility of extensive signal accumulation.
Moreover, Release 14 added the possibility to provide indication to the device about whether or not cell specific reference signals (CRS) and PRS from a TP are configured in such a fashion so that time of arrival based on the combination of PRS and CRS is possible.
In summary, the 3GPP Releases 13 and 14 work on indoor positioning enhancements has introduced several features that bring benefits for positioning of devices indoors, as well as outdoors. WiFi, Bluetooth and uncompensated pressure can be measured by devices and correlated to databases and reference pressure information to enable positioning. Positioning in combined cells where several TPs share the same physical cell identity is enabled based on the introduced PRS and TP IDs. Finer reporting resolution brings positioning accuracy improvements where the report resolution is limiting. Multipath propagation is a challenge for adequate positioning, and significant improvements are provided by the introduced reporting of additional paths for OTDOA. There is also support for dedicated positioning layers or systems based on terrestrial beacons, and support for combined PRS and CRS measurements. 3GPP releases prior to Release 13 is already capable of meeting the emergency call 80-percentile positioning accuracy of 50m, and the new features in Releases 13 and 14 will further improve the accuracy and availability of positioning.