Positioning Techniques for Mobile Devices in LTE

Global Positioning System (GPS)

Determining a deviceís position is traditionally based on satellite-based position estimation using Global Navigation Satellite Systems (GNSS), like the Global Positioning System, short GPS. Today the majority of all modern mobile devices such as smart phones and tablets have an integrated GNSS receiver. GPS satellites send signals containing information the mobile device can use to calculate the time the signal needed reach the device. To estimate a 3D position properly the receiver needs to have an unobstructed line of sight to at least four satellites. In indoor areas, line of sight reception of the low-power radio signals transmitted from the medium-earth-orbit satellites cannot be guaranteed, resulting in inaccurate location. This is one of the drawbacks of only using GNSS. Another problem is that in urban areas the Time To First Fix (TTFF) sometimes takes a very long time because buildings hinder GPS signals from being received, which results in a battery drain for UEs.

Positioning techniques in LTE

There are many different techniques to obtain the position of a UE (User Equipment). These techniques are mostly divided in network-based, UE-based, UE-assisted and network-assisted approaches. However, these techniques can also be used in combination with each other. In Network-based approaches the network does all the work i.e. it measures the necessary values and uses them to calculate the position of the UE. In UE-based approaches UE does all the calculations and no changes to the network are required. In UE and network assisted techniques, network and the UE work together to first measure and then calculate the deviceís position. Usually the UE measures the data needed for location calculation, then the network does the calculation. These approaches are useful as the UE has less calculation power than the network. In case of UE based techniques, UE may determine its location without assistance from network. For e.g., in case of A-GNSS, if UE may figure out its location from GPS data, without getting assistance from network. Network based techniques cannot be directly used by UE to determine its location unlike GPS, however UE can request network to provide UEís location through LPP (LTE Positioning Protocol) or SUPL (Secure User Plane Location).

Assisted Global Navigation Satellite Systems (A-GNSS)

A-GNSS (also known as Assisted-GPS or A-GPS) improves the startup performance of a GPS based system. LTE includes support for Assisted Global Navigation Satellite System (AGNSS). The GNSS incorporates multiple satellite positioning systems viz. Galileo,,GPS and modernized GPS,,GLONASS,Quazi-Zenith Satellite System and Space Based Augmentation System (SBAS). With conventional GPS systems, the GPS receiver in the mobile device is solely responsible for receiving satellite signals and computing its location. In a typical A-GPS implementation, the standalone GPS facilities of the phone are augmented by data provided by the network, termed ìAssistance Dataî, which includes information the mobile GPS receiver can use to accelerate the process of satellite signal acquisition. Two types of assistance data are provided to improve the positioning speed and accuracy performance:

• Data assisting the measurements: e.g. reference time, visible satellite list, satellite signal Doppler, code phase, Doppler and code phase search windows;
• Data assisting position calculation: e.g. reference time, reference position, satellite ephemeris, clock corrections.

A-GPS provides excellent accuracy, and as compared with stand-alone GPS, it can reduce the UE GPS start-up and acquisition times, increase the UE GPS sensitivity, allow the UE to consume less power on the handset with the GPS receiver put in Idle mode when it is not needed. Assistance data can be provided by the LTE network for both GPS and GLONASS satellites. The location of the UE has to be known in advance (for example through TDOA). The UE then can find the GPS signals much faster and is sometimes able to get a GPS signal in areas where usually it would not work. In general, AGPS provides the highest accuracy of any network-enabled technique and works well outdoors and in scenarios where a reasonably good view of the sky is available.

Enhanced cell ID

The CID (Cell Id) methods are based on the Cell Of Origin (COO) concept, where a UE is located within the coverage area of its serving eNB (eNodeB), typically to the specific cell/sector within the eNB. Although this method is the least accurate one, it is the easiest to implement, and highly scalable. To improve accuracy, E-CID was introduced in LTE. ECID method has low accuracy (50~1000 m) depending on the size of the cell, but it is easier to implement than other methods, and is generally available across diverse vendor products and networks. In addition to the use of geographical coordinates of the serving eNB, the position of the UE is estimated more accurately by performing measurements on radio signals. UE measurements which can improve the accuracy of the location estimate using this method includes E-UTRA carrier Received Signal Strength Indicator (RSSI), Reference Signal Received Power (RSRP) etc. E-UTRAN measurements which can be used in the Cell ID methods include the eNB Round Trip Time (RTT) and the Angle of Arrival (AoA). E-CID can be executed in three ways, using different types of measurements:

• E-CID with estimation of the distance from one eNB.
• E-CID with measuring the distance from three eNBs.
• E-CID by measuring the Angle-of-Arrival (AoA) from at least two eNBs, better would be to use three eNBs.

In the first two cases the possible measurements can be: RSRP, a standard quality measurement for UEs; or TDOA (Time Difference of Arrival) and the measurement of the Timing Advance (TADV) or Round Trip Time (RTT). In the first case the position accuracy would be just a circle. Method number 2 and 3 provide a position accuracy of a point, while measuring more sources. For case 1 and 2, the measurements are taken by the device, and are therefore UE-assisted. For case number 3 the measurements are taken by the base station and are therefore network assisted. For estimating the distance RTT and TADV can be used. But to measure the direction AoA measurements should be used.

Observed Time Difference Of Arrival(OTDOA)

OTDOA is a downlink positioning method in LTE, which is based on measuring the difference in the arrival times of downlink radio signals, from multiple base stations. Each LTE cell transmits reference signals and it is the arrival time of the reference signals that the UE compares. The difference in the timing is reported by the UE to the network. Network combines the timing differences with its knowledge of the positions of each cellís antennas, to calculate the UE position. At least four cells need to be measured by the UE.

The measurement conducted between a pair of eNBs is defined as the reference signal time difference (RSTD). This position estimation is based on measuring the Time Different Of Arrival (TDOA) of special reference signals, embedded into the overall downlink signal, received from different eNBs. With 3GPP Release 9, Positioning Reference Signals (PRS) have been introduced as the cell specific reference signals were not sufficient for positioning. The cell specific reference signals cannot guarantee the required high probability of detection, based on simulations, which have shown that the detection is guaranteed in only 70% of cases. That too in an interference free environment, which is not possible in real world implementation. The PRS is periodically transmitted along with the cell specific RS in groups of consecutive downlink sub frames.

Uplink Time Difference Of Arrival†(UTDOA)

UTDOA utilizes uplink Time Of Arrival (ToA) or TDOA measurements performed at multiple receiving points. Measurements are based on Sounding Reference Signals (SRSs). The advantage is that the UE does not need any additional hardware or softwareThe method uses time difference measurements based on Sounding Reference Signal (SRS), taken by several eNBs, to determine the UEís exact location. The network has to be equipped with exactly time-synchronized Location Receivers/Location Measurement Units (LMUs) and a Location Calculation and Control Center to calculate the position of the handset.

RF Fingerprinting

Fingerprinting (also known as Pattern Matching or Database Correlation Method (DCM)) is a commonly known method which is not explicitly standardized in LTE, but is included in LTE specifications. In this method a UEís position is figured out by using measurements made by the eNB, the UE, or a combination of both, and these measurements are then compared with the already stored fingerprints in a RF map to estimate the position. RF map is typically based on detailed RF predictions or site surveying results.

Comparison of various positioning techniques

Conclusion

This article introduced different positioning techniques used in LTE networks and a summary of pros and cons of these techniques. The existing positioning techniques are used in numerous applications that will likely be expanded in the future, in response to new demands. As its quiet expensive to introduce and implement new methods, the industry inclination is to use hybrid technologies (for e.g. AGNSS with OTODA) or improve upon the existing technologies.

References

• 3GPP TS 36.171 v11.0.0 (2012-09) Requirements for Support of Assisted Global Navigation Satellite System (A-GNSS) requirements analysis
• M. Kottkamp, et al., LTE Release 9 Technology Introduction White paper, Rohde & Schwarz, December 2011
• 3GPP TS 36.305, Evolved Universal Terrestrial Radio Access Network (E-UTRAN), Stage 2 Functional Specification of User Equipment (UE) Positioning in E-UTRAN (Release 11), December 2012>/li>
• 3GPP TS 36.300, Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN) Overall Description, Stage 2 (Release 11), December 2012
• 3GPP TS 36.306, Evolved Universal Terrestrial Radio Access (E-UTRA), User Equipment (UE) Radio Access Capabilities (Release 11), December 2012
• Sassan Ahmadi LTE-Advanced, Published by Academic Press, 2013
• Federal Communications Commission (FCC), Wireless E911 location accuracy requirements, June 2011. https://www.fcc.gov
• Cisco Blog – Explosive Growth in Mobility and Location-based Marketing (https://blogs.cisco.com/wireless/explosive-growth-in-mobility-and-location-based-marketing)

Photos Courtesy

• M. Kottkamp, et al., LTE Release 9 Technology Introduction White paper, Rohde & Schwarz, December 2011
• Sassan Ahmadi LTE-Advanced, Published by Academic Press, 2013