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Location service is an integral component of wireless technologies. Mobile devices, with their radio signal measurement and real‐time communication capabilities, are uniquely qualified to perform this mission critical service. Jointly managed by the mobile device and its attached cellular network, Location service acquires and disseminates a device’s location information to authorized applications, supplementing a wide variety of location‐based services. This essential service has been incorporated into most 2G and 3G networks, and it will be featured prominently in LTE networks and beyond.
Location service in LTE aims to provide a cost‐effective solution to support the continuing proliferation of LBS applications. In addition to increasing accuracy and reducing response time, LTE implements a flexible quality of service scheme to cope with the expected growth in LBS usage volume and to fulfill future application requirements. To satisfy this demanding and fluid obligation, multiple positioning methods – used in either standalone or hybrid mode – are adopted to provide consistent results across different coverage environments: rural or urban, indoor or outdoor.
Besides optimizing the economics of location services, LTE carriers also have regulated mandates to report devices’ positioning during E911 emergency procedure with stringent requirements on accuracy, uncertainty, and confidence level. There are also other drivers pushing the evolution of LTE’s location service. By leveraging LTE’s high data rate and low latency with accessible location information, next-generation location services can open up ample new business opportunities, ranging from location‐based applications to vehicle tracking services.
LTE positioning methods
LTE utilizes several positioning methods to determine connected devices’ geographical and velocity information. The network can select one or more methods based on device capabilities, operating environments, and LBS application QoS requirements. The main QoS parameters include positioning accuracy and response time. In addition to horizontal/vertical geographical coordinates and velocity information, LTE positioning reports should also include uncertainty and confidence level to provide better interpretation of results.
Global navigation satellite system refers to the traditional satellite‐based geo‐spatial positioning systems, such as GPS. However, without assistance from the cellular components, locking onto faint signals from dozens of satellites requires long acquisition times and heavy power consumption. Moreover, satellite‐only measurement techniques exhibit significant limitations from line‐of‐sight problems for indoor and certain urban environments. Conversely with A‐GNSS, the network assists its device’s positioning by sharing known environmental information to the mobile device, or user equipment, in order to reduce satellite dependency and speed up acquisition time. Data such as reference time, visible satellite list and code phase search windows can increase a UE’s GNSS measurement sensitivity.
There are two modes of A‐GNSS: mobile‐assisted mode and mobile‐based mode. In the mobile‐assisted mode, user equipment would perform the measurement and forward the data points to the network for final position calculation. Whereas in the mobile‐based mode, network would provide relevant data such as reference position, satellite ephemeris, and clock corrections to assist user equipment in calculating its own position.
User equipment can quickly obtain its approximate positioning by looking up its serving cell’s identity and its corresponding location information. This is commonly referred to as the “CID” method. While straightforward, this approach is ineffective when user equipment is operating in cells with large coverage area, particularly in rural areas where cells typically are miles in radius. To provide more granular location information, 3GPP Release 9 defined an “Enhanced‐Cell ID” positioning method where a device uses CID information in conjunction with the estimated distance to its serving LTE base station, or ENodeB, based on the detected radio signal’s round trip time, reference signal received power, and/or angle of arrival. Using all of this information together would allow a device to identify its position relative to the serving ENodeB and infer its precise geographical coordinates accordingly. If possible, the device may also perform similar RF measurement on its neighboring DNodeB(s) to further refine its positioning through multilateration.
Observed time difference of arrival is a positioning method that uses reference signals from three or more time‐synchronized adjacent ENodeBs to calculate a device’s position based on the received signal time difference. Each measurement of a pair of downlink transmission describes a line of constant difference (hyperbola) where a device may reside. By measuring a minimum of two pairs of downlink transmission (involving a minimum of three ENodeBs), a device’s precise point of location can be identified.
In Release 9, measuring time difference of arrival is only applicable for the downlink signal since the device’s uplink transmit power is limited and may not reach multiple DNodeBs. However, uplink time difference of arrival positioning method has been adopted in Release 11 to utilize uplink sounding reference signals’ time of arrival differences at different ENodeBs.
Additional methods
There are other possible positioning methods besides the three primary ones previously described. RF fingerprinting is a technique that locates user position by mapping the device’s RF measurement onto a previously surveyed RF map. By matching obtained measurements with existing data from the network, it may be possible to derive a device’s position.
Self‐learning is also conceivable by updating a database with the latest positioning results obtained from OTDOA and A‐GNSS methods. For example, adaptive enhanced cell identity is a method that combines RF fingerprinting with other measurements such as timing advance, RSTD, AoA and RSSI. It then automatically builds up a cell’s database tagged with measured radio properties to assist future device positioning.
Location services in operation
While the ENodeB appears to be the primary entity guiding device positioning during location service, it does not actually manage location services. Instead, the ENodeB simply facilitate location service message exchanges between the device and the evolved packet core. Other than its messenger role, the ENodeB merely serves as a reference source for RF measurement. LTE’s location services operation principally involves close coordination amongst three key network entities –“location service target,” “location service server” and “location service clients.” The communication protocols between these entities may occur over the control plane and/or the user plane. The information communicated includes security parameters, positioning capabilities, assistance data, measurements and estimates, and error-handling commands.
Control plane signaling is mainly defined by LTE location protocol, which is a point‐to‐point protocol specified in 3GPP TS36.355. The communication over the user plane, on the other hand, runs over the application layer using secured user plane location protocol, which is developed by the Open Mobile Alliance and is radio access network agnostic.
Location service target refers to a mobile device whose position is of interest. As described above, a location service target may perform RF measurements or even calculate its own position. However, its behavior is closely guided by an location service server using LTE positioning protocols.
Location service targets can communicate with location service servers via the control plane or the user plane. In the case of control plane, LPP messages are carried over Layer 2 signaling radio bearers, which ENodeB then forwards to MME and E‐SMLC. Location service messages on the user plane would be carried over Layer 2 data radio bearers and are forwarded by ENodeB to SLP/SPC via packet data network gateway. In SUPL terminology, location service target may be referred to as SUPL enabled terminal.
Location service server
Besides coordinating with location service targets, location service servers also interact with location service clients to authorize access, gather requirements, perform positioning‐related tasks, and disseminate positioning reports.
On the control plane, evolved‐serving mobile location center acts as the location service server entity. It is analogous to serving mobile location center in a GSM network or standalone SMLC in a UMTS network. The E‐SMLC manages the overall coordination and the resource scheduling required in positioning a device attached to the LTE network. Besides interacting with the device via LPP, this entity controls ENodeB using LPP annex protocol and communicates with MME using location service application protocol. During mobile‐assisted measurements, E‐SMLC is also responsible for the calculation of final device location and the estimation of achieved accuracy.
If LPP occurs over the user plane, SUPL location platform would be in charge of authentications between SET and third-party location service client(s). The SLP then uses Internal location protocol to interface with SUPL positioning center, which is responsible for positioning related functions: security, assisted data delivery, reference retrial and positioning calculation.
Location service client and privacy
A location service client may be a software or hardware entity that makes use of device positioning information. A location service client can be external to the network or reside inside the location service target itself. In LTE, there may be multiple location service clients accessing a location service target’s positioning report in series or in parallel. The network will attempt to satisfy each location service client’s positioning request based on their specified QoS, subject to the network conditions.
Recall in the location service server section, a location service client that inquires device location over the user plane is first authenticated by SLP before corresponding with SPC to obtain the desired location information. In the control plane case, gateway mobile location center acts as the gatekeeper before forwarding a location servce client’s positioning requests to E‐SMLC via MME. There are also additional provisions for privacy controls based on configurable settings and user consents. These procedures ensure that only authorized parties have access to the sensitive user location information.
Conclusion
LTE technology provides an integrated location service solution within its network architecture, making use of multiple positioning methods to address potential application requirements for varying environmental conditions. While location service offers promises of enhanced user experiences and new business opportunities, significant challenges still lay ahead for the LTE network operators and the equipment manufacturers. Because of the sensitivity to radio conditions and location service’s critical role in emergency LBS, device suppliers, network equipment manufacturers and network operators will all be required to conduct extensive lab and field testing that cover a wide range of scenarios to ensure regulatory compliance and an overall high quality of experience for the end users.
