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Jul 16 2012

A Primer on Location Technologies in LTE Networks

Long Term Evolution (LTE) technology offers diverse methods for obtaining and delivering user location that help Communication Service Providers (CSPs) monetize and optimize their networks. How can CSPs select the most effective method to support the unique requirements of location-based services (LBS) solutions? Globally, the wireless market is evolving to 4G LTE, which offers very high performance, a rich set of services and a robust ecosystem to support a wide variety of mobile broadband applications. Location information is essential to the success of a rapidly growing number of these applications. As a result, CSPs who can support LBS gain valuable opportunities to monetize their network resources. To date, only a small percentage of CSPs have pursued these opportunities over legacy networks, although Application and Content Providers (ACPs) have actively built Over-the-Top (OTT) LBS that use the network solely for transport, without sharing service revenues. Now, the migration to LTE technology offers CSPs a variety of new ways to support LBS and strengthen their competitive differentiation with ACPs - even across multi-vendor networks. To deploy the most effective LBS solutions, CSPs first need to identify the unique requirements of each service and then choose the best methods to satisfy these needs. This article supports the selection process by providing a brief overview of typical service requirements, key advantages and disadvantages of solution elements, and a tabular comparison of all these characteristics. It also provides an illustrative use case that examines how LTE location methods could be applied to a dynamic pricing service.

Step 1: Identifying service requirements

Some LBS are targeted at individual users, while others are focused on aggregated user behavior that will be of value to enterprises and ACPs. In very general terms, the services can be grouped into the following categories.

  • Regulatory-based applications, such as emergency call services
  • Consumer applications, such as social networking, service finder and navigation services
  • Business applications, such as fleet management or social networking tailored to business needs
  • Third-party applications that enable merchants or ACPs to target specific end users for their advertising or services
  • Network operator applications that help detect internal problems or optimize networks and business capabilities

Because LBS have widely diverse requirements, CSPs must carefully identify and prioritize a particular service’s needs before choosing methods of gathering and delivering location information. The following list summarizes some of the most important general requirements:

  • Providing the required degree of location accuracy
  • Rapidity of response to a location request
  • Scalability to support a large number of user locations
  • Support for real-time, periodic or event-triggered location updates
  • Ease of implementation
  • Battery consumption for mobile devices
  • Impact on network resources
  • Differentiation of the network provider service from ACP offerings

Step 2: Choosing a method of determining location

A network provider needs to select an appropriate method for determining the location of a user device, based on the service requirements that have been identified. (Table 1 compares the advantages and disadvantages of all the methods discussed in this section.) Primary modes of operation — There are three primary network-enabled modes of operation for determining user location:

  • In the device-based mode, the user device obtains location measurements with assistance from the network and calculates the user’s position.
  • In the device-assisted mode, the user device provides positioning measurements, which the network uses to calculate the user’s location.
  • In the network-based mode, the network calculates the user’s position without involving the device.

The following location technologies support one or more of the above modes. Assisted Global Positioning System (AGPS) —The network assists GPS-enabled mobile devices to improve the performance of their GPS receivers by providing it with assistance information such as reference time, visible satellite list, reference position, and satellite ephemeris. In general, AGPS provides the highest accuracy of any network-enabled technique. However, it is not reliable indoors or in dense urban or high-rise building environments, in which case AGPS can be supplemented with other techniques such as Observed Time Difference of Arrival (OTDOA) or Enhanced Cell ID (ECID) described below. Cell ID (CID) — A user device is localized to its serving eNode B (eNB), typically to the specific cell/sector within the eNB. Although this method is the least accurate, it is easiest to implement, highly scalable, and has high availability. ECID — Position is localized to a finer level compared to CID, using additional radio-related measurements. This method has low accuracies (50 -1000m) depending on the size of the cell, but is easier to implement than most other methods and is generally available across diverse vendor products and networks. OTDOA — This method uses the measured timing of downlink signals received from multiple eNBs to locate the user device in relation to neighboring eNBs. In dense urban and indoor environments, OTDOA can be used to supplement AGPS, provided the user device can detect position reference signals (PRS) from three or more eNBs. Uplink time difference-of-arrival (UTDOA) — Defined as part of 3GPP Release 11, UTDOA will be available later than the other techniques. As a key benefit, it offers minimal impact on air interface resources.

Step 3: Choosing a delivery method for location data

The LTE system provides several methods for delivering location information when it is determined. Each has certain capabilities, strengths and constraints. Thus they should be chosen to match the unique requirements of a specific service. (Table 2 compares the advantages and disadvantages of all the methods discussed in this section.) User plane method — Very little interaction with underlying wireless access technologies is required, so this method can be used ubiquitously across LTE and legacy networks. It also scales well, making it well suited to support commercial LBS, with less complexity, cost and impact on the network than the control plane solution. Based on OMA SUPL standards, this solution relies on user plane data bearers to transfer location-assistance and positioning-related messages between the SUPL-enabled terminal (SET) and the SUPL Location Platform (SLP). A SUPL client is required on the user device. The solution’s lack of direct connection to the access network is a disadvantage for supporting positioning methods, like ECID and OTDOA, which require assistance data from the network. In addition, because this solution can only enable single-user location requests, it is not suitable for applications that need periodic reporting on all user locations within a cell or zone. Depending on the location technology used, the User Plane solution can support high accuracies to meet the needs of the majority of LBS applications targeted at the individual user, such as turn by turn navigation, geofencing, and fleet management. Control plane method — Defined in the 3GPP R9 standards, this solution transfers positioning data over the control plane, between the user device, the eNB and the positioning elements (Evolved Serving Mobile Location Center (e-SMLC)/Gateway Mobile Location Center (GMLC)). This provides easy access to assistance data from the network and enables more reliable service performance than the user plane solution. And because it does not require a client on the user device, it can be used to support emergency services on LTE networks for limited service devices. On the other hand, this solution has less scalability than the user plane, only supports single-user location requests and is more difficult to interwork with legacy networks. Per-call measurement data (PCMD) method — This proprietary near real-time Alcatel-Lucent diagnostics tool offers CSPs all active users’ location data along with other call/connection information that can be used for generating wireless heat maps, network planning and optimization, Minimizing Drive Tests (MDT) and more efficient mobile advertising. For example, heat maps correlating location to usage can be used to determine traffic hot spots or coverage holes that can be addressed with RF design and/or network planning. Network-provided (ULI) method — This standards-based solution supports application enablement, providing a way for third-party applications to get location information. However, it will not deliver the level of granularity offered by user plane or control plane solutions. It reports location changes to a Policy and Charging Rules Function (PCRF) or Online Charging System (OCS) through location-event triggers. Then the data can be exposed to external applications using Application Programming Interfaces (APIs) — or to IP Multimedia Subsystem (IMS)-based applications using a PCRF Rx interface.

Home Subscriber Server (HSS) method — This method is particularly useful for IMS core and application entities that already have an existing Sh interface to the HSS that needs network-based user location data for location-based charging, LI, emergency or other services.

Illustrative Use Case

The location determination and delivery methods described above provide general guidelines on their capabilities and how well they might match different applications depending on the intended use and characteristics of the application. A specific use case for a Dynamic Pricing service is described in this section to illustrate how the guidelines above can be used for selecting the right location methods.

Dynamic Pricing: Dynamic pricing service is a good example of an application enablement solution, which permits CSPs/third parties to have secure access to network-based location information to deploy a new LBS. In this case, it uses real-time network-load information to offer pricing discounts or other special offers to selected end users.

To target the right users, the service needs periodic updates on per-cell utilization levels, along with data that identifies users within the cell. Therefore, the key location requirement for this use case is to obtain the cell level location of all service subscribers, along with cell congestion information.

When considering which LTE data delivery methods would be suitable for this use case, it’s clear that the user plane, control plane and HSS methods are not appropriate because they are single-user solutions and also can have device dependencies depending on the location technology used.

The network-provided ULI method is the best alternative, as it is a standards-based device independent option that can provide location information for all or targeted active users in a cell.

A growing opportunity

As the vast number of applications and use cases that rely on location capabilities continues to expand, LTE offers CSPs new ways to benefit from this market opportunity. The LTE network’s diverse methods can be matched to the specific requirements of each service by leveraging the capabilities of the specific network elements (such as eNB, Mobility Management Entity, SUPL Location Platform/e-SMLC/GMLC, and PCRF) required to support the service envisioned. This open and flexible architecture allows network resources to be monetized by supporting a wide array of location based services.

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About Suma Cherian
Suma Cherian received a Master’s degree in Electrical Engineering from Rutgers University, New Jersey. She has been working at Alcatel-Lucent since 1996, doing systems engineering and architecture work for packet, wireless and next generation networks and applications. She is currently working in the MGR (Multi-Generation Radio) organization in the areas of End-to-End LTE location solutions and LTE SON.