LTE positioning technology and test solution design

The development of mobile positioning technology
 
Nowadays, smartphones have become popular throughout society, and a large number of mobile applications have become an indispensable part of people's lives. More and more mobile applications use mobile positioning technology, whether it is local search applications or various commercial information publishing applications, not to mention numerous traffic navigation applications. It can be said that the application of location-based services (LBS) is already one of the most popular mobile applications.
 
The development of mobile positioning technology has gone through several stages. The initial positioning technology based on the service cell (such as CELL-ID) can quickly locate, but it is not accurate enough. The subsequent GNSS (Global Navigation Satellite System) positioning technology based on satellite signals can accurately locate, but it is slightly inconvenient because the time to first fix (TTFF) is too long due to the need to search for satellites. The most widely used of these is the GPS global positioning system in the United States. Until later, the two were combined to produce A-GNSS (Assisted GNSS) technology. The mobile terminal first obtains positioning assistance data through the mobile network to achieve rapid satellite search, and then calculates the position through GNSS signals. Compared with pure GNSS positioning, A-GNSS can achieve positioning faster, so it has become the most important mobile positioning solution.
 
However, in terms of mobile communications, LTE is coming. In some developed countries (such as the United States), LTE has already begun commercial use. Although China is still in the 3G era, research and experiments on LTE are in full swing. It can be said that LTE is already a general trend. LTE has also further improved the requirements for terminal positioning. The
 
3GPP LTE Release 9 specification defines three mobile phone positioning technologies: ECID, A-GNSS and OTDOA. Relatively speaking, OTDOA is a relatively new technology. It does not require the use of GNSS signals, but uses a positioning principle similar to GNSS to measure the arrival time difference (RSTD) of two or more base station reference signals (RS) to calculate the location of the mobile phone when the location of each base station is known (Figure 1).
 
In fact, OTDOA already exists in WCDMA, but WCDMA is not a synchronous system. The clock error between base stations makes the deployment of OTDOA costly and therefore cannot be commercialized. LTE is a synchronous system, which is conducive to the use of OTDOA. Since OTDOA does not rely on GNSS signals, it makes up for the defect that GNSS cannot be positioned indoors. OTDOA has become another mandatory LTE positioning technology for operators besides GNSS.


Figure 1: OTDOA positioning.
 
LPP positioning protocol
 
In addition to defining the positioning technology of LTE, 3GPP Release 9 also defines a new positioning protocol LPP (LTE positioning protocol) (Figure 2). LPP can fully support the positioning technologies used in LTE (including ECID, A-GNSS and OTDOA), and it also supports the hybrid positioning technology of A-GNSS+OTDOA.


 
Figure 2: LPP general positioning protocol.
 
Based on the principle of determining a point with four spheres, the mobile phone needs four reference points when positioning. With the support of the LPP protocol, in areas with good satellite visibility (such as suburbs), the mobile phone can locate by measuring four satellite signals; in areas with poor satellite visibility (such as the city center), the mobile phone can locate by measuring two visible satellite signals and two LTE base station signals; and indoors where satellites are not visible, the mobile phone can achieve positioning through OTDOA (Figure 3). The advantage of LPP is that it can ensure that mobile terminals can achieve positioning in various environments.


 
Figure 3: LPP supports A-GNSS, OTDOA and A-GNSS+OTDOA hybrid positioning.
 
As a universal positioning communication protocol, LPP is mainly used to exchange positioning assistance data and positioning information between the network and the terminal. In fact, it can be used in both the control plane and the data plane. Relatively speaking, the implementation of the control plane requires the use of dedicated control channels and will significantly increase the cost of the mobile network, because multiple network elements need to be upgraded in software and hardware to support these positioning-related control plane signaling. Therefore, the user plane implementation is more likely to be used in commercial applications. [page]

SUPL (Secure User Plane Location) Protocol

The application of LPP in the user plane is implemented through the SUPL (Secure User Plane Location) protocol (Figure 4). LPP messages, as the payload of SUPL messages, are the actual carriers of positioning information. As a vivid metaphor, SUPL messages are like envelopes, and LPP messages are the letter paper inside. In the SUPL network architecture, the network element SLP (SUPL Positioning Platform) is responsible for processing all SUPL messages. These SUPL messages interact with the terminal in the data link through P-GW and S-GW in the LTE network, and SLP obtains positioning assistance data through the E-SMLC interface.


 
Figure 4: SUPL network architecture.

SUPL 2.0, as an advanced version of SUPL, was created along with LTE. The initial version of SUPL, SUPL 1.0, does not support LTE networks, while SUPL 2.0 can support LTE, as well as the LPP positioning protocol, as well as new positioning technologies such as OTDOA and WIFI. At the same time, SUPL 2.0 is also compatible with all existing positioning protocols and technologies. For example, in addition to LPP, it also supports the common positioning protocols (such as RRLP, RRC, and IS-801) that are currently widely used in 2G and 3G networks (Figure 5). Therefore, as long as SLP and SMLC can support any of the common positioning protocols, SUPL 2.0 can be deployed in the network, regardless of whether it is a 2G, 3G, or 4G network.

This flexibility of SUPL 2.0 is very important, especially in the initial stage of LTE deployment, because LTE deployment cannot be achieved overnight and must be smoothly transitioned from the existing network. The compatibility of SUPL 2.0 with various protocols ensures a smooth and seamless transition from various standard networks to LTE networks.

 
Figure 5: SUPL 2.0 supports multiple common positioning protocols.
　　
New Features of SUPL 2.0

In addition to its technical advantages, SUPL 2.0 also adds a variety of new features. It can be used in various commercial applications or public services, becoming a new business growth point in the future. Here are two of the new features:

Regional event trigger function: Define a location area (such as a cell, multiple cells or a custom-shaped geographic area), and the mobile terminal is required to report its location when entering, leaving, being inside or outside (Figure 6). Figure

 
6: An example of regional event trigger function.

This trigger function can be initiated by the network or the mobile phone, but both parties need to agree on the trigger condition. It is expected that this function will be widely used in various applications such as location-based advertising, services that require pay-per-use, property or child supervision services, etc.

Emergency call location function: When a user dials an emergency call number, the mobile phone is forced to calculate and report the location. North America has a government-mandated enhanced 911 telecommunications service. By dialing the simple number 911, no matter who you are, as long as you are in North America, you can get a variety of rescue services including police, fire, and medical services in a timely manner. The secret here is that the emergency call service center that answers the 911 call can not only understand the type of rescue the caller needs from the voice, but also the caller's location through text or maps.

In 2G and 3G networks, this service is based on the control plane solution. For operators, this system requires a lot of infrastructure costs because a large number of LMUs (location measurement units) need to be deployed on the network side. Of course, many operators are also trying to achieve this through terminal-based SUPL solutions, but the SUPL 1.0 protocol at the time did not have a corresponding network element to connect the emergency call occurring in the control plane and the positioning function occurring in the user plane to support such a demand. Therefore, except for the United States, other countries are cautious about this issue.

Now the emergence of LTE and SUPL 2.0 has made the emergency call positioning function based on the user plane possible, because SUPL 2.0 introduces a functional entity called E-SLP (Emergency SLP), which can cooperate with the IP Multimedia Sublayer (IMS) in the LTE network to realize emergency call positioning. With the maturity of technology and the reduction of deployment costs, it is believed that more and more countries will require emergency call positioning functions, because when people are in urgent need of rescue from the government, social public institutions or others, the mobile communication devices at their fingertips are almost the only tools they can rely on.

Spirent LTS Positioning Test Solution

Positioning business is currently the fastest growing business. When the LTE era arrives, new demands are put forward for positioning services in both commercial applications and public safety services. Whether outdoors or indoors, accurate positioning is required all day long, and a seamless transition is also required based on existing technologies. It can be said that A-GNSS+OTDOA, LPP and SUPL 2.0 are key technologies to achieve these goals, and they constitute the new content of positioning technology in the LTE era. However, this comes at the cost of increasing complexity in mobile terminal and network system technology. This means more R&D costs and more complete consistency and performance tests for terminals and networks. This requires the joint efforts of operators, equipment manufacturers, test equipment manufacturers and the entire industry.

In the field of positioning testing, Spirent Communications, as a world-leading test equipment supplier, has provided a complete set of LTE positioning test solutions. Today, the "8100 LTS Mobile Device Positioning Test System" covers complete tests of OTDOA, LPP and SUPL 2.0 for LTE on the basis of the original 2G and 3G positioning tests.