Probably you have already heard of LBS and LCS, but perhaps you did not associate those ‘names’ to the topic. And if you still do not work or deal with LCS/LBS, you will certainly do some day. Especially if you work in IT or Telecommunications area, where this subject is increasingly present.
We’re talking about location services.
With the advance of services such M2M (Machine to Machine) and IoT (Internet of Things), location services are each day more present in our lives.
For this reason, it’s worth to have at least a good understanding of its concepts and operation.
Then let’s get to today’s tutorial on this subject?
Networks Location Services were originally known as LBS (Location Based Services): location-based services on the device. By knowing the location of the mobile device, the network may offer commercial services in accordance with the same.
Currently these services are known as LCS (Location Services), as standardized by 3GPP.
Actually both refer to the same thing and can be deployed in GSM (2G), UMTS (3G) and LTE (4G) networks. From now on, we will refer to these services only as LCS.
Beyond commercial services, the LCS allows the emergency calls location requirements to be met (such as E112 services, 190 or 911).
Location services (LCS) are location based services, with the goal of obtaining information of where the mobile is (location information). With the standardization of the format of the location information (eg latitude and longitude) the operators can offer different types of services.
And these services can be used in several ways, as for pricing, legal requirements as intercept, location services, emergency call services, among others.
Standardization includes aspects such as reliability, priority, security, and privacy.
In addition it also takes into account the technology used to obtain the information, which can be based on the network (network-based), or based on the mobile (mobile-based).
- In the case of technology network-based, the operator must install equipment that can perform this function.
- In the case of mobile-based, the location information is obtained by the mobile itself, such as through a GPS (Global Positioning System) chip therein.
A common way to represent these aspects is through a graph of Accuracy versus Availability. Depending on the technology used, a certain percentage of calls must be located within a certain distance.
In the 90s, the US government, through the FCC (Federal Communications Commission) mandated that all emergency calls (E911) to begin to meet specific criteria for location and reliability.
Currently, these requirements are:
- For network-based location: 67% of calls should be placed with an accuracy of at least 150 meters, and 95% within 300 meters.
- For location mobile-based: the requirement is slightly higher – 67% of calls must be accurate to at least 50 meters, and 95% within 150 meters.
Over time, however, it became apparent that the location of emergency calls based on mobile GPS was not as efficient: if the mobile is in an indoor environment, or even in dense-urban, it will have problems with direct line of sight.
Problems like this are making the FCC in the United States and other entities around the world require operators higher efficiency (or accuracy) in these locations.
But regardless of the obligation to meet government bonds (which would be already a good reason), it is also very interesting for the operator to explore other aspects that LCS has to offer (as aggregated services that generate revenue).
For that, the methods for location of mobile device are increasingly evolving.
The principle of locating a mobile is quite intuitive: obtaining the distance between a mobile device and a reference. For this, various kinds of measures can be used, depending on their availability. For example, the cell measurements or from GPS satellites. With these measures the location can be obtained (by calculation, we can even consider the differences between various measures of various elements).
The more measurements, the more accurate the location. The radio signals have a known fixed rate (c = speed of light). The propagation time (t) are measured by the network. Then: Distance = c * t.
But to better understand the main existing location methods, let’s first analyze a figure with a basic network (2G, 3G, 4G) in terms of the key elements involved / necessary for LCS. In red we have the elements that may be present, and is specifically this purpose. Soon will become clearer – we’ll see how are used these new elements and protocols.
There are different positioning methods, some based on the mobile and some based on network.
The most common are:
Besides these three most widely known and used methods, there are other methods such as RFPM (Radio Frequency Pattern Matching).
Note: although we’re talking about different methods, applied at different times and scenarios, the methods are more complementary than contradictory to each other.
A-GPS Method (mobile-based)
The mobile-based location is the simplest to understand. The mobile makes a connection with satellites (of course: it must have an active GPS chip) and obtains the location information.
There is however an important information: in the A-GPS (Assisted GPS) method, the mobile does not make the initial connection directly to the satellite (which would be time consuming), but it gets the data from its server cell – which in turn already has the the satellite information stored. As a result, the mobile connection is much faster.
CELLID Method (network-based)
As most impressive example of network-based location, we have the CELLID method. Through the communication that the mobile already has with the network (either in Idle state or Connected state), it is possible to obtain location data.
The solutions based on CELLID has some variations. For example, the CELL-CENTER type is the simplest to be obtained – but also less precise. The error is the coverage radius, usually from hundreds of meters to kilometers.
The CELLID + RTT method ensures a little more accuracy. Using the result of two measurements, the TOA (Time Of Arrival) can be calculated according to the formula:
TOA = ( [ RTT ] – [ UE Rx-Tx time difference ] ) / 2
With this time, we can calculate the distance between the mobile and the cell. When the mobile is in soft handover, the mobile position can be accurately calculated considering the TOA overlap circles (in each cell).
TDOA Method (network-based)
Another method based on measurements of network, and also widely used is TDOA, which uses measurements of TOA (Time Of Arrival) of the mobile signal measured by the network.
To measure that signal, it is necessary to install a new network element, the LMU (Location Mounted Unit).
Current methods comparison
We can update the chart which we saw above, the requirements of Availability versus Accuracy with the main methods.
When possible (available), the best option to increase the accuracy and reliability is to use a hybrid approach, ie, incorporating the help of all methods to increase overall efficiency.
One option is for example, first try to get the location with A-GPS. If unsuccessful, uses the TDOA. This is a good example of complementary solutions for different areas, since the A-GPS is the best option in remote or rural areas, and the TDOA has excellent efficiency in urban and suburban areas.
In addition, other methods have been developed, such as location based on Beacons or Tags, and also through Wi-Fi.
Another method worth mentioning is the RFPM.
RFPM method (network-based)
The RFPM was developed with the aim of measuring the movements of mobile, mapping in a geo-referenced database information from MMR (Mobile Measurements Reports) with information such as signal strength, signal to noise ratio and delay.
Requires no new equipment neither modification of existing equipment.
Comparison of Current and Future methods
Again, we can update the chart of the requirements of Availability versus Accuracy with current methods along with the more recently developed.
Each method has its applicability, noting also that some methods can work together, increasing accuracy and reliability.
Control Plane x User Plane
From the planes point of view (User or Control planes), we have two ways to convey signaling information from the mobile.
The form which is an integral part of the network is to flow in the Control Plane.
- In this case, we have a low bandwidth consumption, along with high security and integrity. This makes the use indicated in cases of emergency calls or network planning/monitoring.
The other way (User Plan) works at the highest level of data connectivity, provided by the physical network.
- In the user plane, the bandwidth consumption is highest, and in addition we have problems with integrity. But the method is more directly associated (remembered) when it comes to providing location information for aggregate location-based services.
Specific LCS elements
Returning to our figure of a basic network (2G, 3G, 4G) highlighting the new elements with specific functions of location, it is easier to understand.
- LMU (Location Mounted Unit): equipment required in each cell to enable the calculation of the OTDOA (based on the network location).
- SMLC (Serving Mobile Location Server): server used for the locations calculation. It can calculate with information from LMU (where it is available), or measures of the network itself, such as TA (Timing Advance).
- GMLC (Gateway Mobile Location Centre): server that, as the name implies, serves as gateway to the LCS services. Although not shown in the figure, communicates with the HLR (Home Location Register), HSS (Home Subscriber Server), VMSC (visited Mobile Switching Centre), SGSN (Serving GPRS Support Node) and the MSC (Mobile Switching Centre) corresponding functions in sending requests for location and receiving location estimative.
Control Plan Communication Protocols between the mobile and the SMLC, defined by 3GPP:
- RRLP (Radio Resource Location services Protocol): used in GSM networks.
- TIA 801: used in cdma2000 networks.
- RRC (RRC Position Protocol): used in UMTS networks.
- LPP (LTE positioning protocol): used in LTE networks.
User Plan Protocols:
- SUPL (Secure User Plane Location): used in A-GPS.
Finally, an example of an LCS architecture in a LTE network topology, which have an E-SMLC (SMLC Evolved) directly connected to the MME, and the GMLC.
However, further study of the above figure is beyond the scope of our tutorial today.
The development and use of localization techniques should be increasingly present in future networks. The requirements that were previously present only in the x-y plane, are now already in three dimensions (including the z axis – height).
While the future is not here (demand for new applications), we already have enough options to use LCS:
- Public and Private Safety: possibility of application in various cases of public and private security. For example, in a car accident, the LCS service could even save a life. And in the case of equity security it represents a another extra help.
- Value Added Services, Media & Content: according to the location, a number of services can be offered to users, such as users who are in a certain region (a shopping, a stadium).
- Network Planning and Optimization: the LCS services can be used to improve efficiency in several areas of engineering itself as an aid in locating points that require new sites (Planning) or points that can be improved with optimization.
- Internal Network Functions: as information to the internal network algorithms, with dynamic and optimized resource allocation.
- M2M and IoT: increasingly present in applications like Machine to Machine and Internet of Things.
Besides those listed above, the number of possible uses of location information is even greater, the limit being the emergence of new applications and solutions itself.