GPS World, March 2011

EXPERT ADVICE FIGURE 2 LPP elementary procedures and messages In LPP terminology target is the end user device to be positioned FIGURE 3 Example of a typical LPP positioning session abnormal situations FIGURE 3 shows a sample positioning session using all the procedures Assume that the server has received a location request for a given target UE and that the server can exchange messages with the UE that is lower protocol layers can provide the transport for the LPP level messages The first transaction of the location session is the capability exchange LPP Request Provide Capabilities This information exchange makes the server aware of the UE positioning capabilities GNSS support supported cellular network measurements Based on this information the server can make a decision on the positioning method to be used based on both UE capabilities and the requested quality of position response time accuracy The actual location information request is carried in LPP Request Location Information message whether position or measurements are requested and or allowed and for instance which GNSSs are allowed to be used It also carries other reporting instructions such as periodicity and required response time Having received this message the target begins its positioning activities In a typical scenario this activity triggers a request for the assistance data For instance if the server requests the GNSS based position and the target does not have the latest ephemerides the target will request those with the LPP Request Provide Assistance Data mechanism transaction 3 Having received the ephemerides the target can position itself quickly without needing the data broadcasts from the satellites and report the location information back to the server in LPP Provide Location Information message Other supporting information such as reference location reference time and ionosphere model may also be provided to the target FIGURES 4 and 6 summarize the contents of LPP Provide Location Information and LPP Provide Assistance Data messages respectively in the gray boxes The LPP Provide Location Information contents can be roughly divided into four categories one category for each positioning method assisted GNSS observed TDOA and ECID and one category for providing the location estimate In the A GNSS category the UE based on the server commands either reports the raw code and carrier phase measurements UE assisted mode or information regarding the provided PVT estimate OTDOA and ECID function only in UEassisted mode in LPP The LPP Provide Assistance Data reflects the same structure and categorization Similarly to Provide Location Information message one can see in the assistance data message GNSS specific assistance as well as OTDOAspecific assistance However there is no ECID specific assistance due to it being available only in UE assisted fashion For OTDOA there is assistance but only to assist the UE in the measurement process not for positioning purposes for instance eNodeB positions cannot be transferred in the assistance data User Plane and Applications RRLP RRC and LPP are natively control plane positioning protocols This means that they are transported in the inner workings of cellular networks and are practically invisible to end users In the control plane their main purpose is to reliably provide the emergency call positioning capability However there is obviously demand for positioning services for locationbased end user applications To address this in 2003 the Open Mobile Alliance started to work with Secure User Plane Location SUPL 10 protocol that brings the same location capabilities to user plane application domain over IP networks as RRLP RRC LPP bring to control plane One design principle of SUPL was not to re invent the wheel thus RRLP RRC LPP are being re used in the user plane domain for positioning OMA SUPL specifies a bearer protocol that carries a 3GPP defined positioning protocol and provides security authentication privacy and charging mechanisms SUPL 10 is already commercially deployed and SUPL 20 is now being deployed globally FIGURE 5 shows the OMA SUPL 20 protocol stack which illustrates the re use of 3GPP positioning protocols over IP networks The security is provided by the standard transport layer security TLS and the user plane location protocol ULP is the wrapper for the 3GPP positioning protocols The vast majority of SUPL 20 deployments will use RRLP as the positioning protocol SUPL 30 currently being defined will no longer support RRLP RRC LPP GPS World March 2011 www gpsworld com 10