9.5 U-TDOA Positioning Procedures

3GPP43.059Functional stage 2 description of Location Services (LCS) in GERANRelease 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

9.5.0 General

Following the receipt of a location request message from the BSC, the U-TDOA capable SMLC interrogates the BSS for the RF channel information associated with the MS to be located. The SMLC uses this information to task the LMUs at the serving and surrounding cells. The LMUs are tasked to measure the identified RF channel(s) and thus provide a time reference from different LMUs. The time-of-arrival information from the tasked LMUs is returned to the SMLC where the MS location is calculated.

9.5.1 U-TDOA Positioning in CS Domain for A/Gb-mode

9.5.1.1 General Procedures

The U-TDOA location method uses the uplink energy transmitted by an MS to make a location determination. If the MS was in the dedicated mode, carrying subscriber traffic prior to the beginning of the location process, the energy associated with this subscriber traffic can be used to locate the MS. If the MS was placed in the dedicated mode by the MSC specifically for location determination purposes, either the SDCCH or TCH can be used for U-TDOA location purposes.

9.5.1.2 CS U-TDOA Messages and Procedures on the Lb Interface

The following section describes the positioning procedure for CS U-TDOA location determination on the Lb interface.

Figure 36: CS U-TDOA Positioning Procedure

1. The SMLC sends a BSSMAP-LE Connection Oriented Information message to the BSC that contains the embedded BSSLAP U-TDOA Request message. The U-TDOA Request message may contain the delta timer value. The BSC starts the delta timer, received or internal, immediately after sending the U-TDOA Response message to the SMLC. The purpose of this timer is to define the maximum time during which the BSC supervises the location request.

2. The BSC sends a BSSMAP-LE Connection Oriented Information message to the SMLC that contains the embedded BSSLAP U-TDOA Response message. The U-TDOA Response message contains the physical channel information (frequencies, hopping sequence, channel type, time slot, sub-channel number, etc.), the intended Power-Up Starting Time (if the power-up procedure for U-TDOA is supported), the MS power, the cell identifier, and the TA. If frequency hopping is used, the U-TDOA Response message also includes the frequency list. The U-TDOA Response message also contains the ciphering key (Kc) if ciphering is used on the air interface and the version of the applied A5 ciphering algorithm (A5/x). The Kc is ciphered if sent from the SMLC to any LMU. The SMLC and any LMU with which it interacts shall also be mutually authenticated. These requirements shall be met using a security mechanism meeting the capabilities of the Zb interface of NDS/IP (TS 33.210) or TLS (RFC 2246). The LMU installation shall meet the same physical security requirements as a BTS installation. For locations on channels that are not ciphered, the algorithm identifier will show the same.

3. If the power-up procedure for U-TDOA is supported, the BSC may use the normal power-control command to order the MS to go to the maximum power allowed by the BTS. After the MS has been at full power for a period of one second, the BSC shall return to normal power-control. During the time the MS is ordered to the maximum power, the BSC may temporarily suspend uplink DTX, if in use.

9.5.1.3 RR Procedure effecting the CS U-TDOA channel description

The location determination process is not an instantaneous event and it can take a few seconds to collect and calculate location determination related data. If changes happen to the last reported channel description and the location determination is not complete, an updated channel description needs to be sent to the SMLC.

The BSC considers the location determinations complete if; it receives a BSSAP-LE Perform Location response message; or the delta timer expires; or it receives a valid BSSLAP message for a new positioning method.

The RR procedures that effect the U-TDOA channel description are listed in Table 9.5.1. The “Treatment” column lists the appropriate BSSLAP message to be sent by the BSC to the SMLC. The Reset message is defined in 3GPP TS 48.071 and shall contain the updated channel description. After sending the Reset message the BSC shall restart the delta timer and continue supervision of the location request.

Table 9.5.1: RR Procedures affecting the CS U-TDOA channel description

RR Procedure in Dedicated Mode	Treatment	Comments
Channel assignment procedure.	Reset
Handover procedures (intra-BSS).	Reset	For successful intra-BSS handover.
Frequency redefinition procedure.	Reset	Only meaningful in the case of frequency hopping.
Packet request procedure while in dedicated mode.	Reset	For DTM, when an existing CS connection is modified as PS resources are added in order to comply with MS frequency/time domain restrictions.
Packet downlink assignment while in dedicated mode	Reset	For DTM, when an existing CS connection is modified as PS resources are added in order to comply with MS frequency/time domain restrictions.
Channel mode modify	Reset

If the BSC receives the BSSLAP U-TDOA Request message during one of the identified RR procedures in Table 9.5.1, it will complete the ongoing RR procedure and then respond with the BSSLAP U-TDOA Response message.

9.5.1.4 BSC Error Handling during CS U-TDOA Positioning Procedure

There are three (3) BSSLAP messages defined to handle error scenarios that occur during the U-TDOA location process: Reset, Reject and Abort. Please refer to 3GPP TS 48.071 for the messages’ details. The BSSLAP Reset message is used to update the U-TDOA channel description as outlined in 9.5.1.3.

In Table 9.5.2, all identified RR procedures are listed that result in the BSSLAP Abort message to be sent from the BSC to the SMLC. The Abort message is only sent if the U-TDOA location determination is not complete. The BSC considers the location determinations complete if; it receives a BSSAP-LE Perform Location response message; or the delta timer expires; or it receives a valid BSSLAP message for a new positioning method.

Table 9.5.2: RR Procedures resulting in BSC Error Handling

RR Procedure in Dedicated Mode	Treatment
Handover procedure (inter-BSS).	Abort
Handover to UTRAN procedure.	Abort
Handover to CDMA2000 procedure.	Abort
RR connection release procedure.	Abort

If the BSC receives the BSSLAP U-TDOA Request message during one of the identified RR procedures in Table 9.5.2, it will respond with the BSSLAP Abort message.

If the BSC is unable to perform the U-TDOA positioning for other reasons than those related to the items listed in Table 9.5.1 and Table 9.5.2, it will respond to the BSSLAP U-TDOA Request message with the BSSLAP Reject message.

9.5.2 U-TDOA Positioning in PS Domain for A/Gb-mode

9.5.2.1 Introduction

The U-TDOA location method uses information transmitted by an MS to make a location determination. The initial state of the MS will be identified and will dictate the procedure to be followed in the location process. If the MS was in the packet transfer mode, sending uplink RLC/MAC blocks prior to the beginning of the location process, the energy associated with this continuing uplink data can be used to locate the MS. If the MS was previously idle in the uplink direction and placed in the active state by the SGSN specifically for U-TDOA location determination purposes, it is necessary to cause the MS to send uplink information using the Packet polling procedure (see 3GPP TS 44.060).

An uplink block of data containing the PACKET CONTROL ACKNOWLEDGEMENT message is equivalent to any other RLC/MAC block for U-TDOA location purposes; i.e. one uplink RLC/MAC block is equivalent to one execution of the Packet polling procedure. This applies only to the lowest numbered timeslot in the case of a multi-slot uplink TBF. The Polling Repetition information element in the U-TDOA Request message defines the total number of RLC/MAC uplink blocks required to achieve the desired location QoS within a recommended period of two seconds, including any PACKET CONTROL ACKNOWLEDGEMENT message received due to the execution of a Packet polling procedure.

9.5.2.2 General Procedures

The U-TDOA location method procedures depend on the initial condition of the MS. If the MS is initially in packet idle mode the Packet Polling method shall be applied as described in sub-clause 9.5.2.2.1. When the MS is initially in the packet transfer mode it may or may not be sending uplink data. If the MS is not sending uplink data the Packet polling procedure shall be applied. The application of the U-TDOA location method in the packet transfer mode is described in sub-clause 9.5.2.2.2.

9.5.2.2.1 MS in packet idle mode

For an MS that is in packet idle mode, application of the U-TDOA location method requires that a single timeslot downlink TBF be established. This downlink TBF shall be used for repeated executions of the Packet polling procedure in order to cause a mobile to transmit for a time sufficient to achieve the requested level of location accuracy. The number of repetitions of the Packet polling procedure required to achieve the desired level of accuracy shall be indicated in the U-TDOA REQUEST message sent from the SMLC to the BSS.

The BSS shall execute the indicated number of Packet polling procedures after an implementation dependent interval to allow assignment of the Location Measurement Units (LMU) to the indicated ARFCN and timeslot(s). The RRBP field shall be used to schedule the resulting PACKET CONTROL ACKNOWLEDGEMENT messages. The BSS must indicate a PACKET CONTROL ACKNOWLEDGEMENT response containing the TLLI by setting the TYPE OF ACK information element to RLC/MAC control block in all PACKET POLLING REQUEST messages associated with U-TDOA positioning.

If the MS is allocated an uplink TBF prior to completion of the required number of Packet polling procedures, the BSS may suspend the Packet polling procedure and send a Reset message to the SMLC containing the physical channel information for the allocated uplink TBF. Following sending of a Reset message, the BSS shall reset the Polling Repetition counter to zero and restart the U-TDOA positioning procedure after an implementation dependent interval.

The downlink TBF established for U-TDOA location purposes should be used if a single timeslot downlink TBF is required during the execution of the U-TDOA location procedure. If a multi-slot downlink TBF is required during the execution of the U-TDOA location procedure, the assignment of this TBF may be delayed until the completion of the U-TDOA location procedure, otherwise the BSS shall send a Reset message to the SMLC and reset the Polling Repetition counter as described previously.

If both an uplink TBF and a downlink TBF are required during the execution of the U-TDOA location procedure, the Packet polling procedure may be restarted and the uplink RLC/MAC blocks can be used for U-TDOA location as described subsequently.

9.5.2.2.2 MS in packet transfer mode

If only a downlink TBF exists it shall be used to execute the Packet polling procedure, on the lowest numbered timeslot transmitted before the last PACKET POLLING REQUEST/PACKET CONTROL ACKNOWLEDGEMENT cycle has been completed, the BSS shall not set the FBI bit in the RLC/MAC header of the last data block. The TBF shall be terminated after the last cycle of the Packet polling procedure using a PACKET TBF RELEASE message from the BSS. If the MS is allocated an uplink TBF prior to completion of the required number of Packet polling procedures, the BSS may suspend the Packet polling procedure and send a Reset message to the SMLC as described in sub-clause 9.5.2.2.1.

If only an uplink TBF exists and RLC/MAC blocks are available for transmission (on the lowest numbered timeslot for a multi-slot TBF), those blocks shall be used to locate the MS using the U-TDOA location method. If the number of uplink blocks pertaining to the uplink TBF is insufficient to satisfy the requested number of uplink data blocks within an implementation dependent period recommended to be two seconds, the Packet polling procedure shall be executed on the existing uplink TBF for the balance of the requested blocks. The lowest numbered timeslot shall be used for the Packet polling procedure if the existing uplink TBF is a multi-slot TBF. The uplink TBF shall not be terminated until the Packet polling procedures have been completed.

If both an uplink and downlink TBF exist, either TBF may be used for MS location using the U-TDOA location method as described previously. The TBF should not be terminated until the Packet polling procedures have been completed.

9.5.2.3 PS U-TDOA Messages and Procedures on the Lb Interface

The following section describes the positioning procedure for PS U-TDOA location determination on the Lb interface.

Figure 37: PS U-TDOA Positioning Procedure

1. The SMLC sends a BSSMAP-LE Connection Oriented Information message to the BSS that contains the embedded BSSLAP U-TDOA Request message. The U-TDOA Request message contains the required number of received uplink RLC/MAC blocks, repetitions of the RLC/MAC Packet polling request procedure or combination of both. The inclusion of this Polling Repetition information element in the U-TDOA Request message indicates that the location determination shall occur in the PS domain.

2. The BSS sends a BSSMAP-LE Connection Oriented Information message to the SMLC that contains the embedded BSSLAP U-TDOA Response message. The U-TDOA Response message contains; the physical channel information (frequencies, time slot, TFI, TLLI, start time, etc.); the MS power; the cell identifier; and the Timing Advance. For MS without an existing uplink or downlink TBF the BSS establishes a downlink TBF, if one does not currently exist. The BSS monitors any uplink TBF until the requested number of RLC/MAC blocks has been received, executes the specified number of Packet polling procedures on the lowest numbered timeslot in the case of a multi-slot TBF or a combination of both as described in clause 9.5.2.1. The BSS releases any downlink TBF established solely for U-TDOA location.

9.5.2.4 RLC/MAC Procedure affecting the PS U-TDOA TBF description

The RLC/MAC procedures that effect the U-TDOA channel description are listed in Table 9.5.3. The “Treatment” column lists the message to be sent by the BSS to the SMLC. The Reset message is defined in 3GPP TS 48.071 and shall contain the updated channel description. After sending the Reset message the BSS shall wait for an implementation dependent interval to allow reconfiguration of the LMUs, start the U-TDOA location process from the beginning and continue supervision of the location request.

Table 9.5.3: RLC/MAC Procedures affecting the PS U-TDOA channel description

RLC/MAC Procedure	Treatment	Comments
Packet Timeslot Reconfigure	Reset
Packet Access Reject	Reset	Access retry cases
Cell Reselection	Reset	MS originated (intra-BSS)
Packet Cell Change Order	Reset	Network originated (intra-BSS)

If the BSS receives the BSSLAP U-TDOA Request message during one of the identified RLC/MAC procedures in Table 9.5.3, it will complete the ongoing RLC/MAC procedure and then respond with the BSSLAP U-TDOA Response message. The Reset message must be sent after completion of the listed RLC/MAC procedure if that procedure must be executed during an ongoing U-TDOA location event.

9.5.2.5 Error Handling during PS U-TDOA Positioning Procedure

In Table 9.5.4, all identified RLC/MAC procedures are listed that result in the BSSLAP Abort message to be sent from the BSS to the SMLC. The Abort message is only sent if the U-TDOA location determination is not complete. The BSS considers the location determinations complete if; it receives a BSSAP-LE Perform Location response message; or it receives a valid BSSLAP message for a new positioning method.

Table 9.5.4: RLC/MAC Procedures resulting in Error Handling

RLC/MAC Procedure	Treatment	Comments
Cell Reselection	Abort	MS originated (inter-BSS)
Packet Cell Change Order	Abort	Network originated (inter-BSS)
Packet Pause	Abort
Packet Access Reject	Abort	Cases without access retry indication

If the BSS is unable to perform the U-TDOA positioning for other reasons than those related to the items listed in Table 9.5.3 and Table 9.5.4, it will respond to the BSSLAP U-TDOA Request message with the BSSLAP Reject message.