12 Uplink-Time Difference Of Arrival (U-TDOA) positioning method

25.3053GPPRelease 17Stage 2 functional specification of User Equipment (UE) positioning in UTRANTS

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

12.1 General

The U-TDOA method calculates the location of a transmitting UE by using the difference in time of arrival of signals at different LMUs. The time required for a signal transmitted by a UE to reach a U-TDOA capable LMU is proportional to the length of the transmission path between the UE and the U-TDOA capable LMU. The U-TDOA method does not require knowledge of the time the UE transmits nor does it require any new functionality in the UE.

Figure 12.1: Ideal Signal Collection

Figure 12.1 shows a time domain representations of two perfectly matched signals as received from the same UE by two U-TDOA capable LMUs in ideal conditions, including high signal-to-noise ratio and no multipath distortion. Because the receivers are at different distances from the transmitting UE, the signals are out of phase; they do not correlate.

Figure 12.2. Ideal and Attenuated Waveforms

Figure 12.2 shows the theoretical effects of increased distance, multipath distortion, and interference on a received signal. The top signal was received under ideal conditions. The lower one represents what that signal would look like in more realistic conditions. It is attenuated to a level much closer to the noise floor. Nevertheless, there is a correlation with the original signal.

Cross-correlation is a statistical technique that compares the waveforms of two signals point for point. Identical signals in perfect phase would yield a correlation coefficient of 1. In Figure 12.3, even though the waveform in (b) is attenuated compared to (a), the alignment would still yield a very high correlation, say 0.999.

Figure 12.3. Using Cross-Correlation to Compute Time Delay

Figure 12.3 shows (a) a signal received by a U-TDOA capable LMU that is close to the transmitter and (b) the same signal received by a second U-TDOA capable LMU farther away. The latter signal is attenuated and perhaps distorted. It is also out of phase with the original signal because it was received at a later time. The correlation coefficient between (b) and (a) would be quite low. However, by shifting the (b) curve to the left, it is possible to find a position in which the correlation coefficient is maximized (shown by (c)). For this position, the amount of shift required is equal to the arrival time difference of the signal between the two U-TDOA capable LMUs.

Figure 12.4. Deriving the Hyperbolic Function for Time Delay

In this model (Figure 12.4), Rx1 and Rx2 represent fixed locations of two U-TDOA capable LMUs in a network. The pairs of intersecting circles centered around Rx1 and Rx2 have radii such that each radius represents the time, t_TR, it takes a signal to travel from a UE transmitter to each U-TDOA capable LMU (that time being unknown). The difference between radii is a constant equal to the difference between t_TR1 for the first U-TDOA capable LMU and t_TR2 for the second U-TDOA capable LMU. t_TR1– t_TR2 is the value that the U-TDOA system measures. When you plot and connect the points of intersections between each pair of circles, the resulting shape is a hyperbola . The hyperbola defined by the reference site in comparison to another site constitutes a baseline. If you constructed a similar hyperbola for another pair of U-TDOA capable LMUs receiving transmissions from the same UE, that hyperbola would intersect at two points with the first hyperbola, yielding two possible locations for the UE. A third such hyperbola (Figure 12.5) would yield a unique location for the calling phone, so that a minimum of four reception sites is needed to obtain the unique location estimate.

Figure 12.5. Intersection of Three Hypothetical Hyperbolas Calculated from Time Delay Differences

In the U-TDOA method, the reference measurement is the one that represents the U-TDOA capable LMU site that collects the signal with the highest quality. That site is usually the site closest to the UE, and normally generates the highest signal level as well as the highest SNR. In addition, it presumably exhibits the lowest multi-path error. Therefore the reference time delay is the one against which time differences are measured. Any errors introduced into the time delay measurement yield inaccuracies in the intersection point. Since the speed of light is approximately 0.3 m/nsec in air, a nanosecond error in time delay measurement introduces 0.3 meters of error.

The more hyperbolas that are calculated, that is, the more cooperating U-TDOA capable LMUs used in the calculation, the more accurate is the estimate of the UE’s position. This is another way of saying that statistical confidence improves with an increase in sample points. When many sites enter into the calculation, the effects of a large time delay measurement error at a single site are minimized.

12.2 U-TDOA SAS architecture

The overall network architecture is similar to the previous architecture and is illustrated in Figure 12.6. The only difference is that the U-TDOA capable LMU are connected directly to the SAS via an overlay network.

Figure 12.6 – U-TDOA SAS Architecture

The impact on the SAS centric approach due to the presence of the Iur is FFS.

12.3 RNC based U-TDOA positioning for Cell_DCH and Cell_FACH RRC states

The U-TDOA positioning method can be performed when the UE is in either the CELL_DCH or CELL_FACH RRC states. If the UE is in the CELL_PCH or URA_PCH state the SRNC shall first bring the UE to the CELL_FACH or CELL_DCH states so that the U-TDOA measurements can be performed by the LMUs and to provide the current serving Cell ID if the UE was in the URA_PCH state.

12.3.1 UE in CELL_DCH state

The SRNC may invoke the U-TDOA positioning method using the PCAP: Position Calculation Request message that contains the Cell ID and RF channel information for the UE being positioned when that UE is in the CELL_DCH state. The SAS executes the U-TDOA positioning method and returns the location and optional velocity, or appropriate error indication to the SRNC using the PCAP: Position Calculation Response message as described below.

Figure 12.7: RNC initiated U-TDOA positioning procedure in CELL_DCH state

1. The operation begins with an authenticated request for positioning information about a UE from an application in the core network being received at the SRNC in a RANAP: Location Reporting Conrol message. The request from the CN may be a request for on-demand or periodic reporting.The SRNC may invoke the U-TDOA positioning method via the Iupc interface.

2. The SRNC sends a PCAP: Position Calculation Request message to the SAS containing the contents of the RANAP Location Reporting Control message, the RF channel information and Cell ID for the UE being positioned. This information indicates that the UE is in the CELL_DCH state.

3. The SAS configures the U-TDOA capable LMUs, analyzes the returned information and calculates the UE position and optionally, velocity.

4. The SAS returns the UE position and optionally the velocity or error indication to the SRNC in a PCAP: Position Calculation Response message.

5. The SRNC returns the UE position and optionally the velocity, to the CN in a RANAP: Location Report message.

6. If periodic reporting was requested by the CN at step 1, steps 2 to 5 may be repeated until the desired amount if reports has been attained, or the procedure is cancelled by UTRAN or CN.

12.3.2 UE in CELL_FACH state

The SRNC may invoke the U-TDOA positioning method using the PCAP: Position Calculation Request message that contains the contents of the RANAP Location Reporting Control message, Cell ID and RF channel information for the UE being positioned. If the UE is in the CELL_FACH state the SRNC, after sending the Position Calculation Request message, shall execute a procedure that causes the UE being positioned to transmit a certain number of pre-coded bits within a certain period of time. The procedure described in steps 6-8 in subclause 12.4.2.1 below is offered as an example that could be used for the U-TDOA positioning method when the UE is in the CELL_FACH state.

The SAS executes the U-TDOA positioning method and returns the location or appropriate error indication to the SRNC using the PCAP: Position Calculation Response message as described below.

Figure 12.8: RNC initiated U-TDOA positioning procedure in CELL_FACH state

1. The operation begins with an authenticated request for positioning information about a UE from an application in the core network being received at the SRNC in a RANAP: Location Reporting Conrol message. The request from the CN may be a request for either on-demand or periodic reporting. The SRNC may invoke the U-TDOA positioning method via the Iupc interface.

3. After sending the Position Calculation Request message the SRNC shall execute a procedure that causes the UE being positioned to transmit a certain minimum number of pre-coded bits within a recommended maximum time interval of three seconds. The number of bits and actual time interval should be related to the required location accuracy, the propagation conditions in the serving cell and the LMU configuration for the SAS and may be determined in an implementation dependent manner. As an example of a method to cause the UE to transmit, the SRNC may configure the UE to transmit RLC Status PDUs as described in subclause 12.4.2 below.

4. After a configurable time interval the SAS configures the U-TDOA capable LMUs, analyzes the returned information and calculates the UE position and optionally, velocity.

5. The SAS returns the UE position and optionally, velocity to the SRNC in a PCAP: Position Calculation Response message.

6. The SRNC returns the UE position and optionally, the velocity to the CN in a RANAP: Location Report message.

7. If periodic reporting was requested by the CN at step 1, steps 2 to 6 may be repeated until the desired amount if reports has been attained, or the procedure is cancelled by UTRAN or CN.

12.4 Optional SAS initiated U-TDOA positioning for Cell_DCH and Cell_FACH RRC states

This subclause describes the U-TDOA positioning procedures in the CELL_DCH and CELL_FACH states when the location method to be used is determined by the SAS.

12.4.1 UE in CELL_DCH state

FDD Mode:

Data (either signalling data (DCCH) or user data (DTCH)) transmitted by a UE in CELL_DCH state will always be sent on the DCH (Transport Channel). The DCH in turn is mapped to the DPDCH (Dedicated Physical Data Channel) by the physical layer. The DPDCH is identified by the allocated frequency, the channelization code and the scrambling code.

The DPDCH always has an associated DPCCH (Dedicated Physical Control Channel). The DPCCH carries control information relevant to the physical layer. The DPCCH is under the same scrambling code, but uses a different spreading code.

While the DPDCH is active only during actual transmission of user data, the DPCCH is continuously transmitted as long as the UE is in the CELL_DCH state. The UE transmits either the DPCCH and the DPDCH, or solely the DPCCH – the scrambling code allocated to the UE can be received continuously as long as the UE is in the CELL_DCH state.

When the UE is in CELL_DCH state, the allocated scrambling code can be used to identify the UE for U-TDOA positioning determination. No additional interference is generated to locate the UE using the U-TDOA positioning method when in the CELL_DCH state.

TDD Mode:

Data (either signalling data (DCCH) or user data (DTCH)) transmitted by a UE in CELL_DCH state will always be sent on the DCH (Transport Channel). The DCH in turn is mapped to the DPCH (Dedicated Physical Channel) by the physical layer. The DPCH is identified by the allocated frequency, the channelization code(s)/timeslot(s) and the scrambling code associated with the cell that the UE is attached to (the scrambling code is cell-specific)..

The DPCH is active:

– during actual transmission of user data

– during the transmission of special bursts [26].

Special bursts are generated periodically when the UE has no data to send.

When the UE is in CELL_DCH state, the allocated uplink scrambling code assigned to the UE’s cell and the channelisation code(s) / timeslot(s) can be used to identify the UE for U-TDOA positioning determination. No additional interference is generated to locate the UE using the U-TDOA positioning method when in the CELL_DCH state.

12.4.1.1 Message Flow

Figure 12.9 U-TDOA message flow, UE in the CELL_DCH state

1) The procedure starts when a RANAP: Location Reporting Control message requesting a geographical UE position is received at the SRNC. The RANAP Location Reporting Control message may contain periodic location information (amount of reports and reporting interval).

2) Location Reporting Control message parameters, including any periodic location information, plus UE capability information plus Cell ID are forwarded to the SAS in the PCAP: Position Initiation Request message.

3) Based upon the service type (emergency service, etc), and the requested QoS, the SAS initiates a U-TDOA position, and sends a PCAP: Position Activation Request message indicating U-TDOA to the SRNC. The SRNC chooses to bring the UE to the CELL_DCH state if not already in this state.

4) The SRNC returns U-TDOA channel information in a PCAP: Position Activation Response message to the SAS.

5) The SAS configures the U-TDOA capable LMUs to perform measurements. The U-TDOA capable LMUs measure the [FDD: DPCCH and possibly DPDCH] [TDD: DPCH].

5a) If periodic reporting was requested in step 2, the SAS obtains a position, and optionally a velocity estimate using the U-TDOA capable LMU measurements and returns it to the SRNC in a PCAP: Position Periodic Result message. The SAS continues to send PCAP: Position Periodic Result messages to the SRNC until the requested amount of reports has been attained or the procedure is cancelled by UTRAN or CN. The SAS sends the final location information to the SRNC in a PCAP Position Initiation Response message. The SAS may repeat steps 3 and 4 at any time to obtain or verify the U-TDOA channel information. The SRNC returns the individual position estimates and optionally the velocity estimate to the CN in a RANAP: Location Report message.

6) If periodic reporting was not requested in step 2, the SAS obtains a position, and optionally a velocity estimate using the U-TDOA capable LMU measurements and returns it to the SRNC in a PCAP: Position Initiation Response message.

7) The SRNC returns the position estimate and optionally the velocity estimate to the CN in a RANAP: Location Report message.

12.4.2. UE in CELL_FACH state

In the CELL_FACH state the mobile is sharing a common uplink channel and is usually not sending data in the uplink direction. It is not possible to rely on the user application to cause the UE to transmit. For the U-TDOA location method the SRNC must force the UE to transmit.

As an example, this may be achieved by sending a RRC Radio Bearer Reconfiguration message that causes the UE to periodically send a RLC: Status PDU to the SRNC for an Acknowledged Mode DCCH. When sufficient repetitions have occurred to meet the requested location QoS the RRC Radio Bearer Reconfiguration message would be used to return the UE to the default state. In the TDD mode, the DCCH signalling radio bearer carrying the status PDUs shall be mapped to the RACH transport channel, not the USCH.

12.4.2.1 Message flow

Figure 12.10 Example U-TDOA message flow, UE in the CELL_FACH state

2) Location Reporting Control message parameters, including any periodic location information, plus Cell ID plus UE capability information are forwarded to the SAS in the PCAP: Position Initiation Request message.

3) Based upon the service type (emergency service, etc), and the requested QOS, the SAS initiates a U-TDOA position, and sends a PCAP: Position Activation Request message indicating U-TDOA to the SRNC. The SRNC chooses to bring the UE to the CELL_FACH state if initially in the CELL_PCH or URA_PCH state.

4) The SRNC returns U-TDOA channel information in a PCAP: Position Activation Response message to the SAS.

5) The SAS configures the U-TDOA capable LMUs to perform measurements. The U-TDOA capable LMUs measure the PRACH or PCPCH bursts associated with the UE being positioned.

6) The SRNC, after sending the PCAP Position Activation Response message, shall execute a procedure that causes the UE being positioned to transmit a certain number of pre-coded bits within a certain period of time. The number of pre-coded bits and period of time should be based upon (e.g. set equal to) a recommended number of bits and recommended time interval defined by the SAS in the PCAP Position Activation Request message invoking U-TDOA positioning. As an example, the SRNC could reconfigure the radio bearer so that the UE begins to send RLC status PDUs every 100 mS by sending the UE a Radio_Bearer_Reconfiguration message with the TimerStatusPeriodic IE configured for 100 milliseconds.

7) The UE begins to send RLC status PDUs every 100 milliseconds.

8) After expiration of the configurable T_rach timer the SRNC sends the UE a Radio_Bearer_Reconfiguration message with the TimerStatusPeriodic IE set to NA in order to stop the periodic status reporting.

8a) If periodic reporting was requested in step 2, the SAS obtains a position, and optionally a velocity estimate using the U-TDOA capable LMU measurements and returns it to the SRNC in a PCAP: Position Periodic Result message. The SRNC returns the position estimate and optionally the velocity estimate to the CN in a RANAP: Location Report message. When the periodic reporting interval transpired, the SAS may send a new PCAP: Position Activation Request message to the SRNC and steps 3 to 8 may be repeated until the requested amount of reports has been attained or the procedure is cancelled by UTRAN or CN. The SAS sends the final location information to the SRNC in a PCAP Position Initiation Response message. The SRNC returns the individual position estimates and optionally the velocity estimate to the CN in a RANAP: Location Report message.

9) If periodic reporting was not requested in step 2, the SAS obtains a position, and optionally a velocity estimate using the U-TDOA capable LMU measurements and returns it to the SRNC in a PCAP: Position Initiation Response message.

10) The SRNC returns the position estimate and optionally a velocity estimate to the CN in a RANAP: Location Report message.