What is OTDOA | How OTDOA works, OTDOA applications, benefits
This page covers OTDOA (Observed Time Difference of Arrival) basics. It mentions how OTDOA works in LTE system and mentions applications of OTDOA. The advantages or benefits of OTDOA are also mentioned. The links to comparison between OTDOA with GPS and U-TDOA are also mentioned.
There are three techniques used in LTE to determine the position of UE.
• A-GNSS (Assisted Global Navigation Satellite Systems)
• OTDOA (Observed Time Difference of Arrival)
• ECID (Enhanced Cell ID)
These methods are used for emergency services as well as for commercial applications (maps, advertisements). We will see OTDOA method used for LBS (Location Based Services). There are two types of OTDOA viz. downlink OTDOA and uplink OTDOA (i.e. U-TDOA). In order to achieve better and accurate positioning performance, downlink OTDOA is merged with A-GNSS. The transmission from eNBs (i.e. Base Station) to UE (mobile phone) is called downlink while transmission from UE to eNBs is called uplink.
• In downlink OTDOA, UE measures the downlink signals and sent back the results to network.
Based on the feedback from UE, network determines position of UE. Downlink OTDOA is specified in 3GPP Release-9.
• In uplink OTDOA, eNBs receive the uplink transmission from UE and determines position of UE. Uplink OTDOA (U-TDOA) is specified in 3GPP Release-11.
How OTDOA Works
• OTDOA uses hyperbolic multilateral algorithm to determine time of arrival of downlink signals
from multiple eNBs (includes 1 serving eNB and 2 or more neighbour eNBs).
• TOA from neighbour eNBs are substracted from reference or serving eNB to form OTDOA's.
• Each time difference (or range) will determine a hyperbola. The point where these hyperbolas intersect is the desired position of the UE. At least 3 timing measurements are essential to estimate UE coordinates viz. (X, Y) or Latitude/Longitude.
• Figure-1 depicts this method. As shown UE measures τ1, τ2 and τ3 with its internal time base. The measurement from eNB is taken as reference and two OTDOA's are constructed which gives t2,1 = τ2 - τ1 and t3,1 = τ3 - τ1.
• TOA measurement has certain uncertainty and this results into uncertainty in the measurement of UE position. This is shown by grad area where two hyperbolas intersect.
• LTE uses special cell specific reference signal which is known by PRS ("Positioning Reference Signal"). It is QPSK modulated and occupies specific position in the consecutive LTE subframes. This will avoid collision with CSRS (Cell specific reference signal) and overlapping with PDCCH channel. This is shown in the figure-2 below.
• The measured time difference of arrival (TDOA) of PRS at UE from serving eNB and neighbour eNBs is known as RSTD (Reference Signal Time Difference). The RSTD measurements are sent back by UE to the network (i.e. eNBs).
• Using RSTD report and known fixed positions of transmitting antennas of eNBs, position of UE is determined.
The figure-2 depicts LTE PRS (Positioning Reference Signal) mapping in the LTE frame. It is for normal cyclic prefix with 4 transmit antenna ports. All the squares are resource elements having frequency domain index as k and time domain index as l. R6 denotes resource elements of PRS. It is mapped in block of 12 subcarriers over 12 or 14 OFDM symbols. It is mapped in 4 consecutive subframes.
PRS based subframes are designed to provide low interference. It has reuse factor of 6 compare to 3 used in RS and hence it provides improved hearability of transmissions from neighbour eNBs.
Applications of OTDOA (Observed Time Difference of Arrival)
Following are the applications of OTDOA:
• It is used as positioning method in all the LTE enabled mobile phones.
• It is used for E911 emergency services in the USA.
• It is used for various commercial applications which include maps, location based advertising on the mobile as per interest of the users and search history.
• It is used to protect the national borders.
• It is used to keep law enforcement in place in critical situations.
• It is used to protect critical infrastructure.
• It is used for the safety of public and for their security.
Advantages of OTDOA | Benefits of OTDOA
Following are the benefits or advantages of OTDOA (Observed Time Difference of Arrival):
• It is very popular among all the positioning techniques due to its accurate location measurement.
• In LTE, it is very easy to measure the RSTD (Reference Signal Time Difference) and hence estimate the UE position. LTE incorporates PRS ( Positioning Reference Signal) in the frame structure which helps in very accurate and effective determination of the UE position.
• It is used in the situations where A-GNSS is either not available or its service is limited.
• Often OTDOA is combined with A-GNSS to obtain better positioning performance.
• The OTDOA measurement method is not affected in the presence of obstructions in the form of building made of concrete/steel structures.
• It works effectively at various places viz. indoors, high buildings, forests etc.
GPS RELATED LINKS
LTE Tutorial related links
What is LTE?
EARFCN vs frequency
FDD vs TDD
UE cell search UE network entry Timers PSS vs SSS LTE RS DMRS SRS LTE Channels BW vs FS vs RB Zadoff chu sequence LTE Advanced eNodeB Measurements UE Measurements SC-FDMA Vs OFDMA LTE Security