OTDOA: Observed Time Difference of Arrival Explained

This page covers the fundamentals of OTDOA (Observed Time Difference of Arrival). It explains how OTDOA works within the LTE system, its applications, and its advantages. Links to comparisons between OTDOA with GPS and U-TDOA are also provided.

There are three main techniques used in LTE to determine the position of a UE (User Equipment):

• A-GNSS (Assisted Global Navigation Satellite Systems)
• OTDOA (Observed Time Difference of Arrival)
• ECID (Enhanced Cell ID)

These methods are used not only for emergency services but also for commercial applications like maps and targeted advertising. Let’s explore how OTDOA is used for Location Based Services (LBS).

There are two types of OTDOA: downlink OTDOA and uplink OTDOA (also known as U-TDOA). To achieve better and more accurate positioning performance, downlink OTDOA is often merged with A-GNSS.

Downlink refers to the transmission from eNBs (Base Stations) to UEs (mobile phones), while uplink is the transmission from UEs to eNBs.

• Downlink OTDOA: The UE measures downlink signals and sends the results back to the network. Based on this feedback, the network determines the UE’s position. Downlink OTDOA is specified in 3GPP Release-9.
• Uplink OTDOA: eNBs receive the uplink transmission from the UE and determine the UE’s position. Uplink OTDOA (U-TDOA) is specified in 3GPP Release-11.

How OTDOA Works

• OTDOA utilizes a hyperbolic multilateration algorithm to determine the time of arrival of downlink signals from multiple eNBs (including one serving eNB and two or more neighboring eNBs).
• The Time of Arrivals (TOAs) from neighboring eNBs are subtracted from the reference (serving) eNB to form OTDOAs.
• Each time difference (or range difference) defines a hyperbola. The point where these hyperbolas intersect is the estimated position of the UE.
• At least three timing measurements are necessary to estimate the UE’s coordinates (X, Y) or Latitude/Longitude.
• As shown in Figure-1, the UE measures τ1, τ2, and τ3 with its internal time base. The measurement from the eNB is taken as a reference, and two OTDOAs are constructed, giving t2,1 = τ2 - τ1 and t3,1 = τ3 - τ1.
• TOA measurements have inherent uncertainty, which leads to uncertainty in the measurement of the UE’s position. This is represented by the shaded area where two hyperbolas intersect.
• LTE uses a special cell-specific reference signal known as PRS (“Positioning Reference Signal”). It’s QPSK modulated and occupies a specific position in consecutive LTE subframes. This avoids collisions with CSRS (Cell-Specific Reference Signal) and overlapping with the PDCCH channel. This is illustrated in Figure-2 below.
• The measured Time Difference of Arrival (TDOA) of the PRS at the UE from the serving eNB and neighboring eNBs is known as RSTD (Reference Signal Time Difference). The RSTD measurements are sent back by the UE to the network (eNBs).
• Using the RSTD report and the known fixed positions of the transmitting antennas of the eNBs, the UE’s position is determined.

Figure-2 depicts the LTE PRS (Positioning Reference Signal) mapping in the LTE frame for a normal cyclic prefix with 4 transmit antenna ports. Each square represents a resource element, with ‘k’ indicating the frequency domain index and ‘l’ the time domain index. R6 denotes resource elements occupied by the PRS. It’s mapped in blocks of 12 subcarriers over 12 or 14 OFDM symbols and distributed across four consecutive subframes. PRS-based subframes are designed for low interference, featuring a reuse factor of 6 compared to the 3 used in RS, thus improving the “hearability” of transmissions from neighboring eNBs.

Applications of OTDOA (Observed Time Difference of Arrival)

Here are some common applications of OTDOA:

• Positioning method in all LTE-enabled mobile phones.
• E911 emergency services in the USA.
• Various commercial applications, including maps and location-based advertising tailored to user interests and search history.
• Protection of national borders.
• Law enforcement in critical situations.
• Protection of critical infrastructure.
• Public safety and security.

Advantages of OTDOA | Benefits of OTDOA

The following are the advantages or benefits of OTDOA (Observed Time Difference of Arrival):

• It’s a popular positioning technique due to its accurate location measurement capabilities.
• In LTE, measuring the RSTD (Reference Signal Time Difference) is straightforward, allowing for accurate estimation of the UE’s position. LTE incorporates the PRS (Positioning Reference Signal) in its frame structure, which helps in very accurate and effective determination of the UE position.
• It’s valuable in situations where A-GNSS is either unavailable or has limited service.
• OTDOA is often combined with A-GNSS to enhance positioning performance.
• The OTDOA measurement method is not affected by obstructions like buildings made of concrete or steel structures.
• It functions effectively in diverse environments, including indoors, high-rise buildings, and forests.