Part 1 discussed the state of location services in current wireless networks lamenting the fact that many phone apps get their location information from internet services rather than the wireless network.  In Part 2 we review location features in LTE Release 9. 

The positioning methods that are specified in Release 9 include:

  • Assisted GPS (A-GPS) based positioning method which involves multiple satellites. Other satellite systems besides GPS are possible e.g. GALILEO
  • Observed Time Difference of Arrival (OTDOA)  a downlink positioning method in R9 which involves multiple cells
  • Enhanced Cell ID (E-CID) which includes both uplink and downlink cell coverage-based positioning techniques and is mainly concerned with the serving cell

The GPS system currently has 31 satellites in orbit. All are time synchronized and each broadcasts a timing signal and its absolute position. The GPS receiver in the UE needs to lock on to the signals of at least three satellites and measure relative delays in order to calculate to latitude and longitude. With four satellites in view altitude can also be determined.  It is also possible to calculate velocity by tracking these signals. Before making the measurements the UE has to find the satellites. The length of time it takes to do this from scratch, known as time to first fix (TTFF) can be 30 seconds or more. To improve the performance therefore, the LTE network assists the UE by sending it information about satellite orbit positions and message transmission times. This greatly reduces the TTFF. Either the UE or the LTE network may calculate the position.



OTDOA (Observed Time Difference of Arrival) is an LTE downlink positioning method. The concept is similar to GPS in that the UE measures signal delays from multiple transmitters. Each LTE cell transmits reference signals and it is the arrival time of these that the UE compares. The difference in the timing is reported by the UE to the network. Combining the timing differences with its knowledge of the positions of each cell’s antennas allows the network to calculate the position.  At least four cells need to be measured by the UE.  Every cell in LTE transmits cell-specific reference signals (CRS) and these may be used for delay measurements. However, the position accuracy often suffers if we use CRS due to the inherent interference in a single frequency system. To improve the accuracy new reference signals known as positioning reference signals (PRS) are introduced in Release 9. The transmission of the PRS in interfering cells can be turned off thus allowing the mobile to hear the signals of each cell more clearly. With this method the LTE network will calculate the position.

The Enhanced Cell ID (E-CID) method provides refinements to the basic Cell ID positioning of Release 8.  The first of these enhancements uses timing measurements to calculate how far away the mobile is from the eNB. This distance is half the round trip time (RTT) times the speed of light. If the UE is connected, both the serving cell and the UE will measure the timing difference between Rx subframes and Tx subframes. The UE will report its measurements to the eNB and the eNB will calculate the RTT.

            RTT = eNB(Rx – Tx) + UE(Rx – Tx)              

With knowledge of the eNB’s coordinates and antenna height the position of the UE can be calculated.



E-CID can be further improved with the addition of a feature known as Angle of Arrival (AoA). The eNB estimates the direction from which the UE is transmitting using a linear array of equally spaced antenna elements. Reference signals received from the UE at any two adjacent elements are phase rotated by an amount which depends on the angle of arrival, the carrier frequency and the element spacing.



Since no positioning method is perfect, LTE Release 9 supports the flexible use of multiple positioning methods. GPS and OTDOA can provide very good position estimates under “optimum” conditions. For GPS this often precludes urban and indoor locations where severe multipath or shadowing can result in poor performance. We have all experienced “satellite lost” announcement just when we need our navigator most. OTDOA positioning may not be too reliable when cell sizes are large and the UE cannot find four or more good cells. LTE R9 makes it possible to deploy and make use of a combination of methods. To misquote Mr. Lincoln, “You can find some of the people all of the time, you can find all of the people some of the time, but you can’t find all of the people all of the time” – if you use only one positioning method that is. The LTE network can accumulate performance data for all methods in every location and use this as the basis for method choice. For example, initially the network knows the cell ID and the statistics database can tell us that A-GPS is preferred (e.g. rural cell) or OTDOA is preferred (e.g. Downtown office building).  Then, if the first method fails to meet the accuracy requirement the network can choose the second most preferred method. So in LTE the algorithms in the network can make use of the network’s evolving knowledge of the environment to improve the performance of the positioning system. Accurate knowledge of the location of a mobile phone, anytime, anywhere can be achieved!

In Part 3 we will examine the performance of Release 9 positioning methods and review their evolution in Release 11.