Positioning Using LTE Signals

GNSS is excellent in open-sky conditions, but in many everyday situations such as traveling in urban canyons or being inside buildings, too few GNSS space vehicles (SV) are visible to get a position fix. An alternative is then desirable, and can be provided by positioning using the signals from cellular base stations. Of particular interest are the new signals and positioning possibilities from LTE cellular network operators, since the LTE coverage is expected to be high in cities and other well-populated areas. Furthermore, to accommodate the need for increasing data rates network operators are configuring their LTE downlink bandwidth to be as wide as possible, providing good resolution of different multipath components, which also assists positioning. A portable experimental setup was built to perform measurements and to gather knowledge about the overall performance of positioning with LTE signals. It consists of a universal software radio peripheral (USRP) N210 that is synchronized to a GPS-locked Rubidium frequency standard. A personal computer (PC) acts as an overall system controller and as a recording unit, storing LTE data samples together with GNSS sentences from a u-blox LEA-6T module. A Matlab-based algorithm does the complete post-processing, extracting pseudoranges for the LTE BS, and calculating the position solution. The results of determining the position of a car driven on a route around the town of Rapperswil, Switzerland show the potential of the positioning approach, using only available LTE signals. Even with the basic system the root mean square (RMS) value of the absolute error in a position using LTE compared to the actual position using GPS is 43 m, demonstrating that the CRS signal of the LTE standard is well suited as a fall-back alternative to GNSS in environmentally challenging situations. LTE SIGNALS SUITABLE FOR RANGING The LTE standard defines two signals that are considered suitable for range measurements, namely the Positioning Reference Signal (PRS) and the Cell-Specific Reference Signal (CRS)[1]. As the name suggests, the PRS was specifically designed for positioning purposes, while the CRS is actually used to determine the phase reference for coherent demodulation of the downlink data. The two signals are generated in the same way and therefore exhibit identical autoand cross-correlation properties. The PRS is transmitted in up to 6 consecutive subframes, which repeat every 160 ms – 1280 ms, with the number and interval depending on the higher-layer configuration. To increase the quality of the range measurements, the non-PRS subcarriers do not bear any data in OFDM symbols containing the PRS. As a result the available transmit power is Table 1: List of possible downlink bandwidth configurations Bandwidth 1.4 MHz 3 MHz 5 MHz 10 MHz 15 MHz 20 MHz Resource Blocks (RB) 6 15 25 50 75 100 Subcarriers 72 180 300 600 90