ION GNSS 2012
Session A2: Algorithms & Methods 2: Navigation & Supplementing GNSS

Title: Analysis of Positioning Capabilities of 3GPP LTE
Author(s): J.A. Del Peral-Rosado, J.A. Lopez-Salcedo, G. Seco-Granados, SPCOMNAV, Universitat Autonoma de Barcelona, Spain; F. Zanier and M. Crisci, European Space Agency/ESTEC, The Netherlands
Date/Time: Wednesday, September 19, 2012, 4:23 p.m.
Room: 204 (NCC)

New positioning capabilities demanded by emerging applications are leading the renovation of Global Navigation Satellite Systems (GNSS). For instance, the introduction of mass-market GNSS receivers in mobile phones and portable devices compromises their robustness when operating in challenging environments, such as deep urban or indoor scenarios. Certainly, the presence of blocking obstacles and propagation disturbances prevents GNSS receivers from observing the expected perfect clear-sky conditions that were assumed in the nominal design of the system. But, also the issue of legal mandates for the location identification of emergency calls (i.e. the E911 in the US, the E112 in Europe and 110 in China) has motivated the compliance of positioning accuracy requirements. Thus, huge efforts are placed on the enhancement of GNSS receivers for any possible working condition. As a standalone receiver, researchers propose advanced signal processing techniques, exploit multiple GNSS constellations or apply new antenna designs. Nevertheless, constraints imposed by the scenario, such as interference or multipath impairments, or by the receiver, such as complexity, limit the application of these countermeasures. Then, complementary systems are usually proposed to assist the operation of GNSS systems. Traditionally, assistance data provided by an external source, such as a cellular network, or inertial navigation systems (INS) has been used to achieve the required performance in harsh environments. Moreover, other alternatives, such as the hybridization with signals of opportunity (SoO) or backup systems, have received recent attention.

A notable example of signal of opportunity or backup system is the Long Term Evolution (LTE) specified by the 3rd Generation Partnership Project (3GPP) consortium. The LTE system is presented as the next generation of mobile communications after GSM (Group Special Mobile) and UMTS (Universal Mobile Telecommunications System) predecessors. Thus, LTE has been received with special attention by network providers, and it is being deployed around the world with a high coverage. The signaling structure adopted for the LTE downlink transmission is a multi-carrier signal, i.e. modulated with Orthogonal Frequency Division Multiplexing (OFDM). Multi-carrier signals are the preferred option for any kind of wireless communication system: wide area network (LTE, WiMAX), local area network (IEEE 802.11g), broadcast (DVB-T/H, DAB) and personal area networks. This signal waveform offers spectral efficiency, robustness against multipath and frequency-selective fading, higher data rates, and flexible resources allocation, with respect to single-carrier. Nevertheless, the use of these technologies and, in general, multi-carrier signals has been basically focused on data transmission and no much attention has been paid on their positioning capabilities. This trend, however, is changing, and the LTE is the first wireless system that explicitly incorporates multi-carrier signals with positioning capabilities, by standardizing a dedicated pilot signal for navigation purposes, i.e. the positioning reference signal (PRS). Therefore, our interest is focused on the study of the positioning capabilities of this LTE multicarrier signal.

The LTE standard specifies synchronization signals (SSs) and reference signals (RSs). The synchronization signals are used for cell search and acquisition of the signal, and the reference signals are mainly used for channel estimation. These signals are based on an OFDM type modulation with different time-frequency distribution and can be used as a signal of opportunity for ranging purposes. Nevertheless, LTE follows the typical frequency reuse factor of a cellular network, which is of one. Thus, the received serving cell signal interferes with the received neighbour cell signals producing inter-cell interference, and resulting in the near-far effect. In order to obtain ranging measurements of the neighbour cells, the LTE standard in Release 9 specifies the mentioned positioning reference signal (PRS), which is especially dedicated for positioning purposes and mitigates the near-far effect, due to a higher frequency reuse factor. The PRS signal is allocated in positioning subframes that have a defined periodicity and are shared with another reference signal (i.e. cell-specific RS). The sophistication of this signal is even higher when the network mutes the PRS transmissions of certain base stations (i.e. PRS muting), in order to further reduce the inter-cell interference. Since the PRS signal can be configured with different parameters, the network provides assistance data to the user. Then, the user obtains timing measurements and sends them to the location server of the network, which computes and delivers the position to the user.

Our interest is focused on the assessment of the final performance of LTE positioning capabilities in order to increase GNSS localization robustness in challenging environments. For that purpose, the root mean square error (RMSE) of the timing in additive white Gaussian noise (AWGN) is firstly considered. The maximum achievable performance in the AWGN channel is evaluated with the Cram‚r-Rao Bound (CRB) for time delay estimation. Then, the LTE cellular network scenario is simulated by considering the cell layout and the system budget (i.e. power level, propagation losses and inter-cell interference). In this interference channel, the final position of the user is calculated. Finally, a characteristic multipath model is introduced to assess the LTE performance in realistic and challenging conditions.

Concluding, this paper will present the characteristics of LTE multicarrier signal in terms of positioning capabilities, analysing preliminary performance in AWGN, interference and multipath channel.

ACKNOWLEDGMENTS The content of the present article reflects solely the authors´ view and by no means represents the official ESA view.

Previous Abstract Return to Session A2 Next Abstract