Jose A. del Peral-Rosado

Achievable localization accuracy of the positioning reference signal of 3GPP LTE

Robustness of nominal Global Navigation Satellite Systems (GNSS) performance can be enhanced by means of complimentary systems, such as the Long Term Evolution (LTE). Particularly, the LTE standard specifies a dedicated downlink signal for positioning purposes, i.e. the positioning reference signal (PRS). This paper presents the achievable localization accuracy of the PRS signal for different interference LTE scenarios by means of the Crámer-Rao bound (CRB) for time delay estimation, in order to assess the LTE positioning capabilities.

Survey on Robust Carrier Tracking Techniques

In most wired and wireless systems, carrier tracking is an essential task that allows the receiver to precisely synchronize with the carrier of the incoming signal. Stringent carrier tracking requirements are imposed in systems that are sensitive to carrier mismatches, such as orthogonal frequency division multiplexing (OFDM), digital communication receivers employing high-order constellations, and terrestrial- or satellite-based positioning systems, just to mention a few. In the recent years, even more critical requirements are being imposed due to the emergence of new applications and services that are pushing traditional systems to operate in much more challenging conditions than the ones for which they were originally designed. The presence of severe fading, signal outages, abrupt phase changes and high user dynamics, are currently compromising the validity of well-known and long-established traditional carrier tracking techniques, thus calling for the development of new robust carrier tracking algorithms. In this paper, we provide a detailed survey on the five main strategies that can be adopted to cope with the technical challenges of robust carrier tracking. These strategies range from some basic optimizations of current tracking loops, to the use of Kalman filter-based architectures, or the application of innovative carrier tracking techniques based on particle filters or compressive sensing. We will also review some open-loop techniques, which are widely adopted in burst-mode communications receivers, as an alternative and potential candidate solution for robust carrier tracking in harsh conditions.

Comparative results analysis on positioning with real LTE signals and low-cost hardware platforms

Long Term Evolution (LTE) networks are rapidly deploying around the world, covering the needs of high data rates demanded by many applications. Still, less attention is paid on the positioning capabilities specified in the LTE standard. Thus, an experimental LTE positioning receiver is presented to assess the positioning accuracy in commercial LTE deployments. This receiver is based on a software defined radio (SDR) and a low-cost radio-frequency (RF) front-end, such as the universal software radio peripheral (USRP) or a DVB-T dongle with the Realtek RTL2832U chipset. These two platforms are then used to capture and post-process real LTE signals generated in the laboratory. The positioning results obtained show the viability on the use of this experimental SDR LTE positioning receiver with low-cost hardware platforms for commercial LTE networks.

Joint maximum likelihood time-delay estimation for LTE positioning in multipath channels

This paper presents a joint time-delay and channel estimator to assess the achievable positioning performance of the Long Term Evolution (LTE) system in multipath channels. LTE is a promising technology for localization in urban and indoor scenarios, but its performance is degraded due to the effect of multipath. In those challenging environments, LTE pilot signals are of special interest because they can be used to estimate the multipath channel and counteract its effect. For this purpose, a channel estimation model based on equi-spaced taps is combined with the time-delay estimation, leading to a low-complexity estimator. This model is enhanced with a novel channel parameterization able to characterize close-in multipath, by introducing an arbitrary tap with variable position between the first two equi-spaced taps. This new hybrid approach is adopted in the joint maximum likelihood (JML) time-delay estimator to improve the ranging performance in the presence of short-delay multipath. The JML estimator is then compared with the conventional correlation-based estimator in usual LTE conditions. These conditions are characterized by the extended typical urban (ETU) multipath channel model, additive white Gaussian noise (AWGN) and LTE signal bandwidths equal to 1.4, 5 and 10 MHz. The resulting time-delay estimation performance is assessed by computing the cumulative density function (CDF) of the errors in the absence of noise and the root-mean-square error (RMSE) and bias for signal-to-noise ratio (SNR) values between −20 and 30 dB.

Joint channel and time delay estimation for LTE positioning reference signals

The ranging performance of the Long Term Evolution (LTE) positioning reference signal (PRS) is enhanced with respect to traditional correlation-based approaches in multipath channels. For that purpose, a joint maximum likelihood (ML) channel and time delay estimation is introduced for the PRS signal. The estimation can be implemented by using the least-squares (LS) criterion that benefits from the multicarrier flexible waveform. Preliminary results are shown with a comparison of the root-mean-square error (RMSE) of this ML estimator and the corresponding Cramér-Rao Bound (CRB) expression for a specific urban pedestrian channel model.

Evaluation of the LTE positioning capabilities under typical multipath channels

The Long Term Evolution (LTE) is a mobile communication standard that is receiving significant attention, and especially offers positioning capabilities by specifying a dedicated downlink signal, i.e. the positioning reference signal (PRS). Thus, this technology can improve the location of mobile terminals operating in harsh environments, such as urban or indoor scenarios. This paper presents a study of the impact of the channel on the positioning capabilities of LTE with respect to the signal bandwidth. For that purpose, typical channel models, such as those recommended by the International Telecommunication Union (ITU), are used to obtain timing error distributions by means of the histogram of maximum likelihood estimates. The results obtained represent the worst-case scenario since the applied estimation process does not consider the presence of the multipath channel. The dependency of the timing error distributions with respect to the type of channel model is also analysed.

Kalman filter-based architecture for robust and high-sensitivity tracking in GNSS receivers

Blockage of the line-of-sight (LOS) component is one of the main problems of GNSS receivers when operating in harsh working conditions. This is the case, for instance, of urban canyons, where GNSS receivers in moving vehicles are subject to sudden fading events with more than 20 dB of signal attenuation. In these circumstances it is not possible for traditional receivers to keep track of the received signals, thus failing to provide reliable position fixes. The problem is aggravated when hardware implementation constraints need to be satisfied. For instance, as in target applications that involve embedding GNSS capabilities into ARM processors of mobile devices. Therefore, high-sensitivity GNSS receivers with reduced complexity and power consumption are mandatory. The contribution of this paper is the proposal and analysis with synthetic signal of a Kalman filter-based architecture for robust and high-sensitivity tracking. The use of extended correlations mitigates the measurement noise and allows the tracking operation under these harsh conditions. The results obtained validate the strategy proposed to operate at a C/N0 (carrier-to-noise ratio) as low as 15 dBHz.

Downlink synchronization of LTE base stations for opportunistic ToA positioning

Long Term Evolution (LTE) signals are a very good complement to Global Navigation Satellite Systems (GNSS) in urban environments, due to their attractive positioning capabilities. However, the network-based positioning methods defined in the LTE standard hinder the use of these signals for ranging in an opportunistic way. This is mainly due to the lack of synchronization between LTE base stations (BSs) in most of current network deployments. To circumvent this limitation, this paper proposes a method to synchronize LTE BSs using time-delay and frequency tracking loops and a-priori known receiver position. The main considerations on the use of this method for opportunistic time-of-arrival (ToA) positioning are discussed. Using real LTE signals emulated in the laboratory, positioning results are obtained with a software receiver under a dynamic trajectory, validating the use of the proposed synchronization for opportunistic ToA positioning.

Floor Detection with Indoor Vertical Positioning in LTE Femtocell Networks

Vertical positioning is nowadays a topic of high interest in indoor environments, due to the stringent requirements on indoor location accuracy for emergency calls, such as E911. Thus, indoor positioning techniques are investigated to achieve floor detection in order to fulfill these legal mandates. The use of Long Term Evolution (LTE) heterogeneous networks is an attractive solution due to the combination of communications and positioning capabilities. This paper provides an overview of the existing floor detection techniques. Practical results using Global Navigation Satellite Systems (GNSS), inertial sensors, barometer, and LTE signals are obtained in an experimental LTE femtocell network, within a deployment in a two-story building. Probabilities of floor detection above 67% of the cases are found for the positioning solutions assessed.

Software-Defined Radio LTE Positioning Receiver Towards Future Hybrid Localization Systems

The appropriate positioning characteristics and fast deployment of Long Term Evolution (LTE) systems makes this technology a promising candidate towards future satellite and terrestrial hybrid scenario. Nevertheless, further studies on the achievable LTE positioning capabilities are still necessary for commercial or realistic network deployments. This paper describes a tool that provides standalone localization in LTE networks. The tool is a software-defined radio (SDR) receiver working at the LTE physical layer, performing acquisition, tracking and positioning. The SDR LTE positioning receiver is validated in a static scenario, showing position errors below three meters in the absence of multipath for a signal bandwidth of 1.08 MHz.