Massimo Crisci

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.

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.

Code smoothing for BOC ambiguity mitigation

The most recent generation of Global Navigation Satellite Systems (GNSS) are implementing Binary Offset Carrier (BOC) modulation. These signals are expected to provide not only better precision in the estimation of the signals delay and phase but also more robustness to multipath effects. The advantage of BOC signals is that the main lobe of the correlation is very narrow, but on the other hand they present side lobes. For high-order signals, the amplitude of the side lobes can be similar to the amplitude of the main one or even exceed it under specific scenarios. Some techniques to mitigate the code ambiguity exploit the fact that BOC signals can be understood as the sum of two BPSK signals. Even though these techniques achieve their objective, they lose the robustness against multipath and increase the tracking noise. This paper presents a new combination between the time delay estimated by these kind of techniques and the time delay estimated using the full BOC. The idea of the combination is the same as the carrier smoothing but instead of using the carrier measurement, two code measurements are combined. Since the delay introduced by the ionosphere is the same, or very close, using the Full-BOC and the two-BPSK techniques, as it will be shown in this paper, the smoothing time can be large values, compared with the common carrier smoothing time. Several simulations of the new code smoothing strategy for different scenarios are presented in this paper.

Detection and mitigation of non-authentic GNSS signals: Preliminary sensitivity analysis of receiver tracking loops

A myriad of applications are based on the positioning and timing information provided by GNSS systems. Protecting the system against any kind of interference has been always on the focus of researchers and manufacturers. Unintentional and intentional interferences are two well known ways to mislead the results of GNSS receivers. However, it has been recently, with the mass-market production of Software Defined Radio (SDR) equipment and cheap hardware signal generators, when retransmission of delayed signal replicas or the generation of a modified version becomes even more feasible. For this reason, this paper attempts to perform an initial sensitivity assessment of the receiver tracking stage against this singular kind of disturbance. This analysis is critical for the development of appropriate techniques to detect and mitigate the effects of non-authentic signals reception.

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.

Assessment of the Feasibility of GNSS in C-Band

There is an interest in exploring the possible use for GNSS applications of the frequency band 5000 – 5030 MHz. Considering the increasing number of GNSS systems sharing the Lband spectrum and the power flux density limitations affecting the GNSS reserved frequency bands, there is an interest to look at new frequency bands. C-band GNSS frequency band is attractive because of the expected less polluted spectrum, the reduced ionospheric errors and the possibility to transmit with an higher PFD from the satellite as well the possible utilisation of higher gain receiver antennas. The main drawback is represented by the higher path losses and the smaller bandwidth available for GNSS services. The paper reports on analyses made on the type of coverage (global vs. regional) and on the possible transmission schemes (continuous vs. burst transmission coupled with beam-hopping/steering). Innovative directions are explored and compared to current LBand capabilities to allow achieving the required performances within the reasonable onboard payload and user terminal constraints. To this purpose, we investigate potential advantages deriving from future GNSS satellites carrying combined L & C-Band payloads. Signal design trade-offs consider Cramer Rao lower bound, multipath and out-of-band emission performance. Detailed link budgets for the different configurations are used as a design/analysis tool, encompassing, among others, receiver architecture performance.

Cloud GNSS receivers: New advanced applications made possible

The widespread deployment of GNSS (Global Navigation Satellite Systems) is pushing the current receiver technology to its limits due to the stringent demands for providing seamless, ubiquitous and secure/reliable positioning. This fact is further aggravated by the advent of new applications where the miniaturized size, low power consumption and limited computational capabilities of user terminals pose serious concerns to the implementation of even the most basic GNSS signal processing tasks (e.g. as in Smart-City or IoT applications). This work presents a paradigm shift for the implementation of next-generation GNSS receivers by taking advantage of Cloud computing platforms, thus leading to the concept of Cloud GNSS receiver.