Francesca Zanier

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.

Fundamental issues in time-delay estimation of multicarrier signals with applications to next-generation GNSS

Multicarrier (MC) signals, now well-known and widely adopted in the field of terrestrial wireless communications, are being adopted also in the field of satellite communications with the DVB-SH standard. The main motivation of MC signaling is its intrinsic ronbustness to multipath (frequency-selective) channels. On the other hand, multipath is known to be one of the dominant sources to the error budget in global navigation satellite systems (GNSSs), mainly due to the bias it arises in time of arrival (TOA) estimation. From this viewpoint, it can be argued that MC signals could be adopted in a long-term horizon as ranging signals in GNSSs. In this paper we focus onto the issue of TOA estimation for Filter Bank Multicarrier Modulation (FBMCM) signal. FBMC is preferred as opposed to the well-known OFDM for its stricter spectral limitation and its frequency orthogonality of the subchannels. TOA estimation for FBMCM signals is investigated in terms of Cramer-Rao bound (CRB), with particular emphasis on the comparison with the case of single carrier modulation. As a corollary, it is also shown that MC signals with uneven power distribution can be adopted as an easy implementation of a cognitive positioning system (CPS).

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.

Spread-Spectrum Continuous-Phase-Modulated Signals for Satellite Navigation

Spread-spectrum continuous-phase-modulated (SS-CPM) signals are investigated with application to satellite navigation. In particular, focus is devoted to the special subclasses of SS-CPM with semi-integer modulation index h greater than one. This constitutes a novelty with respect to traditional navigation signal design on one hand (which is based on direct-sequence spread-spectrum (DS-SS) signals with rectangular pulse) and with traditional communication SS-CPM with h>;1 on the other. Main output is that setting the modulation index h<;1 to semi-integer values allows the SS-CPM signals to behave like having subcarriers, exactly in the same way as it happens for the binary offset carrier (BOC) modulation. The main advantage is that CPM ensures a constant envelope at the transmission side, thus being appealing when these features are required at the system level. After a description of signal properties, focus is devoted to ranging code synchronization, by exploitation of the theory on time-delay estimation (TDE). A few delay-lock loop schemes are also presented, based on an offset quadrature phase-shift keying (OQPSK) approximation of the signal at the receiver side, achieved by Laurents decomposition at the receiver side. Proposed loops performance are analyzed in terms of Cramer-Rao bound (CRB), rms tracking error, and multipath robustness. An application of SS-CPM for a two-rate-service scheme is proposed.

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.

Non-binary spread spectrum signals with good delay-tracking features for satellite positioning

This paper derives an algorithm to design new signal-in-space formats to be possibly used in next-generation constellations of satellites. Considering recent advances in spread spectrum signal generation and detection, it is possible to investigate new strictly band-limited non-binary waveforms which aim at minimizing the error on time delay estimation (TDE), and thus at maximizing positioning accuracy. This paper elaborates on a criterion proposed by the authors to improve TDE accuracy for the additive white Gaussian noise channel, focusing on multipath aspects. Preliminary theoretical analysis on a two-ray multipath channel is provided to support the proposed algorithm.

Criteria to Improve Time-Delay Estimation of Spread Spectrum Signals in Satellite Positioning

Positioning accuracy in satellite navigation systems depends on time-delay estimation (TDE) between satellite transmitted codes and local receiver replicas. This paper explores the fundamental limits of TDE accuracy of spread spectrum signals making use of estimation theory. In particular, this contribution derives some criteria to improve positioning accuracy in the additive white Gaussian noise (multipath-free) scenario, focusing on the (satellite) transmitter side of a direct sequence spread spectrum (DS-SS) system. Three different solutions based on the minimization of the variance of the TDE are presented. The first method derives a design criterion for the shaping pulse format. The second approach outlines a method to design binary pseudorandom spreading sequences. The third solution considers a joint shaping pulse-spreading code optimization to derive band-limited, non-binary spreading waveforms. Performance of the proposed solutions is compared with that of existing DS-SS signals for current satellite positioning systems. Possible countermeasures to the effects of multipath propagation are also discussed.

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.