Abdelmonaem Lakhzouri

Highly efficient techniques for mitigating the effects of multipath propagation in DS-CDMA delay estimation

Delay estimation in direct-sequence code-division multiple-access (DS-CDMA) systems is necessary for accurate code synchronization and for applications such as mobile phone positioning. Multipath propagation is among the main sources of error in the DS-CDMA delay estimation process, together with multiple access interference and non-line-of-sight (NLOS) propagation. This paper provides a review of main delay estimation techniques, existing in the literature so far, which are able to cope with multipath propagation, together with our novel delay estimation techniques proposed in the context of DS-CDMA systems. The performance of all these techniques is compared through analysis and simulations, considering also their relative computational complexity and required prior information. Starting from the traditional delay locked loops (DLL) and their improved variants, we discuss several recently introduced delay estimation techniques able to cope with multipath propagation. The characterization of these methods is given in a unified framework, suited for both rectangular and root raised cosine pulse shapes. The main focus in the performance comparison of the algorithms is on the closely-spaced multipath scenario, since this situation is the most challenging for achieving diversity gain with low delay spreads and for estimating LOS component with high accuracy in positioning applications.

Feedforward delay estimators in adverse multipath propagation for Galileo and modernized GPS signals

The estimation with high accuracy of the line-of-sight delay is a prerequisite for all global navigation satellite systems. The delay locked loops and their enhanced variants are the structures of choice for the commercial GNSS receivers, but their performance in severe multipath scenarios is still rather limited. The new satellite positioning system proposals specify higher code-epoch lengths compared to the traditional GPS signal and the use of a new modulation, the binary offset carrier (BOC) modulation, which triggers new challenges in the delay tracking stage. We propose and analyze here the use of feedforward delay estimation techniques in order to improve the accuracy of the delay estimation in severe multipath scenarios. First, we give an extensive review of feedforward delay estimation techniques for CDMA signals in fading channels, by taking into account the impact of BOC modulation. Second, we extend the techniques previously proposed by the authors in the context of wideband CDMA delay estimation (e.g., Teager-Kaiser and the projection onto convex sets) to the BOC-modulated signals. These techniques are presented as possible alternatives to the feedback tracking loops. A particular attention is on the scenarios with closely spaced paths. We also discuss how these feedforward techniques can be implemented via DSPs.

Extended Kalman filter channel estimation for line-of-sight detection in WCDMA mobile positioning

In mobile positioning, it is very important to estimate correctly the delay between the transmitter and the receiver. When the receiver is in line-of-sight (LOS) condition with the transmitter, the computation of the mobile position in two dimensions becomes straightforward. In this paper, the problem of LOS detection in WCDMA for mobile positioning is considered, together with joint estimation of the delays and channel coefficients. These are very challenging topics in multipath fading channels because LOS component is not always present, and when it is present, it might be severely affected by interfering paths spaced at less than one chip distance (closely spaced paths). The extended Kalman filter (EKF) is used to estimate jointly the delays and complex channel coefficients. The decision whether the LOS component is present or not is based on statistical tests to determine the distribution of the channel coefficient corresponding to the first path. The statistical test-based techniques are practical, simple, and of low computation complexity, which is suitable for WCDMA receivers. These techniques can provide an accurate decision whether LOS component is present or not.

Selection of the multiple-dwell hybrid-search strategy for the acquisition of Galileo signals in fading channels

The acquisition of CDMA signals in adverse mobile communication channels has been studied for more than five decades. However, the introduction of new standard proposals for the future European satellite system (i.e., Galileo) and for the modernized GPS has triggered new interest in fast and reliable acquisition strategies for CDMA systems with very high spreading factors (e.g., length of 10230 chips or higher). Typically, the double-dwell serial search strategies have been preferred for CDMA signal acquisition. Few papers have addressed also the problem of hybrid and parallel search strategies, but typically, the choice of the best number of dwells has not been discussed. The goal of this paper is to introduce a generic method for the computation of the mean acquisition time (MAT) for multiple-dwell hybrid-search acquisition blocks and to compare the performance of several multiple-dwell structures for CDMA systems with high spreading factors. It will be shown here that increasing the number of dwells does not always increase the performance from the point of view of the MAT. We also discuss the influence of various parameters on the selection of the multiple-dwell strategy and we present simulation results for a realistic Galileo signal.

Indoor fading distributions for GPS-based pseudolite signals

During the past few years, accurate positioning of terminals has received much attention in wireless communication. One reason is that of the requirement for emergency call positioning imposed by the authorities. The positioning algorithms based on the Navstar satellites have limitations in the indoor environment because the signal experiences severe attenuation in the indoor environment. The characteristics of the indoor propagation is still not well understood. In many cases, it is assumed that with the deployment of pseudolites, indoor navigation can show better performance. In this paper we analyze the indoor channel propagation using pseudolites, based on the measurement data collected in different scenarios.

On second order statistics of the satellite-to-indoor channel based on field measurements

Wireless positioning has received increased attention during the past few years, where several wireless applications have been envisaged and promoted such as the E-911/E-112 regulations. The positioning needs to be carried out in all the environments covered by the wireless communication services, including the most constraining areas such as dense urban areas and obstructed indoor environments. The most known positioning system is the global navigation satellite system (GNSS), which demonstrated quite reliable positioning capabilities when the receiver is in direct view with the sky. However, in indoor environments the signal characteristics are not well understood yet and positioning capabilities are quite poor. Therefore, understanding the multipath propagation indoors and fading characteristics are quite important to make GNSS works indoors. In this paper, we describe measurement-based modeling results of the level-crossing rate (LCR) and average duration of fades (ADF) with the purpose of giving further insight on the real satellite-to-indoor channel characteristics

A highly efficient generalized Teager-Kaiser-based technique for LOS estimation in WCDMA mobile positioning

Line-of-sight signal delay estimation is a crucial element for any mobile positioning system. Estimating correctly the delay of the first arriving path is a challenging topic in severe propagation environments, such as closely spaced multipaths in multiuser scenario. Previous studies showed that there are many linear and nonlinear techniques able to solve closely spaced multipaths when the system is not bandlimited. However, using root raised cosine (RRC) pulse shaping introduces additional errors in the delay estimation process compared to the case with rectangular pulse shaping due to the inherent bandwidth limitation. In this paper, we introduce a novel technique for asynchronous WCDMA multipath delay estimation based on deconvolution with a suitable pulse shape, followed by Teager-Kaiser operator. The deconvolution stage is employed to reduce the effect of the bandlimiting pulse shape.

Constrained deconvolution approach with intercell interference cancellation for LOS estimation in WCDMA system

In most positioning techniques, two or more non-serving base stations (BS) are involved in the location procedure. The position is usually derived from the estimates of the time of arrival of the line-of-sight (LOS) components from these different BSs. In the downlink transmission, the received signal strength from a remote BS is quite low, especially when the mobile is close to the serving BS, and also it is composed of a sum of multiple propagation paths, which may arrive at sub-chip delay intervals, generating closely spaced multipaths. For accurate location estimates, some measures should be taken to solve the closely spaced paths and diminish the intercell interference. In this paper, we introduce a constrained deconvolution approach aided with an intercell interference estimation scheme to solve the closely spaced paths and to enhance the estimation of the delay of the first arriving path from the desired BS.

LOS estimation in overlapped multipath WCDMA scenarios via adaptive threshold

Mobile positioning is a topic of high interest in current mobile communication systems. The position location is usually derived from the estimates of the distances between mobile and the base stations, which are obtained from the time-of-arrival of the line-of-sight (LOS) component. In time-varying multipath fading channels, the LOS component is not always present, and even when it is present, it might be obstructed by one or several components arriving within short delay (i.e., shorter than the width of the main lobe of the pulse shaping function). This situation of overlapped paths is one of the main sources of error in LOS estimation. We introduce here an adaptive threshold for estimating the LOS component in the presence of overlapped multipath components for wideband CDMA (WCDMA) positioning applications. Our threshold technique is used in conjunction with Teager-Kaiser (TK) based delay estimation. The TK estimator was previously introduced in the context of RAKE receiver and it was shown to be an efficient method for solving closely spaced multipaths.

Estimation of closely-spaced paths via particle filters for WCDMA positioning

Multipath delay estimation is an important and challenging issue for any mobile communication system. Severe transmission conditions in fast changing environment, such as strong interference, shadowing or closely spaced multipaths may affect the capability of estimating correctly the channel parameters. In mobile positioning, accurate estimates of line-of-sight delay with a resolution less than one chip are needed to locate correctly the mobile receiver. Another challenging issue in multipath delay estimation in CDMA networks is the nonlinear dependency of the channel with respect to the multipath delays. In this paper, we introduce a particle-based sequential Monte Carlo filter for joint estimation of channel coefficients and delays in multiuser closely spaced paths WCDMA environment. The simulation results show that the proposed algorithm is stable, fast converging, and outperforms the classic extended Kalman filter-based approach.