This paper proposes a hybrid scheme for user positioning in an urban scenario using both a Global Navigation Satellite System (GNSS) and a mobile cellular network. To maintain receiver complexity (and costs) at a minimum, the location scheme combines the time-difference-of-arrival (TDOA) technique measurements obtained from the cellular network with GNNS pseudorange measurements. The extended Kalman filter (EKF) algorithm is used as a data integration system over the time axis. Simulated results, which are obtained starting from real measurements, demonstrate that the use of cellular network data may provide increased location accuracy when the number of visible satellites is not adequate. In every case, the obtained accuracy is within the limits required by emergency location services, e.g., Enhanced 911 (E911).

This paper presents an analytical approach to evaluate the error probability of orthogonal frequency-division multiplexing (OFDM) systems subject to carrier frequency offset (CFO) in frequency-selective channels, characterized by Rayleigh or Rician fading. By exploiting the Gaussian approximation of the intercarrier interference (ICI), we show that the bit-error rate (BER) for quadrature amplitude modulation (QAM) can be expressed by the sum of few integrals, whose number depends on the constellation size. Each integral can be evaluated numerically, or, in Rayleigh fading, by using a series expansion that involves generalized hypergeometric functions. Simulation results illustrate that the theoretical analysis is quite accurate, especially for Rayleigh channels.

We analyze the effects of the nonlinear distortions, introduced by the transmitting high-power amplifier (HPA), on the performance of the linear decorrelating multiuser detector in direct-sequence code-division multiple-access (DS-CDMA) downlink systems. By assuming and motivating the Gaussian distribution of the nonlinear distortion noise, the symbol-error rate (SER) and the total degradation (TD) has been derived theoretically in additive white Gaussian noise (AWGN) and frequency-flat Rayleigh fading channels. Simulation results show that the analytical model is quite accurate in many scenarios.

The linear minimum mean-squared error (MMSE) detector for direct-sequence code-division multiple-access (DS-CDMA) systems relies on the inverse of the covariance matrix of the received signal. In multiuser environments, the matrix ill-conditioning may produce large performance degradation. In order to cope with this effect, we propose a modified MMSE detector based on the covariance matrix tapering (CMT). This regularization technique modifies the estimated covariance matrix, thereby reducing its eigenvalue spread. Two different tapering matrices are introduced. The performance comparison with other regularization techniques is carried out by simulations, which show the effectiveness of the proposed technique.

The aim of this paper is to examine the effects of the nonlinear distortions introduced by high-power amplifiers (HPAs) in the downlink of direct-sequence code-division multiple-access (DS-CDMA) systems. By modelling the nonlinear distortion noise as coloured Gaussian noise, a semi-analytical expression for the symbol-error rate (SER) of linear multiuser detectors (LMDs) in frequency-selective fading channels has been derived. Simulation results confirm the effectiveness of the theoretical approach.

Abstract- This paper analyses the effects of the nonlinear distortions introduced by high-power amplifiers (HPAs) on the performance of the linear decorrelating multiuser detector (MUD) in synchronous direct-sequence code-division multiple-access (DS-CDMA) systems. By assuming and motivating the Gaussian distribution of the nonlinear distortion noise, the system bit-error rate (BER) has been derived theoretically in additive white Gaussian noise (AWGN) and frequency-flat Rayleigh fading channels. Simulation results are provided in order to validate the theoretical analysis for ideally predistorted amplifiers. I.

This paper deals with the implementation of the sample rate conversion subsystems of the digital front-end, at intermediate frequency (IF), and of a synchronization subsystem in the framework of a software radio receiving terminal. A perviously proposed architecture is adopted and optimized for the realization of a multimodal terminal, the main operational mode of which is UMTS, but also capable to interface with GSM or IS-95. In particular, a class of multiplierless filters in conjunction with reduced complexity FIR filters is chosen for the rate conversion. The selected approach for synchronization is a modified MAX/TC algorithm that exploits dedicated sync channels, for the UMTS case. Its performance in terms of acquisition probability has been studied either in a classic AWGN channel or in a multipath environment. The algorithms have been optimized considering an FPGA based implementation, where power consumption and occupied area strongly constrain designer choices.

The minimum mean square error (MMSE) multiuser detector has received great attention as the optimum linear solution for reducing the multiple access interference (MAI) in DS-CDMA systems. In asynchronous frequency selective channels the covariance matrix estimation errors may introduce BER performance degradation. The aim of this paper is to outline such degradation and to improve the MMSE receiver performance by convenient covariance matrix estimations. Two different approaches are introduced. Simulation results are shown in order to validate the effectiveness of the proposed techniques.

Orthogonal frequency-division multiplexing (OFDM) baseband signals may be modeled by complex Gaussian processes with Rayleigh envelope distribution and uniform phase distribution, if the number of carriers is sufficiently large. The output correlation function of instantaneous nonlinear amplifiers and the signal-to-distortion ratio can be derived and expressed in an easy way. As a consequence, the output spectrum and the bit-error rate (BER) performance of OFDM systems in nonlinear additive white Gaussian noise channels are predictable both for uncompensated amplitude modulation/amplitude modulation (AM/AM) and amplitude modulation/pulse modulation (AM/PM) distortions and for ideal predistortion. The aim of this work is to obtain the analytical expressions for the output correlation function of a nonlinear device and for the BER performance. The results in closed-form solutions are derived for AM/AM and AM/PM curves approximated by Bessel series expansion and for the ideal predistortion case.

This paper analyses the performance of orthogonal multicarrier DS-CDMA systems for personal communications. The basic system operates using the orthogonal multicarrier technique combined with direct sequence code division multiple access. The non-stationary behaviour of the channel is tracked by a periodic sounding. This technique leads to relatively simple receivers that can be implemented with current VLSI and DSP technology. Taking into account the challenges related to an up-link implementation, the system performance in the down-link is investigated both for synchronous and asynchronous DS-CDMAs. In particular, the schemes which work with Walsh sequences (in the synchronous case) are compared with the schemes operating by binary Gold sequences and 4-phase near-optimal sequences (in the asynchronous case). The comparison is carried out on the basis of significant parameters such as bit error rate, spectral efficiency and multipath counteraction for different system configurations.

This paper presents an analysis of the performances of a digital mobile radio link based on DS-CDMA technique and the RAKE receiver. The adopted signature sequences are complex and characterized by different correlation properties, that lead to different performances in terms of bit error rate and spectral efficiency. Moreover, a preliminary analysis of the performances under non-linear channel behaviour and different modulation schemes is provided.>