Saverio Cacopardi

Theoretical analysis and performance of OFDM signals in nonlinear AWGN channels

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.

Effects of HPA nonlinearity on frequency multiplexed OFDM signals

The paper analyzes the performance of the downlink channel of a multimedia interactive service system which transmits the desired information by the frequency multiplexing of several OFDM signals compliant with the DVB-T standard. The effects of the nonlinear distortions introduced by a high power amplifier on the system performance are evaluated both in terms of the bit error rate (BER) degradation in AWGN channels and of the spectral regrowth. The performance comparison to the case of a single DVB-T signal as well as the benefits of an ideal predistortion is also considered by comparing analytical results to computer simulations.

Digital linearizer for RF amplifiers

Broadcast technology is at the beginning of a new era. It is characterized by the intensive use of the most advanced digital modulation formats (8VSB, QAM, OFDM) in combination with high power RF amplifiers. To date the linearity required for these digital formats has only been accomplished in cumbersome low efficiency class A amplifier or even more cumbersome feed-forward systems. A potentially more efficient and cost effective approach is the combination of nonlinear power amplifiers and a predistortion technique capable of compensating for the nonlinear amplifiers. Digital predistortion will provide a highly linear output and improved efficiency. Itelco has developed a digital adaptive base band predistorter to provide for improved performance and cost. The technique is independent of the modulation type, the output frequency, or the signal bandwidth. Furthermore the capability of automatic adaptive predistortion to compensate for the environment (temperature, power supply variations, aging, and even operation during replacement of a faulty module) is highly desirable.

DSP based OFDM demodulator and equalizer for professional DVB-T receivers

This work treats the design of base-band processing subsystems for professional DVB-T receivers using 1600 MIPS, fixed point DSPs. We show the results about the implementation of OFDM demodulation, channel estimation and equalization functional blocks, for the 2k/8k modes. The adoption of general purpose DSPs provides a great flexibility in the development of different configurations of the receiver. Furthermore it enables the feasibility of adaptive equalization schemes, where the quality of the channel estimation varies according to both channel characteristics and speed of the channel variations. The 16/32 bit fixed point architecture leads to a very low implementation loss, and a careful optimization of the pipeline architecture of the processor allows the receiver to obtain short processing delays. The flexibility of the software approach along with the obtained performance make the proposed implementation very interesting in professional and high-end receivers for interactive, multimedia applications of DVB-T.

GNSS/Cellular Hybrid Positioning System for Mobile Users in Urban Scenarios

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).

JPEG2000 and MJPEG2000 transmission in 802.11 wireless local area networks

In this paper the transmission of JPEG2000 and motion JPEG2000 codestream in 802.11 WLAN environment is analyzed. The issues related to the transmission of JPEG2000, using non-reliable protocols as UDP-RTP, are investigated and a proper protection scheme for dealing with such an environment is presented, along with the obtained performance with different systems parameters. The proposed technique addresses a wide range of multimedia applications such as wireless PC to digital projector multimedia presentations, point to multipoint digital slide projection, wireless video-surveillance systems.

A full-rank regularization technique for MMSE detection in multiuser CDMA systems

In multiuser code-division multiple-access (CDMA) environments, the ill-conditioning of the covariance matrix of the received signal may degrade the performance of minimum mean-squared error (MMSE) detectors, especially when few samples are available for the covariance matrix estimation. In order to mitigate this performance degradation, we propose a full-rank regularized MMSE detector based on the covariance matrix tapering (CMT) technique. Simulation results show the effectiveness of the proposed technique at high signal-to-noise ratio (SNR).

Probability of error of OFDM systems with carrier frequency offset in frequency-selective fading channels

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.

Performance comparison of multicarrier DS-SS radio access schemes for WLAN using measured channel delay profiles

In this paper we propose two different MCM (multicarrier modulation)-DS-SS radio access schemes for WLANs and we evaluate their performance in terms of bit error rate and frame error rate by using measured channel delay profiles, in different indoor environments. By comparing the obtained performance with the results of the DS-SS physical layer of the IEEE 802.11 standard, we point out some important architectural aspects and the potential limiting factors of the performance.

Combined OFDM-CDMA configuration for multimedia wireless applications

This paper analyses the applicability of OFDM modulation when combined with CDMA for multimedia wireless systems operating in an indoor environment. We demonstrate the motivations and benefits deriving from this combination. Particular emphasis is given to channel equalisation techniques, designed to face different problems arising in the up-link and in the down-link. In particular, an equalisation technique based on channel inversion, combined with carrier interleaving and adaptive suppression of noisy carriers, is proposed in the down-link, while a pre-equalisation approach is used in the up-link. The analysis provides both BER performance and computational cost of the most important procedures that are to be implemented within multimedia terminals. The computational cost analysis is carried out using commercial DSP platforms.