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The aim of this paper is to examine the effects of the nonlinear distortions introduced by the high-power amplifier in the downlink of direct-sequence code-division multiple-access systems. By modeling the nonlinear distortion effects as a signal power loss and a superimposed colored Gaussian noise, we derive the symbol-error rate (SER) of the minimum mean-squared error (mmse) detector in additive white Gaussian noise channels. Successively, we make use of a semianalytical approach to obtain the SER performance of linear multiuser detectors, such as the RAKE, decorrelator, and mmse receiver, in frequency-selective fading channels. Simulation results confirm the effectiveness of the theoretical approach. Index Terms--Direct-sequence code-division multiple-access (DS-CDMA), downlink, multiuser detectors, nonlinear distortions.

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 provide 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.

Broadcast technology is at the beginning of a new era. It is characterized by the intensive use of the most advanced digital modulations 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 non linear power amplifiers and a predistortion technique capable of compensating for the non linear 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.

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. Index Terms--CABSINET, communication system nonlinearities, DVB-T, non linear distortions, OFDM, predistortion.

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. Index Terms--Communication system nonlinearities, complex gaussian processes, nonlinear distortions, OFDM, predistortion.

In this paper we propose to take advantage of the energy link margin that can exist of satellite connections to enrich the DVB-S services with WEB-like interactive services. The exploitation of such margin is obtained by using multiresolution modulation techniques. The system architecture analysed is asymmetrical, composed of a satellite forward link and a narrowband terrestrial reverse link. The ATM is adopted to support different QoS for different type of information to be delivered. The satellite propagation delay and the traffic and congestion control of the ATM suggest to modify the slow start and the congestion avoidance of the TCP. Our approach is based on the combination of a fixed window flow control at transport layer with the ATM traffic and congestion control. Our analysis shows that the system performance is satisfactory if some bounds of the TCP buffer size are respected and the spacing of the RM cells is within a given range of values.