Joint Impact of Frequency Synchronization Errors and Intermodulation Distortion on the Performance of Multicarrier DS-CD
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Joint Impact of Frequency Synchronization Errors and Intermodulation Distortion on the Performance of Multicarrier DS-CDMA Systems Luca Rugini Dipartimento di Ingegneria Elettronica e dell’Informazione (DIEI), Universit`a degli Studi di Perugia, Via G. Duranti 93, 06125 Perugia, Italy Email: [email protected]
Paolo Banelli Dipartimento di Ingegneria Elettronica e dell’Informazione (DIEI), Universit`a degli Studi di Perugia, Via G. Duranti 93, 06125 Perugia, Italy Email: [email protected] Received 5 August 2003; Revised 19 January 2004 The performance of multicarrier systems is highly impaired by intercarrier interference (ICI) due to frequency synchronization errors at the receiver and by intermodulation distortion (IMD) introduced by a nonlinear amplifier (NLA) at the transmitter. In this paper, we evaluate the bit-error rate (BER) of multicarrier direct-sequence code-division multiple-access (MC-DS-CDMA) downlink systems subject to these impairments in frequency-selective Rayleigh fading channels, assuming quadrature amplitude modulation (QAM). The analytical findings allow to establish the sensitivity of MC-DS-CDMA systems to carrier frequency offset (CFO) and NLA distortions, to identify the maximum CFO that is tolerable at the receiver side in different scenarios, and to find out the optimum value of the NLA output power backoff for a given CFO. Simulation results show that the approximated analysis is quite accurate in several conditions. Keywords and phrases: multicarrier DS-CDMA, carrier frequency offset, intermodulation distortion, Rayleigh fading.
1.
INTRODUCTION
In the last years, several multicarrier code-division multipleaccess (MC-CDMA) schemes [1] have been proposed by combining orthogonal frequency-division multiplexing (OFDM) and direct-sequence code-division multiple-access (DS-CDMA), with the goal of incorporating the advantages of both techniques. Specifically, the low-complexity equalization of cyclic prefixed OFDM systems and the multiple-access interference (MAI) mitigation capabilities offered by DS-CDMA systems make MC-CDMA techniques attractive for future mobile broadband communications [2]. Differently from single-carrier systems, one of the main problems of multicarrier schemes is the high sensitivity to frequency synchronization errors [3]. Indeed, the carrier frequency offset (CFO), which models the frequency mismatch between the transmitter and receiver oscillators, generates intercarrier interference (ICI), thereby destroying the frequency-domain orthogonality of the transmitted data.
The presence of a nonlinear amplifier (NLA) at the transmitter, which introduces both intermodulation distortion (IMD) and out-of-band interference, is another relevant source of impairment in multicarrier systems. Indeed, multicarrier signals are characterized by a high peak-toaverage power ratio (PAPR) [4], and hence they are significantly distorted when, in order to improve the power efficiency, the NLA working point is chosen close to the saturation point. Obviously, the typical impairments of mult
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