Analysis of the Performance Degradation in Uplink MC-CDMA Systems with Doppler Induced Frequency Offsets Ivan Cosovic 1 , Michael Schnell 1 , Miroslav Dukic 2 , Andreas Springer 3 1 German Aerospace Center (DLR), Institute of Communications and Navigation, 82234 Wessling, Germany, Phone: +49-8153-282853, Email: ivan.cosovic@dlr.de / michael.schnell@dlr.de 2 Department of Communications, Faculty of Electrical Engineering, University of Belgrade, 11120 Belgrade, Serbia and Montenegro, Phone: +381-11-3218-305, Email: dukic@etf.bg.ac.yu 3 Institute for Communication and Information Engineering, University of Linz, 4040 Linz, Austria, Phone: +43-732-2468-9725, Email: a.springer@icie.jku.at Abstract—In this paper, uplink Time Division Duplex (TDD) Multi-Carrier Code-Division Multiple-Access (MC-CDMA) sys- tems applying pre-equalization to combat channel impairments are considered. Especially, the performance degradation caused by the Doppler induced frequency offsets among the different mobile users is analyzed in detail. Based on this analysis different frequency interleaving schemes are proposed and their potential to reduce the performance degradation is investigated. It turns out, that a frequency interleaver which is allowed to skip a few, well-defined subcarriers is capable of reducing the influence of Doppler induced frequency offsets significantly causing a very little loss in bandwidth efficiency. I. I NTRODUCTION Multi-Carrier Code-Division Multiple-Access (MC- CDMA) [1], [2] is well recognized as a very promising candidate for the air interface of future mobile wireless downlink systems. However, when considering MC-CDMA for uplink transmission some problems arise. In particular, channel estimation and equalization as well as time and frequency synchronization are more difficult and require more complex algorithms. One of the possibilities to mainly overcome the problems of channel estimation and equalization is to apply channel pre-equalization at the MC-CDMA transmitter as described in [3], [4]. In this paper, the influence of Doppler induced frequency offsets on the performance of uplink time division duplex (TDD) MC-CDMA applying pre-equalization at the transmitter is analyzed in detail. Additionally, countermeasures based on thoroughly designed frequency interleaving schemes are proposed. Sensitivity of uplink MC-CDMA systems to frequency offsets has been considered in several papers [5], [6], but both, an in-depth analysis of the influence of misaligned users on each other and proposals for reducing the resulting performance degradation are not yet available. Throughout the paper, a slowly varying time-variant channel is assumed. Thus, channel pre-equalization techniques are applicable to the uplink TDD/MC-CDMA transmission which are based on the channel estimation results from the downlink transmission [3], [4]. With TDD mode the effects of the channel on the uplink signal can be pre-equalized at the mobile station based on the channel estimation from the downlink frame. The desired user is assumed to be synchronized to the receiver frequency, while all other users are misaligned in frequency. Different frequency interleaving schemes are in- vestigated and their performances are analyzed. A novel skip- carrier interleaver is introduced which significantly improves the performance of an uplink MC-CDMA system in the presence of frequency offsets between the different users. The paper is organized as follows. The TDD/MC-CDMA uplink transmission system considered throughout this paper is described in Section II. In Section III, both the influence of misaligned users on each other is analyzed and frequency interleaving schemes are investigated. The performance of the uplink TDD/MC-CDMA system in the presence of Doppler induced frequency offsets and the performance gains achieved through the proposed skip-carrier interleaver are presented in Section IV. Finally, Section V summarizes the results. II. TRANSMISSION SYSTEM A time synchronous uplink MC-CDMA system with K active users is considered. Fig. 1 depicts the block diagram of the considered uplink transmitter of mobile user k, k = 0,...,K - 1. The uplink transmitter is based on the MC- CDMA system concept with M -Modification as proposed in [2]. Note, the M -Modification allows each user to transmit M data symbols simultaneously using an additional Orthog- onal Frequency-Division Multiplexing (OFDM) component IEEE Communications Society 0-7803-8533-0/04/$20.00 (c) 2004 IEEE