Performance Evaluations of Multi-Carrier CDMA System with Cyclic Shifted Scramble Code in Multi-path Fading Channel Masato SAITO, Akihiro OKUDA, Minoru OKADA, and Heiichi YAMAMOTO Graduate School of Science and Technology, Nara Institute of Science and Technology (NAIST) 8916–5 Takayama-cho, Ikoma, Nara, 630–0192, Japan Tel.: +81–743–72–5342, Fax: +81–743–72–5349, Email: saito@is.naist.jp Abstract—In this paper, we evaluate the error rate of side information identification in multi-path fading environments. In previous study, we proposed Peak-to-Average Power Ra- tio (PAPR) reduction method for Multi-Carrier CDMA (MC- CDMA) signals named Cyclic-Shifted Scramble Code (CSSC). The CSSC utilizes a scramble code and pilot channel to reduce the PAPR of MC-CDMA signals. Transmitter sends the scramble code with some amount of cyclic shifts, which can be recognized as a side information. Therefore, the receivers must extract the side information as accurate as possible. One problem of the method is destruction of orthogonality amongst pilot channel and user channels. The problem degrades the performances of both side information identification and data transmission (bit error rate and packet error rate). From the numerical results, CSSC performs well even in multi-path fading environment and is comparable to the case of perfect scramble code estimation. I. I NTRODUCTION Multi-Carrier Code Division Multiple Access (MC-CDMA) systems are expected as one of promising techniques for Beyond 3rd Generation (B3G) or 4th generation (4G) mobile communications. This is because MC-CDMA systems are ro- bust against multi-path fading channel and are highly efficient in cellular systems [1]–[4]. As a big issue of MC-CDMA system and other multi-carrier modulation technologies, it is recognized that the generated signal performs wide dynamic range of amplitude and sometimes generates high peak. Since such drawbacks require energy inefficient amplifiers such as linear amplifiers, it is very important to effectively reduce the Peak-to-Average Power Ratio (PAPR) of MC-CDMA signal to achieve energy efficient systems with the merits above mentioned. To increase the data rate and to enhance the user capacity, the number of subcarriers should be large in MC-CDMA systems. However, PAPR tends to increase with the number of subcarriers. Amplification of MC-CDMA signals including high PAPR without nonlinear distortion needs linear amplifiers with inefficient energy consumption. If PAPR of the signals can be reduced as low as possible, it enables to employ energy efficient nonlinear amplifiers with minimum signal distortion. To reduce the PAPR of MC-CDMA signals, a lot of re- searches were studied so far. One of the simplest approach to reduce PAPR of multi-carrier signals may be clipping [5]. The clipping, however, causes signal distortion, out-of-band emis- sion, and filitering for suppressing undesired emitted spectrum. Since clipping can be easily combined with other methods, we don’t treat the method in this study and concentrate on distortionless PAPR reduction method. For MC-CDMA uplink, Popovi´ c evaluated the PAPR prop- erties of several kinds of channelization codes [6]. Chan- nelization code is employed for user identification. Nobilet et al., Gar´ ıa-Armada et al., and Saito el al. evaluated the PAPR properties of MC-CDMA both uplink and downlink for various channelization codes [7]–[9]. On the other hand, we have analyzed the effect of a scramble code on the PAPR performance, and concluded that a scramble code eliminates the differences amongst the PAPR properties of channelization codes [10]. Selected Mapping (SLM) for MC-CDMA systems [11], Code Selection Peak Power Reduction (CSPPR) method [12], and Partial Transmit Sequence (PTS) [13], [14] have been proposed for reducing the PAPR of MC-CDMA system. Those methods also have problems such as no consideration of scramble codes, the limitation of available number of users, requirement of transmitting side information, or hardware complexity. Few researches about PAPR reduction of MC- CDMA signals consider scramble codes. As a PAPR reduction method suitable for MC-CDMA systems which employ both channelization and scramble code, we proposed Cyclic-Shifted Scramble Code (CSSC) method in previous study [15]. Since CSSC is based on SLM, the ability of PAPR reduction is almost the same as that of SLM. In CSSC, a scramble code is utilized as a random phase rotation vector of subcarrier for SLM in the transmitter by cyclic shift. When cyclic shifted scramble code is multiplied by spread and multiplexed user data symbols, the action changes the time- domain signal waveform and also its peak power. Selection of the signal with adequate phase of scramble code enables to reduce PAPR of MC-CDMA signals. At the receiver, pilot symbols which are recognized by the receiver beforehand are utilized not only estimating channel parameters, but also estimating the amount of the cyclic shift, that is, the phase of the code. In Additive White Gaussian Noise (AWGN) channel, we have confirmed that the estimation of the phase works well within an allowable error rate which means the error of shift estimation degrades quite little the resultant BER per- 74 2006 IEEE Ninth International Symposium on Spread Spectrum Techniques and Applications 0-7803-9780-0/06/$20.00 ©2006 IEEE