IEICE TRANS. COMMUN., VOL.E88–B, NO.2 FEBRUARY 2005 427 PAPER Special Section on Multi-carrier Signal Processing Techniques for Next Generation Mobile Communications Proposal of Clipping and Inter-Modulation Noise Mitigation Method for OFDM Signal in Non-linear Channel Pisit BOONSRIMUANG †a) , Student Member, Kazuo MORI † , Member, Tawil PAUNGMA †† , Nonmember, and Hideo KOBAYASHI † , Member SUMMARY One of the disadvantages of using OFDM is the larger peak to averaged power ratio (PAPR) of the time domain signal as com- pared with the conventional single carrier transmission method. The OFDM signal with larger PAPR will cause the undesirable spectrum re- growth and the larger degradation of bit error rate (BER) performance both due to the inter-modulation products occurring in the non-linear ampliﬁer at the transmitter. The clipping method in conjunction with the Decision Aided Reconstruction (DAR) method is well known as one of the solutions to improve the BER performance with keeping the better PAPR perfor- mance. However, the DAR method is proposed to mitigate only the clip- ping noise and not for the inter-modulation noise. In this paper, we propose the Improved DAR (IDAR) method, which can mitigate both the clipping noise and inter-modulation noise on the basis of DAR method. The pro- posed method enables the eﬃcient usage of transmission power ampliﬁer at the transmitter with keeping the better PAPR and BER performances. This paper presents various computer simulation results to verify the per- formance of proposed IDAR method in the non-linear channel. key words: OFDM, non-linear channel, PAPR, clipping, DAR, IDAR 1. Introduction The orthogonal frequency division multiplexing (OFDM) technique has been received a lot of attentions especially in the ﬁeld of wireless communications because of its eﬃcient usage of frequency bandwidth and robustness to the multi- path fading [1], [2]. From these advantages, the OFDM technique has already been adopted as the standard trans- mission technique in the wireless LAN systems and the ter- restrial digital broadcasting systems [3]–[5]. The OFDM is also considering as one of the candidate transmission tech- niques for the next generation of mobile communications systems [6]. One of the disadvantages of using the OFDM signal is that its time domain signal has the larger peak to averaged power ratio (PAPR) as compared with the conventional sin- gle carrier modulation method [7]. The larger PAPR would cause the undesirable spectrum re-growth and the degrada- tion of bit error rate (BER) performance both from the inter- modulation products occurring at the non-linear ampliﬁer at the transmitter. Manuscript received May 28, 2004. Manuscript revised August 25, 2004. † The authors are with the Department of Electrical and Elec- tronic Engineering, Faculty of Engineering, Mie University, 514- 8507 Japan. †† The author is with the Department of Telecommunication En- gineering, Faculty of Engineering, King Moungkut’s Institute of Technology of Ladkrabang, Thailand. a) E-mail: pisitb@com.elec.mie-u.ac.jp To overcome the above problem, a clipping method was proposed to improve the PAPR performance of OFDM signal [7]. Although this method can improve the PAPR performance relatively, the BER performance would be de- graded at the receiver due to the clipping noise. The Decision-Aided Reconstruction (DAR) method was pro- posed to mitigate the clipping noise with keeping the better PAPR performance [8]. However, the DAR method was pro- posed on the assumption that the number of IFFT points is equal to the number of sub-carriers, that is the ideal Nyquist sampling frequency. The number of IFFT points is how- ever required to be larger than the number of sub-carriers because of zero padding, which enables the usage of sim- ple analogue ﬁlter to reject the aliasing occurring at the D/A converter. From this fact, the DAR method proposed in [8] is unable to employ in the actual OFDM system. To solve this problem, we proposed modiﬁed DAR method of using pre- ﬁlter, which can apply the actual OFDM system operating at the over sampling frequency [9]. The feature of proposed method is that the error signal due to the clipping noise is re- constructed by using the frequency domain signal instead of using the received time domain signal proposed in [8]. By using the frequency domain signal in the reconstruction of error signal, it is possible to compensate the distortion error due to the band limitation of pre-ﬁlter. These DAR methods including the modiﬁed DAR method can only mitigate the clipping noise and not for the inter-modulation noise due to the non-linear ampliﬁer. The BER performance would be degraded severely when the in- put back-oﬀ (IBO) of non-linear ampliﬁer is taken at near to the saturation region, even though the PAPR performance is improved relatively by using the clipping method. The sim- ple method to reduce the inter-modulation products is to set the IBO at the linear region. However, this method degrades the power eﬃciency of non-linear ampliﬁer, and has serious problem on battery consumption especially for the cases of mobile terminal and portable wireless LAN terminal. In or- der to maximize the power eﬃciency of the transmitter, the non-linear ampliﬁer is typically forced to work at near its saturation region, which will lead to the degradation of BER performance due to the inevitably high non-linearity. In this paper, we propose the improved DAR (IDAR) method [10] in which the clipping noise as well as inter- modulation noise due to the non-linear ampliﬁer could be mitigated on the basis of modiﬁed DAR method in [9]. The salient feature of proposed method is to enable the eﬃcient Copyright c  2005 The Institute of Electronics, Information and Communication Engineers