BLIND TIME DOMAIN EQUALIZATION OF SCFDMA SIGNAL Sosth` ene Yameogo 1 , Jacques Palicot 1 , Laurent Cariou 2 1) Avenue de la Boulaie, 35500 Cesson-sevign´ e, Rennes, France Signal Communication et Electronique embarqu´ ee SUPELEC/IETR ysyameog@supelec.fr , jacques.palicot@supelec.fr 2) France Telecom Rennes laurent.cariou@orange-ftgroup.com Abstract— In this paper a new equalizer for Single Carrier FDMA (SCFDMA)[1][2] is described. This signal is currently proposed for mobile radio uplink communications in 3G Long Term Evolution (3G LTE). In this system, equalization is performed thanks to the addition of a Cyclic Prefix (CP), as in OFDM system, and pilots regularly inserted for channel estimation which leads to the usefull throughput degradation. To reduce this degradation a new structure of blind time domain equalizer, shared in two parts, is proposed in this paper : The filtering itself which is a linear time domain filtering performed before any demodulation device in the receiver side, and the MMSE criterion algorithm performed in the time domain after user selection and demodulation. The results are very convincing both in terms of Mean Square Error and in throughput gain compared to the performances of Zero-Forcing technique(ZF) used in literature. Keywords— SCFDMA, LTE, OFDMA, Equalization, LMS- Algorithm. I. I NTRODUCTION Nowadays, mobile radio system is immersed by more and more services with data rate from few kbit/s up to several Mbit/s. Presently, research beyond 3rd generation mobile radio systems is in progress world wide to enable the future mobile radio system supporting different types of services with different data rates and providing high flexibility and high performance. An important decision for the future mobile radio system is the choice of the multiple access scheme. One possible choice can be the Orthogonal Frequency Division Multiple Access (OFDMA) like in WiMAX (IEEE 802.16), because of its robustness against frequency selective fading channels and high spectral efficiency. But, despite many benefits for high speed data services, OFDMA suffer from high envelope fluctuation in the time domain, leading to large peak-to-average-power ratio (PAPR). Because high PAPR is detrimental to mobile terminals, SC- FDMA has drawn great attention as an attractive alternative to OFDMA and currently proposed for mobile radio uplink communications in 3G Long Term Evolution (LTE)[3][4]. The system, has significantly lower PAPR[5] and therefore can greatly improve the power efficiency. It can be viewed as a new hybrid modulation scheme that cleverly combines the low PAPR of single-carrier systems with the multipath resistance and flexible subcarrier frequency allocation offered by OFDM. However, SCFMA suffers from Inter-symbol Interference (ISI) if a transmission over a frequency selective channel is con- sidered. Therefore for mobile radio applications, SCFMA requires Equalization at the receiver. The only use of Guard Interval(GI) for equalization does not meet the challenges of the future mobile radio system because of the reduction of spectral efficiency. Currently, a zero-forcing(ZF) criterion is used in the system to achieve a frequency domain equalization, inserting regu- larly Pilots for channel estimation. Actually, each frame of SCFDMA signal contains 20 Slots, each slot has 7 SCFDMA symbols where the 4th one is a reference symbol. Therefore 1/7 SCFDMA symbol is used for references leading to useful throughout degradation. In this paper, a new blind time domain equalizer is proposed to reduce this degradation. Because of the loss of linear convolution between the transmitted signal and channel at the receiver side due to the presence of DFT demodulator, and the users demultiplexing in frequency domain, we need a specific time domain equalizer for channel inversion. The specificity of this new equalizer resides on its structure. We proposed a new structure shared in two parts: The filtering itself which is a linear time domain filtering posted in front of the receiver, and the MMSE criterion algorithm performed at the end of the receiver after user selection and demodulation. The results are very convincing both in terms of Mean Square Error and in throughput gain compared to the performances of Zero- Forcing technique(ZF) used in literature. This paper is organized as follows: Section II derives the system model with the different mapping encountered in the literature. Section III states the current equalization applied in LTE. Section IV states time domain equalization problem and provides the new solution we proposed. Simulation results are given in Section V. Finally, conclusions are drawn in Section VI. II. SCFDMA SYSTEM MODEL A. Transmission Chain Fig. 1 illustrates the transmitter and receiver structure of SC-FDMA. The transmitter of an SC-FDMA system first groups the modulation symbols into blocks each containing Q symbols. Next it performs an N-point discrete Fourier transform (DFT) 978-1-4244-2515-0/09/$25.00 ©2009 IEEE