MEMORY CROSSOVER NEURAL NETWORK PREDISTORTER FOR THE COMPENSATION OF MEMORY CROSSTALK AND HPA NONLINEARITY H. Bouhadda 1 , R.Zayani 1 , R. Bouallegue 1 and D. Roviras 2 1 6’Tel, Sup’Com, Carthage University, Tunis , Tunisia 2 LAETITIA, CNAM, Paris, France ridha.bouallegue@supcom.rnu.tn , daniel.roviras@cnam.fr ABSTRACT In [1] and [2], authors proposed two efficient crossover predistortion schemes which are capable to compensate simultaneously HPA nonlinearity and crosstalk effects in MIMO systems. The crosstalk model considered in these papers was memoryless one. However, memory effects of crosstalk can no longer be ignored due to the broadband transmitted signal. Then, in this paper, we demonstrate the effect of memory crosstalk on the Crossover Neural Network Predistorter (CO-NNPD) proposed in [1]. Along, we propose a new crossover predistortion structure based on this conventional CO-NNPD which is capable to enhance good performance in MIMO OFDM systems in presence of HPA nonlinearities with taken into account the memory effects of crosstalk. The Levenberg-Marquardt algorithm (LM) is used for neural network training, which has proven [3] to exhibit a very good performance with lower computation complexity and faster convergence than other algorithms used in literature. This paper is supported with simulation results for the Alamouti STBC MIMO OFDM system in terms of Bit Error Rate (BER) in Rayleigh fading channel. Keywords- STBC, MIMO-OFDM, CO-NNPD (Crossover Neural Network Predistorter), Memory Crosstalk, HPA, nonlinearity, MLP (Multi-Layer Perceptron), LM (Levenberg-Marquardt), NN (Neural Network), MCO- NNPD (Memory CO-NNPD). 1. INTRODUCTION MIMO techniques consist to increase the theoretical data rate proportional to the number of transmit antennas [4], they mitigate fading and significantly improve link quality. For this reason MIMO systems make the object of IEEE norms and they are used in most new technologies especially in 4G wireless systems. OFDM techniques use a set of subcarrier to transmit information (frequency division multiplexing) and they offer high performance in terms of data rate and spectral efficiency [5]. That’s why, the combination of MIMO and OFDM gives the possibility to exploit the advantages of the two techniques. MIMO-OFDM technique has gained wide use in many wireless standards, such as IEEE 802.11n, IEEE 802.16e and 3GPP-LTE [2]. However, like SISO-OFDM MIMO-OFDM exhibits large Peak-to-Average Power Ratios (PAPR), i.e. large fluctuation in their signal envelopes. Indeed, the performance of the transceiver is very sensitive to nonlinear distortions caused by the HPA. The most cost effective solution for the HPA nonlinearity is to implement a digital predistortion [3] which consists to distort the HPA input signal by a predistorter that present the inverse of the amplifier characteristics. Furthermore, in MIMO systems, multiple transmission paths are implemented in the same chipset. These implementations cause the crosstalk between those paths and it affects the signal quality and the predistorter performance [1, 2]. Several techniques have been proposed in literature to suppress the crosstalk effects in the IC design, such as buffering the LO paths [6], grounded guard ring [7], deep trench [7, 8], silicon-on-in-sulator (SOI) substrate [7]. These proposed techniques are able to minimize the crosstalk, although they cannot completely remove it. In [1-2-6, 9], the effects of crosstalk on MIMO systems have been treated. The HPA nonlinearity effects on the performance of systems using MIMO techniques were proposed and analyzed in [10-11,12]. Indeed, it should be noted that only two previous papers [1, 2] have treated the effects of crosstalk on digital predistortion for MIMO systems and they proposed new solutions as a crossover predistorter to compensate simultaneously the crosstalk and HPA nonlinearities. However, the crosstalk model considered in these papers was memoryless one. Yet, in new generation of wireless communications which use OFDM as modulation scheme, memory effects of crosstalk can no longer be ignored due to the broadband input signal. It is worth noting and to the best of authors knowledge that this paper is the first which study the effects of memory crosstalk on crossover predistortion. The performance of the CO-NNPD proposed in [1] are revisited here and the effect of memory crosstalk on it is demonstrated. Along, we propose a new predistortion structure, based on CO-NNPD, to the STBC MIMO OFDM system while taken into consideration the memory crosstalk and HPA nonlinearity. The performance of the new proposed predistortion scheme are analyzed based on BER when the channel is a Rayleigh fading one. The remainder of this paper is organized as follows: 19th European Signal Processing Conference (EUSIPCO 2011) Barcelona, Spain, August 29 - September 2, 2011 © EURASIP, 2011 - ISSN 2076-1465 794