Particle swarm optimization for pilot tones design in MIMO-OFDM systems
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Particle swarm optimization for pilot tones design in MIMO-OFDM systems Muhammet Nuri Seyman1* and Necmi Taşpinar2
Abstract Channel estimation is an essential task in MIMO-OFDM systems for coherent demodulation and data detection. Also designing pilot tones that affect the channel estimation performance is an important issue for these systems. For this reason, in this article we propose particle swarm optimization (PSO) to optimize placement and power of the comb-type pilot tones that are used for least square (LS) channel estimation in MIMO-OFDM systems. To optimize the pilot tones, upper bound of MSE is used as the objective function of PSO. The effects of Doppler shifts on designing pilot tones are also investigated. According to the simulation results, PSO is an effective solution for designing pilot tones. Keywords: MIMO-OFDM, channel estimation, particle swarm optimization
Introduction Recently, to meet the demand on high data rate transmission in communication systems, orthogonal frequency division multiplexing (OFDM) is applied as a modulation scheme. OFDM is a multicarrier modulation technique that operates with specific orthogonality constraints between subcarriers. The orthogonality results a waveform which uses available bandwidth with a high bandwidth efficiency [1]. Also OFDM can be combined with multiple transmit and receive antennas known as multiinput multi-output (MIMO) architecture to improve system capacity and quality of service [2]. However, at the receiver MIMO-OFDM systems require channel state information (CSI) for coherent demodulation and data detection. In order to obtain CSI, blind and training symbol (pilot tones)-based channel estimation techniques are applied. In blind channel estimation technique, CSI is estimated by channel statistics without any knowledge of the transmitted data. But it can suffer from slow convergence in mobile wireless systems because of the time varying nature of channels [3]. In training symbol technique, training sequences that are also called as pilots are inserted into all of subcarriers of OFDM symbols with specific period or * Correspondence: [email protected] 1 Department of Electronic Communication, Vocational High School, Kirikkale University, 71100 Kirikkale, Turkey Full list of author information is available at the end of the article
inserted into each OFDM symbol [4]. Compared with blind technique, pilot-based channel estimation techniques provide better resistance to fast fading and time varying channels [4-6]. However, designing of pilot tones directly affect the performance of channel estimation algorithms. Hence, optimal design for training symbols based on minimizing Cramer Rao lower bound [7], minimizing mean square error (MSE) of estimation [8-10], and maximizing lower bound capacity [11] has been considered in literature. By minimizing Cramer Rao Bound on MSE of channel, the optimal placement of pilot symbols has been considered in [7]. In [8], the number and the placement of pilot symbols and the power allocation
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