Spectrally Efficient OFDMA Lattice Structure via Toroidal Waveforms on the Time-Frequency Plane
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Research Article Spectrally Efficient OFDMA Lattice Structure via Toroidal Waveforms on the Time-Frequency Plane Sultan Aldirmaz, Ahmet Serbes, and Lutfiye Durak-Ata (EURASIP Member) Department of Electronics and Communications Engineering, Yildiz Technical University, Yildiz, Besiktas, 34349 Istanbul, Turkey Correspondence should be addressed to Sultan Aldirmaz, [email protected] Received 2 January 2010; Accepted 29 June 2010 Academic Editor: L. F. Chaparro Copyright © 2010 Sultan Aldirmaz et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. We investigate the performance of frequency division multiplexed (FDM) signals, where multiple orthogonal Hermite-Gaussian carriers are used to increase the bandwidth efficiency. Multiple Hermite-Gaussian functions are modulated by a data set as a multicarrier modulation scheme in a single time-frequency region constituting toroidal waveform in a rectangular OFDMA system. The proposed work outperforms in the sense of bandwidth efficiency compared to the transmission scheme where only single Gaussian pulses are used as the transmission base. We investigate theoretical and simulation results of the proposed methods.
1. Introduction As the demand for mobility and high performance in multimedia services increase, efficient spectrum usage becomes a critical issue in wireless communications. Orthogonal frequency division multiplexing (OFDM) is a multicarrier modulation that has been employed by a number of current and future wireless standards including 802.11 a/g, 802.16 ad, long term evolution (LTE) downlink, and next generation networks. OFDM has been a popular method because of its robustness in frequency selective fading characteristics of broadband wireless systems. OFDM technology transmits data by dividing it into parallel streams to be modulated by subchannels each having a different carrier frequency. Basically, data is carried on narrow-band subcarriers in frequency domain and the carrier spacing is carefully selected so that each subcarrier is orthogonal to the others. There have been numerous studies in the literature to provide efficient spectrum usage in high-performance wireless applications, including video streaming [1, 2]. Different coding methods and modulation types are developed for this task, such as hexagonal QAM structure [3, 4]. In [5], linear combinations of Hermite-Gaussian functions are used in the generation of two orthogonal pulse shapes to increase the system throughput. However, such a linear combination increases the bandwidth of a single subchannel significantly.
Pulse shape design is an important research issue because of the corruption eects of channels, such as Doppler shifts, fading, and noise. Conventional OFDM uses rectangularpulse shapes for each data. Since this pulse shape has sinc shape in the frequency domain, its energy is dispersed to other subcarriers when the channel i
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