Source: https://aaltodoc.aalto.fi/handle/123456789/2686
Timestamp: 2019-04-24 12:24:21+00:00

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Beyond third generation (3G) and fourth generation (4G) wireless communication systems are targeting far higher data rates, spectral efficiency and mobility requirements than existing 3G networks. By using multiple antennas at the transmitter and the receiver, multiple-input multiple-output (MIMO) technology allows improving both the spectral efficiency (bits/s/Hz), the coverage, and link reliability of the system. Multicarrier modulation such as orthogonal frequency division multiplexing (OFDM) is a powerful technique to handle impairments specific to the wireless radio channel. The combination of multicarrier modulation together with MIMO signaling provides a feasible physical layer technology for future beyond 3G and fourth generation communication systems.
The theoretical benefits of MIMO and multicarrier modulation may not be fully achieved because the wireless transmission channels are time and frequency selective. Also, high data rates call for a large bandwidth and high carrier frequencies. As a result, an important Doppler spread is likely to be experienced, leading to variations of the channel over very short period of time. At the same time, transceiver front-end imperfections, mobility and rich scattering environments cause frequency synchronization errors. Unlike their single-carrier counterparts, multi-carrier transmissions are extremely sensitive to carrier frequency offsets (CFO). Therefore, reliable channel estimation and frequency synchronization are necessary to obtain the benefits of MIMO OFDM in mobile systems. These two topics are the main research problems in this thesis.
An algorithm for the joint estimation and tracking of channel and CFO parameters in MIMO OFDM is developed in this thesis. A specific state-space model is introduced for MIMO OFDM systems impaired by multiple carrier frequency offsets under time-frequency selective fading. In MIMO systems, multiple frequency offsets are justified by mobility, rich scattering environment and large angle spread, as well as potentially separate radio frequency - intermediate frequency chains. An extended Kalman filter stage tracks channel and CFO parameters. Tracking takes place in time domain, which ensures reduced computational complexity, robustness to estimation errors as well as low estimation variance in comparison to frequency domain processing.
The thesis also addresses the problem of blind carrier frequency synchronization in OFDM. Blind techniques exploit statistical or structural properties of the OFDM modulation. Two novel approaches are proposed for blind fine CFO estimation. The first one aims at restoring the orthogonality of the OFDM transmission by exploiting the properties of the received signal covariance matrix. The second approach is a subspace algorithm exploiting the correlation of the channel frequency response among the subcarriers. Both methods achieve reliable estimation of the CFO regardless of multipath fading. The subspace algorithm needs extremely small sample support, which is a key feature in the face of time-selective channels. Finally, the Cramér-Rao (CRB) bound is established for the problem in order to assess the large sample performance of the proposed algorithms.
T. Roman, M. Enescu and V. Koivunen. Time-domain method for tracking dispersive channels in MIMO OFDM systems. In: Proceedings of the 2003 IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2003), Vol. 4, pp. IV 393-IV 396, Hong Kong, China, April 2003. [article1.pdf] © 2003 IEEE. By permission.
T. Roman, M. Enescu and V. Koivunen. Joint time-domain tracking of channel and frequency offset for OFDM systems. In: Proceedings of the 4th IEEE Workshop on Signal Processing Advances in Wireless Communications (SPAWC 2003), pp. 605-609, Rome, Italy, June 2003. [article2.pdf] © 2003 IEEE. By permission.
T. Roman, M. Enescu and V. Koivunen. Joint time-domain tracking of channel and frequency offsets for MIMO OFDM systems. Wireless Personal Communications, Vol. 31, Nos. 3-4, pp. 181-200, December 2004. [article3.pdf] © 2004 Springer Science+Business Media. By permission.
T. Roman, S. Visuri and V. Koivunen. Blind frequency synchronization in OFDM via diagonality criterion. IEEE Transactions on Signal Processing, to appear. [article4.pdf] © 2006 IEEE. By permission.
T. Roman, S. Visuri and V. Koivunen. Performance bound for blind CFO estimation in OFDM with real-valued constellations. In: Proceedings of the 60th IEEE Semiannual Vehicular Technology Conference (VTC 2004 Fall), Vol. 6, pp. 3866-3870, Los Angeles, USA, September 2004. [article5.pdf] © 2004 IEEE. By permission.
T. Roman and V. Koivunen. One-shot subspace based method for blind CFO estimation for OFDM. In: Proceedings of the 2005 IEEE International Conference on Acoustics, Speech, and Signal Processing (ICASSP 2005), Vol. 3, pp. 809-812, Philadelphia, USA, March 2005. [article6.pdf] © 2005 IEEE. By permission.
T. Roman and V. Koivunen. Subspace method for blind CFO estimation for OFDM systems with constant modulus constellations. In: Proceedings of the 61th IEEE Semiannual Vehicular Technology Conference (VTC 2005 Spring), Vol. 2, pp. 1253-1257, Stockholm, Sweden, May 2005. [article7.pdf] © 2005 IEEE. By permission.

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