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T. Zemen:
"OFDM Multi-User Communication Over Time-Variant Channels";
Supervisor, Reviewer: E. Bonek, M. Rupp; Institut für Nachrichtentechnik und Hochfrequenztechnik, 2004.

Wireless broadband communications for users moving at vehicular speed is a cornerstone of future fourth generation (4G) mobile communication systems. We investigate a multi-carrier (MC) code division multiple access (CDMA) system which is based on orthogonal frequency division multiplexing (OFDM). A spreading sequence is used in the frequency domain in order to distinguish individual users and to take advantage of the multipath diversity of the wireless channel. The transmission is block oriented. A block consists of OFDM pilot and OFDM data symbols.

At pedestrian velocities the channel can be modelled as block fading. We apply iterative multi-user detection and channel estimation. In iterative receivers soft symbols are derived from the output of a soft-input soft-output decoder. These soft symbols are used in order to reduce the interference from other users and to enhance the channel estimates. We develop an iterative channel estimation scheme for MC-CDMA. The iterative MC-CDMA receiver achieves a performance close to the single-user bound in moderately overloaded systems. The single-user bound is defined as the performance for one user and perfect channel knowledge.

In order to obtain enhanced iterative channel estimates we take advantage of additional information like the estimated mean and variance of the soft symbols, which can be obtained from the decoder output since the used symbol alphabet has constant modulus. Using these information a linear minimum mean square error (MMSE) channel estimator is derived. The iterative receiver achieves enhanced convergence towards the single-user bound with the linear MMSE channel estimator.

At vehicular velocities, the channel can not be treated as block fading for the duration of a data block. Instead, its temporal variation must be modelled adequately. We investigate channel estimation algorithms that do not need the knowledge of complete second order statistics. We assume an upper bound for the Doppler bandwidth only, which is determined by the carrier frequency and the maximum supported velocity. This approach is motivated by the fact that existent wireless channels do not adhere to Jakes' model. First, we deal with time-variant frequency-flat channels. We analyze the Fourier basis expansion, i.e. a truncated discrete Fourier transform (DFT), for time-variant channel estimation. The analysis shows that the windowing due to the block-based transmission leads to spectral leakage and the truncation of the DFT gives rise to an effect similar to the Gibbs phenomenon. Both mechanisms together lead to biased channel estimates.

Slepian's theory of time-concentrated and bandlimited sequences allows a new approach for time-variant channel estimation. It enables the design of doubly orthogonal discrete prolate spheroidal (DPS) sequences with just two parameters; the block length and the maximum Doppler bandwidth. The DPS sequences are used to define a Slepian basis expansion. We give analytic results showing that the bias of the Slepian basis expansion is at least one magnitude smaller compared to the Fourier basis expansion.

The Slepian basis expansion performance degrades for pilot based channel estimation because the orthogonality of the basis functions is lost due to the pilot grid. We tackle this problem by designing a new set of finite sequences that are orthogonal over the pilot index positions but keep their bandlimited and time-concentrated properties. The resulting generalized finite Slepian basis expansion achieves best performance for pilot based time-variant channel estimation which is proven by analytical results and shown in numerical simulations.

We apply the generalized finite Slepian basis expansion for time-variant frequency-selective channel estimation in an MC-CDMA downlink and discuss simulation results. The time-variant frequency-selective channel offers Doppler diversity in addition to multipath diversity. An MC-CDMA system can take advantage of the Doppler diversity through interleaving and coding over a data block. We derive an analytic measure for the Doppler diversity of a time-variant channel and support it by simulation results.

In this thesis, we design an iterative receiver-architecture for an MC-CDMA uplink with multi-user decoding for time-variant mobile radio channels. It is shown that this receiver type reaches the single-user bound up to 2.5dB under full load with N=64 users, at an Eb/N0=14dB, and for mobile users moving with velocities in the range from 0 to 100km/h.

http://publik.tuwien.ac.at/files/pub-et_8800.pdf