OFDM Multi-User Communication Over Time-Variant Channels

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 an 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