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Space-time alignment for asynchronous interference suppression in MIMO OFDM cellular communications and symbol -period estimation for 8-VSB digital TV

Purdue University
Publication Date
  • Engineering
  • Electronics And Electrical
  • Computer Science


This thesis is composed of two parts. The first part addresses multicarrier cellular systems using orthogonal frequency division multiplexing (OFDM). Aggressive frequency reuse increases system capacity but also subjects mobiles to high levels of interference from neighboring basestations. Conventional multiple-input multiple-output (MIMO) systems typically assume the co-channel signals to be synchronous and do not exploit the cyclic prefixes embedded in the OFDM signals. Presented herein are space-time alignment and partial equalization algorithms for cyclic-prefix based systems that are suitable for scenarios where the interferer's cyclic prefix is not time-aligned with the desired signal's cyclic prefix. The algorithms use a two-stage architecture with partial equalization for cyclic-prefix alignment in the time domain followed by simple channel equalization in the frequency domain. The algorithms align the cyclic prefixes of the multiple users, making them synchronous and thereby facilitating interference cancellation on a per frequency bin basis. They also limit the lengths of the effective channels to be no greater than the cyclic prefix length. Simulations demonstrate the effectiveness of the alignment techniques for both coded and uncoded MIMO OFDM systems with the co-channel signals having either diverse or similar power levels. ^ The second part addresses timing recovery in digital receivers. Fast and accurate estimation of the ideal sample times is critical for rapid and stable signal acquisition. Many timing recovery techniques are based heuristically on properties of the pulse shape, which is distorted in the presence of multipath. An open-loop symbol period estimation algorithm is proposed that can be used alone or to speed up symbol timing recovery loops. The proposed algorithm is effective in frequency selective channels, does not require resampling of the digital received signal while estimating the symbol period, does not require training or estimation of the transmitted symbols, and is performed in a computationally efficient manner. ^

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