Robust frequency-domain turbo equalization for multiple-input multiple-output (MIMO) wireless communications

This dissertation investigates single carrier frequency-domain equalization (SC-FDE) with multiple-input multiple-output (MIMO) channels for radio frequency (RF) and underwater acoustic (UWA) wireless communications. It consists of five papers, selected from a total of 13 publications. Each paper focuses on a specific technical challenge of the SC-FDE MIMO system. The first paper proposes an improved frequency-domain channel estimation method based on interpolation to track fast time-varying fading channels using a small amount of training symbols in a large data block. The second paper addresses the carrier frequency offset (CFO) problem using a new group-wise phase estimation and compensation algorithm to combat phase distortion caused by CFOs, rather than to explicitly estimate the CFOs. The third paper incorporates layered frequency-domain equalization with the phase correction algorithm to combat the fast phase rotation in coherent communications. In the fourth paper, the frequency-domain equalization combined with the turbo principle and soft successive interference cancelation (SSIC) is proposed to further improve the bit error rate (BER) performance of UWA communications. In the fifth paper, a bandwidth-efficient SC-FDE scheme incorporating decision-directed channel estimation is proposed for UWA MIMO communication systems. The proposed algorithms are tested by extensive computer simulations and real ocean experiment data. The results demonstrate significant performance improvements in four aspects: improved channel tracking, reduced BER, reduced computational complexity, and enhanced data efficiency --Abstract, page iv

Multiple-Resampling Receiver Design for OFDM Over Doppler-Distorted Underwater Acoustic Channels

Cataloged from PDF version of article.In this paper, we focus on orthogonal frequency-divisionmultiplexing (OFDM) receiver designs for underwater acoustic (UWA) channels with user- and/or path-specific Doppler scaling distortions. The scenario is motivated by the cooperative communications framework, where distributed transmitter/receiver pairs may experience significantly different Doppler distortions, as well as by the single-user scenarios, where distinct Doppler scaling factors may exist among different propagation paths. The conventional approach of front–end resampling that corrects for common Doppler scalingmay not be appropriatein such scenarios, rendering a post-fast-Fourier-transform (FFT) signal that is contaminated by user- and/or path-specific intercarrier interference. To counteract this problem, we propose a family of front–end receiver structures thatutilizemultiple-resampling (MR)branches,eachmatched to the Doppler scaling factor of a particular user and/or path. Following resampling, FFT modules transform the Doppler-compensated signals into the frequency domain for further processing through linear or nonlinear detection schemes. As part of the overall receiver structure, a gradient–descent approachis also proposed to refine the channel estimates obtained by standard sparse channel estimators. The effectiveness and robustness of the proposed receivers are demonstrated via simulations, as well as emulations based on real data collected during the 2010 Mobile Acoustic Communications Experiment (MACE10, Martha’s Vineyard, MA) and the 2008 Kauai Acomms MURI (KAM08, Kauai, HI) experiment

Turbo equalization for multiple-input multiple-output (MIMO) wireless communication systems

This dissertation investigates both of the frequency domain and time domain turbo equalization with multiple-input multiple-output (MIMO) fading channels for radio frequency and underwater acoustic communications. First, a low complexity frequency domain turbo equalization (FDTE) is proposed for the MIMO systems with zero padding (ZP) or cyclic prefix (CP) inserted between the transmitted data blocks and its performance is tested on the real-world UWA communications experiments. Second, as high speed communication system requires efficient bandwidth usage and power consumption, CP or ZP is not transmitted as auxiliary information. An inter-block interference cancelation and CP reconstruction algorithm is developed to re-arrange the channel matrix into a block diagonal one. This improvement makes the FDTE effectively detects the continuous data stream from the high speed UWA communications and its performance has been verified by processing data collected from the UWA communications experiment. Finally, a low complexity soft interference cancelation (SIC) time domain turbo equalizer for MIMO systems with high level modulation is proposed. Compared with the conventional linear or nonlinear turbo equalizers, the proposed SIC turbo equalizer can theoretically reach the bound set up by the ideal match filter and its bit error rate (BER) performance from Monte Carlo simulation achieves a lower error floor as well as a more rapid convergence speed. --Abstract, page iv

Low-complexity iterative receiver algorithms for multiple-input multiple-output underwater wireless communications

This dissertation proposes three low-complexity iterative receiver algorithms for multiple-input multiple-output (MIMO) underwater acoustic (UWA) communications. First is a bidirectional soft-decision feedback Turbo equalizer (Bi-SDFE) which harvests the time-reverse diversity in severe multipath MIMO channels. The Bi-SDFE outperforms the original soft-decision feedback Turbo equalizer (SDFE) while keeping its total computational complexity similar to that of the SDFE. Second, this dissertation proposes an efficient direct adaptation Turbo equalizer for MIMO UWA communications. Benefiting from the usage of soft-decision reference symbols for parameter adaptation as well as the iterative processing inside the adaptive equalizer, the proposed algorithm is efficient in four aspects: robust performance in tough channels, high spectral efficiency with short training overhead, time efficient with fast convergence and low complexity in hardware implementation. Third, a frequency-domain soft-decision block iterative equalizer combined with iterative channel estimation is proposed for the uncoded single carrier MIMO systems with high data efficiency. All the three new algorithms are evaluated by data recorded in real world ocean experiment or pool experiment. Finally, this dissertation also compares several Turbo equalizers in single-input single-output (SISO) UWA channels. Experimental results show that the channel estimation based Turbo equalizers are robust in SISO underwater transmission under harsh channel conditions --Abstract, page iv

Low-complexity soft-decision feedback turbo equalization for multilevel modulations

This dissertation proposes two new decision feedback equalization schemes suitable for multilevel modulation systems employing turbo equalization. One is soft-decision feedback equalization (SDFE) that takes into account the reliability of both soft a priori information and soft decisions of the data symbols. The proposed SDFE exhibits lower signal to noise ratio (SNR) threshold that is required for water fall bit error rate (BER) and much faster convergence than the near-optimal exact minimum mean square error linear equalizer (Exact-MMSE-LE) for high-order constellation modulations. The proposed SDFE also offers a low computational complexity compared to the Exact-MMSE-LE. The drawback of the SDFE is that its coefficients cannot reach the matched filter bound (MFB) and therefore after a large number of iterations (e.g. 10), its performance becomes inferior to that of the Exact-MMSE-LE. Therefore, soft feedback intersymbol interference (ISI) canceller-based (SIC) structure is investigated. The SIC structure not only exhibits the same low complexity, low SNR threshold and fast convergence as the SDFE but also reaches the MFB after a large number of iterations. Both theoretical analysis and numerical simulations demonstrate why the SIC achieves MFB while the SDFE cannot. These two turbo equalization structures are also extended from single-input single-output (SISO) systems to multiple-input multiple-output (MIMO) systems and applied in high data-rate underwater acoustic (UWA) communications --Abstract, page iv

Harvesting time-frequency-space diversity with coded modulation for underwater acoustic communications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2009.Includes bibliographical references (leaves 172-180).The goal of this thesis is to design a low-complexity, high data-rate acoustic communications system with robust performance under various channel conditions. The need for robust performance emerges because underwater acoustic (UWA) channels have time-varying statistics, thus a coded modulation scheme optimally designed for a specific channel model will be suboptimal when the channel statistics change. A robust approach should use a coded modulation scheme that provides good performance in both additive white Gaussian noise (AWGN) and Rayleigh fading channels (and, consequently in the Rice fading channel, an intermediate channel model between the latter two). Hence, high data-rate coded modulation schemes should exhibit both large free Euclidean and Hamming distances. In addition, coded modulation is regarded as a way to achieve time diversity over interleaved flat fading channels. UWA channels offer additional diversity gains in both frequency and space; therefore a system that exploits diversity in all three domains is highly desirable. Two systems with the same bit-rate and complexity but different free Euclidean and Hamming distances are designed and compared. The first system combines Trellis Coded Modulation (TCM) based on an 8-PSK signal set, symbol interleaving and orthogonal frequency-division multiplexing (OFDM). The second system combines bit-interleaved coded modulation (BICM), based on a convolutional code and a 16-QAM signal set, with OFDM.(cont.) Both systems are combined with specific space-time block codes (STBC) when two or three transmit antennas are used. Moreover, pilot-symbol-aided channel estimation is performed by using a robust 2-D Wiener filter, which copes with channel model mismatch by employing appropriate time and frequency correlation functions. The following result was obtained by testing the aforementioned systems using both simulated and experimental data from RACE '08: the BICM scheme performs better when the UWA channel exhibits limited spatial diversity. This result implies that coded modulation schemes emphasizing higher Hamming distances are preferred when there is no option for many receive/transmit hydrophones. The TCM scheme, on the other hand, becomes a better choice when the UWA channel demonstrates a high spatial diversity order. This result implies that coded modulation schemes emphasizing higher free Euclidean distances are preferred when multiple receive/transmit hydrophones are deployed.by Konstantinos Pelekanakis.Ph.D