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 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

Frequency-Domain Turbo Equalization for MIMO Underwater Acoustic Communications

This paper investigates a low-complexity frequency-domain turbo equalization (FDTE) based on linear minimum mean square error (LMMSE) criterion for single-carrier (SC) multiple-input multiple-output (MIMO) underwater acoustic communications (UAC). The receiver incorporates both the equalizer and the decoder which exchange the extrinsic information on the coded bits for each other to implement the iterative detection. The channel impulse responses (CIRs) required in the equalization are estimated in the frequency domain (FD) by inserting the well-designed pilot blocks which are frequency-orthogonal Chu sequences. The proposed SC-MIMO-FDTE architecture is applied to the fixed-to-fixed underwater data gathered during SPACE08 ocean experiments in October 2008, where multiple transducers and hydrophones are deployed in communication ranges of 200m and 1000m, and the channel bandwidth is 9.765625 kHz. The phase shift keying (PSK) signals are transmitted from multiple transducers in various block sizes. The proposed transceiver has been demonstrated to improve the bit-error-rate (BER) performance significantly by processing the QPSK data blocks with block length of 1024 in 200m and 1000m ranges. The average BERs obtained by turbo detection with 3 iterations can achieve approximately 1.4 × 10-4 for the 200m system and 4.4 × 10-5 for the 1000m system

Applying Spatial Diversity to Mitigate Partial Band Interference in Undersea Networks

Many acoustic channels suffer from interference which is neither narrowband nor impulsive. This relatively long duration partial band interference can be particularly detrimental to system performance. We survey recent work in interference mitigation and orthogonal frequency division multiplexing (OFDM) as background motivation to develop a spatial diversity receiver for use in underwater networks. The network consists of multiple distributed cabled hydrophones that receive data transmitted over a time-varying multipath channel in the presence of partial band interference produced by interfering active sonar signals as well as marine mammal vocalizations. In operational networks, many “dropped” messages are lost due to partial band interference which corrupts different portions of the received signal depending on the relative position of the interferers, information source and receivers due to the slow speed of propagation

On the Performance of LDPC-Coded MIMO Schemes for Underwater Communications Using 5G-like Processing

UIDB/EEA/50008/2020This article studies the underwater acoustic (UWA) communications associated with multiple input–multiple output (MIMO), single carrier with frequency-domain equalization (SC-FDE), and with low-density parity-check (LDPC) codes. Low-complexity receivers such as equal gain combining (EGC), maximum ratio combining (MRC), and iterative block—decision feedback equalization (IB-DFE) are studied in the above-described scenarios. Furthermore, due to the low carrier frequencies utilized in UWA communications, the performance of the proposed MIMO scenarios is studied at different levels of channel correlation between antennas. This article shows that the combined schemes tend to achieve good performances while presenting low complexity, even in scenarios with channel correlation between antennas.publishersversionpublishe

On the Performance of LDPC-Coded MIMO Schemes for Underwater Communications Using 5G-like Processing

6F1A-06CB-E82D | Mário Pedro Guerreiro Marques da SilvaN/

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

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