This article investigates a robust receiver scheme for a single carrier, multiple-input–multiple-output (MIMO) underwater acoustic (UWA) communications, which uses the sparse Bayesian learning algorithm for iterative channel estimation embedded in Turbo equalization (TEQ). We derive a block-wise sparse Bayesian learning framework modeling the spatial correlation of the MIMO UWA channels, where a more robust expectation–maximization algorithm is proposed for updating the joint estimates of channel impulse response, residual noise, and channel covariance matrix. By exploiting the spatially correlated sparsity of MIMO UWA channels and the second-order a priori channel statistics from the training sequence, the proposed Bayesian channel estimator enjoys not only relatively low complexity but also more stable control of the hyperparameters that determine the channel sparsity and recovery accuracy. Moreover, this article proposes a low complexity space-time soft decision feedback equalizer (ST-SDFE) with successive soft interference cancellation. Evaluated by the undersea 2008 Surface Processes and Acoustic Communications Experiment, the improved sparse Bayesian learning channel estimation algorithm outperforms the conventional Bayesian algorithms in terms of the robustness and complexity, while enjoying better estimation accuracy than the orthogonal matching pursuit and the improved proportionate normalized least mean squares algorithms. We have also verified that the proposed ST-SDFE TEQ significantly outperforms the low-complexity minimum mean square error TEQ in terms of the bit error rate and error propagation.
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Hardware Implementation of Single-carrier Time-domain Turbo Equalization for Severe Multipath Acoustic Communication Channels
This paper proposes a low-latency FPGA implemen-tation for Turbo equalization to combat very long multipathfading channels where the Intersymbol-interference (ISI) channellength is on the order of 100 taps. Turbo equalization is essentialfor such severe multipath channels, but exhibits very large latencyand high computational complexity due to its sequential anditerative data processing on large-scale matrix arithmetic. Thispaper proposes an FPGA acceleration architecture to exploitthe Hermitian symmetric property of the channel Gram matrixand convolutional nature of Sequential Interference Cancellation(SIC), and successfully implements a linear Turbo equalizerof 100 taps on a Xilinx Zynq UltraScale+ MPSoC ZCU102Evaluation Kit. The architecture is able to support two turboiterations for a 1024-symbol block size and achieve 200 kilo-symbols-per-second (ksps) transmission rate.
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- Award ID(s):
- 1853258
- NSF-PAR ID:
- 10208375
- Date Published:
- Journal Name:
- Ocean
- ISSN:
- 0197-7385
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Receiver nonlinearity gives rise to intermodulation products that are caused by two strong adjacent channel signals called blockers. The nonlinear distortion effects are significantly higher for multiple antenna wideband systems in dispersive environments because third order intermodulation products decreases the signal-to-noise ratio (SNR) at the output of the equalization process. This complicates the demodulation process and increases the bit error rate. This paper considers such nonlinear distortion in the context of space-time shift keying (STSK)-enabled wideband single-carrier systems and proposes an iterative space-time block equalization (ISTBE) framework for frequency domain equalization. We present our design of a practical ISTBE receiver based on the turbo principle and numerically demonstrate that it effectively removes the residual inter-symbol interference while suppressing high-power blockers and the in-band intermodulation distortion that they cause. The proposed system is thus suitable for simple wideband radio frequency front ends operating in the weak nonlinear region and enables adjacent channel spectrum coexistence with heterogeneous transmitters and receivers of different qualities.more » « less
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100 Gb/s NRZ-OOK transmission over 14 km standard single mode fiber in the C-band is demonstrated with a simple intensity modulation and direct detection scheme. The transmission concept utilizes single sideband modulation and comprises a single differential digital-to-analog converter with adjustable phase offset, a new dual electrode plasmonic Mach-Zehnder modulator, a laser at 1537.5 nm, standard single mode fibers, a photodiode, an analog-to-digital converter, and linear offline digital signal processing. The presented SSB concept requires no DSP and complex signaling at the transmitter. The demonstrated SSB transmitter increased the possible transmission distance by a factor of 4.6 compared to a DSB transmitter. We also investigated the equalization requirements. A T/2-spaced feedforward equalizer requires 27 taps to achieve transmission over 10 km with a BER below the HD-FEC limit. In comparison to a DSB transmitter, the SSB transmitter reduced the receiver DSP complexity by a factor of 13.7.
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Accepted Manuscript1.0
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