Frequency Domain Decision Feedback Equalization Research Articles

Doubly-spread channel equalization based on channel shortening in mobile underwater acoustic communications

The shallow water environments and the movement of autonomous underwater vehicles contribute to the distortion of acoustic signals in both the delay and Doppler domains. Numerous studies have focused on eliminating the distortion in underwater acoustic communications but have struggled to find an optimal trade-off between performance and computational complexity, rendering them impractical for marine engineering applications. In our research, we propose a channel-shortening equalization scheme based on frequency-domain decision feedback equalization (FD-DFE) to address this issue. This approach leverages FD-DFE to reduce the number of multipaths and decreases the Doppler spread of the channel. A time-domain decision feedback equalization based on recursive least squares (RLS-DFE) is then employed to eliminate residual delay and Doppler spreads. The mismatch between the block- and symbol-wise equalization is minimized by an overlapping subblock structure. Numerical simulations have demonstrated the effectiveness of this scheme. Error-free transmissions with a data rate of up to 6 kbps were achieved in a reservoir experiment between an autonomous underwater helicopter (AUH) and a shore-base receiving node.

Practical Frequency-Domain Decision Feedback Equalization Based on Expectation Propagation

Practical Frequency-Domain Decision Feedback Equalization Based on Expectation Propagation

Frequency-Domain Decision Feedback Equalization for Single-Carrier Transmissions in Fast Time-Varying Underwater Acoustic Channels

Frequency-domain equalization (FDE) is a computationally efficient method to recover underwater acoustic single-carrier transmissions in a multipath environment. The FDE receivers often operate in a block-by-block manner, assuming that the channel is time invariant during a block duration. This assumption implies that traditional block-based FDE receivers experience performance degradation in fast time-varying channel conditions, since the interfrequency interference (IFI) is often not considered. Here, we focus on the IFI cancelation (IFIC) in single-carrier transmissions. The IFI formulations are developed for a blockwise FDE receiver, where the time reversal (TR) processing is incorporated to alleviate the interblock interference. Based on the formulations, the IFI is estimated by the time-varying impulse responses, which are obtained from an adaptive basis expansion model. A soft two-step IFIC strategy is then employed to address the IFI. In the first step, the main IFI is removed by the time-varying TR processing and explicit IFI cancelation. The residual IFI is suppressed by the frequency-domain decision feedback equalization (FD-DFE) in the second step. To further enhance the communication performance, we develop a soft-symbol-based iterative receiver, referred to as IFIC-FD-DFE. The impulse responses, explicit IFI, and residual IFI are updated in an iterative fashion, based on the soft symbol estimates. The proposed receiver has been tested using the at-sea measurements collected at the Gulf of Mexico in August 2017. Communication packets at four ranges at the carrier frequency of 160 kHz with a symbol rate of 32 kHz have been decoded. The effectiveness of the IFIC-FD-DFE receiver has been demonstrated via comparisons with several FDE schemes in the current literature.

An Improved Iterative Frequency Domain Decision Feedback Equalization for STBC-SC-FDE

This article proposes an improved iterative frequency domain decision feedback equalization(IBDFE) for the unique word(UW) based system of space-time block coded-single carrier frequency domain equalization(STBC-SC-FDE). Compared with the conventional IBDFE, this algorithm uses minimum mean square error(MMSE) equalization and a noise prediction module based on UW instead of the feedback equalization part, while maintain the feedforward equalization. It is shown from the simulation results that the improved IBDFE can achieve satisfactory performance without iterations.

An Improved FD-DFE Structure for Downlink VLC Systems Based on SC-FDMA

Visible light communication (VLC) technology is considered to be an attractive solution for future wireless indoor multimedia communication. To reduce the peak-to-average power ratio, single-carrier frequency division multiple access (SC-FDMA) with time domain asymmetrical clipping is applied for the downlink multi-user VLC system. An improved frequency-domain decision feedback equalizer (FD-DFE) structure is proposed for the SC-FDMA -based VLC system in this letter. Different kinds of subcarrier distributions are considered in the analyses and simulations. Thanks to the clipping noise information on the even subcarriers, which can reduce the decision error, the proposed algorithm provides a lower bit error rate and better transmission performance compared with traditional schemes.

Linear Precoded THP-FDE for Single-Carrier Broadband MIMO Systems

Tomlinson–Harashima precoding with frequency-domain equalization (THP-FDE) is a promising approach for multiple-input multiple-output (MIMO) single-carrier broadband systems. By performing interference cancelation at the transmitter, THP-FDE performs better than frequency-domain decision-feedback equalization (FD-DFE) via eliminating the error propagation problem resulting from incorrect symbol decisions. One of the drawbacks for MIMO THP-FDE systems is that the spatial stream with the largest mean square error (MSE) dominates the error performance. To alleviate this problem, conventional design adopts a sorting algorithm (a permutation matrix) to minimize the worst-case stream MSE. Since the difference of stream MSEs roots in the nonequal diagonal entries of the MSE matrix in the time domain, we are motivated to apply a general linear precoder based on geometric-mean decomposition to balance these MSEs. By jointly optimizing the precoder and equalizer, we are able to minimize the arithmetic MSE, geometric MSE, and the worst stream MSE simultaneously for the MIMO THP-FDE system. Simulation results are presented to demonstrate the superior bit-error-rate (BER) performance of the proposed transceiver architecture for MIMO THP-FDE systems.

Comparative Performance of Complex-Valued B-Spline and Polynomial Models Applied to Iterative Frequency-Domain Decision Feedback Equalization of Hammerstein Channels.

Complex-valued (CV) B-spline neural network approach offers a highly effective means for identifying and inverting practical Hammerstein systems. Compared with its conventional CV polynomial-based counterpart, a CV B-spline neural network has superior performance in identifying and inverting CV Hammerstein systems, while imposing a similar complexity. This paper reviews the optimality of the CV B-spline neural network approach. Advantages of B-spline neural network approach as compared with the polynomial based modeling approach are extensively discussed, and the effectiveness of the CV neural network-based approach is demonstrated in a real-world application. More specifically, we evaluate the comparative performance of the CV B-spline and polynomial-based approaches for the nonlinear iterative frequency-domain decision feedback equalization (NIFDDFE) of single-carrier Hammerstein channels. Our results confirm the superior performance of the CV B-spline-based NIFDDFE over its CV polynomial-based counterpart.

CFO mitigation for uplink SFBC SC-FDMA

To mitigate inter-carrier interference due to large carrier frequency offset (CFO) in an uplink single-carrier frequency division multiple access (SC-FDMA) system, a three-tap adaptive frequency-domain decision feedback equalizer (AFD-DFE) is designed in this paper. Our design exploits the banded and sparse structure of the equivalent channel matrix. The block recursive least squares (RLS) algorithm is used to adapt the AFD-DFE. Consequently, by exploiting the matrix structure in the frequency-domain, the complexity of the block RLS is reduced substantially when compared to its time-domain counterpart. In addition, the design is extended to space-frequency block coded (SFBC) SC-FDMA systems. We show that our proposed AFD-DFE exhibits significant performance improvement when compared to a one-tap AFD-DFE while still enjoying a low computational complexity.

AFD-DFE using Constraint-Based RLS and Phase Noise Compensation for Uplink SC-FDMA

In this paper, we develop a constraint-based block recursive least-squares (CRLS) for an adaptive frequency-domain decision feedback equalizer (AFD-DFE) in uplink single-carrier frequency division multiple access systems. For the AFD-DFE, both the feedforward and feedback filters are implemented in the frequency domain; therefore, the CRLS complexity can be reduced substantially when compared to its time-domain counterpart by exploiting the matrix structure in the frequency domain. The performance of the CRLS algorithm is better than that of the RLS when applied to the AFD-DFE, with no increase in the computational complexity. Our designed AFD-DFE with CRLS not only enjoys a lower computational complexity when compared to the frequency-domain channel-estimate-based minimum mean square error DFE (MMSE DFE), but its performance is also better than that of the MMSE DFE with decision errors (practical case) and is close to the MMSE DFE with correct decisions (ideal case). Moreover, we iteratively compensate the transmitter and receiver phase noise using its properties in the time and frequency domains. Simulation results demonstrate the robustness of our designed AFD-DFE using CRLS.

High spectral efficient W-band optical/wireless system employing Single-Sideband Single-Carrier Modulation

With broader available bandwidth, W-band wireless transmission has attracted a lot of interests for future Giga-bit communication. In this article, we experimentally demonstrate W-band radio-over-fiber (RoF) system employing single-sideband single-carrier (SSB-SC) modulation with lower peak-to-average-power ratio (PAPR) than orthogonal frequency division multiplex (OFDM). To overcome the inter-symbol interference (ISI) of the penalty from uneven frequency response and SSB-SC modulation, frequency domain equalizer (FDE) and decision feedback equalizer (DFE) are implemented. We discuss the maximum available bandwidth of different modulation formats between SSB-SC and OFDM signals at the BER below forward error correction (FEC) threshold (3.8 × 10(-3)). Up to 50-Gbps 32-QAM SSB-SC signals with spectral efficiency of 5 bit/s/Hz can be achieved.

Single-Carrier Space Time Transmission Diversity with Decision Feedback Equalization

In order to suppress inter-symbol interference (ISI), the Frequency domain decision feedback equalization (FD-DFE) algorithm is proposed in this paper for single carrier frequency domain equalization (SC-FDE) combined with Alamouti space time block coding (STBC) systems over frequency-selective channels. We use the cyclic prefix (CP) and two transmit antennas to achieve the significant diversity gain. The feedback equalization part is designed by Mean Square Error (MSE) criterion and the coefficients of filters are all in the frequency domain for the given channel frequency response. The outage and diversity analysis of system is also made to measure the equalization performance. Finally, the simulation results show that the proposed algorithm has better performance than the conventional liner equalization (LE) and hybrid structure decision feedback equalization (HDFE) schemes, and the computational complexity of our method is acceptable.

Frequency Domain DFE for Single-Carrier STBC Block Transmission

Space-time block coding (STBC) is a novel transmit diversity technique that can substantially improve the reliability of wireless communication channel. In this letter, we investigate single-carrier STBC block transmission over frequency-selective fading channel. Based on the minimum mean square error (MMSE) criterion, by exploiting the reliability of tentative decision symbols, we propose a novel suboptimal, low complexity frequency domain decision feedback equalization (FD-DFE) scheme for SC STBC. Simulation results show that the proposed scheme provides better performance than the conventional scheme.

Iterative Frequency Domain Decision Feedback Equalization and Decoding for Underwater Acoustic Communications

Iterative Frequency Domain Decision Feedback Equalization and Decoding for Underwater Acoustic Communications

Iterative Frequency Domain Decision Feedback Equalization and Decoding for Underwater Acoustic Communications

Single-carrier (SC) transmission with frequency-domain equalization (FDE) is today recognized as an attractive alternative to orthogonal frequency-division multiplexing (OFDM) for communication application with the inter-symbol interference (ISI) caused by multi-path propagation, especially in shallow water channel. In this paper, we investigate an iterative receiver based on minimum mean square error (MMSE) decision feedback equalizer (DFE) with symbol rate and fractional rate samplings in the frequency domain (FD) and serially concatenated trellis coded modulation (SCTCM) decoder. Based on sound speed profiles (SSP) measured in the lake and finite-element ray tracking (Bellhop) method, the shallow water channel is constructed to evaluate the performance of the proposed iterative receiver. Performance results show that the proposed iterative receiver can significantly improve the performance and obtain better data transmission than FD linear and adaptive decision feedback equalizers, especially in adopting fractional rate sampling.

Frequency domain decision feedback equalizer for time-reversal space-time block coding

In this paper, a frequency domain decision feedback equalizer is proposed for single carrier transmission with time-reversal space-time block coding (TR-STBC). It is shown that the diagonal decision feedback equalizer matrix can be calculated from the frequency domain channel response. Under the perfect feedback assumption, the proposed equalizer can approach matched filter bound (MFB). Compared with the existing time domain decision feedback equalizer, the proposed equalizer exhibits better performance with the same equalization complexity.

Partial PIC-MRC Receiver Design for Single Carrier Block Transmission System over Multipath Fading Channels

Single carrier block transmission (SCBT) system has become one of the most popular modulation systems due to its low peak-to-average power ratio (PAPR), and it is gradually considered to be used for uplink wireless communication systems. In this paper, a low complexity partial parallel interference cancellation (PIC) with maximum ratio combining (MRC) technology is proposed to use for receiver to combat the intersymbol interference (ISI) problem over multipath fading channel. With the aid of MRC scheme, the proposed partial PIC technique can effectively perform the interference cancellation and acquire the benefit of time diversity gain. Finally, the proposed system can be extended to use for multiple antenna systems to provide excellent performance. Simulation results reveal that the proposed low complexity partial PIC-MRC SIMO system can provide robust performance and outperform the conventional PIC and the iterative frequency domain decision feedback equalizer (FD-DFE) systems over multipath fading channel environment.

Cyclic-delay time-reversal space-time block codes for single-carrier transmission with frequency-domain decision-feedback equalization

Transmit diversity is an important multiple-input multiple-output (MIMO) approach to improve the transmission reliability; however, it is usually designed with maximum-likelihood detection. In this paper, we investigate a simple transceiver structure for single-carrier MIMO systems, in which the cyclic-delay time-reversal space-time block code (CDTR-STBC) is employed at the transmitter, and the frequency-domain decision-feedback equalization (FD-DFE) is used at the receiver. We separate the transmit antennas into two groups, and then, encode the groups by STBC and cyclically delay the data streams within each group. Such processing can mitigate the impact of error-propagation and reduce the signal-to-noise ratio (SNR) loss when FD-DFE is employed, in which SNR is defined as the ratio of the received signal power over the noise power at each receive antenna. To support our proposal, this paper also provides the performance analysis of FD-DFE. It is shown that the proposed scheme can achieve full diversity and full transmission rate for a MIMO system employing arbitrary antennas. Therefore, it has considerable performance improvement over the existing schemes.

Low-Complexity Iterative Frequency Domain Decision Feedback Equalization

Single-carrier transmission with frequency-domain equalization (SC-FDE) offers a viable design alternative to the classic orthogonal frequency-division multiplexing (OFDM) technique. However, SC-FDE using a linear equalizer may suffer from serious performance deterioration for transmission over severely frequency-selective fading channels. An effective method of solving this problem is to introduce nonlinear decision feedback equalization (DFE) to SC-FDE. In this paper, a low-complexity iterative DFE operating in the frequency domain of single-carrier systems is proposed. Based on the minimum-mean-square-error (MMSE) criterion, a simplified parameter estimation method is introduced to calculate the coefficients of the feedforward and feedback filters, which significantly reduces the implementation complexity of the equalizer. Simulation results show that the performance of the proposed simplified design is similar to the traditional iterative block DFE under various multipath fading channels, but it imposes a much lower complexity than the latter.

Low Complex Decision-Feedback Equalization for Time-Reversal Quasi-Orthogonal Space-Time Block Codes

In this paper, we design a practical time-reversal quasi-orthogonal space-time block code (TR-QO-STBC) system for broadband multi-input multi-output (MIMO) communications. We first modify the TR-QO-STBC encoding structure so that the interference between the transmitted blocks can be completely removed by linear processing. Two low complex decision-feedback equalization (DFE) schemes are then proposed. One is built from the frequency-domain decision-feedback equalization (FD-DFE). The derived bi-directive FD-DFE (BiD-FD-DFE) cancels the interference among the successive symbols along the time axis. The other one is the enhanced V-BLAST, which cancels the interference between the real and imaginary parts of the spectral components. They have distinct performance characteristics due to the different interference-cancellation strategies. The underlying orthogonal and symmetric characters of TR-QO-STBC are exploited to reduce the computational complexity. Computer simulations confirm that the proposed equalizers can achieve better performance than the existing schemes.

Multiple-Input Multiple-Output Overlap-Save Frequency-Domain Decision-Feedback Equalization for Single-Carrier Systems in Time-Varying Environments

In this letter, we propose a novel frequency-domain equalization (FDE) scheme for single-carrier multiple-input multiple-output (MIMO) systems over time-varying channels. Based on frequency-domain decision-feedback equalization (FD-DFE), we design a feedforward filter with constraint such that the equalization can be easily realized segment-by-segment with the help of the overlap-save (OLS) method. Since the segment length and block length can be designed independently, our proposal sets relatively short segment length to obtain good performance in time-varying environments, and very long block length to achieve high spectral efficiency. Furthermore, we present two scenarios in the design of filters for MIMO systems.