Single Carrier Frequency Domain Equalization Research Articles

An Improved Orthogonal Matching Pursuit Algorithm for CS-Based Channel Estimation

Wireless broadband transmission channels usually have time-domain-sparse properties, and the reconstruction of these channels using a greedy search-based orthogonal matching pursuit (OMP) algorithm can effectively improve channel estimation performance while decreasing the length of the reference signal. In this research, the improved OMP and SOMP algorithms for compressed-sensing (CS)-based channel estimation are proposed for single-carrier frequency domain equalization (SC-FDE) systems, which, in comparison with conventional algorithms, calculate the path gain after obtaining the path delay and updating the observation matrices. The reliability of the communication system is further enhanced because the channel path gain is calculated using longer observation vectors, which lowers the Cramér–Rao lower bound (CRLB) and results in better channel estimation performance. The developed method can also be applied to time-domain-synchronous OFDM (TDS-OFDM) systems, and it is applicable to the improvement of other matching pursuit algorithms.

Robust equalization for single-carrier underwater acoustic communication in sparse impulsive interference environment

Impulse noise significantly distorts the background noise within communication systems, causing a notable departure from the Gaussian distribution. This distortion, in turn, exerts a detrimental influence on the performance of conventional single-carrier frequency domain equalization techniques. In this paper, we introduce a novel algorithm for joint channel, interference, and symbol estimation within single-carrier systems. This algorithm is grounded in the framework of variational Bayesian inference and possesses the unique capability of simultaneously accounting for the sparsity inherent in the channel and impulse interference. In the course of processing, it leverages multi-parameter probability distributions to effectively model the intricate posteriori probabilities associated with impulse interference vectors and channel vectors. This, in turn, facilitates the computation of maximum a posteriori estimates and subsequently enables the estimation and elimination of interference. We validate the efficacy of our proposed method through both numerical simulations and the processing of experimental data. Our results unequivocally demonstrate the robustness and manageable computational complexity of our approach within the context of impulse interference-prone environments.

An iterative subblock-based receiver for SC-FDE systems in time-varying underwater acoustic channels

Traditional single-carrier frequency domain equalization (SC-FDE) systems assume time-invariant channels that are difficult to realize in underwater acoustic (UWA) communication over block-based durations, leading to inter-frequency interference (IFI). Currently available approaches to resolving this issue involve shortening the block or inserting frequent training sequences at the cost of additional information from the channel and reduced spectral efficiency. This paper proposes an iterative subblock-based receiver algorithm that divides each time-varying block into multiple quasi-static subblocks. The designed receiver has a two-step structure consisting of block processing and cascaded subblock processing, where the former generates prior symbolic knowledge and the latter iteratively weakens IFI. We consider smoothly variant channels between adjacent subblocks and track them using a Kalman filter, leading to a more accurate estimation of the IFI and better equalization. The proposed algorithm was tested through simulations and experiments in Qiandao Lake and the Yellow Sea using blocks of different lengths and numbers of subblocks and elements in the array. The results provide a low bit error rate.

Amplify-and-Forward (AF) Relay Techniques in Wireless SC-FDE Systems to Enhance Diversity Gains

This paper introduces an amplify-and-forward (AF) relaying technique that employs phase dithering and intentional delay within single carrier-frequency domain equalizer (SC-FDE) systems. The proposed relaying technique aims to increase the gain of both frequency diversity and time diversity in slow fading channels. To achieve this, the proposed technique introduces random phase rotation and random intentional delay. The relaying scenario assumes two-hop communication with relaying between the source and destination. It is assumed that many nodes are densely distributed, allowing for many relay nodes to participate in relaying. To verify the performance through computer simulation, the number of relays is 1, 2, 3, 5, 15, and 25, and three modulation coding schemes (MCSs) are considered: QPSK with R=1/3, 16QAM with R=3/4, and 8PSK with R=7/8. The simulation results demonstrate that the proposed relaying technique improves the bit error ratio (BER) performance. The performance improves as the number of relays increases.

Fractionally spaced frequency domain equalization system based on FQPSK modulation

Severe multi-path effects in the process of high-speed data transmission cause inter-symbol interference (ISI). The single carrier frequency domain equalization (SC-FDE) method combining the benefits of Feher-patented quadrature phase shift keying (FQPSK) modulation is raised to solve this problem, including the constant envelope, high spectrum efficiency, and insensitivity to nonlinear distortion. Furthermore, a fractionally spaced frequency domain equalization (FS-FDE) system based on FQPSK modulation is modeled based on the proposed algorithm. Simulation results confirm the superior performance of the proposed model on erasing ISI, comparing to the symbol-spaced FDE (SS-FDE) system based on FQPSK modulation and the FS-FDE system based on QPSK modulation.

Beyond 300-Gbps/λ photonics-aided THz-over-fiber transmission employing MIMO single-carrier frequency-domain equalizer.

We experimentally realized a 320-GHz 320-Gbps/λ terahertz (THz) radio-over-fiber (RoF) system based on a photonics-aided scheme with the help of polarization-division multiplexing (PDM) technology and multiple-input, multiple-output (MIMO) transmission. In this system, the low-complexity MIMO single-carrier frequency-domain equalizer (SCFDE) is implemented to compensate for the polarization-related impairments of the PDM signal, and the demultiplexing performances between SCFDE and the commonly used constant modulus algorithm (CMA) are also compared in this proposed system. After 20-km standard single-mode fiber (SSMF) and 3-m 2 × 2 MIMO wireless link transmission, the bit error rate (BER) of the received 46-GBaud PDM 16-ary quadrature amplitude modulation (16QAM) signal satisfies the soft-decision forward error correction (SD-FEC) threshold with 15% overhead, which corresponds to a record-breaking net bit rate of 320 Gbit/s.

Estimation of IQI for AF Cooperative Single-Carrier Frequency Domain Equalization Systems Using Channel Decoder Feedback

The process of amplify-and-forward (AF) relaying is essential to the improvement of both current and future wireless communication standards. Nevertheless, significant performance loss may be posed by in-phase and quadrature imbalance (IQI) caused by defects in radio frequency components. Prior studies into this research problem were restricted to uncoded broadcasts, even though error-correcting codes are frequently used in real applications. To this purpose, we develop a novel approach applicable to the destination terminal for estimating and compensating for IQI that occurs at the source, relay, and destination terminals. The proposed approach is explored in the context of coded emissions of AF single-carrier frequency domain equalization (SC-FDE) systems. In contrast to other methods for mitigating this radio frequency deflection at each node, the proposed system estimates and compensates for all IQI parameters and channel impulse responses simultaneously. With the use of an iterative expectation–maximization (EM) process, a maximum-likelihood (ML) solution to the problem is computed. At each round, the soft information supplied by the channel decoder is employed to create the a posteriori expectations of the sent data symbols, which are then fed into the estimation process as if they were training symbols. In addition, we address how to use the estimated parameters to perform the task of data detection. The offered predictor and detector exchange soft information in a sequential process, boosting the overall system effectiveness. The simulation results show that the proposed method is not only practicable but superior to the established methods.

The research on channel estimation and signal‐noise ratio estimation based on minimum error entropy kalman filter for single carrier frequency domain equalization system

SummaryIn this paper, the channel estimation and signal‐to‐noise ratio (SNR) estimation technique of single‐carrier frequency domain equalization (SC‐FDE) system under low SNR in aeronautical multipath channel are studied, a SNR estimation algorithm which is easy to implement in engineering and an improved LS channel estimation algorithm based on Kalman filter using minimum error entropy (MEE‐KF) are proposed. This paper first introduces the SC‐FDE system and introduces the principle of MEE‐KF, and then, the channel estimation flow based on MEE‐KF is obtained by combining it with the traditional LS channel estimation algorithm, which makes the estimation results perform better. Simulation results show that after getting more accurate noise variance, the channel estimation results can better follow the changes of the channel after MEE‐KF processing, so as to resist the doppler frequency offset effect and make the channel estimation results more accurate, that is the channel response results of the data part can be closer to the real situation, so that the communication performance of SC‐FDE system has also been greatly improved.

Asynchronous NOMA Downlink Based on Single-Carrier Frequency-Domain Equalization

Asynchronous NOMA Downlink Based on Single-Carrier Frequency-Domain Equalization

Analysis of GFDM in generalized [formula omitted] fading channel: A simple probability density function approach for beyond 5G wireless applications

Generalized frequency division multiplexing (GFDM) is a flexible radio waveform which offers high degree of freedom in varying the number of time slots, number of subcarriers, and pulse shaping filters. In addition to this, it also covers orthogonal frequency division multiplexing (OFDM) and single carrier frequency domain equalization (SC-FDE) as corner cases. A well-known fact is that the performance of a wireless communication system is predominately effected by the characteristics of the fading environment. In this paper, symbol error rate (SER) closed-form expression of GFDM system under the generalized η−μ fading channel is derived. The proposed derivation includes Rayleigh, Nakagami-m, and Nakagami-q fading channels as special cases. A Mote-Carlo simulation test-bed is carried out using MATLAB to compare with analytical results and to validate the derived expressions based on the probability density function (PDF) approach. Further, this paper investigates the performance of the GFDM system by varying various parameters such as fading values, pulse shaping filters, roll-off factor, and modulation order. The detailed analysis suggests that the probability of SER is highly dependent on the above parameters. The results obtained are more useful in evaluating the performance of GFDM based communication system in a generalized manner.

Prefiltered Single-Carrier Frequency-Domain Equalization for Binary CPM over Shallow Water Acoustic Channel

The continuous phase modulation (CPM) technique is an excellent solution for underwater acoustic (UWA) channels with limited bandwidth and high propagation attenuation. However, the severe intersymbol interference is a big problem for the algorithm applying in shallow water. To solve this problem, an algorithm for prefiltered single-carrier frequency-domain equalization (PF-SCFDE) is presented in this paper. The regular whitening filter is replaced by a prefilter in the proposed algorithm. The output information sequence of this prefilter contains the forward information. To improve the performance, the output of the equalizer, combined with the forward information, is used to make the maximum likelihood estimation. The simulation results with minimum-shift keying and Gaussian-filtered minimum-shift keying signals over shallow water acoustic channels with low root mean square delay spread demonstrate that PF-SCFDE outperformed the traditional single-carrier frequency-domain equalization (SCFDE) by approximately 1 dB under a bit error rate (BER) of 10−4. A shallow sea trial has demonstrated the effectiveness of PF-SCFDE; PF-SCFDE had a reduction in BER of 18.35% as compared to the traditional SCFDE.

Low Complexity Single Carrier Frequency Domain Detectors for Internet of Underwater Things (IoUT)s

This paper proposes low complexity detection for internet of underwater things communication. The signal is transmitted from the source to the destination using several sensors. To simplify the computational operations at the transmitter and the sensory nodes, a single carrier frequency domain equalizer is proposed and amplify-and-forward protocols are employed. Fast Fourier transform and use of cyclic prefix are also proposed to simplify these algorithms when compared to time-domain equalization. As precise channel data is difficult to capture in underwater communications, the adaptive implementation of FDE is proposed as a solution that can be employed when the channel experiences a fast doppler shift. The two adaptive detectors are based on the least mean-square and recursive least square principles. Numerical simulations show that the performance of the bit error rate performance of the proposed detectors is close to that of the ideal minimum mean square error.

Adaptation of Code-Domain NOMA to SC-FDE Based Overloaded mmWave Hybrid Massive MIMO

In this letter, we provide a practical framework to resolve whether code-domain NOMA (CD-NOMA) is beneficial when integrated with massive MIMO systems. In order to realize this integration, first, we develop a novel code-beamspace wideband signal model for uplink CD-NOMA in mmWave hybrid massive MIMO systems employing single-carrier (SC) transmission. Then, we apply a state-of-the-art SC frequency domain equalization (SC-FDE) based iterative receiver where the number of radio frequency (RF) chains is limited. Simulation results verify the effectiveness of the proposed architecture in overloaded scenarios. Furthermore, we show that CD-NOMA can enhance the performance of mmWave hybrid beamforming based massive MIMO systems by effectively decreasing the correlation between closely separated user channels in joint code-beamspace.

Joint estimation and detection method based on turbo equalization framework and VAMP

In this letter, we consider the single-carrier frequency domain equalization (SC-FDE) system, and propose a low-complexity joint symbol detection and channel estimation algorithm based on the recently proposed vector approximate message passing (VAMP). Specifically, we leverage VAMP twice to estimate symbols and channels, respectively, in a turbo-like way. Moreover, this algorithm organically combines the gaussian mixture model (GMM), which can accurately simulate the sparse aggregation characteristics of the channel and effectively suppress inter symbol interference (ISI). The simulation results show that compared with the traditional linear minimum mean square error (LMMSE) estimation receiving algorithm and the existing generalized approximate message passing algorithm (GAMP), the designed receiving algorithm has significant advantages in channel estimation normalized mean square error (NMSE) and bit error ratio (BER) performance, where sharing the same order of complexity.

Dictionary-Learning (DL)-Based Sparse CSI Estimation in Multiuser Terahertz (THz) Hybrid MIMO Systems Under Hardware Impairments and Beam-Squint Effect

This work conceives dictionary-learning (DL)-based sparse channel estimation schemes for multi-user Terahertz (THz) hybrid MIMO systems incorporating also non-idealities such as hardware impairments and beam-squint effect. Due to the presence of large antenna arrays coupled with frequency selectivity, beam squint effect is significant in THz systems. Moreover, the manufacturing and calibration errors that inevitably arise during the production of antenna arrays result in hardware impairments such as irregular antenna spacing, mutual coupling and antenna gain/phase errors in practical THz systems. To overcome these problems, this work proposes a DL algorithm to determine the best sparsifying dictionary from the acquired observations for a single-carrier frequency domain equalization (SC-FDE)-based wideband THz system in the presence of hardware impairments as well as the beam squint effect. The dictionary thus obtained is subsequently employed to exploit the sparsity of the MIMO THz channel toward CSI estimation. Furthermore, the Cramér-Rao lower bound (CRLB) is also derived for the joint DL and CSI estimation algorithm, which acts as a benchmark for the mean-squared error (MSE) performance of the channel estimate obtained. The scheme is also extended to SC-FDE-based wideband THz MIMO systems with multiple antenna users. Simulation results are presented to corroborate our analytical findings and also demonstrate the improved performance with respect to the agnostic scheme that ignores the non-idealities.

Iterative Equalization of STSK-Aided Single-Carrier Systems: Design and Analysis With Non-Ideal Power Amplifiers

A plethora of new applications will be enabled by next generation wireless networks. These demand sophisticated medium access control and physical layer protocols in order to fulfill the requirements in terms of spectral efficiency, bit error rate (BER), energy efficiency, reliability, and security, among others. This paper develops an advanced space-time multiple-input, multiple-output (MIMO) solution that builds on the concept of space-time shift keying (STSK) for single-carrier frequency domain equalization (SC-FDE). STSK-aided SC-FDE, or SSF, ensures space-time and frequency diversity in dispersive channels, but can only support suboptimal linear receiver schemes. We therefore introduce a turbo equalizer for SSF in frequency-selective multipath channels. It enables inter-symbol-interference cancellation by means of iterative space-time processing. We coin it the iterative space-time block equalizer (SSF-ISTBE). We provide the mathematical formulation for the proposed SSF-ISTBE framework in non-linear channel conditions. We demonstrate that the SSF-ISTBE achieves a substantial 4 dB gain in terms of BER over the linear minimum mean square error equalizer while significantly reducing the computational complexity of the optimal maximum likelihood SSF. When considering practical transceivers, in particular non-linear power amplifiers, the gain of the SSF-ISTBE becomes more prominent over state-of-the-art approaches, demonstrating its robustness against non-linear distortion.

Increasing Reliability on UAV Fading Scenarios

Unmanned aerial vehicles (UAVs) are the next technology to be incorporated into a telecommunications network to improve command and control on a large scale in both line-of-sight (LOS) and non-line-of-sight (NLOS) conditions. However, there is still room for improvement in terms of reliability. This paper investigates Constant Packet Combining (CPC) and Adaptive Packet Combining (APC) techniques applied to Unmanned Aerial Vehicle (UAV) communication in the presence of large-scale fading, where the channels are subject to sudden degradation for long periods due to obstructions. We use Single Carrier (SC) Frequency Domain Equalization (FDE) combined with the Iterative Block Decision-Feedback Equalizer (IB-DFE) to handle command and control messages mapped for UAV use cases. We present closed-form equations for the equalization design as well as the performance parameters such as Bit Error Rate (BER), the Packet Error Rate (PER), the throughput, the retransmissions amount, the goodput (the transmission rate without the retransmissions quantity), and the outage probability. Then, we analyze the system performance using correlated, independent, and equal channels. There is a trade-off between the overall available power, throughput, and reliability. For instance, more retransmissions result in higher reliability, power consumption and lower goodputs (effective data rates). CPC validates the transmission system and confirms the improvement of BER and PER parameters without energy efficiency optimization. APC is appealing because it can reduce the number of retransmissions for all channels used with the advantage of meeting energy efficiency requirements by adapting the overall power to the scenario experienced by the UAV.

Prototyping of 40 GHz Band Orbital Angular Momentum Multiplexing System and Evaluation of Field Wireless Transmission Experiments

We are developing a spatial multiplexing technique based on orbital angular momentum (OAM) that is capable of 1 Tbit/s point-to-point wireless transmissions for the sixth generation mobile communication system. In this paper, we describe a demonstration of 100 Gbit/s wireless transmission over a range of 100 m that used OAM multiplexing of 15 streams with a 1.5 GHz bandwidth (39-41.5 GHz). The OAM modes of this system were generated using a Butler matrix that allows discrete Fourier transform (DFT) operations to be performed in analog circuits. We designed 8×8 Butler matrices to generate OAM modes by combining hybrid couplers and phase shifters. Since this Butler matrix was connected to 16 element antennas, two 8×8 Butler matrices were connected to make an 8×16 matrix. Furthermore, since these inputs were in 7 OAM mode, one port was terminated to create a 7×16 Butler matrix. It was confirmed that the mode isolation was more than 15 dB in the 1.5 GHz bandwidth. Next, we designed microstrip antennas for a horizontal and vertical polarization uniform circular array (UCA) to radiate the OAM modes. Then, we implemented radio frequency (RF) chains and digital signal processing, including single carrier-frequency domain equalization and adaptive modulation and coding. A transmission experiment conducted in a field line-of-sight environment showed that the system could transmit at 119.45 Gbit/s at a distance of 100 meters, thereby demonstrating the feasibility of wideband OAM transmission in the millimeter-wave band.

SC-FDE Layer for Sensor Networks in Remote Areas Using NVIS Communications

Despite high costs and lengthy deployments, satellite communications have traditionally been used to provide coverage in remote areas. However, given the fact that there is no radio infrastructure available in these areas, Near Vertical Incidence Skywave (NVIS) technology has positioned itself as an attractive alternative to communicate with low-power nodes in remote areas. This type of communication works in the HF frequency range complying with STANAG and MIL-STD standards, which define a physical layer for scenarios that differ from NVIS and low-power communication. The purpose of this paper was to present the definition of a new communication physical layer based on single-carrier frequency-domain equalization (SC-FDE) based on these standards but adapted to the ionospheric communication channel. This physical layer was compared to an OFDM-based layer from a previous study. The experiments performed show that this new approach achieves better results than OFDM in terms of a higher signal quality with a higher specific BER probability. Finally, this layer was also used in the theoretical design of an NVIS gateway to link sensor network devices spanning large-scale remote areas in a secure manner in the context of ubiquitous sensor networks (USN).

Channel Estimation and Equalization for Terahertz Receiver With RF Impairments

The radio frequency (RF) impairments of analog devices have been regarded as an important factor degrading the performance of Terahertz (THz) communications, where in-phase/quadrature (IQ) imbalance and phase noise (PN) are the two typical RF impairments at THz transceiver. In this paper, we investigate the channel estimation (CE) and equalization for THz receiver in the presence of wideband IQ imbalance and PN, where single-carrier frequency-domain equalization is used. Since PN is inserted between channel impulse response (CIR) and wideband IQ imbalance at the receiver, the CIR and IQ imbalance cannot be treated together as an effective channel. Therefore, a novel two-stage CE method is derived to separately estimate the CIR and wideband IQ imbalance parameters, and its corresponding channel equalization method is designed to compensate both wideband IQ imbalance and PN, and equalizes the CIR. Theoretical and simulation results validate the efficiency of our proposed CE, and the proposed equalization method is shown to outperform the state-of-the-art methods.