One-tap Equalizer Research Articles

Comparing the Performance of OFDM and OCDM-based Visible Light Communications: Numerical and Experimental Analysis

Numerical and experimental analysis of visible light communication (VLC) systems based on orthogonal chirp division multiplexing (OCDM) and orthogonal frequency division multiplexing (OFDM) are presented in this paper. Unlike most of the related publications, in this work we numerically compared OCDM and OFDM based VLC systems with the same equalization process, i.e., employing an one tap equalizer (OTE) in both systems. Simulation results in line-of-sight VLC channels show that both systems have the same performance when baseband multicarrier signals are used. The impact of the optical modulation index and the bias current of a previously characterized light-emitting diode were also evaluated. Nevertheless, the experimental demonstrations, in which bandpass signals were used due to the bandpass characteristic of the exploited setup, show that OFDM-based system outperforms the VLC links with OCDM signals, when the OTE equalizer is used in both systems. The performance of the OCDM-based system was improved by using a zero forcing frequency domain equalizer.

Performance evaluation of a simplified power-domain NOMA for visible light communications.

The performance of a visible light communication (VLC) system based on power-domain nonorthogonal multiple access (PD-NOMA) is experimentally evaluated in this paper. The simplicity of the adopted nonorthogonal scheme is provided by the fixed power allocation method at the transmitter and the single one-tap equalization executed before the successive interference cancellation at the receiver. The experimental results proved the successful transmission of the PD-NOMA scheme with three users in VLC links of up to 2.5m, after a proper choice of the optical modulation index. All users achieved error vector magnitude (EVM) performances below forward error correction limits in all evaluated transmission distances. At 2.5m, the user with the best performance reaches an E V M=2.3%.

WITHDRAWN: Reduction of FBMC/OQAM imaginary intrusion compared to OFDM

In comparison to the OFDM (Orthogonal Frequency Division), Filter Bank multicarrying function (FBMC) has superior spectral properties, which complicates operational multiple input and mixed outputs. By extending a symbol in time, we can totally remove the imaginary interference. This gives the ease of applying all MIMO strategies known in OFDM to FBMC. The approach itself is of little complexity since it is based on hadamard matrices. While FBMC spreading allows complicated orthogonality to be restored in one block of communication, interference may be seen from the neighbouring blocks. By inserting a time security slot, the signal-to - interference ratio can also be improved. Furthermore, we examine the effect on the propagation process of a time channel. The estimation of the potential for Bit Error in double selective channels has been improved. In doubly selective channels, that is, time selectivity and frequency selectivity, we study the degeneration of output between OFDM and FBMC. In order to do this, we derive closed-form Bit Error Probability (BEP) terminology for the arbitrary linear modulation methods dependent on one-tap equalizers, in which our general BEP expressions include OFDM and FBMC.

An improved equalization with real interference prediction scheme of the FBMC/OQAM system

FBMC/OQAM transmission system has a better spectral efficiency than OFDM. However, its orthogonality condition is only considered in the real field. In the presence of fading channels, the real orthogonality of FBMC/OQAM might be lost, which calls for new equalization schemes. In this paper, an improved equalizer with real interference prediction (ERIP) scheme of FBMC/OQAM is proposed. We analyze the correlation between the real and the neighboring imaginary interference components taking Doppler shifts into account, and derive the improved ERIP scheme. The simulation results show that the proposed scheme can outperform the original ERIP and the one-tap equalization in time-varying multipath scenarios with an affordable complexity.

Complexity-Reduced Receiver for Universal Filtered Multicarrier Systems

In this letter, a low complexity least squares-based receiver is proposed for universal filtered multicarrier systems. Specifically, the coarse estimate of the transmitted symbol is first obtained by one-tap equalization using the even components of the frequency-domain received signal. The inter-block interference is then removed from the received signal, and the out-of-subband data are punctured. Finally, a complexity-reduced approach is designed to obtain performance improvement based on the overall pre-processed data. A closed-form mean-square error expression and computational complexity analysis are derived to quantify the achievable advantages. Simulation study confirms the effectiveness of the proposed receiver.

MIMO Multicarrier Transmission Over Doubly Selective Channels With Virtual Trajectories Receiver

High mobility and multipath propagation induce Doppler and delay dispersion, respectively. When both phenomena affect the transmitted signal, the communication channel is known as a doubly selective channel (DSC). In multicarrier systems under this scenario, intersymbol interference (ISI) and intercarrier interference (ICI) are ineluctably present, severely degrading the system's performance. Therefore, much effort has been invested in researching convenient waveforms that enable the receiver self-interference cancellation, while making use of the available space, time, and frequency diversity. In this paper, a multicarrier multiple-input multiple-output (MIMO) system for communication in DSCs is presented. By using the virtual trajectory (VT) representation of the MIMO channel, direct conversion of Doppler diversity into additional virtual antennas in the receiver is achieved. This concept is developed for a coherent system with one-tap channel estimation (CE) and equalization algorithms of low complexity as well as for incoherent detection using differential space-frequency block codes (DSFBCs), where pilot signals are not used. For both systems, runtime cancellation and/or inversion of complex interference channel matrices is avoided. Diversity of the doubly selective MIMO channel is exploited without sacrificing compatibility with current multicarrier MIMO systems. The presented system offers robustness in DSCs with minimal extra complexity when compared to conventional approaches. The results demonstrate that MIMO orthogonal frequency division modulation (MIMO-OFDM) can be still implemented in extremely high Doppler scenarios if VT reception is incorporated.

BER Performance Investigation of WPM with Unknown CSI Using Time-Domain MMSE Equalization

Wavelet Packet Modulation (WPM) is an emerging Multi-Carrier Modulation (MCM) technique regarded as a potential alternative to widely used Orthogonal Frequency Division multiplexing (OFDM). OFDM uses Cyclic Prex (CP) and can rely on one-tap equalization. Since WPM has overlapping symbols in the time domain, equalization cannot depend on the use of CP. This paper considers time-domain Minimum Mean-Square Error (MMSE) equalization for WPM with unknown Channel State Information (CSI), where equalizer filter coefficients are computed using known training sequences. The infuence of the transmission overhead due to training sequences on the Bit Error Rate (BER) performance is investigated considering quasistatic time-varying Rayleigh fading channels. Numerical results from computer simulations show that, with moderate overhead due to training sequences, BER performances are comparable with the case of known CSI. Furthermore, WPM is compared to OFDM with one-tap equalization, and is shown to provide better BER performances as compared to OFDM. Finally, this paper demonstrates improvement in Peak-to-Average Power Ratio (PAPR) of the transmitted signal using subcarrier combining for WPM in heterogeneous communication environments, where low-rate and high-rate devices share the same transmission resources.

Detection and Equalization of Set-Partitioned Offset-QAM OFDM in IMDD Systems

We design the detection algorithm for orthogonal frequency division multiplexing (OFDM) based on set-partitioned offset quadrature amplitude modulation (SP-offset-QAM) in adaptively-loaded intensity modulation and direct detection (IMDD) systems. The algorithm mitigates signal-signal beating interference (SSBI), improves the equalization capability of conventional one-tap equalizers, and reduces the complexity and the required length of training sequence in the decoding of multi-dimensional SP-QAM signals. We experimentally demonstrate the adaptively-loaded SP-offset-QAM OFDM over 50-km single-mode fiber to verify the proposed algorithm. It is shown that the modified equalization significantly improves the performance while the SSBI mitigation brings benefits after transmission. The proposed decoding scheme reduces the complexity comparable to that in conventional QAM. It is also shown that the SP-offset-QAM OFDM outperforms the SP-based conventional QAM and conventional offset-QAM OFDM both at back-to-back and after 50-km transmission, when the algorithm is applied to all schemes.

Optimal Filter Length and Zero Padding Length Design for Universal Filtered Multi-Carrier (UFMC) System

Universal filtered multi-carrier (UFMC) systems offer a flexibility of filtering arbitrary number of subcarriers to suppress out of band (OoB) emission, while keeping the orthogonality between subcarriers and robustness to transceiver imperfections. Such properties enable it as a promising candidate waveform for Internet of Things (IoT) communications. However, subband filtering may affect system performance and capacity in a number of ways. In this paper, we first propose the conditions for interference-free one-tap equalization and corresponding signal model in the frequency domain for UFMC system. The impact of subband filtering on the system performance is analyzed in terms of average signal-to-noise ratio (SNR), capacity and bit error rate (BER) and compared with the orthogonal frequency division multiplexing (OFDM) system. This is followed by filter length selection strategies to provide guidelines for system design. Next, by taking carrier frequency offset (CFO), timing offset (TO), insufficient guard interval between symbols and filter tail cutting (TC) into consideration, an analytical system model is established. In addition, a set of optimization criteria in terms of filter length and guard interval/filter TC length subject to various constraints is formulated to maximize the system capacity. Numerical results show that the analytical and corresponding optimal approaches match the simulation results, and the proposed equalization algorithms can significantly improve the BER performance.

Hybrid Micro Genetic Algorithm Assisted Optimum Detector for Multi-Carrier Systems

<p>A low-complexity detection scheme, which consists of a Hybrid Micro Genetic Algorithm (Hybrid- µGA), is proposed for Orthogonal Frequency Division Multiplexing (OFDM) systems. In the absence of orthogonality, intercarrier-interference (ICI) occurs because a signal from one subcarrier causes interference to others. In several environment, the OFDM signal reflections from a far obstacle generate inter-block-interference (IBI) due to long time delays. To avoid these unpleasant effects of IBI and ICI in OFDM system, a Hybrid-µGA detection algorithm is proposed. The proposed detector combines the conventional one-Tap equalizer and the Micro Genetic Algorithm (µGA) search engine. The output of one-Tap equalizer is considered as the input to µGA search engine. Therefore, the µGA starts with some knowledge rather than blindly to speed up the search. Theoretical analysis and simulation results show that the proposed detection Hybrid- µGA scheme substantially improves the performance of OFDM systems. Moreover, its complexity is 10 times lower than the conventional GA.</p>

Low Complexity ZF Receiver Design for Multi-User GFDMA Uplink Systems

This paper presents a low complexity receiver for multi-user uplink transmission using generalized frequency division multiplexing (GFDM). By exploiting the frequency spreading filter in the GFDM, the banded and circulant properties of the frequency transformed modulation matrices of individual users are developed. According to the newly developed properties in the frequency domain, a novel low complexity zero-forcing (ZF) receiver is developed for multi-user GFDM uplink systems in the frequency selective channels. The proposed receiver can be realized by successive discrete Fourier transforms and one-tap equalization, avoiding the huge computation caused by the inverse of the large dimensional equivalent system matrix. Simulation results show that the proposed receiver provides nearly the same symbol error rate as the traditional ZF equalizer with substantially reduced computational complexity.

Performance Analysis and Optimization of DCT-Based Multicarrier System on Frequency-Selective Fading Channels

Regarded as one of the most promising transmission techniques for future wireless communications, the discrete cosine transform (DCT)-based multicarrier modulation (MCM) system employs cosine basis as orthogonal functions for real-modulated symbols multiplexing, by which the minimum orthogonal frequency spacing can be reduced by half compared with discrete Fourier transform-based one. With a time-reversed pre-filter employed at the front of the receiver, interference-free one-tap equalization is achievable for the DCT-based systems. However, due to the correlated pre-filtering operation in time domain, the signal-to-noise ratio is enhanced as a result at the output. This leads to reformulated detection criterion to compensate for such a filtering effect, rendering minimum-mean-square-error, and maximum likelihood detections applicable to the DCT-based multicarrier system. In this paper, following on the pre-filtering-based DCT-MCM model that builds in the literature work, we extend the overall system by considering both transceiver perfections and imperfections, where frequency offset, time offset, and insufficient guard sequence are included. In the presence of those imperfection errors, the DCT-MCM systems are analyzed in terms of desired signal power, inter-carrier interference, and inter-symbol interference. Thereafter, new detection algorithms based on zero forcing iterative results are proposed to mitigate the imperfection effect. Numerical results show that the theoretical analysis matches the simulation results, and the proposed iterative detection algorithms are able to improve the overall system performance significantly.

Advanced Chromatic Dispersion Compensation in Optical Fiber FBMC-OQAM Systems

We report on several methods for the chromatic dispersion (CD) compensation in optical fiber offset-QAM-based filterbank multicarrier (FBMC-OQAM) systems. We show that several equalization structures, initially proposed for wireless FBMC-OQAM systems, can also be applied to optical FBMC-OQAM systems to enhance the CD tolerance. The different CD compensation algorithms are numerically validated and compared to the conventional one tap equalizer in a 30-Gbaud optical FBMC system, in terms of performance and complexity. Considering a 1-dB optical signal-to-noise ratio penalty at a bit error rate of $3.8\times 10^{-3}$ and 256 subcarriers, the results show that the maximum CD tolerance of the frequency spreading method can be enhanced by a factor 10 and 30 for 4-OQAM and 16-OQAM modulations, respectively, compared to that of the conventional one-tap equalizer, at the cost of higher complexity. Even though the other CD compensation methods provide a reduced CD tolerance compared to the frequency spreading method, they require less complexity and hence can be good alternatives.

256-QAM Interleaved Single Carrier FDM for Short-Reach Optical Interconnects

In this letter, we experimentally demonstrated a 40-Gbit/s 256-quadrature amplitude modulation (256-QAM) interleaved single-carrier frequency-division multiplexing (I-SC-FDM) over 10-km standard single-mode fiber (SSMF) for optical interconnects. I-SC-FDM has lower peak-to-average power ratio and lower computational complexity of transmitter compared with orthogonal frequency division multiplex- ing (OFDM). High-level QAM can achieve high spectral efficiency; however, it is susceptible to both linear and nonlinear distortions. The frequency-domain one-tap equalizer and the time-domain decision-feedback equalizer with recursive least square and Volterra algorithm are jointly employed to eliminate the distortions for the 256-QAM I-SC-FDM. After 10-km SSMF, I-SC-FDM has better performance than localized SC-FDM and OFDM. Therefore, I-SC-FDM shows the potential for short-reach optical interconnects.

Filter Bank Multicarrier Modulation Schemes for Future Mobile Communications

Future wireless systems will be characterized by a large range of possible uses cases. This requires a flexible allocation of the available time-frequency resources, which is difficult in conventional orthogonal frequency division multiplexing (OFDM). Thus, modifications of OFDM, such as windowing or filtering, become necessary. Alternatively, we can employ a different modulation scheme, such as filter bank multi-carrier (FBMC). In this paper, we provide a unifying framework, discussion, and performance evaluation of FBMC and compare it with OFDM-based schemes. Our investigations are not only based on simulations, but are substantiated by real-world testbed measurements and trials, where we show that multiple antennas and channel estimation, two of the main challenges associated with FBMC, can be efficiently dealt with. In addition, we derive closed-form solutions for the signal-to-interference ratio in doubly-selective channels and show that in many practical cases, one-tap equalizers are sufficient. A downloadable MATLAB code supports reproducibility of our results.

OFDM and FBMC-OQAM in Doubly-Selective Channels: Calculating the Bit Error Probability

Filter bank multi-carrier (FBMC) is a modulation technique with enhanced spectral properties compared with orthogonal frequency division multiplexing (OFDM). In this letter, we investigate the performance degeneration of OFDM and FBMC in doubly-selective channels, that is, time-selectivity and frequency-selectivity. For that, we derive closed-form bit error probability (BEP) expressions for arbitrary linear modulation methods based on one-tap equalizers, with OFDM and FBMC being special cases, covered by our general BEP expressions. We validate our calculations by Monte Carlo simulations and investigate the BEP error if the interference is approximated as Gaussian noise.

Cyclic Prefix-Based Universal Filtered Multicarrier System and Performance Analysis

Recently proposed universal filtered multicarrier (UFMC) system is not an orthogonal system in multipath channel environments and might cause significant performance loss. In this paper, the authors propose a cyclic prefix (CP) based UFMC system and first analyze the conditions for interference-free one-tap equalization in the absence of transceiver imperfections. Then the corresponding signal model and output signal-to-noise ratio expression are derived. In the presence of carrier frequency offset, timing offset, and insufficient CP length, the authors establish an analytical system model as a summation of desired signal, intersymbol interference, intercarrier interference, and noise. New channel equalization algorithms are proposed based on the derived analytical signal model. Numerical results show that the derived model matches the simulation results precisely, and the proposed equalization algorithms improve the UFMC system performance in terms of bit error rate.

New Designs for Reduced-Redundancy Transceivers

Block-based transceivers are widely used in wireless communication systems, mostly due to their well-defined structure and blockwise encoding. Interblock interference caused by superpositions of delayed signal copies is among their main physical-layer challenges, which is commonly eliminated with redundancy between adjacent data-blocks. Besides, channel equalization is also employed to further mitigate multipath channel interferences. The chosen amount of redundancy, however, may be overestimated, which opens an opportunity for reduced-redundancy superfast transceivers, whose key features include high spectral efficiency and low computational cost. Although superfast approaches aim at low complexity, proper equalizer-coefficient design and update procedures are still very intricate tasks, and most approaches are rather computationally cumbersome. This paper addresses such problem by proposing a new design strategy for reduced-redundancy block-based transceivers, which provides semi-blind equalization with data-selective update, in addition to the possibility of using redundant parallel equalizer structures, which are based on the use of fast Fourier transforms and one-tap equalizers. Simulation results indicate that the proposed methodology is effective and also able to achieve a performance comparable to what is obtained with state-of-the-art techniques, depending on the target application.

Iterative estimation of doubly selective underwater acoustic channel using basis expansion models

An iterative channel estimation scheme is proposed for orthogonal frequency division multiplexing (OFDM) systems over doubly selective underwater acoustic channels. The channel estimator is developed based on the basis expansion model (BEM) with two different types of basis functions, complex exponential (CE) basis and discrete prolate spheroidal sequences (DPSS). Considering the different Doppler characteristics for each cluster, we use cluster-specific parameters in the BEM for the most significant taps, such that the number of unknowns during channel estimation can be considerably reduced. The frequency domain received samples are expressed in terms of the model coefficients of the most significant taps. The channel estimator operates in an iterative, decision-directed fashion. At the first iteration, it utilizes only pilot symbols. After the first iteration, the estimator also uses the symbol decisions produced by a linear minimum mean squared error (LMMSE) block equalizer, in addition to the pilot symbols. We verify the bit-error-rate (BER) performance of the OFDM system over a simulated underwater acoustic channel based on Monterey-Miami Parabolic Equation modeling. It is shown that the LMMSE equalizer with the BEM-based channel estimators and the one-tap equalizer with perfect channel state information have similar BER performances. Both the proposed BEM-based channel estimation schemes perform much better than the basic matching pursuit method, a classic sparsity-aware channel estimation technique.

Design of multi-carrier transceivers based on modified discrete fourier transform filter bank

In order to improve the poor frequency selectivity and spectral leakage of traditional Orthogonal Frequency Division Multiplexing(OFDM) multi-carrier transceiver,multi-carrier transceiver based on modified Discrete Fourier Transform(DFT) filter bank was introduced. The design problem of it was formulated as nonconstrained minimized optimization of perfect reconstruction error of modulated filter banks,and the modified window method was taken to design the prototype filter in the modulated filter banks and linear optimization was used to optimize the objective function. Compared to the traditional window method,a spline function was exploited in the transition band of ideal filter of the modified window to eliminate the Gibbs effect. The simulation results show that the designed filter bank with the proposed method gives a better performance in terms of stopband attenuation,and reconstruction error,and the resulted multicarrier transceiver a better Symbol Error Rate(SER) performance in Quadrature Phase Shift Keying(QPSK) modulation and the 3GPP TS 25. 104vehicular multipath and one-tap frequency-domain equalization.