Nonlinear Equalization Technique Research Articles

LSTM networks enabled nonlinear equalization in 50-Gb/s PAM-4 transmission links.

This paper proposes a nonlinear equalization technique enabled by long short-term memory (LSTM) recurrent neural networks. The proposed technique is implemented at the end of offline digital signal processing. And two approaches utilizing the LSTM network are experimentally tested and demonstrated in transmission of a 50-Gb/s four-level pulse amplitude modulation intensity modulation direct detection link over 100-km standard single-mode fiber. The first approach uses the LSTM network-based equalizer to directly categorize the received signal into four amplitude levels, and the second approach uses the LSTM network to estimate signal noise for compensating the received signal. The experimental results show remarkable performance improvement of the proposed method over conventional linear equalizers, and significant enhancement at high launch power compared with Volterra filtering. Also, the proposed method reveals better short-time universality.

Unsupervised Support Vector Machines for Nonlinear Blind Equalization in CO-OFDM

A novel blind nonlinear equalization (BNLE) technique based on the iterative re-weighted least square is experimentally demonstrated for single- and multi-channel coherent optical orthogonal frequency-division multiplexing. The adopted BNLE combines, for the first time, a support vector machine-learning cost function with the classical Sato or Godard error functions and maximum likelihood recursive least-squares. At optimum launched optical power, BNLE reduces the fiber nonlinearity penalty by ~1 (16-QAM single-channel at 2000 km) and ~1.7 dB (QPSK multi-channel at 3200 km) compared to a Volterra-based NLE. The proposed BNLE is more effective for multi-channel configuration: 1) it outperforms the “gold-standard” digital-back propagation and 2) for a high number of subcarriers the performance is better due to its capability of tackling inter-subcarrier four-wave mixing.

PSO tuned ANFIS equalizer based on fuzzy C-means clustering algorithm

Wireless communication always suffers from intersymbol interference (ISI), multipath fading, co-channel interference (CCI). Equalization techniques try to combat these phenomena to recover distorted data at receivers. Most techniques need a separate process to estimate channel profile before equalization. In this paper, Particle Swarm Optimization tuned Adaptive Neuro Fuzzy Inference System (PSO-ANFIS) based channel equalizer, which is capable of system identification, estimation and equalization of wireless communication channels, is proposed. PSO-ANFIS equalizer uses the training data and employs fuzzy C-means (FCM) clustering to model a wireless communication channel without knowledge of channel dynamics. The PSO-ANFIS equalizer was simulated on a mobile communication model with ISI, CCI and the Additive White Gaussian Noise. The used training method and FCM provided the best regression of system modeling to fit to wireless channel. The performances of PSO-ANFIS equalizer were evaluated and compared to the equalizers of Maximum-Likelihood Sequence Estimation, Recursive Least Squares, ANFIS, Functional Link Artificial Neural Network, PSO-Variable Constriction Factor and Artificial Neural Network-PSO in terms of BER–SNR. The simulation results showed that the performance of the PSO-ANFIS equalizer with FCM clustering gives the best performance among the mentioned nonlinear equalization techniques.

랜덤 심볼열과 결정 궤환을 사용한 자력 등화 알고리듬

Abstract Non-linear equalization techniques using decision feedback structure are highly demanded for cancellation of intersymbol interferences occurred in severe channel environments. In this paper decision feedback structure is applied to the linear blind equalizer algorithm that is based on information theoretic learning and a randomly generated symbol set. At the decision feedback equalizer (DFE) the random symbols are generated to have the same probability density function (PDF) as that of the transmitted symbols. By minimizing difference between the PDF of blind DFE output and that of randomly generated symbols, the proposed DFE algorithm produces equalized output signal. From the simulation results, the proposed method has shown enhanced convergence and error performance compared to its linear counterpart. Key Words : Decision feedback, Blind equalization, PDF, Random symbols, ITL. * 교신저자 : 김남용(namyong@kangwon.ac.kr)접수일 11년 11월 02일 수정일 (1차 11년 12월 26일, 2차 11년 12월 29일) 게재확정일 12년 01월 05일

A nonlinear electrical equalizer with decision feedback for OOK optical communication systems

In this paper we introduce a nonlinear equalizer using the radial basis function (RBF) network with decision feedback equalizer (DFE) for electronic dispersion compensation in optical communication systems with on-off-keying and a direct detection receiver. The RBF method introduces a non-linear equalization technique suitable for optical communication direct detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF with DFE out performs the RBF without DFE and achieves similar results provided by maximum likelihood sequence estimator.

10-Gb/s Optical Fiber Transmission Using a Fully Analog Electronic Dispersion Compensator (EDC) With Unclocked Decision-Feedback Equalization

A 10-Gb/s data transmission over optical fiber is limited in reach due to optical dispersion phenomena that results in inter-symbol interference. Electronic dispersion compensators employing linear and nonlinear equalization techniques are a compact, cost-effective, and adaptive alternative to optical dispersion compensating techniques and devices that are often bulky, expensive, and not adaptive. The decision-feedback equalizer (DFE) is a type of nonlinear equalizer that can effectively compensate for the nonlinear dispersion effects exhibited by fiber channels. However, at multigigabit/second data rates, the implementation of the DFE is challenged by the first feedback-loop latency requirements and process technology speed limitations. This study demonstrates a novel analog approach and circuit architecture to perform decision-feedback equalization at multigigabit/second data rates. The analog decision-feedback equalizer (ADFE) consists of a four-tap linear analog feed-forward filter and a one-tap nonlinear analog feedback filter and is implemented in a 0.18-mum CMOS process. The ADFE is completely differential and a broadband single-differential converter is designed for the ADFE front-end to interface with the single-ended output of a photodetector. The circuit is unclocked and uses current-mode logic techniques. The ADFE is used to extend the transmission distance of multimode fiber at 10-Gb/s to 300 in and of single-mode fiber to 120 km.

Radial Basis Function Network Equalizer for Optical Communication OOK System

In this paper, we introduce a nonlinear equalizer using the radial basis function (RBF) network for electronic dispersion compensation in optical communication systems with on-off keying and a direct-detection receiver. The RB method introduces a nonlinear equalization technique that is suitable for optical communication direct-detection systems that include nonlinear transformation at the photodetector. A bit error rate performance comparison shows that the RBF equalizer outperforms the conventional linear feedforward equalizer. In addition, it is shown that, in optically amplified systems, the RBF equalizer improvement is increased even further. Finally, the feasibility of the RBF method is validated by experimental results.

An estimation technique for nonlinear distortion in high-density magnetic recording channels

Nonlinear distortions, which limit detector performance at high recording densities, are known as the nonlinear transition shift and partial erasure. The nonlinear transition shift is known to be reduced substantially by the precompensation technique, while the partial erasure requires sophisticated nonlinear equalization techniques. In this paper, a new calculation intensive adaptive technique is presented that uses stochastic gradient method to minimize the mean squared error between actual playback signal samples and modeled signal samples. The effectiveness of the proposed adaptive approach to separately estimate these two nonlinear parameters uniquely is demonstrated by computer simulation.

On the use of tentative decisions to cancel intersymbol interference and nonlinear distortion (with application to magnetic recording channels)

We develop a theory of the error rate of receivers that use tentative decisions for adaptive cancellation of intersymbol interference (ISI). We formulate precise conditions under which tentative decisions can be effectively used to cancel linear or nonlinear ISI, and verify the predictions of the theory by Monte Carlo simulations on four typical channels. In the case of magnetic recording channels, we show that the conditions for effective cancellation are satisfied by the precursor interference generated by inductive read heads, resulting in an improvement of the noise margin of 2 dB at the output of a symbol-by-symbol detector or 1 dB at the output of a Viterbi decoder at a bit error rate of 10/sup -9/. This technique has been incorporated in an experimental VLSI partial response maximum-likelihood (PRML) receiver, and the results confirmed by laboratory measurements. We also show that no improvement is obtained when this technique is used to compensate nonlinear distortion, which is another common impairment of magnetic recording channels, and we establish fundamental limits for the improvement achievable by any nonlinear equalization technique in magnetic recording.

Adaptive Equalization Techniques for Digital Video Recording Systems

An adaptive automatic equalizer applicable to high-rate data recording systems such as digital VTRs is proposed. This equalizer is constructed on the nonlinear equalization technique called linear cancellation and on probabilistic decoding techniques. Two particular advantages are noted: that equalization with logical error correction is performed and that the variable part of the equalizer can be composed solely of digital circuits. It has been confirmed that sufficient eye-pattern opening can be achieved using this equalizer at an operation speed of 46 Mbits/sec.