Tap Coefficients Research Articles

Burst-mode digital signal processing for passive optical networks based on discrete multi-tone

Driven by the ever-increasing capacity demands, digital signal processing (DSP) has been first applied to improve the performance of a 50G passive optical network (PON). The main challenge is implementing the burst-mode DSP to deal with the upstream burst signal. This paper proposes a burst-mode DSP using preambles designed for the 50G PON based on entropy-loading discrete multi-tone (DMT). The burst-mode DSP mainly includes the burst-mode timing recovery with the initial timing offset and the burst-mode frequency-domain equalizer with the initial tap coefficients. The experimental results of the 50G DMT PON demonstrate that the burst-mode DSP can achieve fast convergence based on the ∼27ns preamble with 800 samples. The proposed burst-mode DSP makes the DMT PON highly feasible to support non-residential point-to-multi-point applications such as passive optical local area networks.

Empowering quadrature mirror filter bank architectures: dynamic-grey wolf optimization approach for elevated filter orders

The present study introduces a revised version of the Grey Wolf Optimization (GWO) algorithm called Dynamic-GWO, which integrates a dynamic parameter adaptation mechanism for population diversity enhancement. This mechanism implements the real-time adjustment of control parameters throughout the optimization procedure. The algorithm's effectiveness is demonstrated by its consistent performance across a variety of intricate benchmark functions. Further, the suggested approach is employed to determine the eminent filter tap coefficients of the Quadrature-Mirror-Filter (QMF) bank that enables the configuration with filter orders of 120 and 150, which have yet to be achieved using existing methods. The uniqueness of the proposed approach lies in its capability to overcome the inherent difficulties associated with higher-order filter design, including increased filter length and computational complexity. The primary objective function is to minimize the algebraic sum of three types of distortion: pass-band error, excess power of stop-band, and squared error of the overall transfer function at the quadrature frequency ω q = π / 2 , which generally occur in the procedure of Near-Perfect-Reconstruction (NPR) QMF bank design. In addition, the measured maximum reconstruction errors are 1.303 × 10 − 15 , 2.93 × 10 − 16 and 2.18 × 10 − 06 dB, and aliasing distortions are A L = 85.80 , 128.49 and 153.52 dB for filter orders N = 100, 120 and 150, correspondingly. The proposed algorithm yields significant improvements with an 86.68% reduction in the ripple factor and a 10.45% increase in attenuation compared to the baseline GWO for filter order 120, underscoring its effectiveness in enhancing filter performance. The observed outcomes indicate that the Dynamic-GWO algorithm consistently surpasses the baseline GWO and other Nature-Inspired Algorithms (NIAs) regarding computational ease and convergence speed.

Microwave fractional Hilbert transformer implemented in a dispersion-tailored few-mode fiber

We experimentally demonstrate, for the first time to our knowledge, a microwave fractional Hilbert transformer in a few-mode fiber using a transversal filtering approach. The filter taps are provided by a tunable true-time delay line that is realized by exploiting the spatial dimension of a dispersion-engineered double-clad step-index few-mode fiber. Both the fractional order and operational bandwidth of the fractional Hilbert transformer can be continuously tuned by adjusting the tap coefficients and varying the operational optical wavelength, respectively. The magnitude and phase response for different fractional orders, ranging from 0.17 to 1.00 that correspond to phase shifts of 15° to 90°, are measured. Operational bandwidths of 7.4 to 10.6 GHz are demonstrated for a classical Hilbert transformer. Real-time temporal fractional Hilbert transform of a Gaussian-like pulse is also performed. Our results are in good agreement with theory, validating the viability of our approach for implementation of microwave fractional Hilbert transformers.

A 16-Gb/s 3-tap adaptive DFE in 12-nm FinFET CMOS technology

This paper introduces a 3-tap half-rate adaptive decision feedback equalizer (DFE) and a 2-D eye-opening monitor (EOM). A newly developed regenerating sampler (RG-sampler) with smaller setup time and ck2q time is integrated into the proposed DFE, enhancing its performance. To ensure greater robustness, the sign-sign least-mean-squares (SSLMS) algorithm with a pattern filter is employed to adapt DFE tap coefficients. The proposed 2-D EOM accurately captures the shape of the eye diagram using adjustment steps of 1.95ps and 7 mV in the horizontal and vertical directions, respectively. Additionally, by reusing analog components within the adaptive DFE and 2-D EOM modules, significant reductions in silicon area and power consumption are achieved. Implemented in 12-nm FinFET CMOS technology, the transceiver integrated with adaptive DFE and 2-D EOM operates at a data rate of 16 Gb/s over a channel with 30 dB loss at 8 GHz frequency. The proposed adaptive DFE and 2-D EOM occupy a total area of 0.00808 mm2. Measurement results demonstrate that following DFE adaptation, the eye height is 77 mV, and the eye width is 29.25ps at 16 Gb/s.

Toward Full Passive Internet of Things: Symbiotic Localization and Ambient Backscatter Communication

In this paper, we proposed a symbiotic localization and ambient backscatter communication (SLABC) architecture, which uses existing ambient backscatter communication (AmBC) hardware and received signals to help Internet of Things (IoT) localize target objects. The SLABC can be viewed as a specific realization of integrated passive/symbiotic sensing and communication, which has two mutually beneficial stages: 1) sensing stage with generalized Prony with homologous matching (GPHM) method; 2) communication stage with multi-source constant modulus algorithm (MSCMA) method. To formulate SLABC, we propose a unified TMSC signal model to characterize the complex multi-source, multi-path and multi-reflection symbiotic communication (TMSC) and facilitate the key information extraction. Utilizing the frequency domain expression of the unified signal model and the similarity between multi-path channel coefficients and tap coefficients of equalizers, our proposed GPHM and MSCMA methods are efficient to mitigate the TMSC interference and retrieve the interested information to locate targets and communicate. The proposed SLABC concept updates the symbiotic AmBC IoT with the least cost and reduces the dependence on hardware stacking. The simulation results show that: 1) information extracted by GPHM and MSCMA can help localization achieve high accuracy and reduce the complexity of multi-path interference mitigation; 2) both of the two methods have a positive influence on each other.

Multi-Aperture Coherent Digital Combining Based on Complex-Valued MIMO 2N × 2 Adaptive Equalizer for FSO Communication

Collecting light using multiple small apertures offers a lot of advantages over doing so using a single large aperture, including the effective mitigation of scintillation under strong turbulence, low implementation cost, and ease of scalability. However, the tricky of a multi-aperture system is to combine the multiple branches signals with static skew mismatch and with time-varying characteristics of gain, phase and state of polarization, especially for coherent digital combining of polarization multiplexing (PM) signals. In this paper, a complex-valued multiple-input multiple-output (MIMO) 2N×2 adaptive equalizer is designed to combine multi-aperture signals, and the tap coefficient of each branch is calculated based on the constant modulus algorithm (CMA). This technique relaxes hardware constraints on the time alignment of multi-aperture branch since the skew can be compensated electronically thanks to the proposed blind digital combining algorithm. In addition, laser linewidth within evaluation range smaller than 10 MHz has no effect on the performance of 2N×2 adaptive equalizer. Moreover, the feasibility of the proposed adaptive equalizer for moderate and strong turbulence suppression is verified by a 10-Gbps data rate PM-QPSK modulation offline two- and four-aperture simulated turbulence experiment.

Blind skew compensation and digital combining with widely linear equalizer for multi-aperture coherent FSO communication.

Coherent digital combining technology using multiple small apertures has a lot of advantages over doing so with a single large aperture, including the effective mitigation of deep fading under strong turbulence, ease of scalability, and potential higher collected optical power. However, the in-phase/quadrature (I/Q) imbalance and I/Q skew induced by manufacturing imperfections of the coherent receiver front end, and the time mismatch caused by the unequal length of multi-aperture branches will induce a high OSNR penalty and reduce the digital combining efficiency, especially when the system scales to a larger number of apertures, such as massive aperture system. In this work, a complex-valued multiple-input multiple-output (MIMO) 4N×2 widely linear (WL) equalizer is designed to combine multi-aperture signals. Using WL complex analysis, a general analytical model is derived and it is indicated that multi-aperture channel equalization and combining operations can be achieved simultaneously using a MIMO equalizer as long as appropriate tap coefficients are selected. Moreover, the feasibility of the proposed WL equalizer is verified by a 10-Gbps PM-QPSK modulation and a 20-Gbps PM-16QAM modulation four-aperture offline simulated turbulence experiment. The four-aperture combining efficiency of PM-QPSK exceeds 96% even at a single-aperture extremely low OSNR of -6 dB, and 80% for PM-16QAM at a single-aperture OSNR of 0 dB.

High-resolution reconfigurable RF signal spectral processor.

Recent developments in microwave photonic filters (MPFs) offer superior properties for radio frequency (RF) signal processing, such as large instantaneous bandwidth, high resolution and multifunctional shapes. However, it is quite challenging to realize two or more characteristics simultaneously to meet the diverse needs in complex electromagnetic environment. In this paper, we propose a reconfigurable RF signal spectral processor with both large instantaneous bandwidth and high resolution. In the proposed spectral processor, sufficient taps supplied by an optical frequency comb (OFC) offer a large instantaneous bandwidth to process broadband RF signals. Flexible tap coefficients can be obtained by manipulating an optical spectral shaper (OSS), which provides excellent reconfigurability. This tap-by-tap manipulation is realized with a high resolution of hundreds of megahertz, allowing precise shape configuration of the response. In the experiment, we demonstrate a flat-top response with a wide bandwidth of 7.1 GHz. Reconfigurable features such as tunable bandwidth, adjustable center frequency and diverse shapes are also shown. In particular, the measured frequency resolution of 96.5 MHz demonstrates the ability for precise configuration.

Design and verification of DFE re-decision algorithm for PAM6 high-speed transceiver based on FPGA

In this paper, a decision feedback equalizer (DFE) re-decision algorithm for a 6-level pulse-amplitude-modulation (PAM6) high-speed transceiver is proposed. PAM6 is a two-dimensional (2D) modulation. The four outer points are not used in the 2D constellation diagram to reduce the output power. Although the four outer points will not be modulated, they may be encountered during demodulation. This paper considers the above situation and theoretically analyzes the DFE module under the additive white Gaussian noise (AWGN) channel. Compared with the traditional pipelined DFE structure, the proposed algorithm can help solve the four outer points in PAM6. A serializer/deserializer (SerDes) simulation system is built based on a field programmable gate array (FPGA), and the performance of the proposed algorithm is carried out on the simulation system. According to this proposed algorithm, the symbol error rate (SER) of the PAM6 SerDes system can be reduced, and the system performance can be improved. When the DFE tap coefficient is 0.50 and the signal-to-noise ratio (SNR) is 22.0 dB, the proposed algorithm can additionally correct 8.18 % of erroneous symbols. When the DFE tap coefficient is 1.00 and the SNR is 22.0 dB, the proposed algorithm can additionally correct 17.30 % of erroneous symbols.

PAM-4 Receiver With 1-Tap DFE Using Clocked Comparator Offset Instead of Threshold Voltages for Improved LSB BER Performance

This study presents a wireline pulse amplitude modulation-4 (PAM-4) receiver using the least significant bit (LSB) decoding method that uses the offset of comparators. The proposed LSB decoding method can generate the same output as that of a conventional comparator by effectively adding the desired offset voltage to only one of the differential PAM-4 signals. Because the proposed decoding method replaces a 4-input comparator with a 2-input comparator, it can improve the bit error rate (BER) performance of the LSB by as much as the most significant bit (MSB). The predetermined offset is useful not only for LSB decoding but also for the direct decision feedback equalizer (DFE) operation. The differential amplitude and common-mode voltage (VCM) of the PAM-4 signal vary owing to the direct DFE tap coefficient. The modified comparator can generate an appropriate offset voltage without an adaptation loop or a VCM compensator although the PAM-4 signals are changed depending on the DFE tap coefficient. A prototype is fabricated using 28-nm CMOS technology and tested using a 10.29 dB channel attenuation at 10 GHz. The maximum data rate is 40 Gb/s, and the power efficiency and area of the proposed architecture are 1.58 pJ/bit and 0.039 mm <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{2}$</tex-math> </inline-formula> , respectively.

A STAP anti-interference technology with zero phase bias in wireless IoT systems based on high-precision positioning

Fog computing has been applied to the data processing for the Internet of Things (IoT) based on distributed high-precision Global Navigation Satellite Systems (GNSS). However, the space-time adaptive processing (STAP) interference suppression technology in the system will cause fog computing data deviation that includes carrier phase bias and pseudocode offset. An unbiased STAP technique is proposed to eliminate these deviations. First, it is analyzed that the carrier phase bias and pseudocode offset are caused by the non-linear phase response of the STAP equivalent filter. Then, a coefficient-constrained method based on practical engineering processing is proposed, which can eliminate these deviations by restricting the tap coefficients to be symmetrically equal around the center-tap. Moreover, by analyzing the coherent integral function of the pseudocode after filtering, the tap structure of STAP is modified to eliminate the group offset of the pseudocode without increasing the computational complexity and hardware resources. Finally, the unbiased performance and anti-interference performance of the system are verified by numerical and real data simulations.

RF Frequency Selective Switch by Multiple PMIM Conversions

Nowadays, broadband and multi-channel radio frequency (RF) processing has been widely used in communication, radar, countermeasure, and other applications. At present, multiple-input and multiple-output (MIMO)-oriented microwave photonic signal processing technology is relatively scarce, so this paper proposes an RF frequency selective switch (FSS) based on multiple phase modulation to intensity modulation (PMIM) conversions. PMIM conversion has been used in narrowband microwave photonic filtering in the past. We extend it to a wideband and arbitrarily reconfigurable RF spectrum processing unit through an optical frequency comb and periodic optical filter. Although we use the incoherent combination of a multi-wavelength light source, we can obtain any frequency response including rectangles only by using all positive tap coefficients. Using an optical wavelength selective switch (WSS), we obtain RF FSS, and the spectral resolution of RF FSS is much better than that of optical WSS, which is improved by more than two orders of magnitude. The above principles, including single-channel reconfigurable filtering and multi-channel RF FSS, are verified by experiments. Our technology provides a stable solution for future RF MIMO signal processing.

A 100-Gb/s PAM-4 DSP in 28-nm CMOS for Serdes Receiver

This paper presents a dedicated digital signal process (DSP) for four pulse amplitude modulation (PAM4) SerDes receivers. It is targeted to implement data recovery and adaptive equalization under ultra-high-speed and large channel attenuation with a small area and high power efficiency. The DSP consists of a clock data recovery (CDR), a 16-tap feed forward equalizer (FFE), a 1-tap decision feedback equalizer (DFE), and an automatic adaptation engine. An adaptive least mean square (LMS) algorithm is utilized to make the system more intelligent in calculating tap coefficients of the FFE and DFE. To address the timing limitation associated with traditional digital DFE that cannot handle large amounts of parallel data at a high speed, speculative techniques and a customized 4-to-1 multiplexer (MUX) unit are employed to remove the summation time and reduce the selection time, respectively. A first-order sigma-delta modulator is used to replace the traditional moving average to calculate average voltages, which could prominently save the hardware resources and power consumption. Additionally, the influence of input quantization resolution on the equalization ability is analyzed. Implemented in a 28-nm CMOS, the DSP could compensate for up to 33-dB loss at 100 Gb/s with a power consumption of 7.22 pJ/bit.

Sparse Volterra Equalizer to Mitigate the Nonlinear Distortion in High Power Budget IM-DD OFDM PON

To improve the power budget for intensity-modulation direct-detection (IM-DD) orthogonal frequency division multiplexing passive optical network (OFDM PON), a high power budget downstream scheme by using the Volterra equalizer in digital signal processing (DSP) processing is proposed in this paper. When the launch power is increased, the subcarrier-to-subcarrier beating interference (SSBI) and other nonlinear distortion are well mitigated by the Volterra equalizer. Moreover, to reduce the complexity of receiver DSP unit, the sparse Volterra equalizer is obtained through optimizing full Volterra equalizer employing orthogonal search method. Simulation and experiment results are presented to evaluate the transmission performance and analyze the distribution of taps of Volterra equalizer under different respects. The results show that the number of tap coefficient of sparse Volterra equalizer is reduced by 60% compared with that of conventional second-order full Volterra equalizer.

Optical Multi-Tap RF Canceller for In-Band Full-Duplex Wireless Communication Systems

In-band full-duplex (IBFD) technology has become important for future wireless communications, due to its ability of increasing the radio-frequency (RF) spectrum efficiency. However, the benefits can only be realized if the self-interference (SI) is sufficiently suppressed. RF cancellation is important for IBFD systems due to its ability of SI mitigation. But it becomes more difficult for RF cancellation while the frequency and bandwidth increase. In real scenarios, the response of the multi-path SI channel is not frequency-flat, which also limits the cancellation performance. To solve these problems, we present a photonic-enabled multi-tap RF canceller in this paper. The proposed RF canceller has the ability of cancelling multi-path SI. The tap coefficients are precisely adjusted by optical spectrum processing. A prototype system with 8 taps is demonstrated in this paper. The measured results show 25 dB average cancellation depth over 1 GHz cancellation bandwidth under over-the-air conditions. To verify the signal of interest (SOI) recovery capability of the novel RF canceller, an IBFD wireless communication experiment based on 16-quadrature amplitude modulation (16-QAM) was demonstrated, the SOI was successfully recovered.

Analytical method for joint optimization of FFE and DFE equalizations for multi-level signals

Abstract Channel equalization is the efficient method for recovering distorted signal and correspondingly reducing bit error rate (BER). Different type of equalizations, like feed forward equalization (FFE) and decision feedback equalization (DFE) are canceling channel effect and recovering channel response. Separate optimization of tap coefficients for FFE and DFE does not give optimal result. In this case FFE and DFE tap coefficients are found separately and they are not collaborating. Therefore, the final equalization result is not global optimal. In the present paper new analytical method for finding best tap coefficients for FFE and DFE joint equalization is introduced. The proposed method can be used for both NRZ and PAM4 signals. The idea of the methodology is to combine FFE and DFE tap coefficients into one optimization problem and allow them to collaborate and lead to the global optimal solution. The proposed joint optimization method is fast, easy to implement and efficient. The method has been tested for several measured channels and the analysis of the results are discussed.

Accelerating LMS-Based Equalization With Correlated Training Sequence in Bandlimited IM/DD Systems

As higher symbol rates are utilized in the intensity modulation and direct detection (IM/DD) scheme to meet the unrelenting growth of data traffic, overcoming the inter-symbol interference (ISI) induced by the limited bandwidth has become increasingly crucial. Channel equalization based on digital signal processing (DSP) is an effective solution, where least-mean squares (LMS) algorithm is adopted to adjust tap coefficients. However, the LMS algorithm usually has slow rate of convergence and requires lots of training symbols. This work proposes a novel training sequence to accelerate the LMS-based equalization. A first-order Markov chain (MC) is employed for sequence generation, which introduces correlation between samples and shapes signal spectrum. Compared with the conventional training sequence that consists of independent and identically distributed (i.i.d.) samples and has a white spectrum, the MC sequence enables faster convergence of tap coefficients and mean-squared error (MSE). Moreover, an experimental demonstration of a 43 Gbaud PAM-4 signal shows that the proposed sequence can achieve a lower pre-forward-error-correction (pre-FEC) bit error rate (BER) than that of the i.i.d. sequence with the same length. When the PAM-4 signal is transmitted over a 5-km standard single mode fiber (SSMF) with 6-dB system bandwidth of 10 GHz, more than 70% training sequence length reduction can be attained. When the fiber length is increased to 10 km and the signal suffers from severe power fading, more than 48% reduction can be achieved.

Joint Multi-Branch Weighted Combining and Equalization Detector for Cooperative Communication

In cooperative communication, due to the difference of channel qualities between nodes, the effective combining and processing for the signals from multiple nodes is extremely important to improve the system performance. In this paper, we propose an approach named joint multi-branch weighted combining and equalization detector (JMWC-ED) where all inter-node links are independent frequency selective fading channels, and present the corresponding performance analysis. Compared with the existing approaches, in the JMWC-ED, the adjustment of weighted combining for all branches and the equalization of each branch are synthesized into an integrated detector. The weighted combining of multiple branches and the adjustment of the tap coefficient vector of each branch are not independent of each other, but are implemented jointly. Moreover, the weighted coefficients of all branches are adaptively adjusted in accordance with the corresponding channel qualities, and there is no need to know the channel state informations between nodes. Simulation results validate the adjustment ability of the JMWC-ED to weighted coefficients and the performance analysis for the JMWC-ED. We also show the superior performance of the JMWC-ED over existent counterparts.

Surface acoustic wave photonic filters with a single narrow radio-frequency passband in standard silicon on insulator

Integrated microwave photonic filters are becoming increasingly important for signal processing within advanced wireless and cellular networks. Filters with narrow transmission passbands mandate long time delays, which are difficult to accommodate within photonic circuits. Long delays may be obtained through slow moving acoustic waves instead. Input radio-frequency information can be converted from one optical carrier to another via surface acoustic waves and filtered in the process. However, the transfer functions of previously reported devices consisted of multiple periodic passbands, and the selection of a single transmission band was not possible. In this work, we demonstrate surface acoustic wave, silicon-photonic filters of microwave frequency with a single transmission passband. The filter response consists of up to 32 tap coefficients, and the transmission bandwidth is only 7 MHz. The results extend the capabilities of integrated microwave photonics in the standard silicon-on-insulator platform.

A SERIES RECONFIGURABLE STRUCTURE RESONANT CHOPPER FULLY SOFT-SWITCHED DUAL-ACTIVE-BRIDGE WITH DUAL TANK

Compared to the dual-active-bridge converter, the Double-bridge arrangement thunderous converter (DBSRC) can extend delicate exchanging run. To encourage broaden the soft-switching extend and progress the circuit execution, a modern DBSRC with double tank based on DBSRC is proposed in this paper. This modern converter highlights two thunderous tanks and a tapped transformer, and it can perform superior than the DBSRC by optimized tap coefficient x of the tapped transformer. Its operation guideline, voltage pick up, the soft-switching characteristics, and yield control are analyzed in detail, and compared with the DBSRC. Comes about appear that the pro-posed dual-tank topology has displayed higher voltage pick up, more extensive soft-switching locale, and bigger yield control than the conventional DBSRC when the tap-coefficient x is chosen sensibly. DC-to-DC bidirectional resounding converter to be utilized for bidirectional control exchange applications particularly battery charging discharging applications in electrical vehicles. Its similar to an LLC resonant converter, but on the secondary side of the circuit, an additional inductor and capacitor have been added to make the resonant network symmetric for operation in both forward and backward directions. The switches in the inverting step have zero voltage switching (ZVS). Under ZCS, the rectifier diodes on the secondary side also turn off. ZVS and ZCS reduce losses and enable high-frequency operation, resulting in smaller magnetic elements and filter capacitors, lowering size, weight, and volume while enhancing power density. INDEX TERMS—dual active bridge (DAB), dual-bridge series resonant converter (DBSRC), soft-switching, dual-tank, tapped- transformer, tap-coefficient