Intrinsic Imaginary Interference Research Articles

Combined intra symbol frequency domain averaging channel estimation for polarization division multiplexed coherent optical OFDM/OQAM systems

Intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD) seriously degrades the performance of optical orthogonal frequency division multiplexing offset-quadrature amplitude modulation (O-OFDM/OQAM) systems. Therefore, effective channel estimations are required for such systems to maintain good transmission performance. In the previous publications, both the full loaded (FL) and the half loaded (HL) frequency-domain channel estimation methods for polarization-division-multiplexed (PDM) coherent optical OFDM/OQAM (CO-OFDM/OQAM) systems have been proposed. However, for these frequency-domain methods, the channel frequency-domain responses caused by the correlation of the amplified spontaneous emission (ASE) noise are not considered. In this paper, we systematically discuss the frequency-domain channel transmission model for PDM CO-OFDM/OQAM systems. With the analysis of the correlation of the ASE noise, we propose a combined intra symbol frequency-domain averaging (CISFDA) channel estimation method for PDM OFDM/OQAM systems. Compared with the full-loaded frequency-domain channel estimation (FL-FD) method, the CISFDA method promotes the system robustness against the ASE noise and fiber nonlinear effect significantly. The proposed method is validated by numerical Monte Carlo simulations of the PDM CO-OFDM/OQAM system. Our new algorithm outperforms the FL-FD method in the 36 Gb/s PDM OFDM CO-OFDM/OQAM system, and the transmission distance is improved by 500 km with the bit-error-rate (BER) target at 3.8×10-3.

Filterbank-Assisted Channel Estimation for Coherent Optical FBMC/OQAM System

In this paper, a new channel estimator for optical filter bank multicarrier with an offset-QAM modulation (FBMC/OQAM) system is proposed. The conventional channel estimation techniques that are based on interference approximation method (IAM) preamble with least squares (LS) approach suffers from sensitivity to intrinsic imaginary interference (IMI). Therefore, a filterbank-assisted (FB) technique is proposed to eliminate the residual interference associated with the preamble to improve the performance of optical FBMC/OQAM system. To further enhance the system performance, a thresholding block is added between the synthesis and analysis filter banks at the receiver to suppress the residual interference. Simulation findings show that the proposed channel estimator is more effective than the conventional IAM-R and IAM-C with LS based channel estimation at estimating the optical channel that includes chromatic dispersion and residual IMI.

Design and Validation of the Physical Layer Functions of FBMC/OQAM Transceiver With Improved Residual Phase Error Correction

In this paper, we report experimental results on the development and improvement of the physical layer functions of a filter bank multicarrier with off-set quadrature amplitude modulation (FBMC/OQAM) transceiver using a testbed implemented with programmable software-defined radio (SDR). The developed physical layer approach is similar to the IEEE 802.11a standard, and it has been tested for timing synchronization, carrier frequency offset (CFO) correction, channel estimation and equalization. An investigation into the inherent intrinsic imaginary interference in FBMC/OQAM has been carried out, with initial findings indicating the seriousness of the effect of phase noise (PN). The conventional pilot structure used in IEEE 802.11a cannot be directly applied to FBMC/OQAM, thus, a specially designed pilot scheme is proposed to deal with the residual phase error. Three phase noise estimation schemes have been evaluated, namely phase noise compensation (PNC), extended Kalman filter (EKF) and modified blind phase searching (MBPS) methods. To further suppress the residual interference and enhance the phase error correction, an iterative technique is used after the pilot equalization. The performance of the proposed methods has been evaluated experimentally and via simulation for an indoor environment. The results verify the capabilities of the proof-of-concept transceiver and the developed physical layer functions.

A modified convolutional neural network-based signal demodulation method for direct detection OFDM/OQAM-PON

Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is a promising modulation candidate for passive optical network (PON) due to its high flexibility, better time–frequency focusing characteristic, great resistance to the inter symbol and inter carrier interferences and higher spectrum efficiency compared to OFDM. However, the intrinsic imaginary interference together with linear and nonlinear distortions make it more difficult to recover the transmitted OFDM/OQAM signal at the receiver side. To mitigate the transmission impairments, a modified convolutional neural network (CNN) is utilized to learn the channel state information and the constellation demapping mechanism for OFDM/OQAM-PON. The distorted received signals are equalized implicitly to obtain the transmitted binary bits directly. The simulation results show that the CNN based receiver (Rx) can compensate the linear and nonlinear distortions more effectively compared to traditional pilot-based Rx, especially for the high order modulation long-reach PON.

Combined phase offset channel estimation method for optical OFDM/OQAM

On the basis of coherent optical orthogonal frequency-division-multiplexing (CO-OFDM), CO-OFDM/ offset quadrature amplitude modulation (CO-OFDM/OQAM) improves the spectrum efficiency by utilizing specially designed filter banks to eliminate the cyclic prefix (CP) inserted between consecutive OFDM blocks and decreases the demanding of guard band between two channels, being a promising candidate for the next generation optical transmission link. Due to the relaxation of the orthogonal condition from the complex field to the real field, chromatic Dispersion (CD) and polarization mode dispersion (PMD) bring serious intrinsic imaginary interference (IMI) to an optical OFDM/OQAM system. Channel estimation schemes based on the interference approximation method (IAM) have been proved to be effective in suppressing the IMI with low complexity. For the IAMs, increasing the power of the pseudo pilot could decrease the effects of amplified spontaneous emission (ASE) noise and other frequency domain residual error on the channel estimation accuracy. In most of the previous reported IAM works, pilot blocks were designed according to the structure of the center pilot block loaded, while the two side pilots nulled for simplicity. However, the power of the pseudo pilot under this method did not reach the maximum, resulting in non-optimal channel estimation accuracy. In this paper, channel estimation method based on the combined phase offset (CMPO) is proposed and systematically discussed. For the proposed method, three consecutive full loaded blocks are treated as the pilot blocks and the optimal pilot structure have been obtained by utilizing the maximum likelihood algorithm. As validated by numerous Montel Carlo simulations results, system robustness against IMI, phase noised induced interference, and nonlinear effect have been improved significantly, thanks to CMPO.

P.648 Pre-weaning maternal rat antibiotic exposure leads to long-term neurobehavioral and neurochemical outcome in rat offspring

On the basis of coherent optical orthogonal frequency-division-multiplexing (CO-OFDM), CO-OFDM/ offset quadrature amplitude modulation (CO-OFDM/OQAM) improves the spectrum efficiency by utilizing specially designed filter banks to eliminate the cyclic prefix (CP) inserted between consecutive OFDM blocks and decreases the demanding of guard band between two channels, being a promising candidate for the next generation optical transmission link. Due to the relaxation of the orthogonal condition from the complex field to the real field, chromatic Dispersion (CD) and polarization mode dispersion (PMD) bring serious intrinsic imaginary interference (IMI) to an optical OFDM/OQAM system. Channel estimation schemes based on the interference approximation method (IAM) have been proved to be effective in suppressing the IMI with low complexity. For the IAMs, increasing the power of the pseudo pilot could decrease the effects of amplified spontaneous emission (ASE) noise and other frequency domain residual error on the channel estimation accuracy. In most of the previous reported IAM works, pilot blocks were designed according to the structure of the center pilot block loaded, while the two side pilots nulled for simplicity. However, the power of the pseudo pilot under this method did not reach the maximum, resulting in non-optimal channel estimation accuracy. In this paper, channel estimation method based on the combined phase offset (CMPO) is proposed and systematically discussed. For the proposed method, three consecutive full loaded blocks are treated as the pilot blocks and the optimal pilot structure have been obtained by utilizing the maximum likelihood algorithm. As validated by numerous Montel Carlo simulations results, system robustness against IMI, phase noised induced interference, and nonlinear effect have been improved significantly, thanks to CMPO.

ANN-based channel estimation algorithm of IM/DD-OFDM/OQAM-PON systems with mobile fronthaul network in 5G

Abstract 5G mobile fronthaul (MFH) network requires data transmission with large bandwidth and ultra-low latency. Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) passive optical network (PON) system is the 5G MFH solution with low spectrum efficiency and large bandwidth. The OFDM/OQAM-PON transmission performance will be undermined by intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD) and polarization mode dispersion (PMD). In this paper, we proposed an artificial neural network (ANN) based channel estimation (CE) algorithm, which can effectively reduce IMI by estimation of channel transfer function (TF). Simulation results show that the proposed algorithm can optimize the system performance, compared with the conventional LS method, the proposed algorithm can improve bit error rate optimization capability by an order of magnitude.

Low-Complexity MIMO-FBMC Sparse Channel Parameter Estimation for Industrial Big Data Communications

Industrial applications can produce significant amounts of data that require low delay and high data rate communications. Multiple-input-multiple-output filter bank multicarrier (MIMO-FBMC) communications employing offset quadrature amplitude modulation has been proposed for industrial big data due to its reliability and high spectrum efficiency. One of the difficulties in implementing a MIMO-FBMC system is accurate channel estimation (CE). The main factor affecting the CE performance is intrinsic imaginary interference, and the conventional preamble-based CE is not effective in this case. Thus, in this article, a low-complexity sparse adaptive CE scheme is proposed that is based on a dynamic threshold. This reduces the number of inner product calculations by considering only the columns of the measurement matrix greater than the threshold. Simulation results are presented that show that the proposed scheme is better than other well-known methods in terms of computational complexity and CE accuracy.

Independent Component Analysis for Phase and Residual Frequency Offset Compensation in OQAM Multicarrier Systems

Phase noise in the offset quadrature amplitude modulation (OQAM) multicarrier system results in not only constellation rotation but also crosstalk from the unique intrinsic imaginary interference (IMI). Therefore, the method for phase and residual frequency offset (RFO) compensation should be designed specifically to address this. In this article, we exploit the statistical difference of the OQAM signal and the IMI, and propose a novel independent component analysis (ICA) based method for phase and RFO compensation. It is proved that the signal exhibits the minimal entropy with the probability distribution deviating from the Gaussian one the most when the phase is correctly compensated. Several metrics and a recursive algorithm are proposed to separate the signal and the IMI. Simulations and experiments are performed to verify the proposed theory and to compare the ICA method with modified blind phase search (M-BPS), constellation classification (CC), and Kalman filtering (KL). It is shown that the ICA method exhibits significantly better tolerance to the laser linewidth and RFO than CC and KL, and greatly reduces the complexity compared to M-BPS. Therefore, the proposed ICA method can be the most promising solution for phase and RFO compensation in OQAM multicarrier systems.

Volterra Expansion Based Intra Channel Nonlinear Effect Equalization Method for Optical OFDM/OQAM Systems

Optical orthogonal frequency-division multiplexing offset quadrature amplitude modulation (O-OFDM/OQAM) system relaxes the orthogonal condition of the sub-carriers from the complex domain to the real field. Inter-symbol-interference (ISI) and inter-carrier-interference (ICI) could be suppressed by using filter banks with promising time-frequency-localization (TFL) properties. Therefore, cyclic prefix (CP) inserted between consecutive OFDM blocks could be removed for O-OFDM/OQAM system to improve system spectral efficiency. When passing through fiber channel, O-OFDM/OQAM faces serious intrinsic imaginary interference (IMI) induced by chromatic dispersion (CD), and fiber nonlinear effect, which would deteriorate system performance evidently. Fiber nonlinear effect induced interference is a great impairment for long haul transmission O-OFDM/OQAM system, and can not be equalized directly by using nonlinear equalization method designed for traditional optical OFDM. There is still much room for improvement of nonlinear equalization method for O-OFDM/OQAM. In this paper, we systematically study Volterra expansion based nonlinear equalization method (VENE) for O-OFDM/OQAM. We theoretically deduce simplified Volterra series expansion nonlinear transmission matrix (SVEM) for O-OFDM/OQAM based on mathematic deduction. With SVEM, intra channel nonlinear effect induced distortions could be modeled and estimated. By using specially designed pilot blocks, we obtain approximate solution of SVEM and perform effective VENE with very limited complexities. As shown in multiple Montel Carlo simulation results, nonlinear robustness for O-OFDM/OQAM has been improved significantly thanks to VENE, with various transmission distances and system parameters.

Frame Repetition: A Solution to Imaginary Interference Cancellation in FBMC/OQAM Systems

In this paper, we propose a complex-valued symbol based FBMC/OQAM (C-FBMC/OQAM) system to achieve the self-cancellation of intrinsic imaginary interference. The key idea is the employment of a well-designed repeated frame, in which complex-valued QAM symbols are transmitted instead of only real-valued symbols in the classical FBMC/OQAM systems. It is proven that, intrinsic interference among symbols can be eliminated in the proposed C-FBMC/OQAM systems, and the spectral efficiency is maintained even in the existence of repeated frame. Simulation results shows that the proposed C-FBMC/OQAM system and preamble structures for channel estimation demonstrate improved robustness against frequency selective fading channels compared to conventional FBMC/OQAM systems due to the intrinsic interference cancellation.

Per-Subcarrier RLS Adaptive Channel Estimation Combined with Channel Equalization for FBMC/OQAM Systems

Because of the intrinsic imaginary interference in filter bank multicarrier employing offset quadrature amplitude modulation (FBMC/OQAM) systems, accurate channel estimation remains an open issue, which will be studied in this letter. By using the real-time reconstructed pseudo-data as pseudo-pilots, the authors propose a per-subcarrier recursive least squares (RLS) adaptive channel estimation scheme combined with channel equalization (denoted by RLSACE-CE). The proposed RLSACE-CE scheme not only improves channel estimation performance but also achieves high transmission efficiency, which is validated by simulations and analyses in terms of the normalized mean square error (NMSE) of estimated channels and the bit error rate (BER).

Phase Offset Based Channel Estimation Method for Optical OFDM/OQAM Systems

In this letter, we developed a novel phase offset (PHO)-based channel estimation method for optical orthogonal frequency division multiplexing (OFDM)/offset quadrature amplitude modulation (OQAM). In this approach, the suppression of intrinsic imaginary interference (IMI) induced by both the linear and nonlinear impairment could be improved by increasing the pseudo pilot power with the using of PHO. As demonstrated in the numerous Montel Carlo simulation results, PHO outperforms the interference approximation methods (IAMs) for both the suppressing IMI and combating the fiber nonlinearity induced interference.

Fast channel estimation and equalization scheme for offset-QAM OFDM systems.

We show that the orthogonality between signal and intrinsic imaginary interference (IMI) in offset quadrature amplitude modulation (offset-QAM) orthogonal frequency division multiplexing (OFDM) can still be maintained under a certain condition even when the timing slots of different subcarriers are misaligned. We show that the phase and velocity differences over subcarriers induced by fiber dispersion satisfy this condition. Based on this, we propose a fast channel estimation and equalization scheme without prior channel information including coarse dispersion. We investigate the proposed scheme in a 40-Gbit/s offset-16QAM OFDM experiment and 240-Gbit/s polarization-division-multiplexed offset-16QAM OFDM simulations, both over 1200-km single-mode fiber. It is shown that the proposed scheme gives better performance, reduced overhead for the training sequence, and/or lower complexity than other schemes. We also compare offset-QAM OFDM with the proposed scheme and conventional OFDM, and show that in addition to the elimination of cyclic prefix overhead, offset-QAM OFDM gives better performance and longer transmission reach for a moderate/small number of subcarriers.

Dual-Polarization FBMC for Improved Performance in Wireless Communication Systems

Filter bank multi-carrier (FBMC) offers superior spectral properties compared to cyclic-prefix orthogonal frequency-division multiplexing (CP-OFDM), at the cost of an inherent shortcoming in dispersive channels called intrinsic imaginary interference. In this paper we propose a new FBMC based communication system using two orthogonal polarizations for wireless communication systems: dual-polarization FBMC (DP-FBMC). Using this system we can significantly suppress the FBMC intrinsic interference. Therefore in DP-FBMC all the multicarrier techniques used in CP-OFDM systems such as channel equalization, etc., should be applicable without using the complex processing methods required for conventional FBMC. DP-FBMC also has other interesting advantages over CP-OFDM and FBMC: it is more robust in highly dispersive channels, and also to receiver carrier frequency offset (CFO) and timing offset (TO). In our DP-FBMC system we propose three different structures based on different multiplexing techniques. We show that compared to conventional FBMC, one of these DP-FBMC structures has equivalent complexity and equipment requirements. We compare DP-FBMC with other systems through simulations. According to our results DP-FBMC has potential as a promising candidate for future wireless communication networks.

Compressive Sensing-Based Channel Estimation for FBMC-OQAM System Under Doubly Selective Channels

Filter-bank multicarrier with offset quadrature amplitude modulation (FBMC-OQAM) has been considered as an alternative scheme to orthogonal frequency division multiplexing (OFDM). However, the traditional channel estimation techniques of the OFDM cannot be directly applied to the FBMC-OQAM system because of the real-field orthogonality of the FBMC-OQAM signals. Although traditional channel estimation techniques, such as least square (LS) and minimum mean square error (MMSE), are widely applied to the FBMC-OQAM system via canceling the intrinsic imaginary interference from adjacent data symbols, the LS algorithm is subject to noise enhancement and it results in large mean square error (MSE), while the mmse algorithm needs to know the statistical information of channel in advance. Due to sparsity of the wireless channel, channel estimation is investigated as a compressive sensing (CS) problem. In this paper, we first introduce the coding method to cancel intrinsic imaginary interference for the FBMC-OQAM system. Then, a novel sparse adaptive subspace pursuit (SASP) method is proposed to improve the accuracy of LS channel estimation. Finally, we develop two distinctive algorithms, namely, auxiliary pilot (AP) SASP and coding-SASP, to estimate channel frequency respond (CFR) in the FBMC-OQAM system. The simulation results show that the AP-SASP and coding-SASP algorithms can offer a low complexity and fewer measurements compared with conventional orthogonal matching pursuit (OMP) and regularized OMP (ROMP) methods. Moreover, the proposed AP-SASP and coding-SASP algorithms have a better bit error ratio (BER) performance than the conventional OMP and ROMP methods for FBMC system in the doubly selective channels.

Low overhead equalization algorithm for simultaneously estimating channel and mitigating intrinsic imaginary interference in IMDD-OQAM-OFDM system

Abstract In this paper, we propose a low-overhead equalization algorithm used in the intensity modulation direct detection (IMDD) system with offset-quadrature amplitude modulation orthogonal frequency division multiplexing (OQAM-OFDM) modulation. By utilizing the complex interference approximation (IA) structure and the comb pilot-based frequency domain intra-symbol averaging and intra-subcarrier linear interpolation (CFAL) method, the OQAM-OFDM signal’s intrinsic imaginary non-orthogonal distortions can be mitigated and the low overhead of channel estimation with high estimation accuracy can be achieved. The experiment and numerical simulation are constructed to evaluate the proposed IA-CFAL algorithm from the aspects of bit error rate (BER) performance, mean square error (MSE), dispersion tolerance, time jitter tolerance and complexity. Comparison with the previous algorithms are made to prove its robustness to transmission impairments with low training overhead.

OFDM/OQAM based WDM fiber VLLC system employing improved channel estimation method

In this paper, we propose and experimentally demonstrate an OFDM/OQAM based WDM fiber visible laser light communication (VLLC) system employing improved channel estimation method. The improved channel estimation method is based on conventional interference approximation method (IAM) combined with the discrete Fourier transform (DFT) based channel estimation (IADF-CE). It is used to mitigate the intrinsic imaginary interference (IMI) and noise from the estimated channel response for OFDM/OQAM signal. Enabled by IADF-CE scheme, 1 dB and 1.2 dB power penalty improvement can be achieved in comparison with conventional IAM after transmission over 50 km standard single mode fiber (SSMF) and 4.5 m free space link for 16 OQAM and 4 OQAM modulations, respectively. Furthermore, each user can achieve a maximum aggregate data rate of 3.2 Gbps under the bit error rate (BER) of 2.7×10−2 in the system.

Experimental demonstration of multiple-inputs multiple-outputs OFDM/OQAM visible light communications

We experimentally demonstrate a 2×2 optical multiple-inputs multiple-outputs (MIMO) visible light communications system based on the modified orthogonal frequency-division multiplexing/offset quadrature amplitude modulation scheme. The adjacent subcarrier frequency-domain averaging (ASFA) with the full-loaded (FL) and half-loaded (HL) preamble structures is proposed for demultiplexing and mitigating the intrinsic imaginary interference (IMI) effect. Compared with the conventional channel estimation (CE) method, ASFA offers improved transmission performance. With the FL method, we obtain more accurate MIMO CE to mitigate the IMI effect and the optical noise compared to the HL method.

On design of conjugated transmission scheme for FBMC/OQAM systems with interference cancellation

Compared to OFDM systems with cyclic prefix, filterbank multicarrier with offset quadrature amplitude modulation (FBMC/ OQAM) system is considered as an alternative technology for next generation wireless communication systems. However, FBMC systems suffer from intrinsic imaginary interference caused by the real-field orthogonality destruction when passing through complex-valued fading channels. By analyzing the transmultiplexer's response of FBMC/OQAM systems, in this paper, a simple conjugated transmission scheme is proposed for FBMC/OQAM systems. Following the specific conjugation design, the intrinsic imaginary interference including the intrinsic inter-symbol and the inter-carrier interference can be eliminated at the receiver side through linear signal processing operation. Meanwhile, the proposed conjugated transmission scheme is able to obtain extra linear combination diversity gains for improving the systematic performance of FBMC/OQAM. Simulation results show that the proposed scheme is more efficient than conventional methods, especially in practical application scenarios with large Doppler spread caused by high-speed movement.