Interference Approximation Method Research Articles

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.

Modified frequency-domain channel-estimation based on the dual-dependent pilots for polarization-division-multiplexed coherent optical orthogonal frequency division multiplexing systems with offset-quadrature-amplitude modulation

In the past, several frequency-domain channel-estimation methods were proposed for coherent optical orthogonal frequency division multiplexing systems, which use offset-quadrature-amplitude modulation (CO-OFDM-OQAM). A common challenge for these methods is that the training sequences they use, even though they increase the magnitude of the pseudo-pilots, can make the peak-to-average power ratio (PAPR) of the signal worse. To address this problem, an innovative modified frequency-domain channel estimation method is designed and investigated for a polarization-division-multiplexed CO-OFDM-OQAM system that uses dual-dependent pilots. The operating principle of this new method is described in detail. Then, its PAPR performance and channel-estimation capability are examined using numerical simulations. The proposed method not only ensures a sufficiently large magnitude of the pseudo-pilot but also does not seriously degrade the PAPR of the signal. It is also found that the magnitude of its pseudo-pilot (2.3136) exceeds that of the interference approximation method with real pilots (1.1086), and it is only slightly worse than that of the enhanced IAM with complex pilots (2.6074). The PAPR of the new method (max 11.4 dB) is much lower than that of other frequently used methods. Finally, the channel-estimation capability of the proposed method is validated for both back-to-back and long-distance transmission scenarios. Furthermore, the BER performance of several different channel-estimation methods is also compared. The obtained results confirm the above observations.

Sparse Bayesian Learning Aided Estimation of Doubly-Selective MIMO Channels for Filter Bank Multicarrier Systems

Sparse Bayesian learning (SBL)-based channel state information (CSI) estimation schemes are developed for filter bank multicarrier (FBMC) systems using offset quadrature amplitude modulation (OQAM). Initially, an SBL-based channel estimation scheme is designed for a frequency-selective quasi-static single-input single-output (SISO)-FBMC system, relying on the interference approximation method (IAM). The IAM technique, although has low complexity, is only suitable for channels exhibiting mild frequency-selectivity. Hence, an alternative time-domain (TD) model based sparse channel estimation framework is developed for highly frequency-selective channels. Subsequently, the Kalman filtering (KF)-based IAM and its TD counterpart are developed for sparse doubly-selective CSI estimation in SISO-FBMC systems. These schemes are also extended to FBMC-based multiple-input multiple-output (MIMO) systems, for both quasi-static and doubly-selective channels, after demonstrating the special block and group-sparse structures of the IAM and TD-based models respectively, which are the characteristic features of such channels. The Bayesian Cramér-Rao lower bounds (BCRLBs) and the time-recursive BCRLBs are derived for the proposed quasi-static as well as doubly-selective sparse CSI estimation models, respectively. Our numerical results closely match the analytical findings, demonstrating the enhanced performance of the proposed schemes over the existing techniques.

Channel estimation based on interference cancellation method for FBMC-PON

Based on the traditional orthogonal frequency division multiplexing passive optical network (OFDM-PON) system, filter bank multi-carrier passive optical network (FBMC-PON) eliminates the CP by introducing a synthesis filter, dramatically improving spectral efficiency when the number of symbols is sufficient in one transmission. Meanwhile, thanks to its excellent bit error rate (BER) performance, FBMC-PON becomes one of the significant alternatives to OFDM-PON. In previous practical cases, the interference approximation method (IAM) based channel estimation algorithm, which mainly enhances the power of training sequence, plays superior in FBMC-PON. However, the interference cancellation method (ICM) channel estimation method is a better choice due to the self-cancellation of imaginary interference, especially for short-distance optical fiber access channels. We analyze the four different training sequences and consider that in the short fiber channel of PON, it is more vital to suppress the imaginary interference rather than improve the training sequence’s power. The simulation results show that the BER performance of FBMC-PON based on our proposed ICM training sequence structure is one order of magnitude higher than that of OFDM-PON and half an order of magnitude higher than that of IAM in the case of 20 km fiber. Our proposed channel estimation algorithm only changes the structure of the training sequence and does not increase any additional computational complexity. As far as we know, ICM training sequence structure is verified in FBMC-PON by simulation for the first time. The results show that ICM is the optimal choice of the 4 kinds of training sequence structures. ICM training sequence structure has a very high application prospect in short reach FBMC-PON.

Deep-BiGRU based channel estimation scheme for MIMO–FBMC systems

Deep Learning (DL)–based wireless communication systems have the potential to improve the conventional functions and current architecture of communication systems. In this paper, we propose a novel DL-based channel estimation scheme for multiple-input multiple-output filter bank multicarrier with offset quadrature amplitude modulation (MIMO-FBMC/OQAM) systems called deep bidirectional gated-recurrent unit (BiGRU) scheme. This scheme can easily be applied to a single-input single-output (SISO) system. The proposed scheme is divided into two stages: offline and online. The network is first trained in the offline stage. The prediction of channel information and estimation of the channel matrix using the trained network is then performed in the online stage. The simulation results in terms of the normalized mean square error (NMSE) and bit error rate (BER) demonstrate that, under different time-varying channel models, the proposed DL scheme significantly improves the channel estimation performance of FBMC for single and multiple antennas compared to conventional interference approximation method (IAM) channel estimation methods.

Distribution Line Fitting-Based Channel Estimation Without Guard Symbols for OQAM/FBMC Systems

In this paper, we propose a distribution line fitting (DLF) based channel estimation method for offset quadrature amplitude modulation based filter-bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly analyze the distribution characteristics of demodulated OQAM symbols at the receiver, and derive an intrinsic interference free relation between the real and imaginary parts of demodulated OQAM symbols. Then, based on the above derivation, we design a pilot structure of arbitrary columns of pilot symbols without guard symbols between pilot symbols and data symbols. Finally, the distribution lines can be fitted by utilizing the distribution characteristics of demodulated OQAM symbols and the channel frequency response can be estimated. Simulation results demonstrate that the proposed method with one column of pilot symbols outperforms the interference approximation method (IAM) in high signal-to-noise (SNR) regions and outperforms the pair of pilots (POP) method in all SNR regions in terms of bit error ratio (BER), and the proposed method with two columns of pilot symbols outperforms both IAM and POP in all SNR regions in terms of both BER and complexity.

Efficiency Analysis of a Truncated Flip-FBMC in Burst Optical Transmission

A novel Flip-filter bank multicarrier (Flip-FBMC)-based transmultiplexer (TMUX) with offset quadrature amplitude modulation is proposed to enhance the transmission performance compared to a conventional Flip-OFDM system. Moreover, the possibility to reduce the TMUX response (latency) and increase spectral efficiency is investigated for the first time through a tail shortening method. The proposed design is based on a biorthogonal form for visible light communication (VLC) to increase the flexibility of design requirements. However, spectral efficiency suffers from the ramp-up and the ramp-down at the beginning and end, respectively, of a data burst. Hence, as a penalty, Flip-FBMC imposes 9 more symbols than a Flip-OFDM packet and two factors compared to a DCO-FBMC burst. Hard truncation of the lowest energy tail minimizes latency and limits the system penalty to 2.5 symbols, which is lower than that of DCO-FBCM by 2 symbols. The results show that the prototype filter of the Heisenberg factor ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 1$ </tex-math></inline-formula> ) is highly effective in reducing the energy loss of truncated tails and reduces the symbol error rate (SER). The Flip-FBMC gain over a direct line-of-sight VLC channel is analyzed, and the channel estimation of a truncated burst, which is based on the interference approximation method (IAM) of IAM-C type, exhibits a superior performance of 1.5 dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> SER over the IAM-R method and 1 dB at 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> SNR over a cyclic prefix of 1 point Flip-OFDM. On the other hand, the analysis reveals that IAM-C is slightly impacted by the truncated burst compared to the nontruncated version.

Channel Estimation Based on Interference&amp;nbsp;Cancellation Method for FBMC-PON

Based on the traditional orthogonal frequency division multiplexing passive optical network (OFDM-PON) system, filter bank multi-carrier passive optical network (FBMC-PON) eliminates the CP by introducing a prototype filter, dramatically improving spectral efficiency. Meanwhile, thanks to its excellent bit error rate (BER) performance, FBMC-PON becomes one of the significant alternatives to OFDM-PON. In previous practical cases, the interference approximation method (IAM) based channel estimation algorithm, which mainly enhances the power of train sequence, plays superior in FBMC-PON. However, the interference cancellation method (ICM) channel estimation method is a better choice due to the self-cancellation of imaginary interference, especially for short-distance optical fiber access channels. We analyze the different train sequences and consider that in the short fiber channel of PON, it is more vital to suppress the imaginary interference rather than improve the train sequence’s power. The simulation results show that the performance of FBMC-PON based on ICM is one order of magnitude higher than that of OFDM-PON and half an order of magnitude higher than that of IAM in the case of 20km fiber. Our proposed channel estimation algorithm only changes the structure of the train sequence and does not increase any additional computational complexity.

Channel Estimation and Pilot Symbol Optimization Based on Intrinsic Interference Utilization for OQAM/FBMC Systems

In this paper, we propose a novel channel estimation method and optimize the pilot symbols based on intrinsic interference utilization for offset quadrature amplitude modulation based filter-bank multicarrier (OQAM/FBMC) systems. Specifically, we firstly propose a line fitting based channel estimation method by analyzing and utilizing the distribution characteristics of the demodulated OQAM symbols, where no guard symbols are needed between pilot symbols and data symbols. Then, we formulate a pilot symbol optimization problem by utilizing the intrinsic interferences to improve the accuracy of the line fitting. Finally, we obtain the optimal solution of the optimization problem and present the corresponding pilot structure. Simulation results demonstrate that the proposed method with the optimized pilot structure performs the best among candidate pilot symbols and outperforms the traditional interference approximation method (IAM) with the optimal pilot structure.

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.

Reducing Pilot Overhead: Channel Estimation With Symbol Repetition in MIMO-FBMC Systems

In this article, we aim to reduce the pilot overhead of channel estimation in multiple input multiple output (MIMO) systems employing filter bank multicarrier (FBMC). The key idea is the employment of the concept of symbol repetition in frame structure. Firstly, we propose an MIMO-FBMC system based on block repetition, in which complex-valued symbols are transmitted instead of only real-valued symbols in classic MIMO-FBMC systems without block repetition. Then, we present two low-overhead channel estimation approaches in the block-repetition based MIMO-FBMC system and only two columns of symbols are required as the pilot overhead. However, the existence of the block repetition results in the increased sensitively to the channel coherence time. Then, the two channel estimation approaches are extended into classic MIMO-FBMC systems without block repetition. It is proved that, the proposed methods exhibit significant advantage in terms of pilot overhead, compared with the conventional interference approximation method (IAM) and frequency-division multiplexing (FDM) methods. To evaluate the performances of the proposed approaches, we also derive the Cramer-Rao Bound (CRB) bound, and carry out numerical summations with respects to both mean square error and bit error ratio metrics.

A Modified Interference Approximation Scheme for Improving Preamble Based Channel Estimation Performance in FBMC System

Filter bank multi carrier (FBMC) is considered a competitive waveform candidate for 5G that can replace orthogonal frequency division multiplexing (OFDM). However, channel estimation (CE) is a big challenge in FBMC because it suffers from intrinsic interference which is due to the orthogonality of the subcarrier functions in the real field only. In this paper, we investigate a proposed modified interference approximation scheme (M-IAM) by approximating the intrinsic interference from the neighboring pilots to accommodate the complex channel frequency and thus improving CE performance besides simplifying its processing. The M-IAM scheme has larger pseudo pilot magnitude than other conventional preamble schemes, namely the interference approximation method (IAM) with its versions (IAM-C) and (E-IAM-C); in addition to the novel preamble design (NPS). In addition, the proposed (M-IAM) scheme is characterized by the lower transmitted power needed. The CE performance of the M-IAM is investigated through 512 and 2048 subcarriers via different types of outdoor and indoor multipath fading channels that are time-invariant such as IEEE 802.22, IEEE 802.11, Rician, and additive white Gaussian noise (AWGN), as well as time varying channels such as Rayleigh and Vehicular A (Veh-A). Simulation results demonstrate that the proposed M-IAM scheme achieves a lower bit error rate (BER), lower normalized mean square error (NMSE) and lower peak-to-average power ratio (PAPR) over the conventional preamble schemes under the aforementioned channel models. The proposed scheme has the advantage of saving the transmitted power, a requirement that could match 5G low power requirements.

Experimental demonstration of blind equalization for OFDM/OQAM-VLC system

Orthogonal frequency-division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) in direct-detection system obtains many benefits such as free of cyclic prefix (CP) and high side-lobe suppression ratio in comparison with CP-OFDM signal. Unfortunately, for OFDM/OQAM signal, it needs an extra interference cancellation process to eliminate the imaginary intrinsic interference (IMI) when using one-tap frequency equalization known as interference approximation method (IAM). Blind equalization without a pilot overhead for OFDM/OQAM signal is proposed and experimentally demonstrated in visible light communication (VLC) system with limited modulation bandwidth. Experimental results show that blind equalization for OFDM/OQAM-VLC can effectively eliminate the IMI effect and can exhibit better symbol error rate on different subcarriers. Furthermore, compared with conventional IAM, it exhibits 3.2 and 2.7 dB overall improvement in Q2-factor using blind equalization based on the constant modulus algorithm and constant norm algorithm, respectively. In addition, the system can achieve a total data rate of 3.96 Gb / s over 5-m free space link with a bit error rate lower than hard-decision forward error correction limit of 3.8 × 10 − 3.

Time Domain Channel Estimation for MIMO-FBMC/OQAM Systems

In this paper, a general preamble-based time domain channel estimation model for multiple-input multiple-output (MIMO)-filter bank multicarrier (FBMC) system based on offset quadrature amplitude modulation (OQAM) is derived. In contrast to the conventional frequency domain interference approximation method (IAM)-based channel estimation model, the derived time domain model does not require symbol time to be sufficiently longer than the maximum channel delay spread, viz. the derived time domain model does not require the channel to be frequency flat. Based on the time domain model, the weighted least squares (WLS) and minimum mean square error (MMSE) channel estimators for MIMO-FBMC/OQAM systems are investigated. The Cramer–Rao lower bound for the WLS, MMSE and IAM estimators are derived. It is shown numerically that all the estimators achieve their respective bounds, and the proposed WLS and MMSE estimators significantly outperform the existing IAM channel estimator. The comparison of the bit error rate performance of the aforementioned estimators also validates our work.

An improved least squares channel estimation algorithm for coherent optical FBMC/OQAM system

In this paper we propose an improved least squares (LS) based channel estimator for 60 Gbps optical filter bank multicarrier with off-set quadrature amplitude modulation (FBMC/OQAM) system by employing an interference suppression technique based on discrete Fourier transform (DFT). The interference approximation method (IAM) is used to combat the inherent intrinsic interference in FBMC/OQAM system, however, residual intrinsic interference still exists in the system and it has significant impact to the LS channel estimator. The proposed technique suppresses the residual intrinsic interference and channel noise effects in the estimated optical channel impulse response by adaptive optimal channel energy criterion. Simulation results show that, the improved channel estimator is capable of estimating the channel that includes attenuation and chromatic dispersion, and suppressing the residual intrinsic interference and channel noise more effectively than the conventional LS-based channel estimator. The proposed approach is an appropriate alternative for optical FBMC/OQAM system with good trade-off between complexity and performance.

Low-Complexity Coverage Analysis of Downlink Cellular Network for Combined LOS and NLOS Propagation

Recent works on coverage analysis distinguish between the line-of-sight (LOS) and non-LOS links, as they have a significant channel-gain difference. However, analyzing the effect of complex wireless channels while distinguishing between them requires highly complex computation. Herein, we propose an interference approximation method to reduce the computational complexity of such analysis comparing with previously proposed ones. Furthermore, this proposal enables coverage analysis with an only one-fold integral expression while ensuring accurate analysis for various blockage models and parameters, which is validated through simulation.

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.

Sparse Preamble Design for Polarization Division Multiplexed CO-OFDM/OQAM Channel Estimation

In this paper, we consider the polarization division multiplexed (PDM) coherent optical-orthogonal frequency division multiplexing/offset-quadrature amplitude modulation (CO-OFDM/OQAM) system. The problem of channel estimation for this system is investigated. Firstly, we introduce the sparse preamble channel estimation method for PDM CO-OFDM/OQAM and then propose a novel sparse preamble pattern. The interference approximation method is also taken into account for comparison. Simulation results, which are in accordance with the analysis, show that the proposed sparse preamble not only achieves significant gains in terms of bit error rate, but also offers lower peak-to-average power ratio and preamble overhead compared to the available solutions.

Efficient frequency-domain channel equalization methods for dual-polarization orthogonal frequency-division multiplexing/offset quadrature amplitude modulation-passive optical network

We present dual-polarization orthogonal frequency-division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) transmission for passive optical network (PON) with intensity modulation and direct detection, which has high spectral efficiency and high robustness against chromatic dispersion (CD) and polarization mode dispersion (PMD). The frequency-domain optical fiber channel transmission model for dual-polarization OFDM/OQAM-PON with the CD- and PMD-induced intrinsic imaginary interference (IMI) effect is systemically deduced. The intrasymbol frequency-domain averaging (ISFA) and minimum mean-squared error (MMSE) with the full loaded (FL) and half loaded (HL) preamble structures are used to mitigate the IMI effect. Compared with the conventional interference approximation method, the ISFA and MMSE offer improved receiver sensitivity. For channel estimation, the FL method is more effective than the HL method in mitigating the IMI effect and optical noise.