Orthogonal Frequency Division Multiplexing/offset Quadrature Amplitude Modulation Research Articles

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.

Enhanced Optical OFDM/OQAM for Visible Light Communication Systems

Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) has the advantage of high spectral efficiency due to non-requirement of cyclic prefix (CP) and guard band. However, the high direct current (DC) used in OFDM/OQAM is power inefficient for visible light communication (VLC). In this letter, an enhanced OFDM/OQAM (EO-OFDM/OQAM) scheme is proposed for VLC, which is inspired by the idea of the pulse-amplitude-modulation based hybrid optical OFDM (PHO-OFDM) scheme. In this system, the half of subcarriers of the normal branch carry useful data while the remaining subcarriers are selected to carry negative clipping noise instead of useful data for improving the power efficiency, and a flexible power allocation method is introduced. The simulation results show that compared with the PHO-OFDM, the proposed scheme has a better bit error rate (BER) performance under the same average bit rate without additional operations of interference regeneration and cancellation.

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.

A novel PAPR reduction algorithm for DCO-OFDM/OQAM system in underwater VLC

In the underwater visible light communication (VLC), the photon propagation scattering effect is obvious and the multipath effect is enhanced as the link distance increases. This paper uses orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) of filter bank multicarrier technology (FBMC), which is based on time frequency localization (TFL) filter, to overcome inter symbol interference (ISI) and inter carrier interference (ICI). Since light-emitting diode (LED) modulation needs positive modulation signal, we use the direct current (DC) bias to add on OFDM/OQAM signal so that the signal is positive. But, as the OFDM/OQAM is also multicarrier modulation system, the instantaneous fluctuation of OFDM/OQAM signal is bigger than single carrier modulation system and the nonlinear distortion is very serious. Therefore, this paper puts forward the modified overlapped selective mapping (M-OSLM) suppression algorithm to restrain the peak to average power ratio (PAPR), and optimizes the range of data symbol which affects on current data symbol by taking the characteristic of filter bank into consideration so that decrease PAPR. Thus, the instantaneous volatility of OFDM/OQAM signals is reduced in order to select the appropriate DC bias and reduce the clipping distortion. The simulation results show that compared with DC-biased cyclic prefix optical OFDM (DCO-CP-OFDM), DC-biased optical OFDM/OQAM (DCO-OFDM/OQAM) has a better overcoming ability on ISI of underwater wireless optical scattered channel. Meanwhile, the performance of PAPR reduction is greatly improved by M-OSLM algorithm. The signal-to-noise ratio (SNR) is improved by 3 dB averagely compared to DCO-CP-OFDM at the same bit error rate (BER).

Adaptive EKF Based Estimation Method for Phase Noise in CO-OFDM/OQAM System

Recently, dynamic optical network has attracted wide concern for its high efficiency and flexible configurations. Known for high spectral efficiency and flexible allocations of frequency resources orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) is a promising scheme for future dynamic networks. Phase noise estimation (PNE) and compensation are key technologies for maintaining the performance of coherent optical OFDM/OQAM system. In this paper, a simplified phase noise (PN) model for OFDM/OQAM under channel effect is deduced according to distribution feature of intrinsic interference. Some blind PNE methods are studied and their specific implementation process for OFDM/OQAM are presented. Based on PN model and Kalman filter theory, we propose a new adaptive extended Kalman filter (AEKF) blind scheme to meet the demand of flexibility in dynamic networks. Numerical results show that AEKF can adjust the implementation complexity of PNE according to the varying laser linewidth. When the commercial laser linewidth is 200 kHz, its time complexity is only 1/3 of that of modified blind phase search with feedback loop. Besides, AEKF can achieve a stable and reliable PNE performance under varying subcarrier allocation. AEKF can achieve a stable normalized linewidth tolerance over $1.5\times 10^{-3}$ under different subcarrier allocations.

Training‐based frequency synchronisation and highly frequency selective channel estimation for OFDM/OQAM systems

This work addresses frequency synchronisation and channel estimation for highly frequency selective orthogonal frequency division multiplexed systems with offset quadrature amplitude modulation. A two-step approach is proposed, wherein a coarse carrier frequency offset (CFO) estimator, which does not require channel knowledge, is designed in the first step by exploiting the correlation between the received and training signals. A time-domain model is derived for the joint estimation of fine CFO (FCFO) and highly frequency selective channel in the second step. In contrast to the frequency domain model in the literature, the derived time-domain model does not require the channel to be frequency flat. Based on the time-domain model, the weighted least squares and minimum mean square error estimators are investigated to jointly estimate the FCFO and frequency selective channel. The Cramer-Rao lower bounds (CRLBs) are derived for the proposed estimators, and are used to evaluate the performance of the joint FCFO and channel estimation in the presence of inter-symbol-interference due to the overlapping of adjacent time-domain orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) symbols. Simulation results demonstrate achievability of the CRLB bounds and performance gain of the proposed estimators over the state-of-the-art existing estimators.

Phase Rotation to Avoid Imaginary Interference Leakage in Multi-user MIMO-OFDM/OQAM Systems

Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is a powerful substitute for OFDM owing to its higher spectral efficiency and lower spectral sidelobes. However, in OFDM/OQAM systems the receiver must recover a purely real component from a complex signal, as only an orthogonal condition is provided in the real field, resulting in imaginary interference leakage in multiple input multiple output systems. In this paper, a phase rotation method to avoid imaginary interference leakage by confining intrinsic interference to the imaginary component of the signal is proposed. The method is first applied in solving the interference problem on neighboring subcarriers; thereafter, the application scope is expanded to solving interference on multiple subbands in multi-user systems. Simulation results verify the effectiveness of the proposed method, which has the additional benefit of low receiver complexity.

Analysis of the Time-Frequency Localization Property of the Filter Banks for Optical OFDM/OQAM Systems

Orthogonal frequency-division multiplexing offset-quadrature amplitude modulation (OFDM/OQAM) relaxes the orthogonal condition from the complex domain to the real field. Therefore, the cyclic prefix (CP) inserted between consecutive optical OFDM blocks, for combating the inter-symbol interference (ISI) induced by chromatic dispersion (CD) and polarization-mode-dispersion (PMD), could be removed for optical OFDM/OQAM to promote the system spectral efficiency. The ISI and the inter-carrier-interference (ICI) could be combated by using the filter banks with good time-frequency localization (TFL) property. Thus, the accuracy of channel estimation and system transmission performance largely depend on the TFL property of the filter banks for optical OFDM/OQAM. Recently, numerical experimental demonstrations and theoretical discussions of optical OFDM/OQAM have been reported. However, to our best knowledge, discussions on the TFL property of the filter banks have not been studied for optical OFDM/OQAM to date. In this paper, we systematically analyze the TFL property of the filter banks for optical OFDM/OQAM for the first time. We evaluate the TFL property of the filter banks with energy concentration, orthogonal basis, and isotropic property. Three typical filters as the Gaussian filter, the extended Gaussian function (EGF) filter and the isotropic orthogonal transfer algorithm (IOTA) filter have been employed for comparison. According to our theoretical analysis, with the use of the IOTA filter, optical impairments and channel estimation complexities could be decreased evidently thanks to the promising isotropic property. As shown in the numerous Montel Carlo simulations of the coherent optical OFDM/OQAM, system robustness against IMI, phase noise induced interference (PNII) and nonlinear effect could be improved evidently with the use of the IOTA filter.

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.

Equalisation, despreading, and beamforming scenarios of complex spread‐OFDM/OQAM in single‐input multi‐output channels

Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is a promising multicarrier system, which can achieve a better bandwidth efficiency than cyclic prefix (CP)-OFDM. However, OFDM/OQAM suffers from an inherent interference. Since in OFDM/OQAM, the data symbols are real-valued, the pure imaginary self-interference can be eliminated by removing the imaginary part of the received signal. On the other hand, in the literature, it is shown that even for complex-valued data symbols, the self-interference can be eliminated if a spreading code is employed. The resulted scheme is called spread-OFDM/OQAM. In this study, we extend spread-OFDM/OQAM for single-input multi-output channels. In case of multiple antennas, at the receiver after demodulation, the following processes must be performed: equalisation (to remove the channel effect), despreading of spreaded symbols, and beamforming of multiple antennas. We propose three scenarios to apply the mentioned processes. In the first scenario, despreading, equalisation, and beamforming are performed jointly in one step. In the second scenario, despreading and equalisation are accomplished jointly and then beamforming is performed. In the last scenario, each of those processes is applied separately. We compare the complexity of the proposed approaches. Also their performance is evaluated through simulations in terms of bit error rate.

Channel Estimation Based on IOTA Filter in OFDM/OQPSK and OFDM/OQAM Systems

In this paper, we present a study of bit error rate (BER) for orthogonal frequency division multiplexing/offset quadrature phase shift keying (OFDM/OQPSK) and OFDM/offset quadrature amplitude modulation (OQAM) systems with an isotropic orthogonal transfer algorithm (IOTA) filter. The novel noise suppression method based on an IOTA filter is proposed to reduce the error of channel estimation caused by additive white Gaussian noise (AWGN). The OFDM/OQPSK and OFDM/OQAM systems do not insert the guard interval (GI) and pilots in the signal frames, thus they possess transmission efficiency. An analysis was carried out for convolutional coded OFDM/OQPSK and OFDM/OQAM systems in Rayleigh fading channels with generator polynomials and constraint lengths. Compared with conventional OFDM/QPSK and OFDM/QAM systems with the insertion of comb-type pilots, the proposed IOTA filter-based channel estimation method can provide significant energy per bit to time-varying noise power spectral density ratio gains over time and frequency-selective propagation Rayleigh fading channels in OFDM/OQPSK and OFDM/OQAM systems.

A Capacity of OQAM/OFDM System Calculation Method Under the Effect of HPA

In the process of signal transmission of offset quadrature amplitude modulation / orthogonal frequency division multiplexing (OQAM/OFDM) system, high power amplifier (HPA) causes signal distortion due to the limited amplitude of high power signal, which lead to a decrease of capacity of the system. To calculate the system capacity under the effect of HPA, according to the Bussagang theorem, the equivalent signal-to-noise ratio (ESNR) of the signal under HPA is derived. The validity of the derivation is verified by the bit error rate (BER) performance of the system. The capacity mean and capacity variance with the effect of HPA of the system are obtained according to the derivation in the Rayleigh fading channel. This method provides reference for performance analysis and peak to average power ratio (PAPR) reduction of OQAM/OFDM system.

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.

On the time‐frequency symbol density of FBMC/QAM systems

SummaryFilter bank multicarrier (FBMC) systems suffer from an inherent interference, which needs to be eliminated at the detection process. Orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is a well‐known FBMC system, which transmits real‐valued symbols. In OFDM/OQAM, the time and frequency spacing between adjacent transmitted symbols are organized such that the inherent interference becomes pure imaginary and can be removed by a real‐taking operation. Although OFDM/OQAM provides the maximal bandwidth efficiency, it falls short in handling the spatial multiplexing techniques in multi‐input multi‐output channels. In this regard, those modified FBMC systems, which transmit complex QAM symbols (FBMC/QAM) are used to support the spatial multiplexing techniques. In this article, we present a novel matrix formulation for the FBMC/QAM transmission procedure. On the basis of this presentation, we show that the maximal achievable time‐frequency symbol density of FBMC/QAM, with the ability of perfectly removing the interference, is equal to that of the primer OFDM/OQAM.

Efficient frequency‐domain channel equalisation methods for OFDM/OQAM‐PON with intensity modulation and direct detection

In this study, the authors present efficient frequency-domain channel estimation (CE) methods based on the adjacent subcarriers frequency-domain averaging (ASFA) and minimum mean squared error (MMSE) schemes for the orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) passive optical network (PON) system. The complete channel transmission response for OFDM/OQAM-PON with the chromatic dispersion induced intrinsic imaginary interference effect is systemically derived. Compared with the conventional interference approximation CE method, ASFA and MMSE offer improved receiver sensitivity.

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.

Per-Subchannel Joint Equalizer and Receiver Filter Design in OFDM/OQAM Systems

Orthogonal-frequency-division-multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) based multicarrier system achieves better spectral efficiency in comparison with the widely used multicarrier OFDM system. However, the OFDM/OQAM system suffers from intersubchannel interference and intersymbol interference in frequency-selective channels, which cause poor bit error rate (BER) performance. To overcome these challenging issues, in this paper, we propose a per-subchannel joint equalizer and receiver filter design method based on maximizing signal-to-interference-plus-noise ratio (SINR) of each subchannel. An iterative two-step algorithm is developed to design equalizer and receiver filter jointly. At the first step, by assuming that the receiver filter impulse response is identified, an equalizer is designed for each subchannel by maximizing its SINR. At the second step, by assuming that the channel and equalizer impulse responses are known, the SINR of each subchannel is derived based on the receiver filter impulse response coefficients. In this way, the receiver filter of each subchannel is designed independently by maximizing the SINR of that subchannel. The convergence of the proposed iterative design algorithm is proven and also the designing procedure is extended for multi-input-multi-output channels. Simulation results show that the proposed OFDM/OQAM system, in addition of its superior bandwidth efficiency, significantly achieves better performance in comparison with the OFDM system. Moreover, the convergence speed of the iterative SINR maximization algorithm is evaluated by numerical assessments under different scenarios.

Experimental Demonstration of OFDM/OQAM Transmission for Visible Light Communications

We propose a modified orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) scheme for visible light communications (VLC). The OFDM/OQAM VLC system can efficiently boost the data rate, and combat multipath induced the inter symbol interference (ISI) and inter carrier interference (ICI). To combat the effect of intrinsic imaginary interference, intrasymbol frequency-domain averaging and minimum mean squared error (MMSE), combined with interference approximation method, are proposed. The experiment results show that the proposed system offers similar bit error rate performance to that of OFDM, while the bit rate is increased by 9% for the elimination of cyclic-prefix and guard band.

OFDM/OQAM Blind Equalization Using CNA Approach

This paper deals with blind equalization based on the constant norm algorithm (CNA) in orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM). The CNA is designed to fit the complex square constellations with high order. As a consequence, the received signal is reshaped in order to change the real OQAM symbols into complex ones. Although it increases the intrinsic interferences of the OFDM/OQAM signal, it is proved by analysis and through simulations that the CNA with proposed reshaped signal achieves better mean square error (MSE) performance than CMA applied to the received OFDM/OQAM signal. Furthermore, a suboptimal initialization strategy is proposed, which ensures a high convergence speed. The behavior of the proposed blind equalization method is analyzed under synchronization mismatch as well as in time-varying propagation environment.

Study of dual-polarization OQAM-OFDM PON with direct detection

An offset quadrature amplitude modulation orthogonal frequency-division multiplexing (OQAM-OFDM) passive optical network (PON) architecture with direct detection is brought up to increase the transmission range and improve the system performance. In optical line terminal (OLT), OQAM-OFDM signals at 40 Gbit/s are transmitted as downstream. At each optical network unit (ONU), the optical OQAM-OFDM signal is demodulated with direct detection. The results show that the transmission distance can exceed 20 km with negligible penalty under the experimental conditions.