Intra-symbol Frequency-domain Averaging Research Articles

Intra-symbol frequency-domain averaging for turbulence mitigation in optical orbital angular momentum multiplexing.

Optical vortex beams (VBs) possessing helical phase-front have attracted considerable attention in multiplexing communication for their orthogonal orbital angular momentum (OAM) modes. However, the mode-crosstalk and signal jitter caused by turbulence fluctuation are two main challenges in OAM multiplexing communication. Here, we introduce an intra-symbol frequency-domain averaging technology (ISFA) for turbulence mitigation. By equalizing the distorted multiplexing signals, ISFA mitigates the amplitude and phase jitter of received signals without adding system complexity and information redundancy. The experimental results show that VBs are successfully demultiplexed, and the transmission rate reaches 48 Gbit/s. After ISFA, the bit-error-rate of QPSK-OFDM signals is reduced from 1.10 × 10-3 to 6.31 × 10-4, and the error-vector-magnitude (EVM) is reduced from 31.69% to 26.29% under the turbulence strength of Cn2 = 1×10-13m-2/3 and equivalent transmission distance of 200 m. By combining ISFA with MIMO diversity gain, the EVM can be further reduced from 46.70% to 26.70%. These indicate that ISFA is available for turbulence mitigation and compatible with MIMO technology, which may have perspective potential in improving the performance of OAM multiplexing communication.

Frequency-domain averaging-based hybrid channel estimation for short-reach discrete multitone transmission systems

Channel estimation is a key digital signal processing algorithm in optical discrete multitone (DMT) transmission systems. The least-square (LS)-based channel estimation is a simple method, but it is very sensitive to various noises and other interference. The estimation accuracy can be improved using the LS-based intrasymbol frequency-domain averaging (ISFA) channel estimation method. However, there exists inaccurate channel estimation for the edge data subcarriers caused by chromatic dispersion and nonsymmetric averaging windows. We propose a hybrid channel estimation method with LS-based intersymbol frequency-domain averaging and modified ISFA to improve the estimation accuracy of edge data subcarriers and the stability of the bit error rate (BER) performance. The proposed hybrid channel estimation method is experimentally investigated in a short-reach 16-ary quadrature amplitude modulation-encoded DMT transmission system. After 20-km standard single-mode fiber transmission, with the help of the proposed hybrid channel estimation and at a BER of 3.8 × 10 − 3, the receiver sensitivity in terms of the received optical power can be improved by about 1.8 dB compared to that of the conventional ISFA. Furthermore, the proposed hybrid channel estimation has better BER stability.

Integration system of photon-assisted W-band millimeter wave wireless communication and free space optical communication based on OFDM signal transmission

Abstract In this paper, we propose and experimentally demonstrate a novel multi-access orthogonal frequency division multiplexing (OFDM) signal transmission system for free space optical communication and photon-assisted W-band millimeter-wave (mm-wave) communication. This new structure can combine the advantages of the free space optical communication and the W-band mm-wave wireless communication. Besides, the OFDM signal with high spectral efficiency and anti-interference capability can improve the data transmission rate and flexibility. Furthermore, it can adapt to the requirements of different communication scenarios. Utilizing the intra-symbol frequency-domain averaging (ISFA) technique and the discrete Fourier transform-spread (DFT-Spread) technique, 1Gbaud 16-ary quadrature amplitude modulation (16QAM) OFDM signal is transmitted successfully with the distance of 100 m for single-mode fiber (SMF), 100 m for FSO link and 4 m for wireless. The BER is under the soft-decision Forward Error Correction (SD-FEC) threshold of 2 × 10−2. To our knowledge, this is the first time to realize the transmission of OFDM signal in the integration system of free space optical communication and photon-assisted W-band mm-wave communication.

Co-existence of OFDM and FBMC for resilient photonic millimeter-wave 5G mobile fronthaul

This paper presents a novel co-existence of multiple Fifth-Generation (5G) services employing Orthogonal Frequency Division Multiplexing (OFDM) and Filter Bank Multicarrier (FBMC) waveforms in a resilient photonic Millimeter-wave (mmWave) mobile fronthaul architecture. The 5G services are delivered to the remote antenna using shared optical infrastructure and simultaneously upconverted to mmWave frequencies using only one optical local oscillator (LO) for heterodyne upconversion. Radio-over-fiber technology is employed for cloud-radio access network, in which the optical LO used for mmWave upconversion can be placed either at the remote radio head shared by both downlink and uplink signals or remotely delivered from the central office. In addition, the proposed architecture supports the integration of wavelength division multiplexing technology, which enables seamless scalability and management of the photonic infrastructure. As multiple lasers beat to generate the mmWave signals, laser phase noise becomes the dominant system impairment, degrading the detection performance. In order to mitigate the effect, frequency averaging technique is applied to enhance the channel estimation. The comprehensive analysis of different transmission scenarios using industry standard VPI Transmission Maker® simulation platform has demonstrated successful co-existence and transmission of OFDM and FBMC 5G services simultaneously upconverted to 28 GHz, 38 GHz and 60 GHz mmWave signals after 40 km fiber and 2 m wireless. Compared with FBMC, OFDM exhibits better resilience to phase noise with better Error Vector Magnitude (EVM) values at low received powers, but with the application of intra-symbol frequency-domain averaging, the EVM performance of FBMC is comparable to OFDM.

Experimental research of UWB over fiber system employing 128-QAM and ISFA-optimized scheme

Abstract In this paper, an optimized intra-symbol frequency-domain averaging (ISFA) scheme is proposed and experimentally demonstrated in intensity-modulation and direct-detection (IMDD) multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband over fiber (UWBoF) system. According to the channel responses of three MB-OFDM UWB sub-bands, the optimal ISFA window size for each sub-band is investigated. After 60-km standard single mode fiber (SSMF) transmission, the experimental results show that, at the bit error rate (BER) of 3.8 × 10−3, the receiver sensitivity of 128-quadrature amplitude modulation (QAM) can be improved by 1.9 dB using the proposed enhanced ISFA combined with training sequence (TS)-based channel estimation scheme, compared with the conventional TS-based channel estimation. Moreover, the spectral efficiency (SE) is up to 5.39 bit/s/Hz.

Experimental demonstration of an OFDM based visible light communication system using inter-block precoding and superimposed pilots

In this work, we investigate and experimentally demonstrate an orthogonal frequency division multiplexing (OFDM) based high speed wavelength-division multiplexed (WDM) visible light communication (VLC) system using an inter-block data precoding and superimposed pilots (DP-SP) based channel estimation (CE) scheme. The residual signal-to-pilot interference (SPI) can be eliminated by using inter-block data precoding, resulting in a significant improvement in estimated accuracy and the overall system performance compared with uncoded SP based CE scheme. We also study the power allocation/overhead problem of the training for DP-SP, uncoded SP and conventional preamble based CE schemes, from which we obtain the optimum signal-to-pilot power ratio (SPR)/overhead percentage for all above cases. Intra-symbol frequency-domain averaging (ISFA) is also adopted to further enhance the accuracy of CE. By using the DP-SP based CE scheme, aggregate data rates of 1.87-Gbit/s and 1.57-Gbit/s are experimentally demonstrated over 0.8-m and 2-m indoor free space transmission, respectively, using a commercially available red, green and blue (RGB) light emitting diode (LED) with WDM. Experimental results show that the DP-SP based CE scheme is comparable to the conventional preamble based CE scheme in term of received Q factor and data rate while entailing a much smaller overhead-size.

A performance improvement and cost-efficient ACO-OFDM scheme for visible light communications

In this paper, we propose a performance improvement and cost-efficient discrete Hartley transform (DHT)-based asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM) scheme for visible light communications (VLC). The simple one-dimensional modulation constellation and simplified encoding structure reduce the complexity of system considerably. The DHT-spreading technique is employed to reduce peak-to-average power ratio (PAPR) of ACO-OFDM signals. Moreover, the intra-symbol frequency-domain averaging (ISFA) technique is used to increase the accuracy of channel estimation by removing the effect of ambient noise in the VLC channel effectively. To verify the feasibility of the proposed scheme, we study its performance via simulation. This scheme reduces the requirement to the resolution of DAC and increases the tolerance to the nonlinear characteristics of LED, both of which are cost-efficient. At forward error correction (FEC) limit (BER=1×10−3), simulation results illustrate that compared with DHT-based ACO-OFDM without the ISFA technique, our scheme has 3.2 dB and 2.7 dB improvement of the required Eb∕N0 when BPSK and 4-PAM are modulated, respectively.

Unbiased Channel Estimation Based on the Discrete Fresnel Transform for CO-OFDM Systems

In this letter, the deviation of the channel estimator based on intra-symbol frequency-domain averaging (ISFA) for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) is investigated. The deviation-induced estimation error is derived analytically as a function of pulse broadening caused by chromatic dispersion and averaged noise power, and thereby the optimum averaging window size can be determined. To avoid the deviation, we propose an unbiased channel estimation algorithm based on the discrete Fresnel transform (DFnT) for CO-OFDM systems, utilizing the convolution-preservation property of DFnT for intra-symbol averaging. It is shown that the DFnT-based channel estimator converges to the actual channel response under estimation, and achieves better performance than the ISFA estimator, especially in highly dispersive channels. Finally, numerical results are provided to confirm the analysis and its advantages.

Pre-compensation combined with TS-aided and ISFA-enhanced scheme for UWB system

In this paper, a pre-compensation combined with training sequence (TS)-aided and intra-symbol frequency-domain averaging (ISFA)-enhanced scheme is proposed to improve the transmission performance in 64-quadrature amplitude modulation multiband orthogonal-frequency-division-multiplexing ultra-wide band over fiber (64QAM MB-OFDM UWBoF) system. We theoretically analyze and experimentally demonstrate that the proposed scheme is suitable for the 64QAM MB-OFDM UWBoF system in contrast with two other cases: (I) only pilot-aided channel estimation and (II) pilot-aided and pre-compensation combined with ISFA-enhanced channel estimation. The experimental results demonstrate that the performance of system with the proposed scheme can be improved by about 1.25dB and 0.37dB compared with the case I and the case II, respectively, at the BER of 3.8×10−3 after 70km transmission in standard single mode fiber (SSMF).

Application of Volterra Nonlinear Compensation in 75-GHz mm-Wave Fiber-Wireless System

In this paper, we propose and experimentally demonstrate a millimeter-wave (mm-wave) system at 75 GHz with discrete Fourier transform spread (DFT-S) orthogonal frequency-division multiplexing (OFDM) signal transmission. The baseband signal is modulated by a directly modulated laser at the transmitter, and the mm-wave signal is converted to a lower frequency one by an envelope detector at the receiver. The intrasymbol frequency-domain averaging (ISFA) is used for channel estimation and a Volterra nonlinear equalizer is adopted to mitigate the nonlinearity caused by signal–signal beat interference. In this system, a 2-Gbd 16-ary quadrature amplitude modulation (QAM) DFT-S OFDM signal is delivered over 4-m wireless link under the bit error ratio of 3.8 × 10−3. Our experimental results show that the ISFA and the Volterra nonlinear equalizer can improve the system performance by about 2-dB and 1-dB receiving sensitivity, respectively. This is the first time the Volterra nonlinear equalizer has been used in a W-band mm-wave system to improve wireless transmission performance.

Efficient frequency‐domain channel equalisation methods for OFDM visible light communications

The authors present efficient frequency-domain channel estimation methods based on the intra-symbol frequency-domain averaging (ISFA), minimum mean squared error (MMSE) and weighted inter-frame averaging (WIFA) schemes for the orthogonal frequency division multiplexing (OFDM) visible light communications (VLC) system. OFDM-VLC with quadrature phase shift keying, 16- and 64-quadrature amplitude modulation mapping is experimentally demonstrated. Compared with the conventional least square channel estimation method, ISFA, MMSE and WIFA offer improved performance with MMSE offering the best performance in terms of the error vector magnitude but at the cost of high complexity. The authors show that the WIFA can improve the estimation accuracy of time-varying VLC optical channel.

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.

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.

Time-domain channel estimator for polarization interleaving direct-detection orthogonal frequency-division multiplexing passive optical network

We propose an efficient time-domain channel estimation method for polarization interleaving orthogonal frequency-division multiplexing passive optical network with direct detection. Compared with the frequency-domain least-square channel estimator and intrasymbol frequency-domain averaging, the time-domain channel estimator with overhead reduction demonstrates better system robustness against transmission impairments, such as optical noise, chromatic dispersion, and polarization mode dispersion.

Direct-detection OFDM-QPSK system performance improvement enabled by DFT spread and receiver-based ISFA algorithm

We experimentally demonstrate a direct-detection orthogonal-frequency-division-multiplexing quadrature-phase-shift-keying (OFDM-QPSK) system that is capable of delivering a 32 Gbaud OFDM-QPSK signal over 7 km single-mode fiber-28 (SMF-28). Intra-symbol frequency-domain averaging (ISFA) channel response estimation is applied to suppress in-band noise, while discrete Fourier transform-spread (DFT-spread) is used to reduce the peak-to-average power ratio (PAPR) of the transmitted OFDM signal. With the aid of ISFA-based channel estimation and PAPR reduction enabled by DFT-spread, the bit-error ratio of the system after 7 km SMF-28 transmission can be improved from 2×10−3 to error-free when the received optical power is −8.5 dBm.

Weighted interframe averaging-based channel estimation for orthogonal frequency division multiplexing passive optical network

Weighted interframe averaging (WIFA)-based channel estimation (CE) is presented for orthogonal frequency division multiplexing passive optical network (OFDM-PON), in which the CE results of the adjacent frames are directly averaged to increase the estimation accuracy. The effectiveness of WIFA combined with conventional least square, intrasymbol frequency-domain averaging, and minimum mean square error, respectively, is demonstrated through 26.7-km standard single-mode fiber transmission. The experimental results show that the WIFA method with low complexity can significantly enhance transmission performance of OFDM-PON.

Adaptive modulation and intra-symbol frequency-domain averaging scheme for multiband OFDM UWB over fiber system

To overcome the unbalanced error bit distribution among subcarriers caused by inter-subcarriers mixing interference (ISMI) and frequency selective fading (FSF), an adaptive modulation scheme based on 64/16/4QAM modulation is proposed and experimentally investigated in the intensity-modulation direct-detection (IM/DD) multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband (UWB) over fiber system. After 50km standard single mode fiber (SSMF) transmission, at the bit error ratio (BER) of 1×10−3, the experimental results show that the power penalty of the IM/DD MB-OFDM UWBoF system with 64/16/4QAM adaptive modulation scheme is about 3.6dB, compared to that with the 64QAM modulation scheme. Moreover, the receiver sensitivity has been improved about 0.52dB when the intra-symbol frequency-domain averaging (ISFA) algorithm is employed in the IM/DD MB-OFDM UWBoF system based on the 64/16/4QAM adaptive modulation scheme. Meanwhile, after 50km SSMF transmission, there is a negligible power penalty in the adaptively modulated IM/DD MB-OFDM UWBoF system, compared to the optical back-to-back case.

Demonstration of DFT-spread 256QAM-OFDM signal transmission with cost-effective directly modulated laser

We experimentally demonstrated a 256-ary quadrature amplitude modulation (256QAM) direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) transmission system utilizing a cost-effective directly modulated laser (DML). Intra-symbol frequency-domain averaging (ISFA) is applied to suppress in-band noise while the channel response estimation and Discrete Fourier Transform-spread (DFT-spread) is used to reduce the peak-to-average power ratio (PAPR) of the transmitted OFDM signal. The bit-error ratio (BER) of 15-Gbit/s 256QAM-OFDM signal has been measured after 20-km SSMF transmission that is less than 7% forward-error-correction (FEC) threshold of 3.8 × 10(-3) as the launch power into fiber is set at 6dBm. For 11.85-Gbit/s 256QAM-OFDM signal, with the aid of ISFA-based channel estimation and PAPR reduction enabled by DFT-spread, the BER after 20-km SSMF transmission can be improved from 6.4 × 10(-3) to 6.8 × 10(-4) when the received optical power is -6dBm.

An improved novel joint channel estimation algorithm for the 112 Gbit/s PDM CO-OFDM system

In this paper, an improved novel joint channel estimation algorithm is proposed for the 112Gbit/s PDM CO-OFDM system. This method combines the advantages of the intra-symbol frequency domain averaging (ISFA) method and the time domain averaging (TA) method, which can realize more accuracy channel estimation. Simulation results show that this proposal could promise a relatively stable performance even under a rapidly time varying environment. It significantly outperforms the ISFA method by 2.2dB when the DGD equals 1000ps, and has the advantages of 0.5dB over ISFA in considering the laser linewidth and frequency offset.

Improving nonlinearity tolerance of 112-Gb/s PDM OFDM systems with coherent detection using BLAST algorithm

The use of Bell Laboratories layered space-time (BLAST) architecture as a digital signal processing algorithm is proposed in this letter. It is aimed at improving the nonlinearity tolerance of a polarization division multiplexing (PDM) coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. The application of this channel estimation algorithm simulates system performance under different dispersion compensation (DC) maps. Simulation results show that, compared with intra-symbol frequency-domain averaging (ISFA) algorithm, at least 5-dB Q-factor improvement is achieved for the PDM CO-OFDM system at 112-Gb/s data rate over an 800-km standard single-mode fiber (SSMF) without DC.