In this article, we review our recent research progresses on the field programmable gate array (FPGA)-based real-time generation and reception of orthogonal frequency-division multiplexing (OFDM) signals for the short-reach direct-detection system and radio-over-fiber (RoF) system. With the low-complexity real-time digital signal processing (DSP) algorithms, a high-definition video signal with a data rate of 2.97 Gb/s transmission over 20 km single-mode fiber (SMF) without using forward error correction (FEC) technique is experimentally demonstrated in an intensity-modulated direct-detection optical OFDM system in the presence of sampling frequency offset (SFO). In addition, a real-time X-band OFDM-RoF system with heterodyne detection was also realized. With the help of Reed-Solomon coding with a multiplesymbol interleaving/de-interleaving scheme, the 16-ary quadrature amplitude modulation (16-QAM) OFDM-RoF signals over 10-m wireless and 2.26-km SMF-28 link was successfully achieved with a post-FEC bit error rate less than 1×10-9.
Recently, visible light communication (VLC) based on light-emitting diodes (LEDs) is considered as a candidate technology for fifth-generation (5G) communications, VLC is free of electromagnetic interference and it can simplify the integration of VLC into heterogeneous wireless networks. Due to the data rates of VLC system limited by the low pumping efficiency, small output power and narrow modulation bandwidth, visible laser light communication (VLLC) system with laser diode (LD) has paid more attention. In addition, orthogonal frequency division multiplexing/offset quadrature amplitude modulation (OFDM/OQAM) is currently attracting attention in optical communications. Due to the non-requirement of cyclic prefix (CP) and time-frequency domain well-localized pulse shapes, it can achieve high spectral efficiency. Moreover, OFDM/OQAM has lower out-of-band power leakage so that it increases the system robustness against inter-carrier interference (ICI) and frequency offset. In this paper, a Discrete Fourier Transform (DFT)-based channel estimation scheme combined with the interference approximation method (IAM) is proposed and experimentally demonstrated for VLLC OFDM/OQAM system. The performance of VLLC OFDM/OQAM system with and without DFT-based channel estimation is investigated. Moreover, the proposed DFT-based channel estimation scheme and the intra-symbol frequency-domain averaging (ISFA)-based method are also compared for the VLLC OFDM/OQAM system. The experimental results show that, the performance of EVM using the DFT-based channel estimation scheme is improved about 3dB compared with the conventional IAM method. In addition, the DFT-based channel estimation scheme can resist the channel noise effectively than that of the ISFA-based method.
A modulation format, orthogonal pulse amplitude modulation and discrete multitone modulation (O-PAM-DMT), is experimentally demonstrated in a hybrid fiber-visible laser light communication (fiber-VLLC) system using a cost-effective directly modulated laser and blue laser diode. In addition, low overhead is achieved by utilizing only one training sequence to implement synchronization and channel estimation. Through adjusting the ratio of PAM and DMT signal, three types of O-PAM-DMT signals are investigated. After transmission over a 20-km standard single-mode fiber and 5-m free-space VLLC, the receiver sensitivity for 4.36-Gbit/s O-PAM-DMT signals can be improved by 0.4, 1.4, and 2.7 dB, respectively, at a bit error rate of 1×10−3, compared with a conventional DMT signal.
We propose a bidirectional hybrid fiber-visible laser light communication (fiber-VLC) system. To reduce the cost of the system, the cheap and easy integration red vertical cavity surface emitting lasers, low-complexity carrier-less amplitude phase modulation format, and wavelength reuse technique are utilized. Meanwhile, the automatic gain control amplifier voltage and bias voltage for downlink and uplink are optimized. The simulation results show that, by using the proposed system, the bit error rate of 3.8×10−3 can be achieved for 16-Gbps CAP signal after 30-km standard single mode fiber and 8-m VLC bidirectional transmission. Therefore, it indicates the feasibility and potential of proposed system for indoor access network.
To improve the performance of channel estimation (CE), a method of joint frame synchronization and data-aided CE using less training overhead is proposed. A 100-Gb/s polarization-division multiplexing coherent transmission system with quaternary phase-shift keying based on the proposed method is demonstrated by simulation. The simulation results show that the proposed method could achieve accurate timing offset and CE in the presence of strong amplified spontaneous emission noise.
A full-duplex multiband orthogonal frequency division multiplexing (MB-OFDM) ultra-wideband over fiber (UWBoF) system is proposed, and bidirectional transmission of a 1.28-Gbps MB-OFDM UWB signal over 50-km standard single-mode fiber (SSMF) is demonstrated. An optical remote heterodyning mixing scheme is employed to generate a 60-GHz optical millimeter wave. Meanwhile, an optical carrier without modulation data is extracted by using a fiber Bragg grating for the uplink MB-OFDM UWB signal transmission. After 50-km SSMF transmission at a bit error rate of 1×10−4, the power penalties are 0.7 dB for a 4 quadrature amplitude modulation (QAM)-uplink and 1.0 dB for a 16QAM-uplink, respectively. The proposed scheme would greatly reduce the cost and significantly improve the spectrum utilization efficiency in the full-duplex MB-OFDM UWBoF systems.
We propose an enhanced 16 Spiral quadrature amplitude modulation (QAM) (16 E-Spiral QAM) scheme to overcome the laser phase noise in a coherent optical orthogonal frequency division multiplexing (CO-OFDM) system. Considering both additive white Gaussian noise and large phase noise, 16 E-Spiral QAM schemes have a better transmission performance compared to conventional 16 QAM CO-OFDM systems. The simulated results show that the required optical signal-to-noise ratio (OSNR) of the proposed 16 QAM is, respectively, 0.8 and 2.3 dB less than 16 Spiral and conventional 16 QAM at a bit error rate (BER) of 10−3 in a back-to-back case. After 800-km transmission over a single-mode fiber, the tolerance for the laser linewidth of the 16 E-Spiral QAM can improve about 30 kHz with an OSNR of 18 dB compared to that of a conventional 16 QAM.
In direct-detection optical orthogonal frequency division multiplexing (OFDM) systems, the high peak-to-average power ratio (PAPR) will cause nonlinear effects in both electrical and optical devices and optical fiber transmission when the nonlinear amplifiers are employed. A new hybrid technique based on carrier interferometry codes and companding transform has been proposed and experimentally demonstrated to reduce the high PAPR in an optical direct-detection optical OFDM system. The proposed technique is then experimentally demonstrated and the results show the effectiveness of the new method. The PAPR of the hybrid signal has been reduced by about 5.7 dB when compared to the regular system at a complementary cumulative distribution function of 10−4. At a bit error rate of 10−4, after transmission over 100-km single-mode fiber with a μ of 2, the receiver sensitivity is improved by 3.7, 4.2, and 5 dB with launch powers of 3, 6, and 9 dBm, respectively.
A channel estimation method is proposed in polarization-division multiplexing single-carrier frequency domain equalization (PDM-SCFDE) optical coherent communications systems. The method utilizes a pair of orthogonal training sequences (TSs) in frequency domain and combines them with a provided reconstructive algorithm. Consequently, it can estimate full PDM channel state information accurately. Meanwhile, only one block of training overhead is required in the method. In this way, it is extremely bandwidth efficient. Two blocks training overhead are needed for the other literature. Based on a 100-Gb/s PDM-SCFDE coherent optical communications system, numerical simulation results show that the bit error rate performance of the proposed method is better than that of other methods with different TSs. Moreover, the proposed method is robust against amplified spontaneous emission noise.
KEYWORDS: Orthogonal frequency division multiplexing, Modulation, Singular optics, Signal detection, Energy efficiency, Optical engineering, Radio over Fiber, Numerical simulations, Bandpass filters, Signal processing
We compared four commonly used interpolation algorithms including linear interpolation, spline interpolation, low-pass interpolation, and time domain interpolation for channel estimation based on pilots in a reversely modulated optical single sideband system with an orthogonal frequency division multiplexing signal. The results show that the spline interpolation method exhibits the best performance.
The blind equalization of minimum-shift keying (MSK) in coherent optical communication is investigated. After the classical constant modulus algorithm (CMA) equalization step, the output of the equalizer does not necessarily coincide with a delayed and rotated version of the input MSK signal. In our MSK coherent optical system, a blind equalization criterion is proposed to eliminate the latter indeterminacy. Simulation results show that the 10-Gb/s MSK system successfully recovers MSK signal from various transmission impairments.
Iterative clipping and filtering (ICF) is a useful technique to reduce the peak-to-average power ratio (PAPR) of orthogonal frequency division multiplexing (OFDM) signals. However, the classical ICF with Fast Fourier Transform/Inverse Fast Fourier Transform requires much iteration to approach a specified PAPR threshold in the complementary cumulative distribution function. To reduce the nonlinear distortion in both electrical and optical devices and in the optical fiber, we propose a novel ICF based on discrete cosine transform/inverse discrete cosine transform to reduce the PAPR in an intensity modulator and direct detection (IM/DD) optical OFDM system. Furthermore, the new technique considerably improves bit error rate (BER) and reduces the PAPR with just few iterations. The experimental results show that the receiver sensitivity at a BER of 1×10 −3 for a 2.5-Gbytes/s OFDM signal and after 200-km standard single-mode fiber transmission has been improved by 1.1, 2.3, and 3.6 dBm with launch powers of 6, 8, and 12 dBm respectively.
KEYWORDS: Orthogonal frequency division multiplexing, Signal to noise ratio, Modulation, Receivers, Data conversion, Signal detection, Adaptive optics, Computing systems, Single mode fibers, Signal processing
In order to overcome the defects of least squares (LS) estimator used to eliminate phase noise in the direct-detection
(DD) optical orthogonal frequency division multiplexing (OOFDM) transmission system, we present an adaptive Least
Mean Square (LMS) estimator which has the same computational complexity as LS estimator. The performance of the
proposed estimator is evaluated in an experimental system with 2.5Gbits/s DD-OOFDM signal transmission over 100km
standard single mode fiber (SSMF). The experimental results show that the performance of the system with LMS
estimator is much better than that with LS estimator.
We have theoretically and numerically investigated polarization-insensitive all-optical wavelength conversion for polarization multiplexing quadrature phase shift keying (QPSK) signals based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) with a polarization diversity and digital coherent detection. The simulation results show that the different frequency spacing between pumps and the signal have serious effects on the conversion efficiency, that there is an optimal SOA injection current by considering a high conversion efficiency and a small pattern effect, and that the pattern effect on polarization multiplexing QPSK signals is much lower than non-return-to-zero signals.
KEYWORDS: Orthogonal frequency division multiplexing, Signal detection, Receivers, Modulation, Signal attenuation, Signal processing, Optical engineering, Modulators, Optical amplifiers, Dispersion
We experimentally investigate the effect of pilot power allocation on the performance of baseband direct-detected optical orthogonal frequency division multiplexing system by comparing five pilot power allocation schemes. The results show that the scheme of increased pilot power allocation had the highest receiver sensitivity and frequency fading tolerance of all. Allocating more power on pilots at higher frequencies can improve the accuracy of channel estimation which leads to a better system performance.
KEYWORDS: Orthogonal frequency division multiplexing, Single mode fibers, Modulation, Radio optics, Receivers, Fiber Bragg gratings, Extremely high frequency, Dispersion, Optical amplifiers, Signal detection
We have experimentally investigated a full-duplex radio-over-fiber system transmitting 2.5-Gbit/s orthogonal frequency-division multiplexing (OFDM) signals with a 40-GHz optical millimeter wave as downlink. Meanwhile the central can be reused wavelength as the uplink connection for transmitting 2.5-Gbit/s on-off keying (OOK) signals. By experimentally comparing the transmission performance of OFDM downstream and OOK upstream signals, it can be seen that the power penalty for the downstream signals is about 1 dB at a bit error rate of 10−3 after transmission over a 50-km standard mode fiber. However, the power penalty for the upstream signals is less than 0.5 dB at a bit error rate of 10−9 after transmission over the same distance.
KEYWORDS: Modulators, Orthogonal frequency division multiplexing, Modulation, Laser sintering, Single mode fibers, Digital signal processing, Passive optical networks, Radio over Fiber, Signal processing, Network architectures
We demonstrate a novel low-cost radio-over-fiber network architecture with one directly modulated laser and one intensity modulator. By employing an orthogonal frequency division mutiplexing modulation format and frequency quadruple scheme, the bandwidth requirement of the devices and the cost of the system is reduced. Experimental results show that the power penalty for the downlink data after transmission over 20-km single mode optical fiber (SMF)-28 is smaller than 1.3 dB, while for the up-link data, the power penalty after transmission over 20-km SMF-28 is smaller than 0.3 dB.
KEYWORDS: Orthogonal frequency division multiplexing, WDM-PON, Radio optics, Modulation, Radio over Fiber, Receivers, Signal detection, Single mode fibers, Optical amplifiers, Optical engineering
Simultaneous wired and wireless link services in a novel optical line terminal for orthogonal frequency-division multiplexing (OFDM)-wavelength-division-multiplexed passive optical network compatible with radio-over-fiber system is proposed and experimentally demonstrated. In the optical line terminal, optical carrier suppression modulation scheme is used to generate a 40-GHz optical millimeter-wave (mm-wave). 2.5-Gb/s OFDM signal is carried by the 40-GHz optical mm-wave. In the base station, we can provide simultaneous wired and wireless services. The downstream OFDM lightwave can be reused to carry 2.5-Gb/s on-off Keying nonreturn-to-zero (OOK-NRZ) upstream data without erasure of the downstream data.
KEYWORDS: Modulation, Eye, Radio optics, Radio over Fiber, Fiber Bragg gratings, Double sideband modulation, Modulators, Single mode fibers, Signal generators, Telecommunications
Three different schemes to generate optical millimeter-wave and wavelength reuse for up-link connection in the radioover-
fiber (ROF) systems have been theoretically and experimentally investigated. We have compared the performance
of the three different schemes for the radio-over-fiber systems considering the cost and configuration of their
architectures. A novel scheme to generate optical millimeter-wave and realize centralize lightwave operation in the
radio-over-fiber (ROF) systems has been demonstrated. This scheme has shown high performance and low cost
compared with the existing schemes.
A hybrid optical access configuration for simultaneously providing wired and wireless
services was proposed. The proposed system can also remote deliver doubled local-oscillator
signal, which can be reused in the uplink. At the central station, the signals used for wireless links
are subcarrier multiplexing and modulated as PSK format, and the wired baseband signals are
directly modulated as NRZ format, then two signals are simultaneously generated by an external
integrated modulator and transmitted on the same wavelength. The doubled local-oscillator signal
is extracted in the wireless links, then used in the uplinks, the recieved eye diagrams are pretty
well.
KEYWORDS: Modulators, Radio optics, Signal generators, Eye, Channel projecting optics, Receivers, Radio over Fiber, Modulation, Oscillators, Optical engineering
We propose and demonstrate a novel radio-over-fiber (ROF) architecture by using only one phase modulator and one optical filter to generate an optical millimeter (mm) signal and realize wavelength reuse for upstream data connection. In the central office, the rf signals are generated by an electrical mixer that mixes the local oscillator (LO) and downstream data, and a phase modulator is used to generate an optical millimeter-wave signal. In the base station, an optical interleaver with two output ports is used to separate the optical carrier and the first-order sidebands. The separated first-order sidebands are used to generate optical millimeter-wave signals with a double LO frequency, while the separated optical carrier is reused for an uplink connection. There is no additional laser source for the upstream data generation in the base station and no dc-bias controller for phase modulator. By this method, we simplify the configuration of the radio-over-fiber system and reduce the cost of the system.
KEYWORDS: Modulators, Signal detection, Double sideband modulation, Modulation, Radio optics, Signal generators, Eye, Phase modulation, Radio over Fiber, Receivers
In this paper, a scheme of optical millimeter (mm)-wave generation by using only one phase modulator has been
investigated experimentally. To generate a mixing signal, the baseband signal is mixed with the radio frequency (RF)
signal using an electric mixer. And the double-sideband (DSB) signal generated from phase modulator driven by the
mixing signal is transmitted to base station via a single mode fiber. The central carrier and the first-order sideband are
separated by an optical interleaver at base station. The two peak modes of the first-order sideband will beat to generate
an mm-wave with a double repetitive frequency of the RF signal when they are detected by an optical receiver. The
transmission performance of the optical mm-wave is analyzed. According to our research, the different delay of two first
sidebands due to the fiber dispersion will lead to power loss of the mm-wave. And the duty cycle of the code is reduced.
It limits the largest transmission distance of the mm-wave. In the experiment, the 20GHz RF signal generates 40GHz
mm-wave and 2.5Gbps NRZ is carried by the mm-wave. For downlink connection, the power penalty is 0.2 dB after
transmission over 20-km.
Optical label switching (OLS) has been regarded as an efficient technique to route and forward IP packets transparently
in the optical layer. In label switching system, orthogonal modulation has shown some advantages, such as its high
spectral efficiency. One example is the combination of amplitude-shift-keying (ASK) label and differential phase-shift-keying
(DPSK) payload. However, differential quadrature phase-shift-keying (DQPSK) payload has not been used in
optical label switching system yet. In this letter, we propose and demonstrate a novel optical label switching scheme by
the combination modified return zero (MD-RZ) and DQPSK for the first time. The transmitter is set up by cascading two
phase modulators (PM) and one dual-arm LiNbO3 modulator (LN-MOD). The signal is first phase modulated by two
PMs. The MD-RZ label is then impressed by the subsequent LN-MOD. Duty cycle and extinction ratio of the MD-RZ
labels are tunable. The DQPSK payload can be used in a 20Gbit/s optical label switching system, with 2-bit per symbol
modulation/demodulation. Within the network, the label information is read, processed and updated, a process known as
label switching, until the burst reaches the desired egress edge node. The use of balanced detection is expected to
improve performance for the payload. The transmission performance of 20Gbit/s DQPSK payload with 2.5Gb/s MD-RZ-ASK
label is also analysed. MD-RZ labeling of a DQPSK payload has been shown to be a promising candidate for the
implementation of OLS system.
We propose a novel scheme to generate duobinary RZ and dark RZ signals. It needs only one differential Mach-Zehnder
modulator (DMZM), which can adjust the operating point of the modulator to generate duobinary RZ and dark RZ
signals easily. It is more cost-effective compared to conventional schemes due to its simple configuration.
We propose a novel scheme to generate return-to-zero differential phase-shift keyed (RZ-DPSK) and carrier-suppressed
RZ-DPSK (CSRZ-DPSK) formats based on the differential Mach-Zehnder modulator (DMZM), which can adjust the
operating point of the modulator and control chirp conditions easily. It is more cost-effective due to its simple
configuration.
KEYWORDS: Modulation, Signal detection, Radio optics, Modulators, Optical filters, Radio over Fiber, Signal generators, Extremely high frequency, Eye, Continuous wave operation
In this paper, a novel scheme to generate millimeter-wave with wavelength reuse by using one modulator has been
proposed. In such a system, a continuous wave lightwave is intensity-modulated via a dual-arm LiNbO3 Mach-Zehnder
interferometer modulator with DC biased at υ π and driven by two complementary modulation signals, which are
composed of a lower frequency RF cosine signal mixed with baseband signals by an electric mixer and a higher
frequency RF cosine signal. The optical carrier is suppressed and the generated optical signal mainly consists of four
sidebands, in which only two sidebands carry baseband signals. At the base station, an optical interleaver is used to filter
out the proposed optical mm-waves from the sidebands and another IL is used to filter out the wavelength reused for
uplink connection from the rest sidebands. The transmission performance of system is analyzed theoretically and
numerically.
We proposed and demonstrated a novel full-duplex radio-over-fiber system using an optical phase modulator and optical
interleaver to generate DWDM optical millimetre wave for carrying downstream data and wavelength reuse for uplink
connection. Since the optical phase modulator we used has not a dc-bias controller, and the optical interleaver is not as
sensitive to temperature as a fiber Bragg grating, the stability of the generated DWDM optical millimeter wave is largely
improved, and the optical power is effectively utilized because the remaining optical carriers with high power have been
reused. Moreover, the limitation of chromatic dispersion is greatly reduced due to avoiding the generation of higher order
sidebands via driving the phase modulator with optimized RF signal. So this system shows cost-efficient configuration
and good performance over long-distance delivery.
KEYWORDS: Modulation, Eye, Modulators, Radio optics, Extremely high frequency, Signal detection, Linear filtering, Signal generators, Single mode fibers, Optical filters
We proposed and experimentally demonstrated a novel method of optical mm-wave generation using only one Mach-
Zehnder modulator (MZM) and a direct-modulator based on the method of optical carrier suppression. Using this
proposed method, the 2.5Gb/s data channel was successfully transmitted over 20 km for downstream with less than 1-dB
power penalty. Therefore the system configure of mm-wave generation can be further simplified and more cost effective.
The transmission performance of system was analyzed theoretically and numerically. According to our research, the
mm-wave generated by this scheme can be immune fiber dispersion, and the configuration of the base station can be
simplified and the cost is thus reduced.
A configuration of transmitter for superimposing DPSK signals on dark RZ signals with tunable duty
cycle was proposed. The transmitter comprises two optical phase modulators and a delayed line Mach-Zehnder
interferometer. The simulation results show that the spectral efficiency is doubled by using this scheme.
We adopt the worst channel equalization (WCE) scheme to compensate polarization mode dispersion (PMD) in wavelength-division-multiplexing (WDM) systems. The degree of polarization (DOP) of single state of polarization (SOP) optical signal is used as the feedback signal of the PMD compensators, and the particle swarm optimization (PSO) algorithm is used as the searching algorithm. Simulation results show that the DOP of RZ format of the worst-performance channels after compensation is greatly improved, and the PSO algorithm is successfully experienced into adaptive multi-stage PMD compensators in a 40-Gb/s optical WDM communication system. The WCE scheme is an effective way for PMD compensation in WDM system.
We propose a polarization mode dispersion (PMD) compensation scheme for wavelength-division-multiplexing (WDM) system, in which two WDM channels share one PMD compensator at the receiver site. The effect of different modulation formats on multi-stage PMD compensators is studied and compared by numerical simulations in 40-Gb/s WDM optical fiber communication system. The degree of polarization (DOP) of single state of polarization (SOP) optical signal is used as the feedback signal in PMD compensators. The particle swarm optimization (PSO) algorithm is used as the searching algorithm in WDM systems. The compensated DOP values of return-to-zero (RZ) format and nonreturn-to-zero (NRZ) format by multi-stage PMD compensators have been increased distinctly compared to the corresponding cases without compensation. It is shown that the PSO algorithm is implemented successfully in adaptive multi-stage PMD compensation in a 40-Gb/s optical WDM system. The compensated eye diagrams for the two channels by multi-stage PMD compensators indicate that the three-stage PMD compensator which eliminates the influence of second-order PMD completely in WDM system takes the best efficiency in the multi-stage PMD compensators for RZ format and NRZ format.
We propose an adaptive polarization-mode dispersion (PMD) compensation system in which the minor axis of the degree of polarization (DOP) ellipsoid in receiver is used as feedback signal, and the length of the DOP ellipsoid is optimized through the particle swarm optimization (PSO) searching algorithm to compensate PMD. The operation of adaptive equalization is successfully demonstrated by 40 Gb/s transmission system numerically. Furthermore, we show that using DOP ellipsoid as feedback signal and PSO as searching algorithm, the first and higher order PMD can be compensated effectively.
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