In the paper, a rotated quadrature amplitude modulation (QAM) based discrete multi-tone (DMT) is proposed and experimentally demonstrated in underwater optical wireless communication (UOWC). After transmission over 2.4-m water channel and 10-m free space, the experimental results show that, compared with conventional 16-QAM DMT, the received optical power (ROP) is improved 3.3dB at the bit error rate (BER) of 10-5 using the rotated QAM based DMT. In addition, for the rotated 32-QAM DMT, the receiver sensitivity can enhance 3.5dB compared with conventional 32 QAM DMT. And compared to DMT with bit loading, it is improved 1dB at the BER of 10-3.
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.
KEYWORDS: Orthogonal frequency division multiplexing, Signal to noise ratio, Modulation, Digital signal processing, Computer simulations, Visible radiation, Light emitting diodes, Wireless communications, Telecommunications, Demodulation
In this paper, to mitigate frequency selective power fading induced by multipath effect, a clustering algorithm based on k-means is proposed for OFDM-VLC system. The simulation results show that, aided by the k-means clustering algorithm, the signal-to-noise ratio (SNR) is improved by more than 7 dB compared to the scheme without k-means clustering algorithm at BER of 3.8×10-3, the hard decision forward error correction (HD-FEC) limit.
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.
A half cycle 64-quadrature amplitude modulation (QAM) Nyquist subcarrier modulation (SCM) polarization division multiplexing (PDM) intensity modulation direct detection optical communication system is experimentally demonstrated. At the receiver, training sequences-based channel estimation and an overlap frequency domain equalization method are proposed to enhance the system performance. The experimental results show that the half cycle 64-QAM Nyquist-SCM PDM signal can be transmitted over 43-km standard single-mode fibers with a bit error rate below the forward error coding threshold of 2.4×10−2.
An improved eighth-order statistics (EOS) blind phase recovery method is proposed for high-order coherent modulation formats. The method uses a multistage feed-forward carrier phase recovery algorithm by combining an EOS blind phase estimate with a maximum-likelihood carrier phase estimate. Experimental results show that the proposed new algorithm can reduce the required computational effort by more than a factor of 3 for a 16-QAM system.
A real-time base-band orthogonal frequency division multiplexing (OFDM) transceiver with symbol synchronization, channel equalization, sampling clock frequency synchronization, and adaptive modulation technique is successfully implemented by field programmable gate arrays and a 2.5-GSps digital-to-analog converter and analog-to-digital converter. The real-time optical OFDM signal at a raw bit rate of 5.156 Gbps within about 1.1-GHz bandwidth transmission over 100-km standard single-mode fiber (SSMF) is experimentally investigated in a simple intensity-modulation and direct-detection system. The experimental results show that the real-time system has a good bit error rate (BER) performance by using an adaptive modulation technique according to the conditions on the subchannels. After 100-km SSMF transmission, at a BER of 1×10−3, the power penalty is <1 dB. Moreover, there is a negligible penalty between the off-line and real-time digital signal processing results.
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 kind of blind polarization demultiplexing algorithm based on low-complexity and fast-converging independent component analysis (ICA) for quadrature amplitude modulation (QAM) coherent optical communications systems is proposed. The polarization demultiplexing is achieved by maximizing the signal’s non-Gaussianity measured by the information theoretic quantity of negentropy. We demonstrate that some approximate nonlinear functions can be substituted for the negentropy and this greatly reduces the computational complexity. An adaptive gradient optimization algorithm and a fast-converging quasi-Newton algorithm are employed to maximize the negentropy. The numerical simulation and experimental results for polarization division multiplexing quadrature phase shift keying/16QAM without neglecting polarization mode dispersion reveal that the proposed ICA demultiplexing algorithms are feasible and effective for coherent optical receivers.
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.
We have investigated novel optical-wireless architecture to provide WiMax orthogonal frequency division multiplexing
(OFDM) signals over fiber access network by simulation. The proposed architecture utilizes direct detection for WiMax
OFDM. The simulation results show that it can transmit a high capacity WiMax OFDM signal over 250km SMF
successfully.
KEYWORDS: Orthogonal frequency division multiplexing, Radio over Fiber, Single mode fibers, Modulation, Signal detection, Telecommunications, Standards development, Binary data, Data conversion, Optical communications
We have proposed and investigated a novel radio-over-fiber (ROF) system transmitting low-density parity-check
(LDPC) coded orthogonal frequency division multiplexing (OFDM) signals based on IEEE802.16 standard by
simulation. The 10-Gb/s WiMax LDPC-coded OFDM signal using 16-QAM format modulated on each subcarrier. The proposed architecture utilizes direct detection for WiMax LDPC-coded OFDM and successfully transmitted a high capacity WiMax LDPC-coded OFDM signal over 200km SMF without any amplification.
KEYWORDS: Orthogonal frequency division multiplexing, Modulation, Radio over Fiber, Single mode fibers, Bandpass filters, Receivers, Telecommunications, Optical filters, Data conversion, Photonics systems
We have proposed and investigated a radio-over-fiber (ROF) system transmitting 100Gb/s orthogonal frequency division
multiplexing (OFDM) signals by simulation. The ROF system can transmit vector modulation formats, such as OFDM
signal. The simulation results show that carrier suppression technique can be used to achieve two 50Gb/s OFDM signals
over 20km SMF successfully.
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: 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.
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.
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