软切换链路下的混合编码与副载波调制方案

为了提高自由空间光/射频（FSO/RF）混合通信链路的性能，采用混合低密度奇偶校验（LDPC）编码与副载波相移键控/正交振幅调制（PSK/QAM）联合调制的方法，对不同传输比例下混合系统的误比特率性能进行了仿真分析，取得了不同信道条件下单链路和混合链路传输方案的误比特率数据。结果表明，在弱中强湍流条件下采用副载波二进制相移键控（BPSK）调制，相比开关键控（OOK）调制可获得大约4.4dB~5.2dB的增益。采用软切换的混合LDPC编码与副载波BPSK/16QAM调制方案，依据链路状态调整比例为1:1和3:1时，不同湍流强度下可获得大约0.3dB~7.4dB的性能增益。这一研究结果对于提高FSO/RF混合通信系统的全天候高效可靠传输性能具有重要的参考价值。     

Low-Chirp 85-Gb/s Duobinary Modulator in InP Using Electroabsorption Modulators

In 2005, Griffin showed that an InP phase-shift-keying/duobinary Mach–Zehnder interferometer (MZI) modulator could achieve good transmission performance at 10 Gb/s, despite modest residual amplitude modulation in the phase modulators in the MZI (ratio of real to imaginary part of index change was $sim$0.1). Here we show that even strong amplitude modulation in the “phase” modulators (ratio of real to imaginary part of index change is $sim$ 1.0) gives good transmission performance. Allowing for strong amplitude modulation allows a significant increase in modulator bandwidth. We demonstrate low-chirp 85.4-Gb/s optical duobinary generation in a fully packaged InP photonic integrated circuit.      

System Performance of High-Order Optical DPSK and Star QAM Modulation for Direct Detection Analyzed by Semi-Analytical BER Estimation

Recently, higher order modulation formats are intensively investigated to further increase spectral efficiency for building next generation optical transport networks. Direct detection receivers are capable of detecting arbitrary modulation formats with differentially encoded phases such as differentially phase shift keying (DPSK) and star shaped quadrature amplitude modulation (Star QAM) formats. In an earlier publication of the authors (M. Seimetz , “Optical systems with high-order DPSK and star QAM modulation based on interferometric direct detection,” J. Lightw. Technol. , vol. 25, no. 6, pp. 1515–1530, Jun. 2007) a system analysis was performed where performance was mainly characterized by eye opening penalties. Here, these investigations are extended. A tool for semi-analytical BER estimation is developed allowing to calculate the BER down to small values such as $10^{-9}$ for a wide range of modulation formats, as well as for different receiver structures. Using this module, the back-to-back OSNR requirements are calculated. CD and SPM tolerances are characterized by optical signal-to-noise ratio (OSNR) penalties at ${hbox {BER}} = 10^{-9}$ . As far as the authors know, this is the first analysis of the transmission characteristics of optical 16DPSK and Star 16QAM based on the BER. Moreover, further novel aspects such as optimization of the optical and electrical receiver filter bandwidths are investigated.      

Multidimensional LDPC-Coded Modulation for Beyond 400 Gb/s per Wavelength Transmission

In this letter, we propose a multidimensional low-density parity-check-coded modulation scheme suitable for use in up to 400 Gb/s per wavelength transmission, using currently available commercial components operating at 40 gigasymbols/s. We show that we can achieve multiples of the current transmission speed with negligible penalty. At the same time, using this scheme, the transmission and signal processing are done at 40 gigasymbols/s, where dealing with all the nonlinear effects is more convenient and the polarization-mode dispersion is more manageable. In addition, we show that using the proposed technique, we can achieve an improvement ranging from 3 dB over 8-quadrature-amplitude modulation (QAM) to 14 dB over 256-QAM, and an improvement of up to 9.75 dB over the 256-3D-constellation at bit-error ratio (BER) of $10^{- 9}$. We also show that we can reach the 400-Gb/s aggregate rate with a coding gain of 10.75 dB at BER of $10^{- 12}$.      

An adaptive modulation scheme for simultaneous voice and datatransmission over fading channels

We propose a new adaptive modulation technique for simultaneous voice and data transmission over fading channels and study its performance. The proposed scheme takes advantage of the time-varying nature of fading to dynamically allocate the transmitted power between the inphase (I) and quadrature (Q) channels. It uses fixed-rate binary phase shift keying (BPSK) modulation on the Q channel for voice, and variable-rate M-ary amplitude modulation (M-AM) on the I channel for data. For favorable channel conditions, most of the power is allocated to high rate data transmission on the I channel. The remaining power is used to support the variable-power voice transmission on the Q channel. As the channel degrades, the modulation gradually reduces its data throughput and reallocates most of its available power to ensure a continuous and satisfactory voice transmission. The scheme is intended to provide a high average spectral efficiency for data communications while meeting the stringent delay requirements imposed by voice. We present closed-form expressions as well as numerical and simulation results for the outage probability, average allocated power, achievable spectral efficiency, and average bit error rate (BER) for both voice and data transmission over Nakagami-m fading channels. We also discuss the features and advantages of the proposed scheme. For example, in Rayleigh fading with an average signal-to-noise ratio (SNR) of 20 dB, our scheme is able to transmit about 2 bits/s/Hz of data at an average BER of 10 ^-5 while sending about 1 bit/s/Hz of voice at an average BER of 10^-2     

Impact of Optical Transmission on Multiband OFDM Ultra-Wideband Wireless System With Fiber Distribution

Multiband (MB) orthogonal frequency-division multiplexing (OFDM) ultra-wideband (UWB) wireless, which provides high data rate access, is required to be distributed by using optical fiber. The performance of MB-OFDM UWB over fiber transmission system is investigated considering optical modulation and demodulation impact. Theoretical analysis of the effect of fiber dispersion, optical transmitter, and optical receiver response on system performance is carried out considering amplitude and phase distortion. Experiments are conducted and verified by our theoretical analysis and good agreement is obtained. It is found that RF modulation index of $sim {hbox {4}}%$ is optimum for optical transmitter with Mach–Zehnder modulator, and optical receiver with Chebyshev-II response is the best for MB-OFDM UWB over fiber. Compared to back-to-back UWB over fiber, optical transmission is mainly limited by laser phase noise converted relative intensity noise and phase distortion induced by fiber dispersion when optimum modulation index is used. Higher modulation index is limited by amplitude and phase distortion to OFDM signal induced by optical transmitter and receiver response nonlinearities and fiber dispersion and the spectral mask. It is also found that highly received optical power is required for transmission of MB-OFDM UWB signal over fiber.      

Full polarization insensitivity of a 20 Gb/s strained-MQWelectroabsorption modulator

We report on a MQW electroabsorption modulator with tensile-strained wells. The device transmission is shown to be fully polarization insensitive, i.e. both in amplitude and phase. The modulation efficiency is over 20 GHz/V (bandwidth higher than 20 GHz and 1 V drive voltage) which is the highest figure of merit reported for any kind of polarization insensitive modulator. Full polarization independence is further demonstrated by 2.5 Gb/s transmission at 1.55 μm over 475 km of standard fiber without penalty at 10^-9 BER whatever the polarization     

Experimental Demonstration and Numerical Simulation of 107-Gb/s High Spectral Efficiency Coherent Optical OFDM

Orthogonal frequency-division multiplexing (OFDM) is a multicarrier modulation format in which the data are transmitted with a set of orthogonal subcarriers. Recently, this modulation format has been actively explored in the field of optical communications to take advantages of its high spectral efficiency and resilience to chromatic and polarization dispersion. However, to realize the optical OFDM at 100 Gb/s and beyond requires extremely high electronic bandwidth for the electronic signal processing elements. In this paper, we investigate orthogonal-band-multiplexed OFDM (OBM-OFDM) as a suitable modulation and multiplexing scheme for achieving bandwidth scalable and spectral efficient long-haul transmission systems. The OBM-OFDM signal can be implemented in either RF domain, or optical domain, or a combination of both domains. Using the scheme of OBM-OFDM, we show the successful transmission of 107 Gb/s data rate over 1000-km standard single-mode fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated OBM-OFDM system is realized in optical domain which employs 2 $times$ 2 MIMO-OFDM signal processing and achieves high optical spectral efficiency of 3.3 bit/s/Hz using 4-QAM encoding. Additionally, we perform numerical simulation of 107-Gb/s CO-OFDM transmission for both single-channel and wavelength-division-multiplexed (WDM) systems. We find that the $Q$ -factor of OBM-OFDM measured using uniform filling of OFDM subbands is in fact more conservative, in particular, is 1.2 dB and 0.4 dB lower than using random filling for single-channel and WDM systems, respectively.      

PMD Compensation in Polarization-Multiplexed Multilevel Modulations by Turbo Equalization

We propose the multilevel maximum a posteriori probability (MAP) detector suitable for use in polarization-multiplexed optical communications using multilevel modulation and coherent detection. The proposed multilevel MAP detection scheme considers two symbols transmitted over two orthogonal polarization states as a two-component symbol. We demonstrate experimentally that the proposed MAP scheme is effective in eliminating the bit-error-ratio (BER) floor phenomenon introduced by the conventional MAP equalizer, which considers the symbols transmitted over both polarizations independently. To improve further tolerance to optical fiber polarization-mode dispersion, we iterate extrinsic soft information between the MAP detector and soft-iterative decoder, in a turbo equalization fashion. Penalty of less than 1.5 dB at 20 Gb/s (and BER of $10^{-6}$ ) for differential group delay of 100 ps is reported.      

Study of BER of 64-QAM signal and OMI-window of feasible operationin analog/digital hybrid SCM transmission systems

In an analog/digital hybrid subcarrier multiplexed (SCM) transmission, carriers have generally been substituted for transmission signals such as AM vestigal sideband (AM-VSB) AM signals and M-QAM signals to evaluate transmission quality. In practical hybrid SCM, however, carriers are modulated by video signals or digital data, and the amplitude of a multiplexed signal composed of these modulation signals is more compressed than that of the carriers. This causes a decrease in the frequency of clipping of the multiplexed signal at the laser threshold. Consequently, the BER of the M-QAM signal in a practical hybrid SCM is lower than that of the experimental results for the same optical modulation index (OMI). However, it is difficult to prepare many practical modulation signals for experiments in a laboratory. Therefore, there is demand for a bit error rate (BER) analysis method for a multiplexed signal that includes the modulation signals needed to sufficiently evaluate the BER and determine the optimum OMI in a practical hybrid SCM. In this paper, we describe such a BER analysis method that can effectively estimate the BER in a practical hybrid SCM. In practical systems, the BER was greatly improved over the BER of a multiplexed signal of carriers. Furthermore, BER degradations due to clipping can be neglected for the AM signals in setting a practical OMI range. We used this analysis method to study the effective OMI range of AM and M-QAM signals. By assuming modulation signals, the OMI range is enlarged and, significantly, the OMI of an AM signal becomes suitable for setting practical values in AM-SCM transmission. This OMI range is more practical than those of former studies     

A high‐speed long‐haul wavelength division multiplexing–based inter‐satellite optical wireless communication link using spectral‐efficient 2‐D orthogonal modulation scheme

In this work, we propose a novel multi‐bit/symbol spectral‐efficient optical orthogonal modulation scheme based on simultaneously modulating differential quadrature phase shift keying (DQPSK)‐polarization shift keying (PolSK) in a 16‐channel wavelength division multiplexing (WDM)‐based inter‐satellite optical wireless communication (IsOWC) system. Through numerical simulations, we demonstrate a reliable transportation of 16 × 100 Gbps information over 25 000 km of transmission range with acceptable bit error rate (BER) using the proposed system. Further, the impact of space turbulences (ie, pointing error losses) on the BER performance of the proposed IsOWC link has been evaluated using numerical simulations. The simulation results report a successful transportation of information up to 2.7 μrad receiver pointing error angle with acceptable performance.     

The influence of cross-phase modulation on optical FDM PSK homodynetransmission systems

A combination of optical frequency division multiplexing (FDM) and phase-shift-keying (PSK) homodyne detection can increase transmission capacity. However, phase sensitive transmission systems, especially repeatered ones, suffer from data-dependent optical amplitude fluctuation that is converted to phase fluctuation by fiber nonlinearity. The authors discuss how this data-dependent amplitude fluctuation affects the error rate performance of optical FDM PSK homodyne detection systems. If only the optical amplitude fluctuation induced by phase modulators is taken into account, the allowable power fluctuation to keep the power penalty at 0.5 dB at a bit error rate (BER) of 10^-10 is below 0.17 mW for BPSK homodyne detection and 0.09 mW for QPSK homodyne detection. However, if only the amplitude fluctuation induced by the fiber chromatic dispersion is taken into account, the allowable number of repeaters to keep a 0.5-dB power penalty due to XPM at a BER of 10^-10 is 1 for BPSK homodyne detection and below 5 for QPSK homodyne detection     

A Novel Scheme to Generate 112.8-Gb/s PM-RZ-64QAM Optical Signal

We propose and experimentally demonstrate a novel scheme to generate eight-level electrical signals for a high-speed 64 quadrature amplitude modulation (64QAM) optical signal. The required optical signal-to-noise ratio at a bit-error rate of $2times 10^{-3}$ for the single- (56.4 Gb/s) and dual-polarization (112.8 Gb/s) return-to-zero 64QAM is 21 and 26.5 dB, respectively.      

Recursive carrier phase tracking for synchronous multilevel 4QAMreceivers

Four-quadrature amplitude modulation (4QAM) allows high spectral efficiency in coherent optical data transmission by exploiting the two phases and the two polarizations of the propagating wave. In this paper the performance of 4QAM is analyzed in the presence of laser phase noise and photodetection noise. A decision feedback phase tracking scheme based on the Kalman filter and leading to a novel hybrid PLL structure is employed and its accuracy evaluated. The computational complexity of the proposed phase estimator is also discussed. The performance of the resulting 4QAM system is evaluated in terms of bit error probability versus the signal-to-noise ratio E_b/N₀, for different ratios of the symbol rate R_s over the laser linewidth B_L     

Demonstration of 20 Gbit/s subcarrier multiplexed transmissionsystem

A 20 Gbit/s subcarrier multiplexed (SCM) optical transmission system is presented. To date, this represents the highest aggregate bit rate SCM transmission that has been demonstrated. Results show that the bit error rate (BER) is less than 10^-12 for a link loss of 29.5 dB, which corresponds to 118 km single mode fibre (SMF). Furthermore, excellent performance is demonstrated after transmission over 82 km of dispersion compensated SMF. Results suggest that the system can support transmission over multiple spans of optical fibre     

Estimation of the Bit Error Rate for Direct-Detected OFDM Signals With Optically Preamplified Receivers

In this paper, we provide a numerical bit error rate (BER) estimation approach for direct-detected orthogonal frequency division multiplexing (OFDM) signals in the presence of optical preamplified receivers. The individual BER of each subcarrier is first computed by considering their electrical signal-to-noise ratio (SNR), and then the ensemble BER is derived simply by taking the average of all the subcarriers' BERs. The calculated BER is verified by the conventional error-counting approach with high precision and is still accurate with higher quadratic-amplitude modulation (QAM) formats, even under the influences of the optical filtering and polarization mode dispersion (PMD) effects. Based on our simulation approach, the required extra power budget for 16- and 64-QAM relative to 4-QAM format are found to be ${sim}$3.8 and 8.2 dB, respectively, at a BER of 10$^{-9}$. Furthermore, we use this approach to compare the receiving sensitivities and PMD tolerances for the previous proposed gapped and interleaved radio-frequency (RF)-tone-assisted OFDM systems. The results show that the gapped OFDM has a better sensitivity while the interleaved OFDM has a more PMD-tolerable capability.      

Weighting Factor Estimation Method for Peak Power Reduction Based on Adaptive Flipping of Parity Bits in Turbo-Coded OFDM Systems

In this paper, we propose a weighting factor (WF) estimation method for peak power reduction (PPR) based on adaptive flipping of parity carriers in a turbo-coded orthogonal frequency-division multiplexing (OFDM) system. In this PPR scheme, the peak-to-average power ratio of a turbo-coded OFDM signal is reduced with adaptive flipping of the phase of the parity carriers corresponding to the WFs. At the receiver, the WFs are estimated at a turbo decoder by exploiting the redundancy of an error-correcting code using no extra side information. The proposed WF estimation method is based on an iterative decoding of the turbo code, i.e., the turbo decoder provides not only error correction capability but the WF estimation function as well. When the proposed WF estimation method is used for the system using a turbo code with constraint length $K = 4$ and a code rate of $R = 1/2$, the instantaneous power of the OFDM signal at the complementary cumulative distribution function of $10^{-4}$ can be reduced by about 2.1 dB through the application of the PPR scheme. When the bit error rate (BER) performance is evaluated as a function of the peak signal-to-noise power ratio (PSNR), the proposed method achieves better BER performance than the case without the PPR in an attenuated 12-path Rayleigh fading condition. The improvements in BER performance as a function of PSNR are about 1.1, 2.0, and 2.1 dB at $hbox{BER} = 10^{-4}$ for turbo-coded OFDM signals using QPSK, 16-state quadrature amplitude modulation (QAM), and 64-state QAM schemes, respectively.      

Proposal to Achieve 1 Tb/s per Wavelength Transmission Using Three-Dimensional LDPC-Coded Modulation

We propose a three-dimensional (3-D) low-density parity check (LDPC)-coded modulation scheme that enables optical transmission beyond 320 Gb/s in aggregate rate using currently available commercial components operating at 40 giga-symbols/s. The proposed scheme introduces significant performance improvement of up to 4.1 dB at a bit-error ratio of 10^-9 over the corresponding two-dimensional scheme. In addition, by using LDPC-coded 1024-3D-constellation it is possible, at least in theory, to achieve beyond a total of 1-Tb/s transmission using transmission equipment operating at 100 giga-symbols/s, once it reaches the maturity of 40-Gb/s systems.     

基于矢量天线的新型三维调制解调方法

借助于矢量天线,提出了一种将信号相位、极化幅度与极化俯仰角参数进行三维调制的方法。给出了极化与相位联合三维调制解调的实现策略,讨论了三维联合调制的星座图和星座点分布方案。推导出了三维联合调制信号误码率的解析表达式并进行了仿真。分析与仿真结果均表明:提出的调制方法具有传输速率高和误码率低的特点。     

Coherent detection of optical quadrature phase-shift keying signals with carrier phase estimation

This paper describes a coherent optical receiver for demodulating optical quadrature phase-shift keying (QPSK) signals. At the receiver, a phase-diversity homodyne detection scheme is employed without locking the phase of the local oscillator (LO). To handle the carrier phase drift, the carrier phase is estimated with digital signal processing (DSP) on the homodyne-detected signal. Such a scheme presents the following major advantages over the conventional optical differential detection. First, its bit error rate (BER) performance is better than that of differential detection. This higher sensitivity can extend the reach of unrepeated transmission systems and reduce crosstalk between multiwavelength channels. Second, the optoelectronic conversion process is linear, so that the whole optical signal information can be postprocessed in the electrical domain. Third, this scheme is applicable to multilevel modulation formats such as M-array PSK and quadrature amplitude modulation (QAM). The performance of the receiver is evaluated through various simulations and experiments. As a result, an unrepeated transmission over 210 km with a 20-Gb/s optical QPSK signal is achieved. Moreover, in wavelength-division multiplexing (WDM) environment, coherent detection allows the filtering of a desired wavelength channel to reside entirely in the electrical domain, taking advantage of the sharp cutoff characteristics of electrical filters. The experiments show the feasibility to transmit polarization-multiplexed 40-Gb/s QPSK signals over 200 km with channel spacing of 16 GHz, leading to a spectral efficiency as high as 2.5 b/s/Hz.