Clipping Distortion Suppression of Directly Modulated Multi‐IF‐over‐Fiber Mobile Fronthaul Links Using Shunt Diode Predistorter

Herein, we demonstrate clipping distortion suppression of directly modulated multi‐IF‐over‐fiber links using a simple shunt diode predistorter. The dynamic range of a directly modulated analog fiber optic link is limited by nonlinear distortions caused by laser‐diode clipping. We investigate the link performance in the context of carrier‐to‐noise and distortion ratio (CNDR) and error vector magnitude (EVM) requirements when supporting LTE‐A services. We also design an analog predistorter with a shunt‐diode structure, and demonstrate experimentally that the predistorter has the ability to suppress clipping‐induced third‐order intermodulation distortions of the link by at most 14 dB. It also improves the CNDR and EVM of the 4‐IF‐multiplexed LTE‐A carriers by 7 dB and 2.9%, respectively.     

Effects of clipping-induced impulse noise in externally modulatedmultichannel AM/M-QAM video transmission systems

The effect of clipping-induced impulse noise in externally modulated multichannel AM/M-QAM video transmission systems is investigated. It is shown that the bit-error-rate of the M-QAM signals in the symmetric double-clipping events is, to the first order approximation, twice the single clipping events under the same conditions. The results are in good agreement with the experimental data for both 1319-nm and 1550-nm externally modulated laser transmitter-based systems     

光纤AM CATV外调制发射技术

就光纤AMCATV外调制发射机的关键问题，即光源的选择、调制器的选择、非线性补偿、限幅失真抑制及CSO和CTB的闭环控制进行了较全面的论述。     

副载波复用外调制系统限幅失真抑制技术

对副载波复用光波系统中几种限幅失真抑制方法进行了论述和分析,特别指出一种性能较好的预编码技术,该技术可作为混合ＡＭ－ＶＳＢ／Ｍ－ＱＡＭ副载波复用旬调制光发射驱动电路的设计依据。     

Fiber-reflection-induced impairments in lightwave AM-VSB CATVsystems

Impairments due to multiple fiber reflections are evaluated for amplitude-modulated (AM) vestigial-side-band (VSB) lightwave cable TV (CATV) systems using directly modulated distributed feedback (DFB) lasers or externally modulated diode-pumped YAG laser transmitters. For DFB lasers, the spectral broadening caused by laser chirp results in a Gaussian optical spectrum with a width near 5 GHz. Square-law detection of multipath signals from two -30-dB reflections results in an effective intensity noise near -150 dB/Hz. For externally modulated YAG lasers, the same multiple reflections result in interference that cannot be described by an effective RIN. Although the total interference power is comparable to that of the DFB lasers, the signal and interference are highly correlated. As a result of this correlation, multipath interference for YAG-based systems cannot be measured using the standard test procedure with unmodulated carriers     

A Turbo Detection Structure for Uplink Coded MC-CDMA System With Distortion Cancellation Capability

In this paper, a modified turbo detector is proposed, which, by combining a standard turbo detector with an iterative reconstruction method, cancels multiaccess interference (MAI) and clipping distortion in uplink coded multicarrier code-division multiple-access (MC-CDMA) systems. For clipping noise cancellation, signal reconstruction methods use nondistorted samples to reconstruct distorted samples; however, in MC-CDMA systems, the MAI causes these methods to not work properly. On the other hand, because of clipping, MAI-cancellation methods do not work properly. In this paper, we propose a joint iterative MAI and clipping noise-cancellation method, which is composed of a standard turbo detection structure and a clipping error estimator in each iteration. Furthermore, we investigate the convergence of the proposed method. Simulation results show that the proposed method improves the performance of coded MC-CDMA systems in the uplink in the presence of clipping distortion.      

Significant performance advantage of electroabsorption modulatorintegrated distributed feedback laser (EML) transmitter in transportingmulticarrier QAM signals

Data are presented that show that, for transporting quadrature amplitude modulated (QAM) radiofrequency (RF) subcarriers in suboctave frequency range, electroabsorption modulator integrated distributed feedback lasers (EMLs) can he modulated with significantly higher (2.5 times) modulation index without any in-band signal distortion as compared to the directly or externally modulated (using LiNbO₃ MZ modulator) distributed feedback (DFB) laser transmitters in the 1.55-μm band. This occurs when the selection of frequency range and biasing the modulator section of an EML is such that the third-order intermodulation distortion is suppressed and the second-order distortion is outside the in-band frequency range. We have used an equivalent of 64- and 256-QAM 40 carriers in 550-800 MHz with the test carrier modulated with 5.063 M symbols per second. For a given bit error rate (BER), the receiver sensitivity was as much as 4 dB (optical) higher with an EML-based transmitter compared to all other transmitters. The results are presented for QAM signals but they are equally applicable for other formats of digital modulation of RF carriers, such as quadrature phase shift keying (QPSK) in suboctave frequency range     

Compensation for fiber-induced composite second-order distortion inexternally modulated lightwave AM-SCM systems using optical-phaseconjugation

Compensation for composite second-order (CSO) distortion due to the interplay of fiber chromatic dispersion and self-phase modulation in externally modulated amplitude modulated subcarrier multiplexing (AM-SCM) systems employing optical-phase conjugation (OPC) is investigated. The simulation results show that significant reductions (>20 dB) of fiber-induced CSO may be achieved using OPC, which results in an increase of the transmission distance in AM-SCM video distribution systems operating near 1550 nm. In addition, a comparison with another compensation technique, the phase modulation technique, is established, showing that OPC is much less sensitive to variations in the fiber length. However it is also calculated that for practical implementation of the OPC-based compensation technique, 25 to 30 dB reduction in the amplified spontaneous emission (ASE) associated with the OPC will be required     

LED clipping distortion compensation in optical wireless communication via multiple transmit LEDs

A method to reduce signal clipping distortion in indoor optical wireless communication systems based on orthogonal frequency division multiplexing (OFDM) modulation is presented in this paper. Compensating the resulting clipping distortion due to LED operational constrains is achieved through the use of iterative signal clipping (ISC) and multiple light emitting diodes (LED) at the transmitter. ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photo diode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion due to clipping is eliminated or significantly reduced based on the considered number of LEDs.     

Interfaces for high speed fiber-optic links: analysis andexperiment

An analysis of directly and externally modulated fiber-optic links is presented. The theoretical analysis is based on the signal flow diagram of the interface circuits to the laser diode, Mach-Zehnder electro-optic modulator, and p-i-n photodiode. System parameters such as gain, noise figure, two-tone intermodulation distortion, and spurious free and compression dynamic range are expressed as a function of frequency and operating point of the laser and external modulator. Two directly and externally modulated fiber-optic links were designed and fabricated to verify the analytical models. The direct modulation FO link was developed at the Ku-band (11.6-12.4 GHz), whereas the external modulation link was at L-band (875-925 MHz). Spurious-free dynamic ranges of 95.8 dB Hz^2/3 and 113 dB Hz ^2/3, respectively, were achieved. The predictions based on the analytical models match the measured results     

Radio-on-Fiber Transport Systems Integration With 622-Mb/s Baseband Transmission

We proposed and demonstrated an architecture by integrating radio-on-fiber transport systems and baseband (BB) transmission with the help of an injection-locking scheme. An externally modulated radio-frequency data stream and a directly modulated BB data signal are simultaneously transmitted over an 80-km single-mode fiber link. Good bit-error-rate performances were achieved in our proposed integrated systems. Such a proposed system is attractive and suitable for long-haul transmission due to its simple and cost-effective architecture.      

A new approach for evaluating clipping distortion in multicarriersystems

Multicarrier signals are known to suffer from a high peak-to-average power ratio, caused by the addition of a large number of independently modulated subcarriers in parallel at the transmitter. When subjected to a peak-limiting channel, such as a nonlinear power amplifier, these signals may undergo significant spectral distortion, leading to both in-band and out-of-band interference, and an associated degradation in system performance. This paper characterizes the distortion caused by the clipping of multicarrier signals in a peak-limiting (nonlinear) channel. Rather than modeling the effects of distortion as additive noise, as is widespread in the literature, we identify clipping as a rare event and focus on evaluating system performance based on the conditional probability of bit error given the occurrence of such an event. Our analysis is based on the asymptotic properties of the large excursions of a stationary Gaussian process, and offers important insights into both the true nature of clipping distortion, as well as the consequent design of schemes to alleviate this problem     

Distortion related to polarization-mode dispersion in analoglightwave systems

Analog transmission in single-mode fiber using chirped sources gives rise to nonlinear distortion when polarization-mode dispersion (PMD) is present. We investigate experimentally and theoretically two mechanisms for this distortion: for chirped sources, PMD in the presence of polarization-mode coupling results in second-order distortion that is proportional to the square of the modulation frequency; when polarization-dependent loss is present, an additional second-order distortion term occurs that is independent of modulation frequency. Both mechanisms give rise to distortion that is time varying due to the sensitivity of PMD to ambient temperature changes. Numerical examples indicate that these effects can limit the capacity of analog systems that use directly modulated semiconductor lasers     

Evaluation of 4-Gbit/s optical fiber transmission distance with direct and external modulation

Transmission characteristics for a recently modulated measured distributed-feedbacked (DFB) laser and an externally modulated DFB laser using a Ti:LiNbO/sub 3/, Mach-Zehnder modulator at 4 Gb/s are discussed. The transmission characteristics are estimated by an advanced eye-pattern analysis method. The maximum measured fiber dispersion with a directly modulated laser is 100 to 140 ps/nm when the chirp power penalty is 1 dB. However, for external modulation, there is no power penalty after transmission over a 2220-ps/nm dispersive fiber. This confirms that external modulation has superior transmission characteristics. The modulation scheme for 4-Gb/s systems in terms of these results is discussed.<>     

Optimization of fiber amplifier SCM lightwave video systems usingdirect and external modulation

An analysis of the design factors involved in optimizing laser transmitters for SCM video distribution networks is presented. This includes optimization of the erbium-doped amplifier, linearized external modulation, and laser overmodulation to increase the optical loss budget. A full nonlinear distortion spectral analysis is employed to determine the dependence of the optimum optical modulation index on the allowable postamplifier loss. A general optimization procedure has been developed for the fiber amplifier and modulator to maximize the postamplifier loss for a specified CNR objective. Results are presented showing a comparison between the distribution capacity of direct and externally modulated SCM-EDFA systems for VSB-AM and FM-SCM transmission systems     

Fundamental nonlinear distortions in analog links with fiberamplifiers

The author analyzes the second-order distortion in a modulated laser-based AM subcarrier multiplexed (AM-SCM) video transport system using fiber amplifiers for power boosting. All three of the involved system elements: the laser, the fiber amplifier, and the transmission fiber, contribute to the system distortion. The laser distortion originates from the internal photon-electron interaction, whereas the fiber amplifier and dispersive distortion are directly related to the intrinsic frequency chirping of the modulated laser. From the distortion magnitude and phase analysis, the author finds that under most situations the distortions have an electronic counterpart and can be electronically compensated. In addition, the system distortion can be minimized with a combination of critically chosen elements that have minimum or counteracting distortion mechanisms. Optically amplified AM-SCM systems can thus be engineered to perform well within the stringent distortion requirements for both 1.3- and 1.55-μm wavelength ranges     

High-Dynamic-Range Wireless-Over-Fiber Link Using Feedforward Linearization

We demonstrate the implementation of feedforward linearization at 5 GHz, which is the highest operating frequency yet reported, with 500-MHz linearization bandwidth having at least 24-dB distortion suppression. Simultaneous reduction of 26-dB third-order intermodulation distortion and 7-dB laser-noise reduction is achieved at 5.2 GHz, leading to enhanced spurious-free dynamic range of 107 dB (1 Hz) in a directly modulated uncooled semiconductor laser for applications in wireless-over-fiber (WoF) systems. This paper also provides detailed analysis on feedforward, theoretical distortion reduction, criteria for component selection, and the effect of dispersion. The effectiveness of feedforward in a multichannel system is demonstrated at 5.8 GHz for fixed wireless systems, such as WiMAX. These results suggest that the feedforward-linearization arrangement can make practical multichannel and multioperator WoF systems.     

Clipping distortion in lightwave CATV systems: models, simulations,and measurements

Theoretical, numerical, and experimental investigations of clipping distortions in CATV systems are reported. The applicability of A.A.M. Saleh's (1989) calculation of the carrier-to-nonlinear-distortion ratio (CNLD) is extended by incorporating a more precise spectral analysis and further analytic results. An effective transfer function model which spectrally resolves the clipping distortion at intermodulation products (IMPs) of all orders and frequencies, and features closed-form analytic calculation of the second- and third-order distortions (CSO and CTB) from basic principles, as well as the CNLD, is introduced. It is found that Saleh's model slightly overestimates the simulation results for the CNLD, while the new model is in essential agreement with the simulation for all three distortion measures. Experimental measurements of the CSO, CTB, and CNLD over a 50-dB range in distortion exhibited excellent agreement with the simulations and the new model over the entire range. The unified nature of the model allows standard CATV, CSO, and CTB measurements to be theoretically connected to the CNLD and clipping boundary     

Subcarrier multiplexing for lightwave networks and videodistribution systems

Applications for subcarrier multiplexing include a variety of analog and digital lightwave transmission systems. An overview of the requirements and capabilities of these systems is presented by describing specific examples of the most popular system types. These examples include multiuser interactive local area networks and multichannel digital, FM, and AM-VSB (vestigal sideband) video distribution systems. Limitations imposed on each by the linearity of directly or externally modulated sources, receiver noise, and relative-intensity noise are discussed     

工作点偏移对光纤AMCATV外调制传输性能的影响

对于光纤ＡＭＣＡＴＶ外调制传输，调制器的偏置工作点设置是非常重要的。通过理论分析得出：当工作点偏离最佳偏置点时，主要对系统的组合二次失真ＣＳＯ（ＣｏｍｐｏｓｉｔｅＳｅｃｏｎｄＯｒｄｅｒｄｉｓ－ｔｏｒｔｉｏｎ）产生影响，并导出相应的公式。对于ＰＡＬ／Ｄ制５５０ＭＨｚＣＡＴＶ系统，要保证ＣＳＯ小于－７０ｄＢｃ，工作点的偏移不能大于１０ｍＶ；实验结果和理论分析一致。