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。     

Rayleigh backscatter effects on 1550-nm CATV distribution systemsemploying optical amplifiers

We analyzed the applicability of externally modulated 1550-nm laser transmitters for trunking and distribution of AM CATV channels using power and in-line EDFA's. The distribution of multiple AM CATV channels over long fiber spans is degraded by the presence of Rayleigh backscatter-induced low-frequency interferometric noise. When the laser source is modulated externally, the low-frequency interferometric noise is mixed and translated around the AM carriers. Furthermore, when isolators are not used with the optical amplifiers, the low end of the broadcast channels could be severely degraded due to doubly amplified Rayleigh backscatter. Employing narrow-linewidth semiconductor or Nd:YAG laser sources at the transmitter will lower the tail of the low-frequency interferometric noise level but will increase the translated noise peak level at each AM carrier. Therefore, the standard CNR measurement techniques, which assumes the noise spectrum is flat, may not reveal the correct video picture quality seen at the customer premises. In this analysis, we compared NCTA RF CNR and baseband video SNR results using CCIR recommended unified weighting filter. We determined that for laser linewidth less than 1 MHz and with long fiber spans, baseband video SNR as opposed to RF CNR measurements should be used to characterize the performance of AM-VSB CATV broadcast distribution systems. Finally, an experimental 78-channel AM-VSB CATV distribution system is constructed employing two EDFA's simulating head-end and hub sites and we compared RF CNR and baseband video SNR measurements using a 700-kHz linewidth externally modulated 1550-nm DFB transmitter     

Numerical simulation of clipping induced distortion in externallymodulated lightwave AM-SCM systems

For the first time to our knowledge the results of numerical simulation, and experimental verification, of clipping induced distortion in externally modulated lightwave AM-SCM CATV systems are presented. Comparison of these results with clipping induced distortion in directly modulated systems show a remarkable outcome: under the same modulation conditions the composite triple beat (CTB) distortion component in externally modulated systems is 6 dB higher than in directly modulated systems     

BER sensitivity of OFDM systems to carrier frequency offset andWiener phase noise

In this contribution the transmission of M-PSK and M-QAM modulated orthogonal frequency division multiplexed (OFDM) signals over an additive white Gaussian noise (AWGN) channel is considered. The degradation of the bit error rate (BER), caused by the presence of carrier frequency offset and carrier phase noise is analytically evaluated. It is shown that for a given BER degradation, the values of the frequency offset and the linewidth of the carrier generator that are allowed for OFDM are orders of magnitude smaller than for single carrier systems carrying the same bit rate     

Effect of channel-estimation error on QAM systems with antenna diversity

This paper studies the effect of channel estimation error and antenna diversity on multilevel quadrature amplitude modulation (M-QAM) systems over Rayleigh fading channels. Based on the characteristic function method, a general closed-form bit-error rate (BER) for M-QAM systems is presented. The effect of the inaccurate channel estimation on the performance for pilot-symbol-assisted modulation M-QAM systems with antenna diversity is investigated. Simulation results for M-QAM (M = 4, 16, 64, 256, etc.) show that the analytical method can accurately estimate the system performance. Moreover, numerical results show that with the antenna-diversity technique, the BER performance improves significantly, especially in perfect channel-estimation cases. It is also found that the channel-estimation error limits the benefit of antenna diversity. By increasing the length of the channel estimator and the amplitude of the pilot symbol, more accurate channel estimation can be achieved, so that the BER performance is improved.     

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     

Performance of directly modulated DFB lasers in 10-Gb/s ASK, FSK,and DPSK lightwave systems

A comparison is presented of the performance of amplitude-shift-keying (ASK), frequency-shift keying (FSK), and differential-phase-shift-keying (DPSK) lightwave systems which operate at 10 Gb/s with directly modulated 1550-nm distributed feedback (DFB) laser transmitters and conventional 1310-nm dispersion-optimized fiber. Computer modeling techniques were used to accurately simulate the amplitude modulation response and the frequency modulation response of DBF lasers. The system performance is evaluated from simulated eye patterns for both direct and heterodyne detection. With the narrow-optical spectral widths of these signal formats, fiber chromatic dispersion limits up to 70 km were obtained for transmission at 1550-nm using conventional 1310-nm optimized fiber     

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     

CSO/CTB performances improvement by using optical SSB filter at the receiving site

We propose and demonstrate an externally modulated national television system committee (NTSC) 77-channel erbium-doped fiber amplifier-repeated system employing an optical single sideband filter at the receiving site. In comparison with conventional externally modulated fiber-optical CATV systems, excellent performances of carrier-to-noise ratio /spl ges/50 dB, composite second order /spl ges/70 dB, and composite triple beat /spl ges/69 dB were achieved for the full channel band over a 100-km single-mode fiber transport. Our proposed system is suitable for the long-haul fiber-optical CATV system.     

Performance comparison between DCF and RDF dispersion compensation in fiber optical CATV systems

An architecture of an externally modulated AM-VSB CATV 77-channel erbium-doped fiber amplifier (EDFA)-repeated system which use the combination of a single-mode fiber (SMF) and reverse dispersion fiber (RDF) as a dispersion compensation device is proposed and demonstrated. Compared to the conventional externally modulated fiber optical CATV systems with or without a dispersion compensation fiber (DCF), excellent performance of composite second order (CSO) /spl ges/ 78 dB accompanied by satisfied carrier-to-noise ratio (CNR) /spl ges/ 50 dB and composite triple beat (CTB) /spl ges/ 65 dB was obtained in the proposed system.     

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.      

Dispersion Effects in Millimeter-Wave Fiber-Radio Systems Employing Direct-Sequence Code Division Multiple Access

We investigate the impact of fiber chromatic dispersion on the performance of an externally modulated millimeter-wave fiber-radio system incorporating a broadband radio direct-sequence code division multiple access (DS-CDMA) scheme. In particular, we investigate the effect of dispersion on DS-CDMA signals with regard to CDMA code rate, center frequency, and modulator chirp. We show that the spreading of the signal spectrum using DS-CDMA can result in significant reductions in data amplitude variations normally experienced in externally modulated millimeter-wave fiber-radio systems. We also demonstrate the successful transmission of a 10-Mb/s data channel over a 39-GHz fiber-radio link employing DS-CDMA and consisting of 25 km of standard optical fiber and a 1-m radio link.     

On the Performance of the MIMO Zero-Forcing Receiver in the Presence of Channel Estimation Error

By employing spatial multiplexing, multiple-input multiple-output (MIMO) wireless antenna systems provide increases in capacity without the need for additional spectrum or power. Zero-forcing (ZF) detection is a simple and effective technique for retrieving multiple transmitted data streams at the receiver. However the detection requires knowledge of the channel state information (CSI) and in practice accurate CSI may not be available. In this letter, we investigate the effect of channel estimation error on the performance of MIMO ZF receivers in uncorrelated Rayleigh flat fading channels. By modeling the estimation error as independent complex Gaussian random variables, tight approximations for both the post-processing SNR distribution and bit error rate (BER) for MIMO ZF receivers with M-QAM and M-PSK modulated signals are derived in closed-form. Numerical results demonstrate the tightness of our analysis     

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.<>     

Error probability for orthogonal space-time block code diversity system using rectangular QAM transmission over Rayleigh fading channels

In this paper, theoretical symbol error probability (SEP) expressions are derived for orthogonal space-time block code (OSTBC) diversity systems employing arbitrary rectangular M-QAM transmission over flat Rayleigh fading channels. Independent fading between diversity channels is assumed. Channel average powers may be distinctive, identical, or mixed with both. The rectangular M-QAM results are extended to square M-QAM, M-PAM, and binary antipodal signaling. All derived expressions are in elementary forms without complicated high-order transcendental functions and unevaluated integrals and, hence, are strictly exact and can be readily simulated by the computer. Moreover, it is shown that mixed Rayleigh fading results can be readily extended to various Nakagami-m fading results. A four-transmit-antenna system with a half-rate OSTBC for 16-QAM signaling is used to demonstrate that the theoretical result is in excellent agreement with the Monte Carlo simulated result. From simulation curves, it is shown that, under the independent channel fading condition, channels with identical powers have better error rate performance than channels with distinctive powers.     

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     

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     

Approaching the capacity of the MIMO Rayleigh flat-fading channel with QAM constellations, independent across antennas and dimensions

In this study we consider the challenge of reliable communication over a wireless Rayleigh flat-fading channel using multiple transmit and receive antennas. Since modern digital communication systems employ signal sets of finite cardinality, we examine the use of the quadrature amplitude modulation (QAM) constellation to approach the capacity of this channel. By restricting the channel input to the M-QAM subset of the complex-plane, the maximum achievable information rate (C/sub M-QAM/) is strictly bounded away from the channel capacity (C). We utilize a modified version of the Arimoto-Blahut algorithm to determine C/sub M-QAM/ and the probability distribution over the channel input symbols that achieves it. The results of this optimization procedure numerically indicate that the optimal input symbol distribution factors into the product of identical distributions over each real dimension of the transmitted signal. This is shown to vastly reduce the computational complexity of the optimization algorithm. Furthermore, we utilize the computed optimal channel input probability mass function (pmf) to construct capacity approaching trellis codes. These codes are implemented independent across all antennas and symbol dimensions and, if used as inner codes to outer low-density parity check (LDPC) codes, can achieve arbitrarily small error rates at signal-to-noise ratios very close to the channel capacity C/sub M-QAM/. Examples are given for a 2-transmit/2-receive antenna (2 /spl times/ 2) system.     

Amplitude density of infrequent clipping impulse noise andbit-error rate impairment in AM-VSB/M-QAM hybrid lightwave systems

Analogue video cavalcade services in cable access television (CATV) networks usually require a very high carrier-to-noise ratio (CNR) in order to provide a good quality picture. In consequence, practical lightwave hybrid amplitude modulation (AM) M-ary quadrature amplitude modulation (M-QAM) systems are found to suffer only infrequent clipping impulse noise, which can be provably modeled as a Poisson arriving pulse train. Based on this knowledge, a new expression for the probability density function (PDF) of the clipping noise at the output of a QAM demodulation matched filter is given which can be numerically evaluated with high accuracy. The bit-error rate (BER) performance prediction is then carried out for M-QAM signals within the hybrid system in the presence of an additive mixture of Gaussian and clipping impulse noise. The agreement between the analytical results and the experimental data is quite good     

FPGA implementation of multiplierless M-QAM modulator

Adaptive multilevel QAM (M-QAM) modulation can increase throughput in a wireless packet data network. A technique is discussed for efficiently realising M-QAM modulators on field programmable gate arrays (FPGAs) using 'multiplierless' finite impulse response (FIR) filters with carry-save addition and canonic signed-digit coefficients. An adaptive M-QAM modulator supporting 4, 16, 64 and 256-QAM is presented