ディジタル多チャンネル周波数多重伝送システムの大容量化に関する研究

摘要

In the 21st century, broadcasting systems are going digital . The Hybrid opticalFiber and Coaxial (HFC) is typically used in the physical layer of the cabletelevision system. The research works in this thesis are motivated by the hope ofimproving the transmission quality of cable television systems by replacingcoaxial trunk lines with optical fiber. It will enhance the transmission capacityof cable television systems and provide high-speed access networks for bothdigital broadcasters and Internet service providers.First , a high density single carrier 1024 QAM transmission scheme for digitalcable TV systems is presented as successful results of feasibility study. About50 Mbps transmission within a single 6 MHz channel , higher spectrumefficiency, is achieved. The BER performance without forward error correction(FEC) of a prototype 1024 QAM modem is 10-4 at the CN ratio of 37 dB. Theimplementat ion loss is less than 1 dB. Also, a 4096 QAM transmission, 60 Mbpsin a conventional cable TV facility is achieved. Robustness against interferenceon the QAM signal is experimentally investigated.Secondly, the statistical behavior of the amplitude probability distribut ion ofintermodulation ( IM) distortion in FDM systems is experimentally investigated.In FDM systems, the non- l inearity of a laser diode or an ampl fier causes anumber of IM, i .e. CSO (composite second order ) , CTB (composite triple beat ) ,and etc. The IM disto t ions impair the digital signal and degrade the BERper formance. In order to f ind out the relationship between the IM distortionsand the BER, the amplitude probability distribut ion is measured. Experimentalresults show that the amplitude distribut ion of interference, including the IMdistor t ion and thermal noise, differsf roma Gaussian distribution. Thedistributions depend not only on i ts power but also on the kind of IM sources(digital modulated or carrier waves) , the number of IM components, and the CNratio. The BER performance varies with IM of both CW and digital QAMcarriers even when the carrier to inter ference noise ratio is the same. Thesimulation results are matched well with the measurement results.Finally, the approximation of probability dist r ibut ion function of CTB fromOFDM signals in the presence of nonlinear devices is demonstrated. In order toclarify the validity of an approximation formula, the amplitude distribution ofCTB distortion and the BER of QAM channel are measured. They are in goodagreement with the formula. On the whole, these research results and findings inthis thesis are applicable for design and implementation of conventional cable TV systems to provide high speed digital video services.