Research On Some Issues About Ultra-wideband Optical/Wireless Access Networks

Under the driven of the global information technology,broadband and wireless are the the two inevitable trend of current communications industry.In this paper,Some issuess in ROF system and DDO-OFDM system are investigated.ROF system can realize ultra-wideband wireless access by transmitting millimeter-wave on optical fiber;DDO-OFDM system can be applied to optical access system effectively for its combination between the low-cost feature of direct detection optical system and the high spectral efficiency characteristics of OFDM technology.The main content of the paper and innovation are as follows:First,Concern for the low-cost networking technology and transmission capacity limition of 60 GHz ROF system,theorytical analysis is conducted on the 60 GHz millimeter waveform transmission performance over optical fiber and wireless.Experimental analysis is conducted to analyze the 10 km SSMF and 1.3m wireless transmission performance of 60 GHz signal with different data rate,and the data rate is chosen as follows: 1.25Gbps?2.5Gbps?3.75 Gbps and 5.0Gbps.The limition factors of ROF transmission performance over optical fiber and wireless are discussed for future 60 GHz system design and commercialization.Second,A simple RSSB scheme using only one intense modulator is propo sed.To enhance the MD,the RSSB scheme with optical carrier suppression(OCS)modulation format is investigated.In the proposed RSSB scheme,bandwidth limit is overcomed by using electrical coupler instead of electrical mixer.Meanwile,OCS formats is used to suppress the optical carrier for the purpose of maximum the modulation power efficiency(MPE).The influnce of MZM DC-bias and modulation depth on RSSB signal are also investigated by numerical simulation.The numerical simulation results show that the optimal MZM DC-bias for the RSSB is the null point.And when system biased at null point,the system Q factor penalty is less than 0.5dB for all MD cases after 100 km SSMF transmission.The proposed schme can realize chromatic dispersion free transmission,the numerical simulation results is consistented with the theorical analysis.Third,Adaptive code technique is proposed to combat the inter subcarrier mixing interferences and frequency selective fading.In the proposed scheme,the coding rate is chosen depended on the performace of subcarriers.And the adaptive code technique is applied to both 60 GHz ROF OFDM system and DDO-OFDM system.Theorotical analysis,numerical simulation and experimental analysis are operated to demonstrate the advantage of adaptive code technique.The results show that the performance of adaptive code technique with 1/4 ratio is improved more than 1.5dB compared with the fixed turbo code with code rate of 0.625 in the case of same length of parity bits.Since different turbo code rate can be obtained by changing the puncturing rate,adaptive code rate technique can be realized simply through traditional turbo encoder.Adaptive code rate technqiue has a strong practical value.Fourth,A IM/DD PolMux-OOFDM system without frequency guard band is proposed,which has a high spectral efficiency and an ability to realize high-speed transmission with low-speed devices.A theoretical analysis of the proposed system is presented in detail.From the theoretical analysis,we can find that t here exist cross-polarization interferences(CPI)?inter-symbol interferences(ISI)and frequency selective fading(FF)in every OFDM signal at the output of the photo-detector.In the PolMux-OOFDM system,CPI can be mitigated by estimating the cross-polarization channel responses and operating de-polarization.As the maximum symbol time offset is less than the length of CP in OFDM,the ISI can be cancelled by removing the CP in OFDM.However,since FF affects the amplitude of the subcarrier,it will introduce a deep fading in the subcarriers with high carrier frequency,and thus FF effects cannot be cancelled or compensated directly by DSP in the later.As a advanced space time block code,polarization time block code can reduce the damage caused by FF effects effectively.Then,we proposed to encoded the subcarriers in high frequency region with PTBC over two consecurtive periods to combat the FF,and the other subcarriers in the two periods of PolMux-OOFDM signal can carry different data information.Experimental analyses are operated at 10.59 Gbps IM/DD PolMux-OOFDM system and 53.8Gbps IM/DD PolMux-OOFDM system.The experimantal results show that the proposed PTBC scheme can combat FF in IM/DD PolMux-OOFDM system in both rate cases.And we can find from the BER curves of the 53.8Gbps IM/DD PolMux OFDM system,the receiver penalty is very small,which demonstrates the feasibility of the practical application of the proposed IM/DD PolMux-OOFDM system.