Research On Key Techniques In Radio-over-Fiber Broadband Wireless Access System

As one of the key technologies in future broadband access systems,ROF(radio-over-fiber)shall play an important role in delivering gigabits/sec broadband wired and wireless data.Because of many advantages,such as high spectrum efficiency,resistence in dispersion and strong immunity to interference,OFDM(Orthogonal Frequency Division Multiplexing)for ROF has recently become a hot research spot,though the high PAPR(peak-to-average power ratio)of OFDM is an unavoidable problem to be resolved.This dissertation has researched the generation of optical millimeter wave(MMW),PON(Passive optical network)architecture compatibility with ROF system,PAPR reduction in an OFDM ROF system,and application of chaos in broadband ROF system,which is an excellent guidance in developing future broadband ROF access network in terms of theorical principle as well as application prosoerity.The complishments achieved are listed below:1.Generation of dual-frequency millimeter wave signal and full-duplex ROF systemA full-duplex ROF system of generating 40G/60GHz dual-frequency millimeter wave signals is proposed by utilizing two PMs(phase modulator).We have theoretically analyzed the principle of generating dual-frequency millimeter wave as well as practically proved the feasibility of signal transmission by simulation.When transmisitting a 2.5-Gbps NRZ(Not Return Zero)BB(baseband)data on 40-GHz and 60-GHz MMW signals over 25-km SSMF(standard single-mode fiber),the power loss is less than 1 dB at a BER(bit-error-rate)of 10-9.Moreover,the BER performance of 60GHz signal are slightly better than that of 40GHz,the reason is that 60GHz signal is generated by one-sideband,whereas the 40GHz signal is generated by two-side band which result in a serious walk-off effect due to dispersion-induced.Depending on application,subscribers should select proper wave frenquencyin terms of bandwidth requirements in practice.A full-duplex 40GHz SSB(Single-side Band)ROF system with bidirectional 10-Gbps 16QAM(16-quadrature-amplitude-modulation)OFDM signals is proposed.We have theoretically analyzed the system work principle as well as proved the feasibility of signal transmission by simulation.Based on the simulated BER curve,the OSNR(Optical Signal Noise Ratio)of uplink is larger than downlink when the BER is the same since the spectrum of reuse wavelength signal at the BS(Base Station)is broaden due to dispersion effects and noise.The influence can be neglected in practice as long as the configuration is reasonably designed.2.Convergence of wired and wireless serverceWe theoretically analyze and demonstrate architecture for delivery of wireless and wired data using one PM and 2.5-Gbps downlink signals carried on optical wave based on optical up-conversion.At the receiver,an optical interleaver with 20-GHz bandwidth is used to separate two 1st-order sidebands for 40GHz MMW signal and centre carrier for BB signal,which can support both wireless and wired mode.The simulated results show that the power loss for both wired and wireless data is only 0.2 dB at a BER of 10-9 over 40-km SSMF.Since the PM does not require complex electrical circuit for controlling bias,the stability and reliability of the system is greatly improved,and the system flexibility is enhanced by wired and wireless modulation.A WDM-ROF-PON(wavelength-divisionmultiplexed radio-over-fiber passive optical network)architecture based on 16QAM-OFDM modulation for bidirectional access networks is proposed and demonstrated.We theoretically analyze the system work principle.In our simulation,the intensity modulation of 10 Gbps 16QAM-OFDM signals are employed for downlink and the demodulated base-band signals by wired and wireless receiver are demonstrated,the optical carrier is reused for uplink.The crosstalk between downlink and uplink is very small.Considering flexibility of received signal,economy of coherent local oscillator signal,and high spectrum efficient of OFDM for uplink,the WDM-ROF-PON architecture will be a promising candidate in next optical network.3.Reduction of PAPR in OFDM Intensity-modulated Direct-detection system and OFDM-ROF systemWe experimentally demonstrate an IM/DD(Intensity-modulated Direct-detection)system reducing PAPR of 16QAM-OFDM signal with SLM(selected mapping)technique.The OFDM signals with minimal PAPR,which can be obtained by controlling rotation factors randomly,are delivered 100-km SSMF.Measured results illustrate the receiver sensitivity and BER performances have good improvements.Then a full-duplex SSB WDM-ROF(wavelength-divisionmultiplexed radio-over-fiber)system with SLM technique for decreasing PAPR is proposed,the PAPR of 16QAM-OFDM downstream signal and upstream signal is low.Simulation results show the proposed method for PAPR reduction can effectively improve the receiver sensitivity and the power penalty of the downlink(uplink)signal is about 2 dB(3dB)at BER of 10-3 after 42km SSMF.4.Appication of Chaos in broadband OFDM-ROF systemWe experimentally demonstrate an IM/DD system reducing PAPR of OFDM signal based on CSLM(chaos and SLM)technique.A novel hybrid chaotic systemis advanced,and it can control generation of phase rotation factors.The research has utilized this method to transmit OFDM signal along 100km long SSMF.Our experimental results show that the receiver sensitivity is improved with launching power increasing.Comparison with traditional SLM technique,only initialization condition as overhead information is transmitted,transmission efficiency is greatly improved,and the redundant information does not increase with the phase sequence increasing.A 256QAM-OFDM ROF system transmitting encrypted image based on chaos is proposed.In the CO(central office),every pixel of Lena image is randomly changed according to encryption pseudo-matrix,which is controlled by chaos in the process of grayscale substitution,and the encrypted image signal is encoded to 256QAM-OFDM format with CSLM(chaos and selected mapping)technique,then the OFDM signal with low PAPR is modulated on 30GHz optical mm-wave and transmission over 25-km SSMF.From the recovery constellation,the constellation points basically gathered near the corresponding emission.From the decrypted results,the original image can be demodulated if the keys and system parameters are corrected.