Research On Key Signal Processing Schemes In Indoor Visible Light Or Coherent Optical Fiber Based Communication Systems

With different transmission media,lightwave communications can be categorized into optical fiber communication and optical wireless communication.On one hand,driven by development of information technologies,conventional optical fiber communication systems are facing the pressure of continuing increase of capacity.Due to advantages including high spectral efficiency,coherent optical communication has been widely used and become the mainstream in long-haul networks and metropolitan area networks.It has also been adopted in optical access networks.As one of key technologies,carrier phase estimation algorithms using digital signal processing(DSP)have been fully developed and come into practical application.However,there are still some issues which need to be investigated for performance improvement.On the other hand,for indoor scenarios within the range of personal activities,visible light communication(VLC)has been a hot research area for indoor wireless access in “the last few meters”.VLC is free of limitation of spectrum licenses.It also features high frequency reuse.Currently,research works of VLC are mainly focused on improving transmission rate in a simple point-to-point case.However,system practicability and feasibility issues in real indoor scenarios are ignored.In indoor scenarios,transmission performance of VLC systems will be severely affected by factors like multi-path channel as well as dimming control,which will also limit the commercialization of VLC.Therefore,we need to rethink the conceptual design of future indoor VLC systems.For example,data transmission,positioning and illumination need to be integrated and supplement with each other.This paper focuses on several key technologies in both indoor VLC systems and coherent optical communication systems,respectively.By using DSP techniques,signal manipulation is conducted to improve transmission performance and facilitate system practicability.The main contributions of this paper are as follows.1.In indoor VLC systems,the characteristics of incoherent orthogonal frequency division multiplexing(OFDM)signals are investigated in the time domain.A time domain reshuffling(TDR)method for both DC-biased optical(DCO-)and asymmetrically clipped optical(ACO-)OFDM are proposed,respectively,as well as its unified design.The implementation of TDR is very simple.By using ingenious preprocessing operations for frequency subcarriers,OFDM signal sequence in the time domain can be reshuffled.Therefore,potential high peaks at the symbol leading edge can be relocated within each OFDM symbol.Results show that TDR can significantly alleviate the impact of inter-symbol crosstalk and improve system bit error rate(BER)performance under partial non-ideal transmission conditions like indoor dispersive channel and insufficient guard interval.2.In indoor VLC systems,this paper proposes a location-based equalization(LBE)concept to overcome the multi-path effect results from multiple LED sources and indoor reflections.A LBE method is further designed for OFDM systems with different implementation schemes.With LBE,the indoor location information of a receiver(Rx),particularly in terms of coordinates,can be utilized to estimate and equalize indoor VLC multi-path channel.In this way,indoor VLC,for the first time,becomes an indoor location-based value-added service.Results show that the proposed LBE scheme can effectively counteract VLC multi-path effect and improve system BER performance.LBE is also robust against both horizontal and vertical location errors for a VLC Rx.3.In indoor VLC systems,an adaptive dimming control mechanism is proposed based on OFDM and pulse width modulation(PWM),which can meet receiver sensitivity requirement.In order to compensate for the reduced data rate caused by dimming,a novel dimming control method is further proposed by combining OFDM and multi-pulse position modulation(MPPM),where excess data transmission is achieved by utilizing MPPM pattern effect.Results show that with dimming control,the proposed scheme can effectively reduce both required M-QAM symbol rate and LED launching power while maintaining reliable communication at a constant data rate.Moreover,the dimming control range of VLC systems can be also extended.4.In indoor VLC systems,this paper proposes a hybrid visible light(VL)and infrared(IR)transmission and dimming control scheme.A low-power IR link is adopted to assist VL downlink.In this way,the “off” periods of dimming control signals can be efficiently utilized to transmit data information without affecting illumination function.Results show that the hybrid scheme can achieve stable transmission while ensuring implementing dimming control to be more simple and energy-saving under low illumicaton levels.Espacially,a basic data link can be maintained in the dark scenario.Therefore,the full dimming range with duty cycle between 1 and 0 can be eventually achieved.5.In coherent optical communication systems,based on decision-aided maximum likelihood(DA-ML)phase estimation,this paper proposes a cold-start method for DA-ML Rxs.By applying Mth-power algorithm to start-up data symbols,initial phase noise can be obtained.Then the Rx will be switched to conventional DA-ML mode for subsequent phase estimation.As a result,comparative BER performance can be achieved as conventional DA-ML.With cold-start,the DA-ML Rx can be started up without training sequences while adding to no extra work load.Therefore,cold-start can be used in burst-mode transmission and multi-Rx scenarios effectively.Results also verify the feasibility and robustness of cold-start.6.In coherent optical communication systems,based on conventional DA-ML algorithm,this paper proposes a flexible DA-ML phase estimation method for M-PSK systems.Considering the time varying property of laser phase noise,weight coefficients are derived based on ML criterion and introduced into conventional phase reference vector.This helps to track the time-variant phase noise and improve phase estimation accuracy.Results show that flexible DA-ML can effectively overcome the block length effect of conventional DA-ML.With no need to carefully find out the optimum block length or the optimum forgotten factor,flexible DA-ML can significantly improve system BER performance and increase laser linewidth tolerance.Through carrying out above research works,it can lay a solid foundation for VLC application in real indoor scenarios.These research works can also make an effective supplement to the practical investigation on phase estimation algorithm in long-haul coherent optical communication systems.