Key Technology Research On Visible Light Transmission And Underground Space Application

With the development of underground resource,human activity is gradually expanding to underground space.The demand for intelligent upgrading of underground space is growing.In underground space,traditional wireless technologies are challenged by limited transmission environment and difficult equipment deployment.As an emerging wireless technology,visible light transmission including visible light communication(VLC)and visible light positioning(VLP)uses light emitting diode(LED)as light source to realize wireless optical communication and indoor positioning.However,in underground environment,the existing LED based visible light transmission system has problems such as high system complexity,limited bandwidth,resource competition between communication and positioning,and difficulties of multi-user multi-rate access.Based on the actual requirements of underground scenarios,this paper investigates low-complexity bandwidth expansion technology,visible light communication and positioning(VLCP)integrated technology.non-orthogonal multiple access(NOMA),and other key technologies.The main research contents and innovation points of this paper are summarized as follows.(1)To address the demands of low-complexity and high-rate transmission in underground space,this paper proposes a separated linear equalizer based on joint optimization to improve the system bandwidth while limiting the complexity.The equalizer is divided into two parts,the pre-and post-equalizers.The amplitudefrequency response of each part is determined by the separation factor and channel characteristics.The separation factor is optimized using an iterative approach with the object of the minimum mean square error.Simulation results show that the spatial complexity of the separated equalizer grows linearly with the growth of the tap coefficients and is lower than that of the nonlinear equalizer.Under the given conditions,the proposed equalizer could provide better or similar bit error rate(BER)performance than that of the nonlinear equalizer for the VLC system.(2)To address the demands of efficient communication and positioning in underground space,this paper proposes an adaptive feedback threshold(AFT)algorithm as a basis of a resource-competition-free VLCP integrated system.The algorithm decomposes the signal into DC and AC components.The DC component is used for power estimation,which reduces the complexity of the positioning algorithm.For communication,the decomposed DC/AC components are provided to the maximum ratio combining algorithm as weighting factor and the branch signal respectively.Simulation results show that the proposed AFT algorithm has high tracking accuracy for DC components in the signal,which means efficient communication and positioning can be surpported.(3)For the multi-user multi-rate access demand of different devices in underground space,this paper proposes a filter gain based serial interference concellation(SIC)-free technique to achieve low-complexity multi-rate baseband VLC-NOMA transmission.The technique uses filter gain to express the signal to interference plus noise ratio(SINR)brought by the filters,equalizers and difference of transmission rate.With the filter gain,the SIC process can be cancelled.Numerical analysis shows that parameters such as roll-off factor and power allocation factor have an impact on the user and system SINR.In addition,an objective function containing the maximum user SINR value and fairness among users is constructed.A three-dimensional particle swarm optimization algorithm is used for parameters optimization.The simulation results show that the proposed low-complexity system can achieve VLC-NOMA transmission without SIC and obtain the same or better BER performance than the traditional VLC-NOMA technology.(4)In order to verify the effectiveness of the visible light transmission system in practical underground scenarios,this paper investigates the automated guided vehicle(AGV)control based on VLC and the location awareness in underground space based on VLP.Using the VLC system compatible with the Ethernet interface,the movement control of AGVs under line-of-sight and non-line-of-sight conditions is evaluated.For the location awareness,the multi-light source positioning based on signature code and areas switching algorithm is used to realize adaptive positioning to enhance the robustness of VLP system.Furthermore,the AFT algorithm is used to realize the VLCP integrated system in mobile environment.The research results show that the visible light transmission system can achieve low BER performance for communication and low accuracy performance for positioning under lower illumination;and high accuracy positioning can be achieved under higher illumination.