Research On Multi-Dimension Performance Of Visible Light Communication System Based On NOMA Mechanism

With the rapid development of various new business requirements and the explosive growth of intelligent terminal equipments,the massive data has brought challenges to the current wireless communication network in terms of spectrum resource utilization.Light-emitting diode(LED)-based visible light communication(VLC)can provide high-capacity,high-bandwidth communication services with its400 THz license-free visible light spectrum resources.Based on this advantage,it can effectively alleviate the scarcity of spectrum resources in current wireless communication networks.Moreover,due to the high reusability of visible light in space and frequency and weak wall penetration charateristic,VLC can be used for secure communication services that supporting high density and large-scale user connection scenario.The research results show that VLC as an effective supplement to radio frequency(RF)communication will become one of the strong supporting technologies for future wireless communication networks.However,the limited modulation bandwidth of the LED device and the attenuation of the line-of-sight(Lo S)link of the optical transmission signal in VLC channel greatly restrict the spectrum utilization and communication range of the LED-based VLC cell.Motivated by this,the non-orthogonal multiple access(NOMA)mechanism is introduced to give full play to the advantages of visible light spectrum resources in this thesis.Meantime,multiple-LED communication cell-based relay cooperation mode is combined to provide a wide-range,multi-user connected communication service for the VLC system.However,the main issues include that NOMA mechanism of user signal superposition increases the difficulty of communication performance analysis.Moreover,the transmitted signal is a non-negative real number optical signal with limited amplitude in the practical VLC system,and its corresponding basis theoretical of channel capacity analysis is different from traditional RF systems.Furthermore,the introduction of relays increases the scope of communication services and improves the quality of service for edge users,while increasing the risk of security information being eavesdropped.In order to tackle these challenge and issues,this thesis investigates the channel modeling algorithm design,the outage probability and the physical layer security performance of the relay-aided and non-relay VLC system based on the NOMA mechanism.Specifically,the main contributions of this thesis are as follows.(1)In order to tackle the problems that the statistical channel model is hard to obtain the channel details and the trade-off between the applicability of existing deterministic channels in scenarios and their computational complexity,a bidirectional ray tracing method based on the conical ray tube model has been proposed.And it is verified to be high-precision and high-efficiency when reducing the simulation complexity compared with the recurrence method.The proposed method can model the channel of the VLC system in any indoor scenario with high computational efficiency.Then,it is applied to the single-input single-output VLC system without a relay and the single-input multiple-output VLC system with the relay cooperation access point.It can be seen that the received light power is mainly depending on the distance from the light source to the user and the field of view of the receiver,and the impulse response due to diffuse reflection can be ignored.Moreover,compared with the channel model of the relay system scenario,the amplitude of the impulse response received from the relay node is about 10 times higher than that received from the source node to the user,and the delay is almost negligible.This result verifies that the relay access point can improve the channel quality.It provides an efficient modeling method for the channel model of the relayaided VLC system.(2)In order to tackle the limited transmission range of VLC systems and the difficulty in guaranteeing the communication service quality of edge users,relay cooperation and NOMA technology are introduced,and the outage performance of the proposed system is deeply studied.First,the probability density function(PDF)and cumulative distribution function(CDF)of the VLC ordered channel gain are deduced by using the statistical probability theory.Then,considering that the visible light signal in the actual system needs to meet the non-negative real number characteristics and the amplitude limit,the outage probability expression of the proposed VLC system is derived based on the bounds of channel capacity with amplitude constraint.Simulation results verify the correctness of the derived theoretical expressions.The results show that the introduction of relay nodes can save the system transmission signal-to-noise ratio 15 d B to achieve the same outage probability performance.Moreover,the optimal parameter design of the relay emission semiangle and deployment height corresponding to the optimal system outage performance is obtained by analyzing the influencing factors of the system outage performance.Furthermore,these results can provide theoretical guidance for the layout of the VLC system with the relay access point.(3)In order to tackle this problem that the security information transmission brought by the introduction of VLC relay cooperative access point,the secure performance is evaluated from the perspective of physical layer security.Firstly,the upper and lower bounds of the security channel capacity under the amplitude constraint are derived from the information entropy.Then,in order to improve the applicability of the system performance analysis method,taking into account the VLC relay scenario of eavesdroppers with Poisson distribution,the PDF and CDF of each user and relay access point are analyzed by using statistical geometric methods.Moreover,the upper and lower bound expressions of the secure outage probability of the proposed relay-aided VLC system are derived.Monte Carlo simulation results verify the validity of the theoretical analysis.Furthermore,the research results show that the secure outage probability of the relay-aided VLC system increases averagely by 47.9% compared with the non-relay system,which indicates that the relay-aided VLC system faces greater physical layer security performance risk while improving the communication service range.In addition,the relations are obtained between the secure outage probability and the impacting factors from relay deployment radius,legimitate user distribution radius and LED emission semiangle.The results show that the secure outage probability of the relay-aided VLC system increases with the deployment radius of the relay node,the distribution radius of the legitimate user and the LED emission semiangle.