Performance Analysis Of Indoor VLC CSMA Networks With Imperfect Carrier Sensing

With the advantage of the vast unregulated spectrum,green energy saving,high data rates and low cost,visible light communication(VLC)has attracted broad attention.The traditional wireless communications based on spectrum radio frequency(RF)technology face the problem of the spectrum resource shortage,which could not meet the needs of high transmission rate and the ultra wide bandwidth of users in the future.VLC is the new green technology to solve the problem.In this paper,we concentrate on the VLC.Recently,the theoretical researches of VLC have attracted much attention.However,research results mostly focus on the techniques of physical layer,the research findings on the techniques of upper link layer are rarely.IEEE 802.15.7 supports multi-point to multi-point VLC,which attracts broad attention over the last two years.IEEE 802.15.7 suggests,it could coordinate channel access process based on the competitive way of carrier-sensing.With the advantage of simpleness,economy and scalability,carrier sense multiple access(CSMA)mechanism is very suitable for applications in the large optical Internet of Things(Io Ts).However,the strong directionality of light and randomness of diffuse reflection which give rise to a severe imperfect carrier sense problem(hidden node and exposed node),bring great challenges to the CSMA process of VLC systems.Therefore,we explore the optical CSMA process in imperfect carrier sense case and analyze the system performance for the multi-point to multi-point VLC uplink communications networks.We consider a multi-point to multi-point large indoor VLC random access network,which separates uplink communication from downlink communication with the time-division technique.Based on stochastic analysis theory,we abstract indoor nodes using diffuse reflection light of the ceiling to complete carrier sense process into the choice problem of independent sets.The carrier sensing is abstracted as the probability statistics problem.The optical hard core point process(OHCPP)is proposed to model the process of random access of communication nodes in the imperfect carrier sense case.Further,we derive the closed-form expression for approximating the outage probability and transmission capacity ofVLC networks.Also,we explore the performance of random access for the VLC CSMA network with imperfect carrier sensing and the choice of optimal carrier sensing threshold.Our works and contributions are summarized as follows.First,we study the intensity of simultaneously active transmitters based on the OHCPP model.According to the first reflection light of the ceiling,we obtain the reflection light power distribution of any LED transmitter and analyze the relationship between the carrier sensing region and reflection light power.Moreover,considering the severe imperfect carrier sensing problem,we propose the OHCPP to depict the feature of indoor VLC CSMA networks which captures the feature of exposed nodes and adds the retaining probability of nodes.And based on the probability theory and mathematics set theory,we derive the intensity of simultaneously active transmitters in VLC with the imperfect carrier sensing.Finally,we explore the characteristics and analyze the effect factors of the intensity of simultaneously active transmitters.Then,we study the interference model and analyze the outage probability and transmission capacity of VLC networks.Due to the strong directionality of light and the complexity of the channel,it is very difficult to build the interference model.In this paper,we analyze the interference from the hidden nodes based on the probability theory,and derive the closed-form expression of outage probability.Thus,we obtain the transmission capacity of VLC networks in imperfect carrier sensing case.The research results show the existence of an optimal carrier-sensing threshold for a given emission angle which maximizes the intensity of simultaneously active transmitters.Our study will provide theoretical basis for the design of VLC communication systems in future.This paper models the random access process of indoor VLC networks with the imperfect carrier sensing in theory and provide the reference to the research on the link layer and future applications.