Energy Adaptive MAC Protocol For IEEE 802.15.7 With Energy Harvesting

In recent years,visible light communication(VLC)has been a research hot in the field of wireless communications with the advantage of lighting and communication.IEEE 802.15.7protocol provides a global standard for short-distance wireless optical communications,and the protocol has a very broad application prospects,such as area lighting,signboards,streetlights,vehicles,traffic signals and so on.In VLC system,the devices are usually battery powered,so the energy of devices is constrained,thus affecting their service life.The research scene in this paper is that the devices are used for smart home,and the devices are embedded in the wall.In this case,the battery can not be replaced.Nevertheless the energy harvesting(EH)technique is an effective solution for the energy constrained network.This paper is mainly considering to harvest the stabilized energy of the visible light everywhere in the room,and we propose an EH model for indoor visible light,which considering the impact on EH due to the distances between devices and the coordinator,the acceptance angle of devices and the obstruction probabilities with different positions.As the visible light has the characteristics of fast decay and linear propagation,the introduction of EH will bring new challenges to the IEEE 802.15.7 protocol.Combining EH with VLC will cause a serious problem,so we will research the problems and provide a solution in this paper,which provides a basis for the performance evaluation and optimization in actual network.In this paper,we first study the indoor visible light EH model.In the indoor visible light scene,we give the reasonable LED layout by the theory analysis and simulation.Considering the distances between the devices and the coordinator,the receiving angle of devices,we propose an indoor visible light EH model.As the devices are deployed in the wall in this paper,we simulate the energy distribution on the walls.Furthermore,due to the visible light characteristic that the optical link is easy to be obstructed,we give an indoor visible light obstruction model,which considering the height distribution of moving objects.And then,we study the media access control(MAC)protocol with EH ability(EH-MAC)in indoor VLC.The unfairness of the throughput exists among devices due to the significant difference of their energy harvesting rates that change with distances,acceptance angles andthe obstruction probabilities.Based on the EH model and the obstruction probability model proposed above,we present an energy adaptive contention algorithm(EAC)to overcome this unfairness problem.The device would adjust its contention window according to its energy harvesting rate.As a result,the device with lower energy harvesting rate will have a shorter contention window to improve its transmission opportunity.On the contrary,the device with higher energy harvesting rate will have a longer contention window to reduce its transmission opportunity.Through the experimental simulation,we can see that the modified MAC protocol can improve the throughput of the devices with lower energy harvesting rate and reduce the throughput of the devices with higher energy harvesting rate,which makes the devices achieve higher fairness.Finally,this paper uses the two-dimensional discrete-time Markov chain to model the backoff process of EH-MAC/EAC.The devices with different energy harvesting rates will choose two different backoff strategies.In this paper,Markov chain modeling is carried out on these two backoff strategies,and the visible light characteristic that the optical link is easy to be obstructed is taken into account in the modeling process.Theoretical analysis shows that our EH-MAC protocol with energy adaptive contention algorithm has a higher fairness of throughput and the result of experiment simulation conforms to the result of theoretical analysis.