Investigation On Random Access And Performance Optimization Of VLC System Aided By Multipacket Reception

With the rapidly increase of communication service types,finite RF spectrum can not meet people's needs.Visible light communication(VLC)becomes the alternative technique for RF communication as its advantages of high bandwidth and unregulated spectrum,and is expected to conquer the restriction of wireless radio frequency communication and provide higher data transmission rate.In recent years,VLC have attracted much attention.Most of the researches concentrate on the techniques of physical layer(PHY)such as modulation and coding and the research on medium access control layer(MAC)is rare.IEEE 802.15.7 standard is present by EEE 802.15 TG7 which defines PHY and MAC for short-range visible light communication suggests four types of random access mechanisms for VLC where the slotted carrier sense multiple access with collision avoidance(CSMA/CA)is the most popular one.To some extent,CSMA/CA is effective to mitigate the collision of uplink random access and to improve system performance.Due to the narrow beams and the directionality of light,the random access mechanism using carrier sensing still can not avoid the existence of hidden devices.Thus,the hidden device problem and the obstruction problem in VLC system cannot be ignored.Currently,there are a crowd of methods to cope with the hidden device problem such as enhancing transmission power,adjusting carrier sensing threshold,etc.In this paper,multipackets reception(MPR)technique is introduced to relieve the hidden device problem.Thus,in the VLC system with MPR in the presence of hidden devices and obstructions,how to design a reasonable analysis model to explore the system performance is significance for further research of VLC.In this paper,we propose a novel random access algorithm based on the IEEE 802.15.7slotted CSMA/CA and model the random access process of devices.Then analyzing and optimizing the VLC system performance.First,in the IEEE 802.15.7 slotted CSMA/CA mechanism,the device backoff,senses and access to compete for channel resources in the contention access phase(CAP).But the random access mechanism of the standard does not take into account the hidden device problem and the obstructions problem,it also does not apply to the VLC system which has a coordinator with MPR capability.Thus,the first step is that under the circumstance of obstructed optical channel and considering the hidden device effect,wepropose a random access algorithm for VLC system with MPR capability and analyze the system performance.We model the packets generation process of unsaturated device with Markov source.And based on the slotted CSMA/CA,we propose a new random access algorithm for the unsaturated device where MPR capability can be used reasonably.In this paper,we utilize Markov chain theory to establish the arrival process model and the random access process model of the unsaturated device.The effects of hidden devices and obstructions,the impact of the arrival rate and MPR capability are mapped into the transition probability of the Markov chain model.Based on the Markov chain analysis model,we derive the system performance metrics,including throughput,packet loss probability and reception power efficiency,then analyze effects of arrival rate,MPR capability,obstructions and hidden devices on the system performance.Secondly,the introduction of MPR to alleviate the hidden device will add the energy consumption,and under the condition of the unsaturated system,MPR capacity can not be fully utilized,resulting in energy dissipation.Therefore,how to adjust the MPR capacity dynamically with the number of active devices,improve energy efficiency and ensure the system throughput is the second problem to be solved in this paper.Since it is stochastic for packets arrival,the number of active devices in this system is variable and unknown in priori for coordinator.In order to optimize the performance of unsaturated system,we first present an estimation algorithm which provide the number of active devices of this system for the coordinator.Secondly,according to the estimation of the number of active devices,we propose a MPR capacity adjustment algorithm and get the optimal MPR capability to achieve maximum reception power efficiency periodically.Then with the optimal MPR capability,we propose a backoff windows adjustment algorithm to adjust backoff windows to achieve the maximum throughput.Thirdly,a tradoff algorithm is presented to adjust the optimal MPR capability and the optimal backoff windows to tradeoff the system throughput and reception power efficiency,so as to optimize the system performance.