Research On Multilevel Demodulation Technique For Optical Camera Communications

Optical Camera Communications(OCC)is a new visible light communication method using camera as receiver,which has the advantages of no spectrum registration,green energy saving,low cost and so on.However,the frame rate of the camera on the receiving end limits the transmission rate of the OCC system.In addition,the LED(Light Emitting Diode)light source is usually only on and off in two states.Even if the receiver adopts the shutter exposure mode to improve the communication rate,the OCC system is still in the application category of low-rate communication.In order to promote the further development of OCC field,it is critical to realize the high-rate transmission of the OCC system.In order to increase the communication rate of the OCC system and improve the performance of the system,a multilevel modulation scheme based on Polit aided K-means(PAK)algorithm is proposed under the rolling curtain exposure mode.The sender uses PWM(Pulse Width Modulation)waves to control the LED lights to send out light signals with four brightness levels.The camera at the receiving end receives four grayscale strips.Each strip represents two bits,so the OCC is twice as fast.In the OCC system,the transmission rate reaches 9.96kbit/s.What's more,the K-means clustering result is very sensitive to the initial clustering center.The innovation of the PAK algorithm proposed in this thesis is that the pilot is introduced into the OCC system.By the pilot information,the receiver can obtain the state of the strips propagated through the optical channel in different environments to overcome the interference of the band caused by ambient light and other noises.Moreover,the sensitivity of k-means clustering algorithm to the initial clustering center is inhibited.The above scheme improves the communication rate and system performance of OCC.For densely deployed OCC networks,multi-channel access is a major challenge when supporting large numbers of users.Non-Orthogonal Multiple Access(NOMA)modulation can be used as a potential candidate technology.NOMA in the power domain enables multiple communication by changing the transmission power,allowing different users to share the same time and frequency.How to apply NOMA technology to OCC system to improve channel utilization and system performance is a problem worthy of attention.On the basis of the above research,this thesis proposes a NOMA multi-channel communications technology based on multilevel OCC including multi-process communications and multi-user communications.This technology superimposes two independent signals with different power levels into one signal through an LED light source,and transmits information for multiple processes or multiple users simultaneously in the form of multilevel modulation.In addition,according to the effective bandwidth and effective capacity theory,this thesis compares the service mechanism of the proposed multi-user communications technology with the delay performance of the random access mechanism.The results show that the delay violation probability of NOMA multi-user communications based on multilevel OCC is lower than that of random service mechanism and has more reliable guarantee under the condition of fair competition or low priority.