Analysis Of Channel Capacity And Secrecy Capacity In Indoor Visible Light Communications

With the rapid development of mobile Internet and the emergence of Internet of things,the abilities of high-speed communication,seamless connection,strong security and low delay commuinication are needed for the mobile communication technology in the future.The traditional radio frequency network can not meet these requirements due to the lack of spectrum resources.Therefore,visible light communications(VLC)have become a hotspot in industry and academia in recent years because of its rich spectrum resources,high data rate,green,no electromagnetic interference and confidentiality.At present,many works have studied the channel capacity and securecy capacity for VLC system.However,the noise in VLC is often assumed to be independent of the input signal in these works.Typical illumination environments in VLC result in large signal-to-noise ratio(SNR).In the high SNR regime,the value of noise relies on the input signal.Furthermore,many works are often assumed the fixed receiver in VLC,few consider the effects of random factors on system performance.the location or orientation of receiver in the actual VLC system is not fixed.Hence,this thesis focuses on the above issues.Firstly,this thesis considers a point-to-point indoor VLC system with signal-dependent Gaussian noise,and derives the channel capacity for the system.Considering both illumination and communication,the non-negative input of VLC is constrained by peak and average optical intensity constraints.Two scenarios are taken into account: one scenario has both average and peak optical intensity constraints,and the other scenario has only average optical intensity constraint.For both two scenarios,the thesis derives closed-from expressions of capacity lower and upper bounds.Specifically,the capacity lower bound is derived by using the variational method and the property that the output entropy is invariably larger than the input entropy.The capacity upper bound is obtained by utilizing the dual expression of capacity and the principle of “capacity-achieving source distributions that escape to infinity”.Moreover,the asymptotic analysis shows that the asymptotic performance gap between the capacity lower and upper bounds approaches zero.And all derived capacity bounds are confirmed using numerical results.Numerical results show that the peak-to-nominal optical-intensity ratio has no effect on the capacity performance when the optical intensity is large,and the capacity performance degrades when the variance of signal-dependent noise is increased.Secondly,this thesis focuses on the physical-layer security for a VLC system with a transmitter,a legitimate receiver and an eavesdropper.In the system,both the signal-independent and signal-dependent Gaussian noise are taken into account.By considering the nonnegativity and average optical intensity constraints,the thesis first derives secrecy capacity lower and upper bounds of the VLC system by using the variational method,the dual expression of the secrecy capacity,and the concept of “the optimal input distribution that escape to infinity”.Numerical results show that the secrecy capacity upper and lower bounds are tight,which validates the derived theoretical expressions.By asymptotic analysis at large optical intensity,it is shown that the performance gap between asymptotic upper and lower bounds is small.Then,adding a peak optical intensity constraint,the exact and asymptotic secrecy capacity bounds are further analyzed.The tightness of the derived bounds is verified by numerical results as well.Furthermore,numerical results show that the signal-dependent Gaussian noise will degrade the system performance,and increasing the nominal optical intensity P or the peak optical intensity A cannot increase secrecy capacity without limitation.Finally,this thesis considers a typical VLC system with a fixed transmitter and a random receiver.Two types of receivers are investigated:(1)those with random location and(2)those with random orientation.Based on the established system model,the statistical characteristics of the channel are obtained,and closed-form expressions of the average channel capacity and the outage probability are derived,respectively.At last numerical results verify the accuracy of derived theoretical expressions.Moreover,the effects of the nominal optical intensity,the dimming target,the transmitter height,the receiver zone’s radius,the outage threshold,and the Lambertian emission order on system performance are also provided.