Research On Energy Efficiency And Spectral Efficiency In Visible Light Communication Systems

With the rapid development of the mobile internet technology,the shortage of spectrum resources faced by the wireless communication technologies has become an urgent problem.In order to alleviate this phenomenon,the high frequency is explored positively.Because visible light communication(VLC)has the advantages of high frequency band,fast transmission speed,high security,high efficiency and easy implementation,it has attracted a lot of attention and become a hot topic in optical communication research fields.Meanwhile,to promote green and low-carbon development,not only spectrum resources utilization but also energy efficiency(EE)should be considered in the next-generation wireless communication system.In order to make a balance between energy efficiency and spectral efficiency(SE),a joint optimization of EE and SE in VLC system is investigated in this paper.Specifically,my contents are summarized as follows.Firstly,two schemes are proposed for the joint optimization of EE and SE in VLC system.For the first scheme,the EE maximization problem is established under SE requirement.On the basis of demonstrating the tradeoff between EE and SE,as well as the formulation of EE,the optimization problem can be formulated as a fractional programming problem,and finally it can be further transformed into a non-fractional optimization problem,and an iterative algorithm is proposed to solve the optimal power.Simulation result shows that blindly increasing power can not achieve higher EE under the premise of satisfying SE requirement.Therefore,it is very important to choose proper amount of power.For the second scheme,a multi-objective optimization problem of EE and SE is formulated,and further transformed into a single-objective one by means of the weighted product scalarization method.In addition,the relationship between EE,SE and the introduced weighted factor is derived and verified.Secondly,for the VLC system with the direct current-biased optical orthogonal frequency division multiplexing(DCO-OFDM),the relationship between EE and SE is investigated from two perspectives,i.e.,linear light emitting diode(LED)and nonlinear LED.For the linear LED,the achievable SE and EE of DCO-OFDM VLC system with bit and power loading is derived and the corresponding closed-form expression of the EE-SE tradeoff is obtained,on the basis of which,the optimal SE to maximize EE is achieved with the help of Lambert W function.Then,effects of the DCO-OFDM VLC system parameters,e.g.,horizontal distance,vertical distance,responsivity of photodiode(PD)and number of subcarriers,on the EE-SE tradeoff are discussed,and simulation results verify the theoretical derivation results.For the nonlinear LED,a nonlinear polynomial model of LED is established.Furthermore,the closed-form expression of SE and EE under high signal-to-interference-plus-noise ratio(SINR)is derived.Then,effect of the nonlinear factor on SE and EE is studied.Finally,simulations are carried out to validate the theoretical results.