Research On Spectral And Energy Efficiency Of Visible Light Communication Systems Based On DCO-CFDM With Finite Alphabet Inputs

The explosion of mobile services has lead to a growing shortage of wireless spectrum resources.Visible light communication(VLC)has become a key technology for wireless communication due to its advantages such as huge unlicensed frequency band,providing high-speed data transmission and lighting at the same time,etc.Introducing orthogonal frequency division multiplexing(OFDM)technology into VLC can help resist the inter-symbol interference and improve the communication capabilities of the system.Among OFDM technologies in VLC,direct current-biased optical OFDM(DCO-OFDM)technology is most widely used due to its high spectral efficiency(SE).In practice,the DCO-OFDM system usually operate with finite alphabet inputs such as quadrature amplitude modulation.Assuming that the time domain signal approximates the classical Gaussian distribution will lead to signal approximation error,as well as the clipping noise.However,up to now,the theoretical bounds of information transmission rates of DCO-OFDM with finite alphabet inputs is yet unknown,thus the resource allocation for the optimal SE and energy efficiency(EE)of the DCO-OFDM system based on the information transmission rate,as well as the resource allocation for the optimal EE of multiple access system are more challenging.Therefore,this thesis mainly focuses on the above-mentioned problems,and the specific research contents are as follows:For the DCO-OFDM system with finite alphabet inputs,the exact information transmission rate without closed-form expression and achievable rate with closed-form expression are proposed,and the resource allocation schemes for optimal SE based on the above two rates are proposed.Specifically,the optimal DC-bias which does not clip the signal is first proposed,and further using the relationship between mutual information and minimum mean-squared error,under both constraints of average optical power and total electrical transmit power,the Multi-level mercury-water-filling power allocation scheme for optimal SE based on the exact information transmission rate is proposed.Numerical results show that the proposed power allocation scheme is related to the channel gain of each subcarrier and the total distributable power,and the proposed scheme outperforms the equal power allocation scheme and water-filling scheme.To achieve the optimal EE of DCO-OFDM system,based on the exact information transmission rate and the achievable rate with closed-form expression,the resource allocation schemes for optimal EE are proposed.Specifically,under the constraints of SE threshold,average optical power and total electrical transmit power,by substituting the optimal DC-bias and using the Dinkelbach-type algorithm,the concave-concave fractional problem is transformed into a series of convex subproblems that converge to the optimal solution of the original problem with a finite number of iterations,and the optimal power allocation scheme is obtained by the interior point algorithm.Numerical results show that the proposed power allocation scheme for optimal EE based on the exact information transmission rate is related to the channel gain of each subcarrier and the SE threshold,and the proposed scheme outperforms the equal power allocation scheme.For the scenario of multi-user access,the theoretical bounds of user information transmission rates of the DCO-OFDMA system with finite alphabet inputs are proposed,and the resource allocation scheme for the optimal EE is further proposed.Specifically,by deriving the optimal DC-bias and fixing the subcarrier allocation,and further using the Lagrangian function and Karush-Kuhn-Tucker(KKT)conditions,the optimal resource allocation scheme which minimizes the total electrical transmit power of the system is proposed under constraints that the user achievable rates with closed-form expression satisfy the user rate thresholds.Numerical results show that the proposed resource allocation scheme is related to the channel gain of each subcarrier and the user rate thresholds,and this proposed scheme outperforms the equal power allocation scheme.There are 22 figures,5 tables and 87 references in this thesis.