Improved OFDM Modulation And Demodulation Schemes In Visible Light Communications~2

Recently, Orthogonal Frequency Division Multiplexing (OFDM) has drawn great attention in Visible Light Communications (VLC), due to its easiness to equalization, high spectrum efficiency and flexibility to resource allocation. The VLC system usually adopts intensity modulation, hence the reality and nonnegativity of the transmitted signal are required. This thesis aimed at the unipolar characteristic of the optical OFDM signals, and proposed several improved schemes. The main contributions are listed as follows.First, this thesis study on the improved receiver for PAM-DMT systems. PAM-DMT is a proposed optical OFDM schemes. By using noise cancellation method or frequency diversity combined method, the bit error rate during the transmission can be significantly reduced. However, the combination of these two methods has never been studied. In this thesis, theoretical analysis and simulation results showed that the combination schemes would not bring more performance gains. This fact also illustrated that these two methods are essentially equivalent, so the receiver design in PAM-DMT system should use only one of the two methods.Second, a novel OFDM scheme called PDO-OFDM is proposed, which simultaneously transmits PAM-DMT signals and DCO-OFDM signals. The conventional PAM-DMT systems has no need to use DC bias and saves the transmitted power. Its shortage is the low subcarrier utilized rate, hence it is not suitable for high data rate transmission. In the PDO-OFDM system, DCO-OFDM signal is added on the PAM-DMT signal, sufficiently utilizing the subcarriers, while the DC bias is considerably low. The analysis and simulations show that, as long as the power allocation and rate allocation are reasonably arranged, the PDO-OFDM will have much better BER performance than the conventional optical OFDM.Third, a novel OFDM scheme called HPAM-DMT is proposed, in which the ACO-OFDM signals is added on the PAM-DMT signals during the transmissions. The ACO-OFDM signals usually has significantly clipping noise at the even subcarriers, but in the HPAM-DMT systems, this clipping noise will not have any interference on the PAM-DMT signals. Further analysis show that HPAM-DMT systems have better performance than PDO-OFDM systems at moderate data rates.Fourth, a novel OFDM scheme called eHPAM-DMT is proposed, which can be seen as adding DCO-OFDM signals on the HPAM-DMT systems. This scheme fully uses the subcarriers while the requirement for DC bias is little, hence it saves transmitted power. Simulation results that it has the best system performance at high data rate communications.Fifth, this thesis also did further research on the improvement about the receivers in PDO-OFDM, HPAM-DMT and eHPAM-DMT systems, based on noise cancellation method or frequency diversity combined method. Simulation results show that the receiver designs can efficiently reduced the bit error rate, while the computation complexity will slightly increase.