| The carrierless amplitude and phase modulation (CAP) technique is a promising technique in visible light communication (VLC) systems. However, the CAP signal will suffer serious inter-symbol interference (ISI) and cross channel interference (CCI) when the timing offset exists, which results in the low timing sensitivity of the CAP receiver. There are two approaches to solve the low-timing-sensitivity problem, i.e. designing low-timing-sensitivity CAP pulses and increasing the timing synchronization accuracy of the CAP receiver. As for the design of CAP pulses, the zero-ISI and zero-CCI conditions of both the time and the frequency domain were derived in previous papers, and a set of low-timing-sensitivity CAP pulse called the NS CAP pulse was proposed, which only lowered the quadrature channel timing sensitivity. This thesis tries to design the CAP pulses by adapting the carrier frequency of the pass band Nyquist puses. As for the synchronization method at the CAP receiver, much work was done upon the analog synchronization approaches. This thesis studies the digital synchronization approaches, which are more stable and accurate than the analog approaches. The main work of this paper is listed as follow:1)The thesis studies the design of low-timing-sensitivity CAP pulses. Compared with the PAM pulses, the CAP pulses not only should consider the ISI problem, but also consider the CCI problem. According to this situation, the thesis firstly gives the idea of the pass band Nyquist pulse based on the conventional CAP pulse, and then analyzed the condition that the carrier frequency of pass band Nyquist pulses should satisfy to meet the zero-CCI condition. Finally, the thesis proposed two new sets of 2-D CAP pulses, i.e. the PO CAP pulse and the 2-D FD CAP pulse, and proposed a new set of 3-D CAP pulse. Simulation results show that the PO CAP pulse and the 2-D FD CAP pulse are less sensitive to the timing offset than the existing CAP pulses, but the conventional and NS CAP pulses will achieve better performances when the timing offset is low;2) The thesis studies the timing estimation algorithm for the CAP signals. Firstly, this thesis proves the valid-ness of the maximum average power timing estimation algorithm in CAP systems. Secondly, this thesis proposed a multi-frequency point O&M algorithm for the pass band PAM signal, which can utilize the higher frequency portion of the received signal to estimate the timing offset. Then, the multi-frequency point O&M algorithm for the pass band PAM signal is extended to the CAP signal and is further improved as for the conventional CAP signal. Simulation results show that the maximum average power timing es-timation algorithm is highly dependent to the sampling frequency of the receiver, the higher the sampling frequency is, the more accurate the performance will be. As for the CAP system that sends pass band pilot signal for timing estimation, using the quadrature channel pass band Nyquist waves will achieve better estimation performance than using the in-phase channel pass band Nyquist waves. As for the CAP system that utilizing the CAP signal to obtain the tractable timing estimation, the PO CAP signal and the 2-D FD CAP signal will achieve the best timing estimation performance by utilizing the quadrature channel triple baud rate frequency O&M algorithm; the NS CAP signal will achieve the best timing estimation performance by utilizing the in-phase channel basic baud rate frequency O&M algorithm; good timing estimation performance can be achieved by using the modified O&M algorithm for the conventional CAP signal, but the performance of the original O&M algorithm is quite limited. Moreover, because of the existence of the CCI, more symbols should be used to achieve an equivalent estimation performance for the CAP signals than for the PAM signals.3) Lastly, this thesis gives a detailed introduction to the implementation of the synchronization module at the conventional CAP pulse based CAP receiver. |
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