Research On Nonlinear Distortion Reducing Technique In OFDM Based Visible Light Communication

OFDM technology can be used in visible light communication(VLC) system to cope with multipath effect and inter-symbol interference(ISI) to realize high speed data communication. However, by the nonlinear influence of power amplifier and optical modulation devices, OFDM based VLC system faces a serious problem of nonlinear distortion. Existing nonlinear distortion reducing technology is mainly based on peak to average power ratio(PAPR) reducing techniques in the conventional wireless communication, and still faces the following prob lems when used in VLC system:(1) performance of existing PAPR reducing techniques is severely limited by the characteristic of VLC system such as signal structure and carrier distribution.(2) Nonlinear distortion is serious under transmitted optical power restriction as lacking of nonlinear distortion method that reckoning both lighting and communication requirement. Aimed at these problems, this paper relies on 863 projects to researching on nonlinear distortion reducing technique in OFDM based VLC system.Firstly, for the serious nonlinear distortion problems caused by high PAPR of Flip-OFDM signal and asymmetrically clipped optical OFDM(ACO-OFDM) signal, a dynamic compensation based nonlinear distortion suppression algorithm for Flip-OFDM and a even subcarriers based nonlinear distortion suppression algorithm for ACO-OFDM are proposed from the perspective of OFDM signal processing. Then, from the viewpoint of the whole system power distribution, a nonlinear distortion suppression algorithm based on powe r allocation is proposed to further suppress nonlinear distortion in these two systems. The main results of this study are as follows:1. A dynamic compensation based algorithm is proposed to reduce the nonlinear distortion in Flip-OFDM based VLC system. Firstly, partial transmit sequence coefficient are selected by minimizing the negative signal amplitude of Flip-OFDM, and the Flip-OFDM signal is clipped to ensure that the signal is in the dynamic operate region of LED. Then, the compensatory signal is calculated according to the positive signal sequence dynamically, and added to the corresponding zero position of negative signal sequence. Finally, positive signals are recovered according to compensatory signals in receiving terminal by combined detection methods of maximum likelihood and decision threshold algorithms, which can reduce nonlinear distortion. Simulation results show that compared with PTS-C lipping algorithm, the proposed algorithm can make the PAPR performance achieves a gain about 1.6d B where the complementary cumulative distribution function(CCDF) is 10-4, and have up to about 5dB improvement for SNR where t he BER is in the order of 10-3.2. An even subcarriers based nonlinear distortion suppression algorithms for ACO-OFDM system is proposed. Firstly prove the conclusion that when clipping the OFDM symbol employing only even subcarriers, the clipping noise is only applied to even subcarriers. Then on this basis, combine the tone teservation method(TR) and clipping method, according to the characteristic that ACO-OFDM system only employ odd subcarriers to transmit, even subcarriers are used to generate a PAPR restrained Signal which is clipped to meet the unipolar characteristic by clipping technology. Thus the system PAPR is reduced without d estroying the unipolar characteristic of the system or affecting data carriers to achieve the purpose of suppressing the nonlinear distortion. Simulation results show that the algorithm can make the PAPR performance of the system achieving a gain about 3.9d B with three iterations where the CCDF is 10-4, and has up to about 1.2dB improvement of PAPR performance where CCDF is 10-4 and 2.0dB improvement of SNR performance where BER is 10-4 when compared with PTS algorithm.3. A power allocation based algorithm for reducing the nonlinear distortion is proposed from the viewpoint of the whole system power distribution for the optical power constraints scene. Firstly, the nonlinear clipping distortion of the system is analyzed. Then the effective signal to noise ratio(ESNR) is defined to measure the clipping distortion, and the clipping distortion analysis is converted into SNR analysis. Finally, the optimal bias signal and OFDM signal power allocation under optical power constraints base on the principle of effec tive SNR optimum is proposed. Simulation results show that the performance of optical OFDM is influenced directly by the power allocation of OFDM signal and bias signal. C hannel SNR value of 24 d B and 27 dB is enough to insure BER is below 10-3 under the optical power constraints of 200 mW and 250 mW respectively in ACO-OFDM system.