Research On PAPR Reduction Based On PTS Techniques In CO-OFDM Systems

Coherent optical orthogonal frequency division multiplexing(CO-OFDM) technique combines the advantages of OFDM technique and coherent optical communication technique,and it is a main candidate technique of the next generation of ultra high speed optical communication systems, which because of its higher spectrum efficiency and stronger anti-dispersion capacity than the traditional optical communication technique. However, the high peak-to-average ratio(PAPR) problem is still a challenge that all systems based on OFDM modulation have to face, and this problem may degenerate the performance of the systems and increase the cost. Thus, PAPR reduction is critical to improve power efficiency and enhance the stability of the systems. Therefore, the techniques of PAPR reduction in CO-OFDM systems are analyzed and compared in this thesis, and the representative partial transmit sequences(PTS) technique is selected as the object of in-depth study. The main achievements are as follows:First of all, the current classification of PAPR reduction techniques is simply introduced and the factors to evaluate the performance of PAPR reduction techniques are analyzed. Then the clipping technique of pre-distortion technique and the selected mapping(SLM) technique of probability technique is analyzed and respectively compared with the conventional PTS technique. The simulation results show that the clipping technique can simply and effectively reduce the PAPR of the signals, while the non-linear process is the main defect of this technique which causes both in-band distortion and out of band radiation. The PTS technique and the SLM technique both belong to probability technique which focuses on reducing the probability of the high peak and will not distort the signals. It is known that the PTS technique is more advantageous than the SLM technique if the amount of computational complexity is the same. Therefore, the PTS technique is selected as the object of further research, analyses and simulation results of this part have a certain reference value when choose the PAPR reduction technique for the particular systems.In the next place, the theory of the PAPR reduction algorithm based on the conventional PTS technique is introduced. An improved two-stage enhanced-iterative-algorithm PTS(TS-EIA-PTS) PAPR reduction algorithm with the low computational complexity, based on the EIA-PTS algorithm that proposed in our laboratory previous study, is proposed to solve the high computational complexity problem of phase factor searching process of EIA-PTSalgorithm. The simulation results show that compared with the original OFDM signal, the TS-EIA-PTS algorithm significantly reduces the probability of high peak. And it can further improve the PAPR reduction performance of the system and gradually approaches to the EIA-PTS curve with the increase of the remaining stage n-d. The proposed TS-EIA-PTS PAPR reduction algorithm can reduce the computational complexity by 18.47% under the circumstance of the original signal partitioned into 4 subblocks at the remaining stage n-d ?5. And the PAPR reduction performance only degrades 0.2 dB when compared with the EIA-PTS algorithm at the4CCDF=10-. Furthermore, it has almost the same bit error rate(BER) performance as that of the EIA-PTS algorithm, and the reduction performance of the computational complexity can be further improved with the increase of the subcarrier number. Therefore, the proposed TS-EIA-PTS algorithm has a higher application value in practical high data rate CO-OFDM systems.At last, aiming at the drawback of the EIA-PTS algorithm which has to transmit the side information, a cyclically shifting EIA-PTS(CS-EIA-PTS) algorithm is proposed in this thesis.This proposed algorithm, which with the interleaved partition method, can reduce computational complexity of the IFFT operation of the EIA-PTS algorithm by utilizing the Cooley-Tukey FFT algorithm. The simulation results show that compared with the pseudo-random partitioned EIA-PTS algorithm, this CS-EIA-PTS algorithm can reduce the computational complexity by 57.03% when the QPSK modulation is applied with the subcarriers N ?128 under the condition of the original signal partitioned into 4 subblocks.The BER performance of the proposed CS-EIA-PTS algorithm without side information almost overlaps together with the EIA-PTS algorithm with perfect side information, while the CS-EIA-PTS algorithm has better BER performance and improves SNR by 0.3 dB at4BER=10-when compared with the EIA-PTS without perfect side information. Moreover,when compared with the pseudo-random partitioned EIA-PTS algorithm and the interleaved partitioned EIA-PTS algorithm, the CS-EIA-PTS algorithm improves the PAPR reduction performance by 0.25 dB and 0.45 dB at4CCDF=10-, respectively. Thus, these results reflect the superiority of the CS-EIA-PTS algorithm in reduction of the PAPR and the computational complexity.