Constant Envelop Optical Orthogonal Frequency Division Multiplexing

This work was supported in part by the National Natural Science Foundation under Grant60932004(Transmission theories and technologies of optical orthogonal frequency-division multiplexing), in part by the National Natural Science Foundation under Grant60877052(Key technologies and basic theories of OFDM signal in ROF system), in part by the National Basic Research Program of China (973Program) under Grant2012CB315605, and in part by the National Natural Science Foundation under Grant61271193, and Grant61072054。In the optical orthogonal frequency division multiplexing (OFDM) systems, there are two factors limiting its development. First, due to the statistical characteristics of the accumulated signal of each sub-carrier in OFDM system, OFDM signal usually has large peak to average power ratio (PAPR) which leads to large fiber nonlinearity, especially in the long-haul transmission. Second, OFDM signal is sensitive to the carrier frequency offset (CFO) and phase noise. Herein, in order to address these two problems existing in OFDM system, we investigate many schemes to reduce PAPR. Also, we reduce the sensitivity of OFDM signal on carrier frequency offset (CFO) and phase noise. Thereby system performance is improved. In addition, in order to address the conflict between the improving communication rates and insufficient bandwidth, how to improve the spectrum efficiency has also been an urgent problem. In this paper, we propose a new modulation format to reduce PAPR, analyze its spectrum efficiency and propose a new scheme to improve its spectrum efficiency.In this paper, we investigate optical OFDM system extensively, especially the therotical analysis of constant envelop OFDM and design of its parameters. In addition, we investigate how to reduce the phase noise. Besides, we propose a new scheme to improve its spectrum efficiency and large amount of simmulations and experiment on these are carried out. The main works of this paper are listed as follows:1. In this paper, we propose a constant envelop minimum-shift keying orthogonal frequency division multiplexing (CE-MSK-OFDM) modulation scheme. This scheme can mitigate the inherent drawbacks of CE quadrature phase shift keying OFDM (CE-QPSK-OFDM) and MSK-OFDM. Advantages of the proposed CE-MSK-OFDM modulation scheme are theoretically investigated. A CE-MSK-OFDM coherent optical experimental system is presented. In our experiment,2.5Gb/s CE-MSK-OFDM signal and2.5Gb/s CE-QPSK-OFDM signal are transmitted over a100km standard single mode fiber (SSMF) without phase compensation to check its advantages in mitigating intercarrier interference (ICI). In addition, performance under large fiber launch power is also measured to check its advantages in counteracting fiber nonlinear impairments. The experimental results show that the proposed system performs well in decreasing ICI and peak-to-average power ratio (PAPR) as the theoretical analysis reveals.2. To improve spectrum efficiency (SE) of minimum shift keying (MSK) in orthogonal frequency division multiplexing (OFDM) system, we introduce multiple modulation indices (h) into CPFSK scheme. We theoretically prove that only multiple of1/2can be used as h in OFDM-CPFSK scheme.0,1/2, and1are good candidates to improve its SE. Therefore, triple-h OFDM-CPFSK scheme is proposed. Implementation of the proposed scheme and its differential decoder are also presented. With h=1/2, the proposed scheme is simplified to minimum shift keying (MSK), a typical CPFSK scheme. To demonstrate the improvement in SE,10Gb/s tri-h OFDM-CPFSK signal and10Gb/s OFDM-MSK signal are transmitted over standard single mode fiber (SSMF). SE of the proposed scheme is about twice as much as that of OFDM-MSK. In addition, Bit error rate (BER) of the proposed scheme and OFDM-MSK are measured to show its improvement in resisting chromatic dispersion (CD) and polarization mode dispersion (PMD).3. In this paper, to improve SE of MSK in CE-OFDM, continuous phase frequency shift keying (CPFSK) with triple modulation-index h is proposed.We theoretically prove that only multiple of1/2can be used as h in CPFSK-OFDM system. Among them,0,1/2, and1are good candidates. Experiment results on electrical spectrum shows that SE of about2bit/s/Hz is achieved. Bit error rate (BER) of tri-h CE-CPFSK-OFDM and tri-h CPFSK-OFDM under large average fiber launch power demonstrates an improvement of tri-h CE-CPFSK-OFDM in resisting PAPR. BER of tri-h CE-CPFSK-OFDM and CE-MSK-OFDM versus fiber length shows a robust performance in resisting CD and PMD.