在直接检测光正交频分复用(DD-OFDM)系统中,同相/正交相(IQ)不平衡造成严重的性能下降。我们搭建了两个分别实现双边带(DSB)和单边带(SSB)调制的背靠背DD-OFDM 系统,并依据差错向量幅度(EVM)和符号差错率(SER)分析比较了上述两系统对IQ 不平衡的容忍度。结果表明,在背靠背情形下,DSB 系统比SSB 系统对IQ不平衡的容忍度要大。我们还搭建了两个传输距离均为40 km 的DD-OFDM 系统,分别实现了DSB 信号和SSB信号的传输,同样依据EVM 和SER 分析比较了这两个系统对IQ 不平衡的容忍度。结果表明,在传输距离为40 km的情况下,SSB 调制比DSB 调制更能增强DD-OFDM 系统在传输信号时对IQ 不平衡的容忍度。

In-phase/Quadrature-phase (IQ) imbalance can result in severe performance degradation in optical Direct-detection Orthogonal-frequency-division-multiplexing (DD-OFDM) systems. We build two optical back-to-back DD-OFDM systems, which implement Double-sideband (DSB) and Single-sideband (SSB) modulation, respectively. The tolerance to IQ imbalance of these two systems is analyzed and compared by using Error Vector Magnitude (EVM) and Symbol Error Rate (SER). It is found that, in the back-to-back case, the DSB system has stronger robustness to IQ imbalance than the SSB System. We further build two optical DD-OFDM systems each with 40-km transmission, which respectively implement DSB and SSB transmission. Similarly, the tolerance to IQ imbalance of these two systems are analyzed and compared in terms of EVM and SER. It is found that, however, in the case of 40-km transmission, the SSB system has stronger robustness to IQ imbalance than the DSB system. As a result, it is concluded that, in the case of transmission, SSB modulation can enhance the tolerance to IQ imbalance of DD-OFDM systems.