Microwave photonic channelization technology converts broadband microwave signals to the optical domain for processing, breaking through the bandwidth limitations of conventional electronics. In general, external intensity modulation based on the Mach-Zehnder modulator (MZM) is employed in microwave photonic channelized links. However, due to the intrinsic cosine response of MZM, several nonlinear distortions occur in the process of electro-optical conversion, including harmonic distortion, intermodulation distortion (IMD), and cross-modulation distortion (XMD). Since the harmonic components can be effectively removed by filters, the IMD and XMD will become the main factors limiting the system's performance. Numerous electronic and optical methods have been proposed to compensate for the IMD in conventional narrow-band links, but they are incapable of suppressing the XMD. In this study, we propose a nonlinear distortion compensation scheme based on digital domain iteration processing, which can suppress the IMD and XMD simultaneously. Moreover, compared with the previous linearization methods, the proposed scheme does not require the construction of complex compensation function models or the introduction of additional hardware devices.

Theoretical analysis shows that an approximate equation can be established between the distorted intermediate frequency signal output and the linear one from each channel. The iteration process can be utilized to approach the linearized output. Specifically, the distorted output in each channel can be selected as the initial value for the first iteration. The initial value is first squared and processed by low-pass filtering. Then, it is split into two paths that are processed by different operators. Finally, the results of the different channels processed by the operator are multiplied, and the distorted output is divided by them to obtain the result of the first iteration. Similarly, the output result of the first iteration can be used for the second iteration. Therefore, the digital compensation algorithm based on iteration can gradually convert the distorted output into a linear result.

A simulation experiment is built to verify whether the simulation results are consistent with the theoretical derivation results. The signal spectra before and after the digital compensation algorithm processing are presented (Fig. 3). It can be found that rare times of iterations are sufficient to suppress the third-order intermodulation distortion (IMD3) and cross-modulation distortion (Fig. 4). With the increase in the fundamental signal power, the power of XMD and IMD3 increases with the slope of one and three, respectively [Fig. 5 (a)]. As the power of the out-of-channel signal increases, the power of the fundamental signal and IMD3 is almost unchanged, while that of XMD increases with the slope of two [Fig. 5 (b)]. According to the simulation experiment, the IMD3 and XMD can be completely suppressed [Fig. 6 (b)] when the parameter is accurate. When the parameter deviation is 5%, IMD3 and XMD have been suppressed by 15 and 16 dB, respectively [Fig. 6 (c)]. The ability of the digital compensation algorithm to suppress nonlinear distortion disappears as the parameter deviation approaches 18% [Fig. 6 (d)].

The linearity in microwave photonic channelized links is mainly limited by the IMD and XMD. In this study, a nonlinear distortion compensation method based on digital domain iteration processing is proposed, which jointly processes the intermediate frequency signal output from each channel in the digital domain and approaches the ideal result of linearization through iteration. It can effectively suppress the IMD and XMD in channelized links. The simulation results show that the method can completely suppress the IMD and XMD when the parameter is accurate. When the parameter deviation is 5%, the IMD3 and XMD can still be suppressed by 15 and 16 dB, respectively.