Microwave photonic technology has an important potential in future high-speed microwave/millimeter-wave communication systems due to its large bandwidth, low loss, and immunity to electromagnetic interference. However, due to the inherent cosine response of the electro-optic modulators, the output signals of the broadband multi-carrier microwave photonic link (MPL) will suffer from nonlinear distortions, mainly including harmonic distortions (HD), cross-modulation distortion (XMD), and third-order intermodulation distortion (IMD3). Since HD can be filtered out by a suitable filter, the XMD and IMD3 are the main factors limiting the system performance. We build a nonlinear distortion model for in-band third-order IMD3 and out-of-band XMD compensation of a broadband MPL. Despite various optical and electrical methods are proposed to compensate for the IMD3, few methods can quickly compensate for both XMD and IMD3 of a broadband MPL spontaneously. Thus, a nonlinear distortion model is presented for compensating the in-band IMD3 and out-of-band XMD in the wideband MPL. This method does not require priori parameters of the system and signals, and a complicated training and iterative optimization process, which is more practical.

We provide a nonlinear distortion model for a broadband multi-carrier MPL. Firstly, due to large frequency differences between the HD signal and the fundamental frequency signal, the HD signal can be easily filtered by a digital filter. Then, the XMD and IMD3 signals are extracted, which are the opposite sign to the fundamental frequency signal. Thus, it is easy to obtain that the cubic power of the XMD and IMD3 signals is also the opposite sign of the fundamental frequency signal. Based on the characteristic, a cost function with a closed-form solution can be constructed, where an optimal linearization coefficient is obtained quickly and adaptively. Finally, this optimal linearization coefficient is introduced to compensate the XMD and IMD3 simultaneously in the digital domain.

Simulation experiments are built to verify the performance of XMD and IMD3 suppression. Figure 2 shows the signal spectra before and after linearization as two-tone signals are received. The XMD and IMD3 are suppressed by more than 35 dB and 29 dB respectively. The power of the fundamental frequency signal is found to remain unchanged, but the power of the XMD term increases linearly with the slope change of 2 (Fig. 3). Additionally, after compensation by the proposed algorithm, all the XMDs are suppressed below the noise and the compensation effect does not decrease with the increasing input fundamental signal power. As the power of the input fundamental signal increases, the powers of the fundamental signal and the IMD3 signal of the pre-compensation in-band signal rise linearly with slopes of 1 and 3 respectively. Meanwhile, the power of the XMD term after linearization increases linearly at a slope of 5. The spurious-free dynamic range of the compensated system is improved by more than 21.5 dB (Fig. 4). According to the simulation experiment, after algorithmic compensation, the error vector magnitudes (EVMs) of single-carrier orthogonal frequency division multiplexed signal (OFDM) and multi-carrier OFDM signals are optimized by 6.1% and 5.9% respectively (Figs. 6 and 7). As multi-carrier OFDM signals with different V_pp are input (Fig. 8), the best compensation effect is at 1 V, and the EVM is optimized by 7.2%.

A nonlinear distortion model is presented for the XMD and IMD3 generated in a broadband multi-carrier MPL. Then based on the characteristic that the XMD and IMD3 signals have the opposite sign to that of the fundamental frequency signals, the out-of-band XMD and the in-band IMD3 can be suppressed. Compared with the traditional XMD and IMD3 compensation methods, this method does not require priori parameters of the system and signals, and a complicated training and iterative optimization process. Simulation results show that the XMD and IMD3 are suppressed by more than 35 dB and 29 dB respectively, and the spurious-free dynamic range is improved by about 22 dB as the multi-tone signal is transmitted. When a multi-carrier OFDM signal is transmitted, the EVM of the signal is optimized from 8.1% to 2.2%.