Joint Pre- and Post-Equalization in Optical MIMO with Multimode Fiber Link

In optical fiber communication, the concept of multiple-input and multiple-output (MIMO) transmission using multimode fiber has gained a lot of attention due to an enhanced fiber capacity. This paper outlines a joint pre- and post-equalization (PPE) scheme for an optical intensity modulation and direct detection MIMO system. For restraining the transmitter's complexity and allowing multi-level modulation for performance benefits, a simple few-tap linear feed-forward equalizer in conjunction to the zero-forcing based post-equalizer is proposed. Using a mathematical optimizer, the pre- and post-equalizers are jointly designed and optimized, which yield a performance gain of 19.95 % compared to the post-equalization only, with a constraint subjects to the maximum transmit power of the laser source. This work offers one of the first investigations regarding the benefit of joint-PPE schemes in an optical MIMO system with a multimode fiber channel, where the inter-symbol interference is assertive due to modal dispersion along with crosstalk. The usefulness of the joint-PPE scheme is tested over two distinct frequency-selective channels using computer simulations. In terms of the bit-error rate (BER) performance, the joint-PPE scheme outperforms the post-equalization only and provides a gain of approximately 2 dB at 10⁻³ BER.