A simple single-polarization heterodyne optical network unit (ONU) receiver with Alamouti-coding based polarization diversity at the optical line terminal (OLT) side is a promising solution for passive optical networks (PON) beyond 50 Gbps. The special equalizer required for data recovery of such system has been demonstrated for 200 Gbps PON downstream using offline processing of serial data. In this letter, we extend such equalizer implementation with parallel processing that is required for real-time implementation. The proof-of-concept experiment for 200 Gbps downstream transmission is demonstrated to show the effectiveness of the proposed digital signal proccesing (DSP). The impact of phase noise and limitation of degree of parallelism are further investigated via numerical simulations.