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We propose a novel coded modulation scheme for coherent optical orthogonal frequency-division multiplexing (CO-OFDM) to achieve high-speed transmission at spectral-efficiencies near the Shannon limit. The proposed coding scheme relies on the concept of bit interleaved coded modulation with iterative decoding (BICM-ID), low-density parity-check (LDPC) codes, and shaped iterative polar modulation (IPM). To further increase the transmission spectral efficiency, reduced guard interval (RGI) CO-OFDM is used, in which fiber chromatic dispersion (CD) is digitally compensated prior to OFDM demultiplexing at the receiver. We experimentally demonstrate the generation and forward error correction (FEC) decoding of a 231.5-Gb/s RGI-CO-OFDM signal with 256-IPM subcarrier modulation, occupying a bandwidth of 20.75 GHz. The coded 256-IPM signal offers a coding gain of 15.1-dB compared to uncoded 256-point quadrature amplitude modulation (256-QAM) at a post-FEC bit error ratio of 10- 15. Transmission was demonstrated over an 800-km ultra-large-area fiber (ULAF) link with a record intrachannel spectral efficiency of 11.15-b/s/Hz.