Optical orthogonal chirp division multiplexing with chirp index modulation

We propose and experimentally demonstrate a modulation technique, termed orthogonal chirp division multiplexing with chirp index modulation (OCDM-CIM), where partial chirps are deliberately unmodulated to reduce interference and energy consumption. OCDM-CIM exhibits the combined advantages of interference rejection from the chirp-spread spectrum in OCDM and energy efficiency (EE) from index modulation, achieving an attractive trade-off among spectral efficiency (SE), EE, and performance by flexibly controlling the number of active chirps per subblock to adapt to different granularity requirements of the system. In addition, a distributed subblocking strategy is introduced to obtain additional coding and diversity gains. Experimental results show that compared to conventional orthogonal frequency division multiplexing (OFDM) with subcarrier index modulation under the same index parameter setting, the proposed OCDM-CIM reduces the peak-to-average power ratio by 2.5 dB at the complementary cumulative distribution function of 10⁻³, enhances optical receiver sensitivity by 2.7 dB, and increases the achievable bitrate by 8.7 Gbps at the bit error rate threshold of 3.8 × 10⁻³ for hard-decision forward error correction. Moreover, an improvement of 25% in EE can be achieved compared to OCDM at the same SE.