LPAM-OTFS and LHPAM-OTFS for IM/DD Optical Wireless Communication Systems

Orthogonal time frequency space (OTFS) is an excellent two-dimensional modulation scheme suitable for optical wireless communication (OWC) systems. However, the conventional pulse amplitude modulated optical OTFS (PAM-OTFS) only modulates the imaginary parts of the Doppler bins, resulting in a loss of spectral efficiency. To address this issue, we first propose a novel efficient layered PAM-OTFS (LPAM-OTFS) scheme, enhancing the spectral efficiency by layering and utilizing the real parts of the Doppler bins. Based on LPAM-OTFS, we further propose a general efficient layered hybrid PAM-OTFS (LHPAM-OTFS) scheme. It can improve the overall performance by providing a more flexible layered method of spectral resource allocation. Theoretical derivation demonstrates that LHPAM-OTFS achieves the same spectral efficiency as LPAM-OTFS and layered asymmetrically clipped optical OTFS (LACO-OTFS) with lower computational complexity. Numerical results indicate that LHPAM-OTFS offers superior peak-to-average power ratio (PAPR) performance and higher power efficiency than its counterparts, which implies better ability to resist nonlinear distortion.