Symbol-interleaved precoding technique for DDO-OFDM systems with the KK receiver

Abstract

Receiver sensitivity of direct-detection optical orthogonal frequency-division multiplexing (DDO-OFDM) systems can be significantly improved by using the Kramers-Kronig (KK) receiver. However, the KK algorithm is sensitive to the high signal-to-average power ratio (PAPR) of the OFDM signal, which may violate the minimum phase condition (MPC) and lead to severe signal distortions during the optical field reconstruction. Channel-independent discrete Fourier transform (DFT) precoding technique can efficiently reduce the PAPR and realize subcarrier signal-to-noise ratio (SNR) equalization, but the PAPR of the precoded signal will increase due to chromatic dispersion. Besides, the relevant researches exhibit that it is hard to accurately reconstruct the complex field from the sampled signals based on KK relation at the low carrier-to-signal power ratio (CSPR) even if the MPC is satisfied. To combat the signal distortions induced by the KK algorithm and improve the bit error rate (BER) performance, we propose and verify DFT/Walsh-Hadamard transform (WHT)-based precoding with a simple symbol-interleaving technique for DDO-OFDM transmission systems. In addition to PAPR reduction, the proposed technique can equalize both subcarrier and inter-symbol SNRs efficiently. The simulated results show that the BER performance can be improved by up to one order of magnitude at a CSPR of 6 dB after up to 1000km single-mode fiber transmission, compared to the conventional DFT precoding technique. It is expected that the proposed technique can be employed to improve the receiver sensitivity of medium-reach DDO-OFDM transmission systems with the KK receiver.