Chirp-Like Polyphase Sequence Based Discrete Circulant Transform Spread OFDM for Bandwidth-Limited IM/DD Systems

We propose and investigate chirp-like polyphase sequence (CLPS) based discrete circulant transform spread (DCrT-S) OFDM in bandwidth-limited intensity-modulation and direct-detection systems. This scheme can control the distribution of residual inter-symbol interference (ISI) and inter-carrier interference (ICI) across subcarriers by adjusting the CLPS's parameters. We also propose a sparse CLPS (SCLPS) scheme, by inserting zeros between the elements of the CLPS, to further balance the effects of the ISI/ICI and the noise and improve the performance by optimizing both the CLPS's parameters and the sparsity. We reveal the optimal design rule of CLPS/SCLPS analytically and investigate the performance of the proposed schemes using two-parameter CLPS in ∼110-Gbit/s simulations and experiments. It is shown that the optimal parameter vector of the CLPS is a Zadoff-Chu sequence that spreads the ISI/ICI more evenly over subcarriers. The performance is further improved by optimizing the sparsity. The optimal CLPS and sparsity are insensitive to the received optical power, the system bandwidth, the length of cyclic prefix, the chirp parameter and the signal data rate, and so can be pre-set without channel information. Both CLPS- and SCLPS-based DCrT-S-OFDM outperform Nyquist PAM, conventional OFDM, discrete-Fourier-transform spread (DFT-S) OFDM, and orthogonal-circulant-transform precoded (OCT-P) OFDM. By using the proposed scheme, the data rate is enhanced by 6.7% and 3.2% compared to DFT-S-OFDM and OCT-P-OFDM at a bit error rate (BER) of 10
⁻³
, while the complexity is reduced to 57.9% of OCT-P-OFDM.