Sidelobe Suppression using Cancellation Sub-Carriers for OFDM Superchannels

We mitigate the inter-channel-interference (ICI) in orthogonal frequency division multiplexing (OFDM) superchannel by adding cancellation sub-carriers (CCs). We experimentally show that CCOFDM outperforms conventional OFDM by 0.4-dB in a dual-polarization 400-Gbps 3360-km OFDM superchannel. Introduction Superchannels based on concatenating lowerrate orthogonal-frequency division multiplexing (OFDM) channels have been demonstrated to provide 400-G and 1-Tb/s transmission1,2. Compared with Nyquist wavelength division multiplexing (N-WDM)3,4, OFDM system can use simple one-tap equalization at the receiver, but have a drawback of higher sidelobe powers5. Unless the sub-carriers on the adjacent bands that constitute the superchannel are carefully controlled in frequency6, this sidelobe power will degrade the system performance. Thus, sidelobe suppression could be a critical issue for the successful deployment of OFDMsuperchannels. Windowing of OFDM symbols (or filtering the OFDM spectrum) is a common method to suppress its sidelobes, but this destroys the orthogonality of the sub-carriers7. In this paper, we propose suppressing the sidelobes by inserting cancellation sub-carriers (CCs) at the edges of the OFDM channels, which are specially designed by using linear least squares algorithm8. The implementation complexity of this approach is small as it only needs to design two or four extra sub-carriers; it also maintains the sub-carrier orthogonality. We experimentally demonstrate a 400-Gb/s 4channel OFDM-superchannel system using the CC technique to reduce inter-channelinterference (ICI). By inserting 4 CCs into a 302 sub-carriers quaternary phase shift keying (QPSK)-OFDM signal, 8-dB sidelobe suppression and more than 0.4-dB performance improvement after 3360-km is successfully demonstrated. Principle of CC-OFDM Fig. 1(a) illustrates the conceptual block diagram of the CC-OFDM system transmitter (Tx) for one sub-channel. After bit-to-symbol mapping and serial-to-parallel (S/P) conversion, the kth block contains N complex data symbols, dn,k. In contrast to conventional OFDM, before the inverse fast Fourier transform (IFFT) operation a few CCs are appended to the edge of the band for the purpose of sidelobe suppression. These CCs carry complex weighting values, gm,k, which are calculated from the data symbols within each IFFT block. The optimal number of CCs has been suggested8 to be 4, thus we chose 4 CCs through our paper. After zero padding, the IFFT size becomes S. The design criterion of CCs aims to minimize the superposition of the sidelobe spectra of the weighted CCs and the original Tx signal, the sidelobe are defined as the spectra range outside the Nyquist frequency, as highlighted in Fig. 1(b). For IFFT block k, the optimization of Fig. 1: (a) Block diagram of a CC-OFDM transmitter. (b) Comparison of QPSK-OFDM spectra with and without CCs (N = 20, the number of OFDM symbol is 60, VPItransmissionMaker simulation with 10 MHz frequency resolution). Transmitter