Sub-band-based transmission for mode-multiplexed optical systems

Abstract

Mode-multiplexed optical transmission is subject to mode coupling and potentially large differential mode delays. In most recent implementations, these effects are compensated for at the receiver by complex adaptive MIMO equalizers. Although frequency-domain MIMO equalization requires a moderate complexity compared to time-domain equalization, the long required FFTs may face implementation issues and yield a relatively slow response to dynamic effects. In this paper, we evaluate an alternate transceiver architecture based on sub-band partitioning, implemented by filter banks, which enables concurrent time-domain equalization. The performance of sub-band and single-carrier schemes are compared using Monte-Carlo simulations. Keywords: sub-band partitioning, space-division multiplexing, MIMO equalization. INTRODUCTIONSpace-Division Multiplexing (SDM) has been accepted as the only transmission technique able to cope with the exponential traffic growth experienced in several segments of data networks, specially in inter and intra data center interconnects. Besides parallel transmission of signals in bundles of single-mode fibers (SMFs), multicore fibers (MCFs) and multimode fibers (MMFs) are important candidate technologies for the SDM transmission media. Signals conveyed by MCFs with uncoupled cores are easy to couple and switch, but require careful crosstalk management. On the other hand, MMFs use typically multiple-input multiple-output (MIMO) processing to separate the signals multiplexed over the multiple orthogonal modes. The complexity of such MIMO equalizers is specially challenging in mode-multiplexed systems with large differential mode delays (DMD) and low or intermediate mode-coupling levels. Although in frequency-domain equalization (FDE) the computational complexity scales with the logarithm of the delay spread, the application-specific integrated circuit (ASIC) area reserved for the fast Fourier transforms (FFTs) may become excessively large [1], and the equalizer may react slowly to dynamic effects.A promising alternative to avoid large FFT sizes is to split the spectrum into smaller sub-bands and carry out source separation and deconvolution in parallel for the individual sub-channels [2]. This idea has been validated in conventional polarization multiplexed systems (PM) transmission using a computationally-efficient structure with polyphase filters (called in the paper a twice-underdecimated filter bank) [3]. The technique has been applied to mitigate chromatic dispersion (CD) [4] and non-linear effects [5]. Polyphase filter bank architectures have also been employed in the context of MIMO equalization for MMFs in [6], where the twiceunderdecimated filter bank architecture was adapted to achieve fast convergence in an orthogonal frequency division multiplexing (OFDM) system with weakly coupled spatial channels. Alternatively, sub-band partitioning in the optical domain has been reported in [7] to mitigate DMD in MMFs and coupled-core MCFs.In this paper, we investigate the performance of the ...

Filter Bank Multi-Sub-Band Transmission for Optical Systems With Mode Multiplexing

“…However, in sub-band systems, in which the channel length is reduced according to the number of sub-bands, the channel becomes shorter, and TDE is still an interesting choice. Therefore, in this paper we extend the results of [1], comparing the MSB-TDE architecture with that of an SC-FDE solution. We also evaluate both systems under dynamic channel conditions, and assess their resilience to laser phase noise.…”

“…We investigated in [1] the performance of the multi-sub-band (MSB) architecture using polyphase filter banks in MIMO SDM equalization with digitally-generated subchannels for a static channel. The performance of a single-carrier (SC) system with time-domain equalization (TDE) was compared with that of an MSB-TDE architecture at the same data rate and bandwidth, using Monte-Carlo simulations.…”