Multilayer equalizer for 2-channel OTDM-PAM-8 transmission system

For implementing a high-speed intensity-modulation and direct detection-based short-reach system, such as data center interconnects, with bandwidth-insufficient transceivers, it is necessary to use multilevel modulation-formatted signals to increase spectral efficiency [1], [2], [3]. N-level pulse amplitude modulation (PAM-N), known as the simplest multilevel signaling method, is considered a promising solution for future interconnects [1]. The use of PAM-4 signals has been standardized in IEEE 400GBASE-LR8 [4], [5], [6]. Nevertheless, implementing beyond a 400 Gigabit Ethernet may require a further increase in spectral efficiency, given by the rapid growth of traffic at a rate between 25% and 80% per year [7], [8], [9]. Consequently, there has been much effort to increase the levels of PAM to eight [2], [9], [10], [11], [12], [13], [14], [15]. To use PAM-8 signals, which require a high signal-to-noise and distortion ratio (SINAD) [16], it is necessary to develop techniques to improve signal quality, such as nonlinear equalizer and probabilistic shaping [11], [16], [17].

To further increase the operating speed of the transmission system, the multiplexing technique can also be used in combination with multilevel modulation [7]. Recently, the feasibility of optical time division multiplexing (OTDM) in short-haul systems has been investigated and experimentally demonstrated [18], [19], [20], [21]. This system could be made more cost-effective by integrating transceivers (e.g., generated using lithium niobate-on-insulator or silicon-on-insulator technology) [18], [19]. In particular, the two-channel OTDM system in combination with a symbol-spaced multiple-input multiple-output (MIMO) equalizer was proposed [19], [20]. However, the symbol-spaced MIMO equalizer requires an OTDM demultiplexer to determine the optimal sampling position (which is difficult to achieve before applying an equalizer when using the PAM-8 signal because of low SINAD) [22]. This necessitates development of a receiver insensitive to the sampling position to apply the PAM-8 signal to the 2-channel OTDM system for the proper operation of the MIMO equalizer.

In this paper, we propose utilizing a multilayer equalizer consisting of a demultiplexer based on half-symbol-spaced feed-forward equalizers (FFEs) and a simplified symbol-spaced 2 × 2 nonlinear MIMO equalizer for the 2-channel OTDM system. This is an extension of our previous work on a 2-channel OTDM system [19]. The demultiplexer, which is the first layer, makes the bit-error rate (BER) performances insensitive to the sampling position. This property enables the 2-channel OTDM system to utilize PAM-8 signals in a stable manner. In addition, it makes the OTDM system robust to the imperfections of the multiplexer (e.g., timing mismatch problem). The simplified nonlinear MIMO equalizer in the second layer can effectively lower the BER floor, which significantly reduces tap length compared to the conventional second-order Volterra equalizer. In the experimental demonstration, the 2 × 75-Gb/s OTDM-PAM-8 signal can be successfully transmitted over a 1.9-km standard single-mode fiber (SSMF) with a BER of <10−3. The multilayer equalizer used in the demonstration consists of a demultiplexer (consisting of two half-symbol-spaced 6-tap FFEs) and the simplified symbol-spaced 2 × 2 nonlinear MIMO equalizer.

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