The gap between on-chip and off-chip communication speed has become wider as the IC process technology continues to shrink in order to increase the chip performance. The speed of on-chip circuit has outperformed the off-chip communication speed. Therefore, the performance threshold of a system which consists of multiple IC's is limited by the off-chip communication speed. I/O interfaces such as PCI-Express, USB 3.0, and DDR3 are designed to bridge the gap by introducing high-speed transceiver system which typically operates at Giga-Hertz range. However, legacy copper interconnect on a motherboard backplane cannot support data rate. As a result, integrity of the signal is impaired with nonideal effects introduced by the channel. Continuous-Time Linear Equalizer (CTLE) is used at the receiver front-end to compensate the high-frequency losses introduced by the channel. The implementation of CTLE is normally limited to first-order. Second-order CTLE offers the advantage of incremental peaking gain when dealing with channel of high losses. Therefore, in this paper, the characteristics and theoretical circuit analysis of first-order and second-order CTLEs are presented. Both equalizers are designed to address a 5-Gb/s data rate transmission. An arbitrary 20-inch channel is used as test bench to compare the performance of the two equalizers. Simulation results show improvement in receive eye voltage opening and insertion loss for second-order CTLE but with degradation in terms of receive eye time opening, jitter, and amplitude noise.
A minimum mean squared error (MMSE) equalizer is a way to effectively increase transmission performance for nonlinear Fourier transform (NFT) based communication systems. Other equalization schemes, based on nonlinear equalizer approaches or neural networks, are interesting for NFT transmission due to their ability to deal with nonlinear correlations of the NFTs' eigenvalues and their coefficients. We experimentally investigated single-and dual-polarization long haul transmission with several modulation schemes and compared different equalization techniques including joint detection equalization and the use of neural networks. We observed that joint detection equalization provides range increases for shorter transmission distances while having low numeric complexity. We could further achieve bit error rates (BER) under HD-FEC for significant longer transmission distances in comparison to no equalization with different equalizers.
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