SerDes: Four Wires Are Better Than Two

Kandou Bus SA has recently been proposing the technique ENRZ (Ensemble Non Return to Zero) for use as the next generation interconnect standard for the 56 Gb/s generation of interconnect interfaces at the OIF (Optical Interconnect Forum). ENRZ is technique where three bits are orthogonally modulated over four correlated wires. It delivers more throughput with lower power consumption than regular NRZ does. The receive eyes after ENRZ’s joint detectors are NRZ shaped and are compatible with off-the-shelf binary DFEs (Decision Feedback Equalizers).
ENRZ is also amenable to SerDes-like implementations that do not require DSP (Digital Signal Processing) and can be implemented as a “turbo mode” of two regular NRZ SerDes. ENRZ requires that the four wires be skew-matched to a similar constraint as the intra-pair skew constraint for differential pairs. An important channel for ENRZ is two loosely coupled differential pairs that are separated by a small gap. The ISI Inter-Symbol Interference performance of ENRZ is much better than that of PAM-4 (Pulse Amplitude Modulation 4-level).

At an OIF panel at OFC 2014 entitled “56 Gb/s Serial: Why, What, When?”, Brian Holden of Kandou Bus articulated the vision of using 4-wire ENRZ links in every portion of the system instead of using traditional differential links. He described how the 56 Gb/s generation that the industry is currently working is highly challenging because of the high baud rate. Beyond that, in the 112 Gb/s generation, there are almost no appealing two-wire alternatives. Because of these twin difficulties, a choice of a four-wire interconnect architecture in this generation can set the industry up for success in the both the 56 and 112 Gb/s generations.

Kandou presented a two generation interconnect architecture for electrical interfaces. That architecture is to use ENRZ at 37.3 GBaud over four wires for all of the interfaces of the 56 Gb/s generation. This interconnect delivers 112 Gb/s over four wires. A standardized bit multiplexing technique will be useful with any of the proposed 56 Gb/s interfaces including ENRZ, given the ubiquity of 25 Gb/s optical interfaces. There are several applications that have differing frequencies on each of the 28 Gb/s legs, notably Fibre Channel.

For the generation following, the ENRZ plus PAM-3 (Pulse Amplitude Modulation 3-level) can be used. That interface would run at 47.1 GBaud and would deliver 224 Gb/s over those same four wires. Neither PAM-4 nor PAM-8 are likely to be good choices for this interface due to their inferior ISI performance and PAM-4’s higher baud rate.

For both generations, the in-package, USR (Ultra Short Reach) interface has different constraints. Several good alternatives exist including 4, 6 and 8 wire alternatives.

Bottom line: a consistent four-wire interface is the optimal choice across the 56 Gb/s generation. Choosing this interface for the 56 Gb/s generation will also set the industry up to succeed in the 112 Gb/s generation. Four wires are better than two.

About Kandou Bus S.A.
Headquartered in Lausanne, Switzerland and founded in 2011, Kandou Bus is an innovative interface technology company specializing in the invention, design, license and implementation of unmatched chip-to-chip link solutions. Kandou’s Chord™ technology lowers power consumption and improves overall performance of semiconductors, unlocking new capabilities in electronic devices and systems. www.kandou.com.

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