Transition-limiting codes for 4-PAM signaling in high speed serial links

As signaling rates move to multi-gigahertz frequencies and multi-level schemes are used to increase bit rates, intersymbol interference becomes a key issue for signal integrity. The paper discusses the use of coding techniques to minimize ISI on channels using embedded clocking and 4-PAM signaling. The coding techniques are designed to increase timing and noise margins, while guaranteeing minimum transition density for clock recovery. The elimination of the worst case pattern sequences reduces peak and mean-square distortion. The results from the coded versus uncoded sequences are compared across several channel environments. The probability of symbol error and the related performance bounds are calculated. Simulations were used to verify the analytical models. The proposed codec blocks were synthesized, so that their implementations could be compared in terms of resulting latency, gate-count and area.