Gigabit simultaneous bi-directional signaling using DS-CDMA

This paper proposes use of orthogonal coding to allow simultaneous communication over a digital link without use of a modulation scheme. The interconnect consists of a complex, lossy and noisy channel (consistent with designs in a Hewlett-Packard computer system) and a novel transceiver. CDMA schemes have been used extensively for spread spectrum communications, especially for multiple access in wireless applications. In general, these systems involve spreading the baseband information over a wider bandwidth using a set of codes. These codes are orthogonal to each other and hence provide a means for multiple access. These coded signals are subsequently modulated with a carrier before being sent on the channel (M.F. Chang et al., Proc. IEEE, vol. 89, no. 4, pp.456-66, 2001; Sergio Verdu, Multiuser Detection, 1998). This is generally achieved either by a BPSK or a QPSK modulation scheme. The modulation allows for reduction of the error probability by optimizing the constellation distances and making the system more robust. If such signals were sent on a wired line it would not be necessary to up-convert the baseband information with a carrier because the resources for frequency allocation are not limited like in air, and baseband transmission is possible. DS-CDMA has been used to design point-to-point wired uni-directional buses (R. Yoshimura et al., IEEE ISSCC 2000, pp. 370-1, 2000) but there has been no prior investigation as to its performance on a simultaneous bi-directional channel. This paper provides such an investigation on a potential high-loss interconnect that would connect two CPUs in a Hewlett-Packard computer system. The channel was chosen to demonstrate performance on longer interconnects in computer systems, which often contain high losses and complex discontinuities. The paper introduces the key concepts used in the design of the link and gives a brief overview. It then describes the channel used, the design and implementation of the transceiver, and concludes with a summary of the design and the results.