Signal Processing for High-Speed Links - Signal Processing Systems - page 310

and HFC, and wireless), through 10 s of meters (Ethernet LAN), to less than a
meter (back-plane and chip-to-chip I/O) [ 29 ] . Interestingly, the structure of data
aggregation and distribution in this network hierarchy has resulted in the highest
data rates in its two extremes, ULH and metro optical (longest) and back-plane and
I/O (shortest), compared to any other type of communication link. Indeed, today,
both back-plane and optical links support data rates in excess of 10 Gb/s. We refer to
such links as high-speed links. Also interesting is the fact that high-speed links have,
in the last decade, seen an extensive application of signal processing techniques, and
VLSI architectures in the design of high-speed transceivers simultaneously for both
link types. This chapter discusses this recent trend and the role of signal processing
and integrated circuits in the design of high-speed links.
While the data rates for both back-plane and optical links exceed those of all
other digital communications media, in some respects, they are among the least
sophisticated, using simple on-off keying (OOK) or non-return to zero (NRZ), and
baseband comparators for symbol-by-symbol clock and data recovery (CDR). For
example, until recently, optical links did not employ any form of equalization,
and back-plane links to this day do not incorporate any physical layer forward
error-correction (FEC). In this respect, these links are quite primitive in that they
haven't leveraged the tremendous advances in statistical signal processing and
communication techniques that have enabled a number of important advances
in both wired and wireless communications such as those in digital voice-band
modems, cellular technologies, such as GSM and CDMA, broadband enabling
technologies, such as cable modems and DSL modems, and OFDM technology in
use in a host of wireless digital transmission standards.
The reason that simple modulation, transmitter and receiver structures were
suitable for high-speed links is because at lower data-rates and short distances, both
the back-plane and optical links act as infinite bandwidth channels. As a result,
optical links have been the transmission media of choice in backbone networks
and are rapidly making in-roads into customer premises, enterprise networks, while
back-plane links dominate system-level interconnect such as those in storage area
networks. However, as data rates in these links rise above a few Gb/s into the 10 Gb/s
range, these links began to exhibit intersymbol interference (ISI) or dispersion in
addition to noise.
Figure 1 shows block diagrams of a high-speed back-plane link and an optical
link. Much of the processing in both links is similar even though significant
differences exist. Serial data d in the back-plane transmitter in Fig. 1 a is clocked

using a transmit phase-locked loop (PLL) and sent to a pre-emphasis filter/driver
to pre-equalize channel ISI. The back-plane channel is typically a copper trace,
typically few tens of inches, running over an FR4 dielectric through vias or stubs
connecting different metal layers and connectors linking line cards to the board. At
the receiver, the channel output is processed by a receive amplifier, which filters out-
of-band noise, and amplifies signal for subsequent processing. The receiver recovers
a symbol-rate clock from the data using one of several clock recovery techniques.

[ k ]