SoCs no panacea for comms infrastructure

This is not to imply, however, that system builders aren't interested in the advantages of SoC integration. Both basestations and switching centers are densely packed with electronics. Telecom system builders do look at devices that will increase channel density without raising power consumption or heat dissipation inside a closed cabinet.

That's certainly the case in central-office switching stations, where manufacturers are interested in increasing the channel de nsity and, in some cases, overlaying data call services such as DSL and analog modems on the existing voice lines. Change here is slow, since it must be made without disruptions to the existing services.

In contrast, cellular basestations must build in "hooks" and accommodations for new services. The migration from second-generation to 2.5- and 3G mobile services will demand multicarrier RF transmission with increased Internet and packet data traffic. Existing basestations must be rewired not only to accommodate more channels but also to digitize RF and baseband information that was previously handled as a high-frequency analog signal.

Contributors to this week's In Focus look into several strategies for integrating base-station and central-office functions into new-generation silicon.

Because of their interface to a 48-volt circuit, codecs and voice-processing systems harness a high-voltage fabrication process. In some cases this requires a bipolar interface; in others, a wide-geome try CMOS (3 and 5 microns) offers a much higher breakdown voltage than deep-submicron CMOS. But as Dru Cabler and Don Beech of voice codec manufacturer Legerity Inc. note in their article, finer-geometry CMOS enables the channel density of voice-processing circuits to transition from two to four, four to eight, eight to 16 and so forth.

With cellular basestations, manufacturers are exploring ways for bipolar processes-particularly silicon germanium-to replace gallium arsenide in RF transceivers. GaAs is relatively inexpensive and offers low noise and good power transfer characteristics at gigahertz frequencies. Power MOSFETs, particularly LDMOS transistors, have replaced the GaAs antenna drivers in cellular basestations operating at 2.1 GHz and below, but everything else in the transmitter-receiver loops is subject to review. BiCMOS, with SiGe as the bipolar implant, is increasingly being explored as the means of obtaining some of the higher channel densities suggested by CMOS, along with a high-frequency bipolar process.

Brad Brannon and Paul Hendriks of Analog Devices Inc. explain the elements involved in the construction of 2.5- and 3G basestations in their contribution. "Within the receive and transmitter chains, processes such as GaAs are very good for low-noise devices," they write, "but these processes are not sufficient for high-density functions such as A/D converters. Conversely, CMOS is an excellent process for data converters, but provides poor noise performance." Brannon and Hendriks call for a "smart partitioning" scheme that selects processes for their optimum behavior in each element of the basestation.

Similarly, in an exclusive online article, PMC-Sierra Inc.'s Laurie Wallace describes a technique for linearizing the output of base-station power amplifiers. The technique-which involves applying at the transmitter predistortion that comes out as a entirely linear signal at the receiver-depends on the application of DSP wave-shaping technology. But this does not imply that DSPs could be integrated on the same chip with the power amp. "In cellular basestation systems, discrete devices are the norm," Wallace says. "Power amplifiers rely on discrete components to perform functions such as crest factor reduction to digital predistortion."

About the only place for deep-submicron CMOS-90-nanometer technology-is in the protocol dissection telcos must perform as they convert the voice-based asynchronous transfer mode protocol packets into Internet Protocol (IP) packets. The Internet functions much like a conveyor belt for packets, putting blocks from multiple senders on the same transmission channel and sorting them at the central office. But ATM and IP use different-size packets, with different headers and address references, and sorting them out requires high-speed processing.

Jeffrey Timbs and Larry Wakeman of Agere Systems explain the options that telcos are con sidering as they migrate their mobile networks from ATM to a packet-based protocol such as IP. "The network interface may need to be provisioned to process compressed speech for either all-ATM, hybrid ATM/IP or all-IP transport techniques," they write.