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4G wireless challenges raise stakes for DSPs

Posted: 16 Aug 2002 ?? ?Print Version ?Bookmark and Share

Keywords:dsp? cdma? adc? soc? fpga?

While the wireless world is starting to adopt 3G technologies defined by CDMA-based systems operating at 200MIPS, researchers are forging ahead to 4G systems whose modulation schemes are not yet set but whose DSP requirements are expected to reach in the multiple Giga-instructions-per-second (GIPS) range.

Industry observers expect that developers will introduce 4G wireless communications systems between 2007 and 2010. Historically, previous systems have seen a change of generation every 10 years or so. Ultimately, though, the success of 4G systems depends on how economically companies can build complex and overblown adaptive arrays, large power amplifiers and giant DSPs.

To make these systems cost-effective, the functions of analog circuits need to be digitized as much as possible and ADC and digital processing needs to occur as close to the antenna as possible, according to Yukou Mochida, senior vice president at Fujitsu Laboratories Ltd. The digital processing section will start being implemented by lining up processing blocks that use DSPs and FPGAs.

Unlike previous generations of wireless devices, where the communications part and processing part existed as separate physical entities, 4G handheld devices and basestations alike will be based on SoCs that combine processing and communicating onto one IC.

Among other requirements, 4G systems demand always-on mobile devices connected to the Internet. Also, although person-to-person communications have dominated conventional communications systems, future communications between people and computers and between computers and other similar equipment, will be much more common than is evident today.

In his IEDM keynote speech last year, Fujitsu's Mochida suggested that if DSPs have increased in power by an order of magnitude in each generation, from 4MIPS through 40MIPS to 400MIPS, then it might take 4GIPS to accommodate 4G applications.

R&D into 4G mobile communications systems is already well established. System requirements for 4G devices include support for minimum transmission speeds of 100Mbps downlink and 30Mbps uplink. But major problems must be overcome if data rates of 100Mbps are to become a reality, Mochida said. Also, signal processing needs to be applied to modems as a countermeasure to severe delay spreads. And, of course, system capacity needs to be high enough to accommodate high-data rate users who might otherwise hinder other users.

According to Mochida, the ideal frequency range for 4G must lend itself to an economical service rollout and offer a high level of mobility. The use of HFs makes an economic rollout difficult, however, because HFs make cell radii extremely small. The success of cost-reduction schemes hinges on the rate of IC integration, yet there are many areas that DSP will find hard to tackle.

Mochida said he expects that 4G baseband sections will be fully digitized. He also said that it will become commonplace for a signal to be digitized using an A/D converter at the baseband entrance, with all processing after that point done digitally. As digitization progresses, it will spread to the modulation/demodulation functionality of the IF section. Eventually, all analog processing will be transformed into digital processing.

Mainstream move

As 2G gives way to 3G, the handset is evolving from a tool for voice communications to a high-speed data terminal. In line with this shift, the range of applications has expanded and now includes video encoding. Throughout this transition, digital processing was applied to more and more sections of baseband processing.

Extrapolating, a throughput of several Gips will be required across the handset. The use of OFDM for modulation is being investigated for 4G, to enable high-speed data services in a mobile environment. "If we attempt to deal with such high-speed processor-intensive modulation entirely in software, it will require a throughput more than 10 times greater," Mochida said.

"Speculating beyond 3G is tough, when we don't really know what 3G will be when it grows up," said Will Strauss of Forward Concepts. "Everybody assumes that video will be the defining technology in 3G handsets and certainly, MPEG4 will be part of them."

Strauss said most of the 4G buzz has been about the air interface. "It will not be WCDMA or cdma2000 but likely will be OFDM and to enable the higher data rates, we will probably have to move to higher RFs, probably above 3GHz (as opposed to present-day 800MHz cellular and 1.9GHz PCS frequencies)." That means the FCC and WARC (the world spectrum allocation/regulatory body) will have to make new frequency allocations, Strauss said.

4G expected arrival

4G devices are not likely to become widely available until about 2010. Strauss predicts that DSP clock speeds will exceed the 3GHz range by then, with many parallel processing elements accounting for hundreds of billions of operations per second. "Naturally, SoC approaches will be the norm," Strauss added.

Industry observers predict that CMOS technology will see a doubling of circuit miniaturization and integration by 2005. This development should secure devices that can enable full digitization of 3G baseband processing. However, an even greater leap forward is required to bring about the ultimate all-digital handsets for 4G.

While CMOS technology is expected to continue its progress in miniaturization, higher speeds and lower power consumption through 2015, there will be a visible gap between the rate of progress in CMOS and that of the handset. This gap will be creating a serious hurdle for the early introduction of 4G, according to Fujitsu's Mochida.

- Nicolas Mokhoff

EE Times





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