Date: Sun Sep 17 2000 - 19:05:41 PDT
[Impressive... from a marketing view point, at least. Made the cover of
EETimes. And you can't beat "digital predistortion" at a cocktail
AT&T, Nortel plot '4G' wireless nets
By Margaret Quan and Patrick Mannion, EE Times
Sep 15, 2000 (2:56 PM)
DENVER — Amid projections of a boom in wireless data, top technologists
from AT&T Labs and Nortel Networks sketched out their first efforts to
define fourth-generation cellular networks in separate presentations
this week. The companies also pointed to key technologies — such as
software radios, wideband transceivers and a new breed of power
amplifier — needed to deliver wireless networks with cellular data rates
of 20 Mbits/second and beyond. Many of those technologies were addressed
by technical papers at the Radio & Wireless Conference (Rawcon) held in
Denver Sept. 10-13.
Rawcon keynoter Al Javed, chief technology officer for access networks
at Nortel (Brampton, Ontario), predicted that more than 1 billion users
will be accessing the Internet wirelessly by 2004.
AT&T demonstrated in New York an asymmetric network it called 4G Access
that combines existing Enhanced Data Rates for GSM Evolution (Edge)
technology for an uplink with wideband orthogonal frequency-division
multiplexing (OFDM) for the downlink. The goal: to speed downloading of
packet data, particularly for streaming audio and video.
"There is a developing industry consensus on focusing on improved
downlink performance [for 4G]," said Michael Bamburak, vice president of
the technology development group at AT&T Wireless. AT&T believes
multicarrier OFDM will provide wireless downlink access at up to 10
Mbits/s for stationary systems and more than 384 kbits/s at 800-kHz
bandwidth in a high-mobility environment.
Deploying the 800-kHz OFDM 4G network will require wideband software
radios and advances in digital signal processors. DSP cores might need
built-in channel-coding cores, and the ability to handle multipath
fading and fast Fourier transforms to extract the multiple tones from
the OFDM signal, said the director of the 4G program, Nelson
Sollenberger, division manager for Wireless System Research at AT&T Labs
(Red Bank, N.J.).
AT&T has already begun a two-phase upgrade of its wireless network on
the way to 4G Access. The first phase will involve deployment of
software at cellular basestations. A second phase, likely to occur in
approximately two years, will involve aggressive hardware deployment of
smart antenna technology being developed by AT&T and its OEM partners.
Cellular data surge
Despite the fact that 4G networks are still several years out, testing
and planning are necessary now if carriers are to meet demand for what
researchers call an intense hunger for wireless high-speed data
services. Forecasts suggest that by 2005, 50 percent of cellular
subscribers will be data-capable and handsets will surpass PCs as
Internet access devices.
"Messaging will be the primary driver of wireless-data adoption over the
next few years," said Dave Jackson, senior wireless analyst for Cahners
In-Stat (Scottsdale, Ariz.). "We estimate that the number of wireless
messages sent per month will balloon from 3 billion in December 1999 to
244 billion by December 2004."
Mobile commerce and location-based services will follow messaging as
growth vehicles, Jackson added. However, In-Stat warns that the success
of wireless data will depend on building out a new infrastructure that
can handle location-based services, high levels of security, new payment
options and detailed billing. "After that, the march toward wireless
data will become a stampede," said Jackson.
Nortel's Javed detailed a feature list for Internet protocol-based 4G
networks with data rates up to 20 Mbits/s. Such nets should sport
aggregate data throughput per cell of 100 Mbits/s (forward); a spectral
efficiency (bits/s per hertz per cell per carrier) of 20 forward, six
reverse; and a dormant-to-active transition time of 0.1 second. These
figures up the ante from 3G Universal Mobile Telecommunications System's
(UMTS) specs of 3.8-Mbit/s throughput, spectral efficiency of 0.8 and
transition time of 2 seconds.
Technologies needed to hit those specs, Javed said, include wideband
receivers, multibeam antenna systems and space-time coding, followed by
better power amplifiers, advanced modems, RF transceivers and multi-user
detection. Papers at Rawcon outlined advances in several of these areas.
Designers lack a formal methodology to implement software radios on
reconfigurable platforms. Researchers at the Mobile and Portable Radio
Research Group at Virginia Tech sought to solve that problem by
proposing in one Rawcon paper a Layered Radio Architecture using
stream-based computing, which allows over-the-air updates and software
"The design is based on an FPGA that was developed in-house called
Stallion, which supports fast run-time reconfiguration," said
Srikathyayani Srikanteswara, one of the researchers. "The stream-based
approach gives a uniform, modular structure to the processing modules
and defines the protocol for interaction between various modules, while
the layered architecture makes it possible to incorporate all of the
features of a software radio while minimizing complexity."
Power amplifiers, meanwhile, could surge to 70 percent of the cost of 4G
basestations without innovative design and channelizer power savings,
Javed said. Techniques such as digital predistortion would compensate
for nonlinearities, a big power dissipater and contributor to poor
"Digital predistortion is a result of accurate modeling of LDMOS power
devices and could replace other compensation techniques such as RF
feed-forward for greater accuracy," Javed said. DSP-based algorithms
could also be used, but the overall goal is to use digital correction
techniques to lower cost and improve efficiency.
Researchers at Stanford University described at Rawcon a method of
maintaining continuous regulation of oscillator phase alignment for the
two decoupled feedback loops in Cartesian feedback. Cartesian feedback
is a linearization technique that holds much promise if stability can be
maintained. But instability and phase error can occur over time due to
temperature and process changes if the system isn't properly adjusted.
The Stanford team outlined a way to regulate the local-oscillator phase
alignment — without the use of DSP techniques. "Poor phase regulation is
one of the major barriers to the use of Cartesian feedback," said
researcher Joel Dawson. "Our system could greatly reduce that barrier
and allow it to operate — maintenance-free — in a hostile environment."
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