Subject: [NEXTEL-1] ISSCC panel considers UWB implementation schemes

From: [email-masked]
Date: Wed, 18 Feb 2004 00:21:37 -0500

18 February 2004

ISSCC panel considers UWB implementation schemes
By Stephan Ohr
EE Times
Feb 16, 2004

SAN FRANCISO " A Sunday evening (Feb.15) panel at the International Solid States
Circuits Conference (ISSCC) described dazzling implementation schemes for UWB
(ultra wideband) transceiver devices with operating data transmissions of 100,
200 and even 480 Mbits/s One speaker, Raj Sengottaiyan, a Motorola SPS vice
president of engineering, described not a laboratory experiment, but a
commercial three chip set, "sampling now."

Such confidence seemed almost out of place in a session, moderated by Rudolf
Koch, director of the RF and wireless engineering group at Infineon
Technologies, at pains to show that UWB implementations could be chancy. Not
only would the bursty data rates be difficult to obtain over a spectrum
extending from 3.1 to 10.6 GHz, but, more significantly, the IEEE standard on
this " IEEE 802.15.3a " was not ratified yet.

The standards approving group was polarized between two alternatives, said
presenter Jeff Foerster, a wireless researcher in
Intel's Corporate Technology Group. One proposal favors single-carrier, Direct
Sequence Code Division Multiple Access

(DS-CDMA) with M-ary Bi-Orthogonal Keying (MBOK, similar to what is now used for
802.11b at 2.4 GHz " only with an extremely wide spread). Another group (the
50-company MBOA alliance, for example) is advocating multiband orthogonal
frequency division multiplexing (MB-OFDM, similar to DSL multitones, except over
a multi-GHz spectrum).

In principle, UWB would allow very high bandwidth data bursts over very short
distances. It would form the basis of personal home networks, suggested
Foerster. It would act as a "cable replacement" technology, similar to
Bluetooth, only transmitting data (like streaming video) between consumer
appliances in 500 MHz channels. The technical challenges were to keep the UWB
signal on the ambient level (below -41.3 dBm/MHz) and, more difficult, keep it
from generating interference on wireless LAN devices supporting 802.11a (and
some cordless phones at 5.8 GHz).

DS-CDMA can use rake and equalizer (either RF sampling or chip-rate sampler
expected), he said. MB-OFDM uses FFT and low-rate code using baseband sampler.
Both would need highly digital implementations necessary for efficient multipath
energy capture, particularly direct conversion architectures with very high
sample rate analog to digital (A/D) converters. Apart from controlling the
spillover that MB-OFDM would likely generate in the 802.11a frequency band, this
technology would likely see regulatory challenges, especially international: UWB
was legal in the US but not yet in other countries.

But no sooner did Jeff Foerster conclude his challenge when Motorola's Raj
Sengottaiyan took the podium to announce the XtremeSpectrum chipset " one that
relies on direct sequencing over the entire UWB. To keep the UWB transceiver
from creating interference in the 802.11a spectrum, Motorola chose to sample in
two regions " 3.1 to 5.15 GHz, and 5.825 to 10.6 GHz " and then correlate the
samples. That was cheaper, in terms of component count and power consumption,
Sengottaiyan insisted, than sampling over the entire UWB and trying to suppress
802.11a interference with a notch filter.

The three-chip set includes a baseband processor, a media access controller
(MAC) with an 1394 interface, and PHY (with separate RF front end). The circuits
were implemented in BiCMOS using SiGe bipolar transistor implants. To affect a
114 Mbits/s data rate, the chipset uses a 12-bit ternary code (i.e., 12-bit
symbol code represents one data bit) with a 4.104GHz PLL and a 1.2 Gbits/s
signal capture circuit. A 24-bit code would ensure higher data integrity,
Sengottaiyan said, but would reduce the data transfer rates to 57 Mbits/s.

The two other circuits presented seemed to represent "work in progress" rather
than commercial parts. Ranjit Gharpurey of the University of Michigan presented
a time-and-frequency Interleaved OFDM Scheme for UWB. Like Motorola, Michigan's
work " underwritten by Texas Instruments " worried about the cost of notch
filters in the 5-GHz region. Though ODFM would look at specific frequency
slices, distributed amplification techniques could be costly, Gharpurey said.

Michigan chose to build circuits for four distinct frequency groups -- 3.168 to
4.752 GHz, 4.9 to 6.0 GHz, 6.0 to 8.1 GHz and 8.1 to 10.6 GHz -- and to perform
128-tone FFTs on QPSK-modulated signals in each band. The channel rates 640
Mbits/s for a data rate of 110 Mbits/s.

Lydi Smani, RF System Design Engineer for STMicroelectronics presented a UWB
transceiver that relied on pulse-position modulation (PPM) with an additional
polarity indicator. The device could obtain a 600 Mbits/s chip rate, Smani said,
using a 20-GHz A/D and a 625MHz clock.  Effectively, the circuit would crab
pulses as short as 20 ps.

Like Motorola, STMicroelectronics worried about the cost of notch filters, and
elected to sample in two regions " 3.1 to 5 GHz and 6 to10.6 GHz. Unlike
Motorola, STMicro is attempting to implement the baseband, the MAC and the RF
circuitry as single chip using STMicro's HCM059 high-performance CMOS process.

While all the transceivers cited the importance of fast-sampling A/D converters,
the disparity of the approaches cast a pall on Jeffrey Foerster's notion of
"coexistence" between UWB, W-CDMA and various 802.11 technologies (a/b/g and n).
In HIS vision, we might have a "cognitive radio," one that would recognize the
transmission technology required for each application and automatically choose
the "best" available according to criteria like peak throughput, power
consumption, or quality of service (QoS).

http://www.commsdesign.com/news/tech_beat/showArticle.jhtml?articleID=17700319

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