The 10 Hottest Technologies
telecoms-mag.com
"Time Division Duplexing: Maximizing Spectrum
Like the CDMA-TDMA-GSM debate in mobile wireless networks,
a similar battle is brewing in fixed wireless: frequency division
duplexing (FDD) vs. time division duplexing (TDD). But unlike the
mobile world, action in the local multipoint distribution service
(LMDS) spectrum, with the exception of two or three notable
carriers, has been a case of all talk and less-than-widespread
deployment. Air interface standards, currently being worked on in the
IEEE?s 802.16 committee to deal with issues such as interference and
interoperability, could help change that, propelling broadband wireless
to a $7.4 billion market for services by 2003, according to the
Strategis Group.
The deployment of TDD systems could be a big boost to the market
as well. While the major broadband wireless deployments from
Winstar and Teligent have been tried-and-true FDD, these and other
carriers, such as NextLink, are taking a close look at TDD for its
spectral efficiency capabilities. TDD, which is being backed by
start-ups Ensemble Communications and WavTrace, was designed
with data traffic--asymmetrical and ?bursty? by nature--in mind. TDD
uses a single channel for simultaneous, two-way communications.
Separation between the transmit and receive functions occurs in the
time domain rather than by frequency (as in FDD). With TDD the
radio?s uplink and downlink operate on the same frequency, but at
different times at a fixed interval and the amount of bandwidth
allocated to each direction is flexible. This circumvents the need for a
large guard band to isolate the upstream and downstream traffic,
increasing the efficiency of the spectrum.
The LMDS spectrum is quite large, so efficiency is not currently a
factor: If service providers have both A and B band channels in a
market that?s a total of 1150 MHz of spectrum, equivalent to about
200 video channels. ?Service providers are really just starting to sign
on customers, so it?s not an issue just yet,? said Andy Fuertes, senior
analyst with Allied Business Intelligence. ?But going forward, as
broadband wireless catches on, then carriers may be thinking a lot
more about spectral efficiency.?
Just as analysts believe there?s enough room for broadband access
and mobile wireless via several technologies, so do they think there?s
room for both FDD and TDD. Whether TDD has a compelling
enough story to dominate the market is up in the air, but the interest
being paid to the technology is positive. NextLink, the largest LMDS
spectrum holder in the country, has chosen WavTrace for its Los
Angeles trial along with three other vendors. The start-up also has a
strong relationship with Harris Corp., which owns nearly 20 percent
of the company. Ensemble has strategic marketing alliances with
Digital Microwave and ADC. " |
