Dallas -- MCI WorldCom Inc. and Sprint Corp. have a way to
go in their efforts to make MMDS a viable broadband-access platform in the mass market.
But they don't have to go as far as it seemed a few months
Last week, Cisco Systems Inc. and City Telecom Hong Kong
Ltd. revealed that the Chinese service provider had successfully deployed the recent Cisco
wireless-product line, which uses a new modulation technique to address line-of-sight and
interference problems in multichannel-multipoint-distribution-service networks.
And representatives of other vendors appearing at the
Wireless Communications Association International engineering symposium here said they,
too, had new solutions that would make life easier for MMDS operators.
But participants at the conference left no doubt that
fixed-wireless-network service providers operating at MMDS and other spectrum tiers still
have many issues to resolve before they succeed in challenging wireline competitors.
The challenges are especially daunting for companies like
MCI and Sprint that are hoping to achieve high penetration in the mass market with MMDS,
which requires techniques to reuse limited spectrum while avoiding interference and
overcoming line-of-sight limitations and other barriers.
Costs, backhaul options for interconnecting transmitter
sites and uncertainties surrounding media-access-control protocols are also major issues,
noted Frank DeNap, director of the Advanced Technology Labs at Sprint. "There's a lot
more to deploying wireless-broadband systems than resolving the RF-interface issues,"
Sprint is looking at the Cisco system and other vendor
solutions that rely on the modulation technique known as "OFDM" (orthogonal
frequency-division multiplexing), as well as the more traditional QPSK (quadrature phase
shift key) and QAM (quadrature amplitude modulation) options, DeNap noted.
But he stressed that the carrier would not wait for new
solutions. "We've got to get out and capture customers," DeNap said.
"That's our No. 1 priority."
Cisco's first OFDM-based product -- which it calls
"VOFDM" (vector OFDM) to designate a second facet to its air-interface
technology -- is a point-to-point version designed to enable connectivity to hard-to-reach
customers and to support robust interlinking of transmitter sites.
So far, the technology is living up to claims of getting
signals to customers who can't be reached by other modulation schemes, City Telecom
managing director Paul Cheung said.
"Despite the complexity of the trial system, we
managed to commission the whole system in less than three days," Cheung said.
"We have successfully established multimegabit wireless links providing voice-over-IP
[Internet protocol], video-on-demand and IP-multicasting applications with limited line of
sight, which is an essential capability for extending the service coverage in a densely
populated urban area like Hong Kong."
OFDM -- which is being used for digital TV in Europe, and
which is gaining support from some broadcasters in this country against the Federal
Communications Commission-endorsed digital-TV standard -- divides a given frequency
channel, such as the 6-megahertz MMDS video channel, into small segments. Each is packed
with the maximum bit load permitted by the conditions within that frequency band.
This way, the amount of bits that can be delivered over the
6-MHz channel is not limited by the poorest-performing frequency segment within the
Cisco's system also uses dual-feed antenna receivers at the
end-user premises to capture and combine signals coming in from separate paths, taking
advantage of multipath reflections to maximize signal strength of antennas that are not
within the line-of-sight coverage.
The company has lined up Broadcom Corp. to supply ASICs
(application-specific integrated circuits) that also contain the MAC component, and it has
support from 10 other vendors, as well.
OFDM isn't the only new technique vying to improve
wireless-broadband spectral efficiency.
Chip manufacturer TelesciCOM Ltd. is bringing a chip to
market based on a newly invented modulation scheme known as "OCDM" (orthogonal
code-division multiplex). The vendor contended that OCDM is a cheaper means of
accomplishing robust performance than OFDM, which requires care to avoid accumulation of
phase noise over multiple RF carriers.
"In terms of phase-noise performance, OCDM is orders
of magnitude better than OFDM for the same data rate," said Doron Koren, a senior
engineering executive at TelesciCOM. He added that the company's system employs
soft-coding techniques to improve the performance of error-correction coding, thereby
gaining a 1-decibel to 2-dB advantage in signal-to-noise at any given bit rate, compared
with conventional error-correction modes.
As DeNap noted, however, operators have to get to market
quickly and make do with the equipment that's currently available. In Sprint's case, this
means deploying in 10 markets by the start of the second half of next year and in another
20 by year's end.
Cisco's point-to-multipoint version of VOFDM won't be
available until well into next year, and TelesciCOM -- while it has unnamed
original-equipment-manufacturer partners -- has an even harder task of making its
technology a viable option against the giants of the systems-networking domain.
Just how far operators might have to go to make do with
what's available was made clear by Robert Saunders, director of wireless planning at
BellSouth Corp.'s BellSouth Entertainment division.
In Atlanta, where the "tree-canopy" barrier
extends to 100 feet above the terrain and the terrain varies by 1,500 feet of altitude,
BellSouth has installed five transmitters to achieve coverage, with four operating at
1,000 watts of power, Saunders said.
"That level of power output poses major challenges for
interference and frequency reuse, but it's what we need to get the signals to the
end-users," Saunders said. The good news, he added, is that the company has come up
with engineering techniques -- including directing transmitter-antenna signals into the
hilly terrain to avoid multipath reflections -- which allow the system to work effectively
across the metro region.
But even with the high power output, BellSouth has had to
improvise means of getting signals into households by using antenna receivers that are
installed in the trees, rather than on rooftops. "Georgia pines make good poles, but
you have to use a very narrow-beamwidth antenna to compensate for the swaying,"
BellSouth -- which is delivering upward of 160 channels of
video, and which may eventually add high-speed data over its five MMDS networks -- is
looking at OFDM to help it deal with multipath problems, Saunders added.
"We're getting a very strong customer response --
higher demand than we anticipated -- and that means we have to find ways to expand our
coverage quickly and cost-effectively," he said. "It's a big challenge."
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