Trade-offs for mobile broadband
individual.com
I don't think this expert quite understands the advantages of CDMA - DPR
June 20, 2000
Eric Wilson, Vice President of Systems
Management, Vyyo Inc., Cupertino, Calif.,
ewilson@vyyo.com
Personal communication services show that
it is possible to provide mobile access and
have been popular among users. Now
third-generation, or 3G, communications
open the possibility of faster data rate
mobile applications, but the spectrum
needed for 3G overlaps some of the
spectrum currently used for fixed wireless
applications.
So it is time to take advantage of the lessons
learned from existing commercial system
deployments of cellular phone and existing
mobile data access systems, such as
Ricochet, cellular digital packet data
(CDPD) and RAM Mobile Data. Taking
advantage of innovative technology while
concentrating on sound RF principles will
provide 3G users with the best service
combination of functionality and cost and
will also encourage optimal use of a limited
resource: spectrum at easy-to-use
frequencies.
Fixed wireless will always be more
spectrum-efficient than mobile access
techniques because mobility requires an
omnidirectional antenna (one that radiates
in all directions). Mobility is possible only
with relatively slow data speeds or with a
very extensive infrastructure. It is
technically and financially unfeasible to
support high data rates in a mass-deployed
mobile system. Of course, "high speed" and
"mass deployed" are in the eye of the
beholder and, more important, in this case
may be traded off for each other.
To be sure, the exponential growth of the
number of Internet users has been amazing;
but even more astounding is the exponential
growth of Internet traffic. This divergence
between the number of users and the
amount of traffic is the result of average
users needing more bandwidth, that is,
higher access speed. More bandwidth is
required because more and better
information, as well as more multimedia
content, are available through the Internet.
The better the Internet gets, the more of it
everyone wants.
People will always want portability and
mobility: the more convenience wireless
engineers give them, the more they want.
These two market realities are forcing the
matter of providing high-speed access in
mobile systems. Major telecommunications
operators have spent billions and are
preparing to spend much more to solve the
last-mile conundrum. However, the
simultaneous physical implementation of
both high speed and mobility for a
reasonable price is not so simple.
High data rates require broad
bandwidths-simple enough. This was
proved more than 50 years ago and is still
true in the new millennium. The 3G mobile
cellular phone systems offer Internet access
with the promise of 2-Mbit/second data
rates. Although these data rates are
achieved quite handily in the wired world
and routinely in the fixed wireless
spectrum-for example, multichannel
multipoint distribution service is 2.1 to 2.7
GHz and local multipoint distribution
service, 24 to 40 GHz-they are a bit harder
to achieve in the architecture of a mobile
system. This is one reason that the 3G
IMT-2000 proposal limits mobile users to
384 kbits/s.
System planners as well as politicians and
FCC commissioners must be cognizant of
the trade-offs involved in high-speed
wireless systems. High data bandwidth
comes either through unsophisticated
modulation with wide spectrum
allocations-which is very expensive-such as
analog FM modulation, or through
advanced, compressed modulation
techniques such as 64 QAM (quadrature
amplitude modulation) and its derivatives.
More sophisticated modulations are used to
reduce the required bandwidth for a given
data rate. However, the more bits per hertz
transmitted, the better the signal-to-noise
ratio (SNR) required to allow accurate
demodulation at acceptable bit error rates
(BER).
At a given path loss, sufficient SNR is
achieved either by narrowing the bandwidth,
which limits the data bandwidth, or
increasing the power of the transmitter. Of
course, narrowing the spectrum bandwidth
improves the noise power, but it also forces
a higher-order modulation to transmit the
same amount of data. Looked at in another
direction, doubling the data rate may be
achieved either by using a higher-order
modulation with the same RF
bandwidth-that is, going from quadrature
phase shift keying to 16 QAM, which
requires roughly 7 dB better SNR-or by
doubling the RF spectrum bandwidth, which
requires 3 dB more transmitter power.
The real-world realities mean that neither
approach achieves the desired outcome of
more data bandwidth at the same power
level, since either the SNR must be
increased to support the more complex
modulation type or the received power must
be higher to overcome the wider noise
power.
Several solutions are available for high data
rate systems. The transmitted power can be
increased; the distance to the reception node
can be shortened, resulting in an increased
number of nodes, and/or directional
antennas can be used so that the transmitted
power is concentrated toward the intended
receiver. All of those solutions will improve
the signal level at the receiver. However, the
first two result in increased background
noise. On one hand, the users in adjacent
cells also must increase their transmission
power. On the other, they must, of course,
be closer to adjacent basestations. Each of
these approaches has a point of diminishing
returns that limits net data throughput in an
area because interference increases from
nearby nodes. The transmitter power must
be increased again to overcome this
interference, or path length must be
shortened even more to overcome the
interference of other users in nearby cells.
That raises several questions. First, how do
cell phones, with their omnidirectional
antennas, operate with large cell sites? The
data bandwidth is narrow, which means the
SNR is improved by narrow spectrum
bandwidths resulting in low-noise power
bandwidths. The data rate for most cell
phone systems is less than 20 kbits/s.
Second, what about spread spectrum
solutions? Deployment experience with
code division multiple access has shown
how difficult that is to do in reality. Spread
spectrum is subject to the so-called
waterfall effect when the traffic load
approaches the upper limit. Once the
number of users exceeds that limit, every
user in a cell experiences quality problems.
Next, what about existing mobile wireless
data services? All of them have either very
narrow data bandwidths or an extremely low
user density. The data rates are advertised to
be 70 kbits/s but experienced rates are
typically less than half that.
Finally, how do fixed wireless systems
operate with high data speeds? They
overcome the path loss matter by using
antenna gain to reduce wasted transmitted
energy. The antennas provide directivity for
the radiated energy concentrating it where it
will enhance the SNR (and BER) rather
than radiating it in all directions. Even a
little antenna gain improves the situation
considerably. An antenna with a 17-dBi
gain at the subscriber site reduces
transmitted power by a factor of 50 for the
same SNR at the receive site. This means
reducing the power from 1 W to 20 mW for
a typical high data rate link. More
important, the radiation is directed toward
the intended receiver rather than
omnidirectionally. The side lobes on a small
directional antenna are typically more than
10 dB below the main beam, thereby
reducing interference to other cell sites. Of
course, mobility is nearly impossible with a
directional antenna since the antenna must
be aimed in the correct direction.
Another thing users want is an option for
self-installation of the access module inside
their homes. This means that the path loss
calculations must take into consideration
the attenuation that would occur through
the building walls when wet from rain, or
through a wall with aluminum-backed
insulation or through a window screen.
Those conditions make the argument for a
directional antenna even stronger. Any
transmitter would need to increase the
output power enough to overcome the
additional losses; therefore, the side lobes of
a directional antenna would go up
proportionally. However, they would still be
lower, by the gain of the antenna, than if an
omnidirectional antenna were being used.
The 2.1-GHz band is more vulnerable to
foliage attenuation and losses through walls
than are lower-frequency systems. Every
experienced cell phone user has noticed how
much better the lower frequencies penetrate
a building-for example, broadcast radio, TV
signals or even 870-MHz digital AMPS cell
phones-vs. 1.8-GHz PCS phones.
Generally, the lower the frequency the
easier it is to receive a good signal or
transmit from inside a building. Using a
high-speed wideband access system at 2.1
GHz as proposed by 3G will be frustrating to
users when inside a building and, once again,
the distance to the access nodes will have to
be reduced, resulting in more nodes and a
high noise floor of interference.
Desired connectivity speeds will continue to
increase, putting more demands on an
access system. Any given connection speed
will be less satisfying to users over time. It is
easier to offer faster speeds to more users
with directional antennas than with
omnidirectional ones.
Everyone wants faster connectivity. Mobile
access is an important business opportunity
with many possible applications. However,
the majority of legitimate mobile
applications do not require high-speed
access. For example, checking traffic,
receiving financial updates or finding phone
numbers, vendor locations, product price
and availability and airline schedules do not
require high-speed connections.
Going back to the principles outlined in the
beginning of this article, the optimal
solution is to use spectrum efficiently
consistent with subscribers' requirements.
One solution is to deploy high-speed, fixed
wireless access using a large number of
micro- and picocells. The microcells are
wireless nodes that bridge a central
community basestation and several
neighborhoods' piconodes. For example,
microcells might address distances of up to
4 km. A layer of picocell wireless bridges
would support path lengths of 400 m. Those
micro- and piconodes could be distributed
based on three criteria: number of
subscribers supported, maximum distance to
the farthest subscriber and total traffic in
the micro- and picocell.
In this scenario, most fixed or transportable
subscribers would be able to find a piconode
that has visibility from any room in their
homes from which they wish to operate.
The microcells could be co-located with
existing cell phone hubs and the picocells
could be roof-mounted nodes, each with a
directional antenna toward the microcell
and four sector antennas to cover the
neighborhood's users.
This scenario allows very-high-frequency
reuse through a large number of micro- and
picocells that may be accessed via small
directional antennas that are user installed.
It takes advantage of the directional
antennas to maintain low amounts of
useless, randomly radiated energy.
Another major advantage of this
multilayered concept is that truly mobile
users may use omnidirectional antennas and
work from picocells that are nearby. Of
course, the system operator will want to
limit the amount of traffic transmitted over
the omni antennas to prevent the noise
floor from rising. That could be done via
differential pricing or restricting the
upstream data rate when using an omni
antenna.
eetimes.com
Copyright c 2000 CMP Media Inc.
By Eric Wilson, Vice President of Systems
Management, Vyyo Inc., Cupertino, Calif.,
ewilson@vyyo.com |
