Subject: [NEXTEL-1] Rappaport On Isolating Interference- It's Not Just "Basic Physics"

From: [email-masked]
Date: Sun, 09 May 2004 10:55:06 -0400

Rappaport On Isolating Interference- It's Not Just "Basic Physics"
Isolating Interference
By Ted S. Rappaport

Wireless Review, May 1, 2000
Find the key to (system design) happiness.

As wireless systems proliferate worldwide, the arch-enemy of
wireless-system designers — uncontrollable interference —hampers
capacity and limits the cost-effectiveness of new sites and system
hardware. When you consider the fact that in five to seven years the
planet will have more than 1.5 billion wireless users, it's clear that
the ability to predict and control interference will be the single most
important technical challenge to wireless usability. Fortunately,
emerging packet-data-transmission formats will provide some tolerance to
interference at the expense of delay. However, with the advent of data
and Internet protocols over wireless, the RF emissions of co-channel and
adjacent-channel users will continue to define the limits of quality and
capacity in any wireless systems.

Numerous technological advances have taken aim at taming interference in
practical cellular-system deployment. One major step was the development
of CDMA technology, which actually induces interference among co-channel
users between base stations and in neighboring cells. How can the
introduction of more interference lead to the control of interference to
the end user? The key is in the baseband processing of CDMA. By
selecting a unique user code, simple circuitry at baseband can detect
the desired user while spreading out the unwanted signals so that they
appear as low-level noise. That CDMA actually improves capacity is borne
out by the fact that most of the leading 3G technologies submitted for
ITU approval use some of the ideas of CDMA.

Other methods, such as TDMA used in IS-136 and GSM, guarantee that only
a specific user occupies a particular RF
channel at any instant of time, thereby restricting the number of
simultaneous users in a cellular system. When used with
standard co-channel cell-system planning, it's possible for TDMA systems
to provide a well-controlled interference
environment, although the interference from users in neighboring
co-channel cells is ultimately the limiting factor in capacity and
quality.

Other multiple access and multilevel keying-modulation strategies, such
as orthogonal frequency division multiplexing, offer
various advantages and show promise for emerging broadband-wireless
systems in a large multi-user interference environment.
No matter what the multiple access or modulation scheme, the rubber
meets the road when service providers are forced to build out quickly,
to keep up with consumer demand (capacity hot spots) and competing
coverage areas. Here, frequency planning becomes difficult due to the
rapid expansion, and all multiple-access technologies become stressed
because of the higher levels of co-channel interference induced by new
base stations, microcells and repeaters. The effect of adding just a
single new base station in an urban core can create horrendous
interference problems in a small area, or can render a control channel
of an incumbent base station useless, due to bleedover. The filtering
characteristics of subscriber phones, the specific radio-propagation
characteristics for all radiating stations, and the effectsof
inter-modulation harmonics in the base-station-transmitter chain all
play into whether or not interference will occur. The great wireless
build-out is happening around the world, but nowhere is the interference
more acute than in the high-capacity urban cores, within and around
metropolitan office buildings, airports and public arenas, which already
are inundated with RF interference.

A secret weapon available to wireless-service providers in a rapid
build-out mode is the ability to archive and simultaneously track the
interference effects of every base station in their networks. By
modeling the RF environment with a 3-dimensional representation of the
physical world, it becomes possible to understand, predict and design
around the RF environment that your own cell-system design creates. This
use of knowledge is a crucial aspect in controlling interference.

Real-Life RF Interference

An example can be seen below, where a 1-by-1-mile urban core is modeled
with six low-power IS-136 microcells and three taller macrocells in the
U.S. 800MHz cellular band. The system has evolved over time, where each
microcell is placed to serve users in a high-density urban environment.
If each microcell has 57 RF voice channels, and three voice users are
assigned per RF channel, then the system under study has nine base
stations with 516 RF channels. The capacity of such a system easily can
support thousands of simultaneous users with little blocking, but in
major metropolitan districts such as Chicago, Hong Kong, Mexico City or
New York, additional capacity may be needed. (See Figures 1 and 2.)

For example, say that an office building wished to install its own
wireless-office-system (WOS) solution for its employees.  Here is the
dilemma: Already there's significant channel reuse in the urban core.
What will happen if additional base stations and antennas are added
within a building? This is where wireless-system-design software can
help. Not only can computer-aided design tools help provide "what if"
analysis of the interaction of new systems within an urban core, but
they can simultaneously track the whereabouts of new RF equipment within
the building, making it easy to locate antennas, leaky feeders and
distribution systems in the future. (See Figure 3.)

This ability to archive and maintain a parts-list inventory for the
service provider and building owner is a free bonus on top of the
important ability of properly locating antennas and making judicious
channel assignments. Figure 4 shows how the C/I is virtually unaffected
on both the forward and reverse link by adding a 10th 57-channel base
station in the building while positioning the omnidirectional antenna in
the center of the building under test. An alternate design, using leaky
feeders within the building of interest, further improves the co-channel
interference ratio, while reducing some coverage in parts of the
building.
Scanning Frequency Use

Still another way to understand and combat interference is to conduct a
rapid scan of the frequency use around the building of
interest. This allows the RF engineer to simply measure, in situ, the
RF-interference environment, and then to design the
in-building or campus system quickly. The article "Getting In" (Wireless
Review, March 1, 2000), shows some simple
methods of modeling the entire RF environment so that a reliable
in-building design can be made. With this "measure and plan"
technique, an RF engineer can be certain that the proper channel
selection and antenna placements can be made. Again, having a
computer-aided design tool that integrates measurements with 3D building
data makes the job of interference control a snap.
Other methods for controlling interference include the use of adaptive
frequency planning, where a careful scan of the entire
frequency band is conducted for the purposes of measuring
RF-interference levels everywhere throughout your market. Once
the scan is completed, algorithms can process the measured data to
determine how many channels, and which specific
channels, are available in a particular market location (say within an
office building). Research at Virginia Tech's Mobile and
Portable Radio Research Group resulted in algorithms that could break
down an existing channelization scheme throughout a
cellular market, so that a specific channel listing could be defined at
existing base stations. This concept of adaptive frequency
planning allows service providers to re-adjust their existing co-channel
cellular-system configurations to better optimize the RF
channels at each transmitter. The value of such an approach allows a
service provider to "reset" its market-wide channelization scheme to
minimize interference, thereby making up for the lack of careful
interference control during a high build-out phase.  An example of a
capacity study, which makes use of these concepts, appears in the
January 1999 issue of the IEEE Vehicular Technology Transactions, where
it was possible to predict the number of available channels throughout a
skyscraper in a major U.S. city.

Other interference-control mechanisms act upon the leakage of
adjacent-channel interference at the base-station receiver. By using new
superconductive-filter technology, it's possible to manufacture very
tight RF channel filters that help reduce adjacent-channel interference
on the reverse link. With portable-phone users in skyscrapers,
macrocells suffer huge amounts of interference on the reverse link
unless in-building systems are used that force subscriber terminals to
use very low power.  This is yet another reason why in-building WOS is
sure to evolve rapidly over the next few years.

One thing is certain about interference: It's difficult to completely
design against. But with judicious channel-assignment schemes and
deployment strategies that use knowledge of the physical 3D radio
environment, it's possible to maximize capacity and the use of RF
assets. Fortunately, new technologies in multiple access, filter design
and computer-aided design and archiving are coming online to help
engineers carry out the great wireless build-out more efficiently and
accurately.

Rappaport ([email-masked]) is Wireless Valley Communications chairman
and Virginia Tech James S. Tucker professor.

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