Allocation technique boosts efficiency, minimizes
interference for wireless internet broadband
Penn State engineers have developed an economical way
to more efficiently manage radio spectrum use and prevent
interference on wireless broadband systems for high-speed
Internet access -- potentially bringing down costs for
consumers. Dr. Mohsen Kavehrad, director of Penn State's
Center for Information and Communications Technology Research
(CICTR), says, "With this technique, service providers could
offer quality service to more homes using only a limited span
of the radio spectrum. And, if providers can squeeze more
customers onto the available bandwidth, it could translate
into lower costs for the consumer." In addition, the approach
promises equipment cost savings since simulations show that
the new scheme maintains performance at top industry standards
with more economical components.
The new approach is detailed in a paper, "Co-Channel
Interference Reduction in Dynamic-TDD Fixed Wireless
Applications Using Time Slot Allocation Algorithms," published
in the October issue of the IEEE Transactions on
Communications. The authors are Wuncheol Jeong, doctoral
candidate in electrical engineering, and Kavehrad.
Kavehrad explains that, currently, high speed Internet
access capable of carrying MP3 files, video, or
teleconferencing is available primarily over wired networks.
However, wireless local loops are being introduced as
broadband alternatives in some test markets. These new
wireless networks are facing serious obstacles in competing
for bandwidth; sometimes, having to share bands with cordless
phones or even microwave ovens. Even when the wireless
providers use licensed bands, they face the prospect of many
customers simultaneously uplinking and downlinking information
across the net, creating co-channel interference.
The wireless local loops work much like cell phones via a
base station that sends the radio signals carrying the
Internet connection out to any customer whose residence or
business is equipped with an appropriate antenna. Unlike cell
phone usage, however, the two-directional uplink and downlink
traffic between the customer and the Internet provider's base
station is more asymmetrical with very little use during
sleeping hours and lots of use when kids come home from school
and download music or play games, for example. Kavehrad says,
"The nature of multimedia traffic is not static in uplink and
downlink directions, as with voice telephony, and the
bandwidth is more biased toward downlink transmissions."
Wireless local loops need both software and hardware that
enables the network to respond to the changes in traffic while
also making sure that every hertz in the available spectrum is
used as efficiently as possible. In addition, the system must
contend with the fact that some incoming interfering signals
are stronger than others.
The solution developed by the Penn State engineers is
software that allows the subscriber signal whose direction of
arrival is subject to a lesser number of strong interferers to
be processed ahead of the ones experiencing the most
interference. In other words, the new strategy is a scheme
that allows avoiding strong co-channel interference by
sequencing the processing of the signals according to the
amount of interference they are experiencing. Since the amount
of interference any subscriber's signal experiences varies
microsecond by microsecond, no subscriber has to wait very
long for a turn.
Kavehrad adds, "The usual techniques employed to suppress
interference use adaptive spatial filters which require
expensive RF components and a large number of computations to
queue the subscribers' signals. However, with our approach, we
need only a simple, cost effective spatial filter and
relatively fewer computations. Our simulations show that the
performance of the new approach and the traditional technique
are comparable. Thus, our strategy shows a practical
compromise between complexity and cost, while achieving the
desired signal quality."
The research was supported by Penn State's CICTR and a
grant from the National Science Foundation.
