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How is Globalstar able to provide such a reliable and dependable
communications service? Here is a brief overview of some elements that
contribute to the exceptional service and coverage of Globalstar
LEO Satellites
A Low Earth Orbit (LEO) is any Earth orbit up to approximately 1,500
kilometers in altitude. Low-Earth-Orbit satellite systems offer
significant advantages over Geosynchronous (GEO) systems for the
delivery of mobile satellite services (MSSs). These advantages result
from an orbit selection that enhances the quality of services to
low-power mobile hand-held and vehicle-mounted user equipment. GEO
satellite systems, located at an altitude of 35,800 kilometers above
the Earth, are best suited for their missions of high-speed data,
television transmission, and other wideband applications.
Mobile users, however, need systems that are specifically tailored to
their needs. The true mobile user requires hand-held and
vehicle-mounted telephone devices with omnidirectional antennas that
are easily portable and can provide instant access to a global
communication system. Furthermore, telephone users desire and require
"telephone quality" transmissions. These users do not want long
propagation delays inherent in GEO systems that produce the echo effect
and also use bulky, expensive, directional antennas which must point or
must be pointed at a satellite.
Also, technical limitations of current GEO satellite systems severely
limit the capacity of such systems to service mobile users. The scarce
spectrum available for MSSs communications requires deployment of
systems that will provide services to the users in a manner that
maximizes the use of spectrum while encouraging a multiplicity of
systems to share the spectrum. GEO systems, as presently configured
with frequency division multiple access-frequency modulation (FDMA-FM),
require inefficient band segmentation to share the spectrum.
Globalstar will provide capacity even when a satellite fails, due to
the rotation of its LEO constellation. A GEO system, by contrast, would
suffer an entire regional system outage if an operational satellite
were to fail. Also, current and planned GEO systems cannot service the
personal communications market due to the expense of supplying adequate
satellite power and practical deployable antenna sizes.
Path Diversity
Path Diversity is a patented method of signal reception that
permits the combining of multiple signals of varying power strengths
into a single, coherent signal. Subscriber terminals will be able to
operate with a single satellite in view, though typically two to four
satellites will be overhead.
Subscriber terminals will communicate with as many as three satellites
simultaneously and, utilizing a rake receiver, will combine those
signals into a single, static free signal. Subscriber terminals will
also alter their power levels to compensate for shadowing and
interference as needed, with an average power output range of between
50-300 mw.
As satellites are constantly moving in and out of view, they will be
seamlessly added to and removed from the calls in progress, thereby
reducing the risk of call interruption. This will enable the Globalstar
system to provide service to a wide variety of locations, with less
potential for signal blockage from buildings, terrain or other natural
features.
CDMA Technology
Globalstar utilizes a version of Code Division Multiple Access
(CDMA) technology based upon the IS-95 CDMA standard to provide high
quality, digitally crisp voice, data, and fax services. This standard
utilizes digital transmission methods in which users share time and
frequency allocations, and are assigned by unique assigned codes. The
signals are separated at the receiver by using a correlator that
accepts only signal energy from the desired circuit. Undesired signals
are ignored as merely being noise. This technology allows a large
number of wireless users simultaneously to access a single radio
frequency channel orthogonally reducing interference, thereby resulting
in a manyfold increase in capacity when compared to analog systems such
as Frequency Division Multiple Access (FDMA).
The CDMA concept is analogous to the situation encountered at a party.
At the "CDMA Cocktail Party," all subscribers are talking in the same
room together simultaneously. This is analogous to the technique used
in CDMA. Imagine that every conversation in the room is being carried
out in a different language that you do not understand. They would all
sound like noise from your perspective. If you "knew the code," the
appropriate language, you could imagine filtering out the unwanted
conversations and listening only to the conversation of interest to
you. A CDMA system must filter the traffic in a similar way. Even with
knowledge of the appropriate language, the conversation of interest may
not be completely audible. The listener can tell the speaker to speak
more loudly, and can also signal other people to speak more softly. A
CDMA system uses a similar power control process.
Globalstar has adapted a combination of FDMA with CDMA and spread
spectrum modulation that enables it to support multiple users
simultaneously, and to share its allocated frequencies with other CDMA
systems. Globalstar believes that CDMA, combined with the path
diversity offered by multiple satellites, results in higher call
quality and fewer dropped calls when handing off between satellites,
while minimizing interference with other users sharing the same
spectrum allocation. |
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