| |
|
|
|
| |
Satellite Systems
– The future and beyond |
|
|
| |
|
|
|
| |
Overview
|
| |
| As cellular systems reach maturity,
many are looking to the future. While the PCS frequencies (1800
MHz – 2000 MHz) are still being built out, the next generation
of cellular communication is being launched. To provide true
homogeneous worldwide wireless coverage with no gaps, LEO (low
earth orbit) satellites are being used. These LEO satellites
orbit the earth in high speed, low altitude orbits with an orbital
time of 70-90 minutes and an altitude of 400 – 700 miles. LEO’s
provide small coverage cells around the size Washington state.
Since LEO’s are not geosynchronous, they are forced to fly complete
orbits, and thus many must exist to guarantee every area is
covered by at least one satellite at all times. Therefore, call
handoff is executed when the satellite moves, not when the person
moves as it is with ground-based cellular systems. |
| |
| Satellite Systems |
| |
| Iridium |
| |
| Motorola has just launched the last
satellite in the Iridium system, a LEO system consisting of
66 satellites with 6 in-orbit spares. Iridium satellites have
an orbital altitude of 485 miles and weigh 1,500 pounds each.
Iridium will allow a person to place a call anywhere in the
world using a device slightly larger than a small hand-held
cell phone. Iridium will begin service this summer. |
| |
| Globalstar |
| |
| Globalstar is a CDMA system headed
up by Qualcomm. This system will have 48 satellites with 8 in-orbit
spares. Each satellite will weigh 1000 pounds and will orbit
at 880 miles. Again, this system will use phones that are just
slightly larger than today’s conventional cell phones and allow
calls to be placed anywhere in the world. Globalstar expects
to have the system launched in 1998 and will begin to provide
service in 1999. |
| |
| Teledesic |
| |
| Teledesic is the most promising and
furthest off of the LEO systems. Teledesic is a partnership
of many large players in this industry, including Motorola,
Microsoft, Boeing, Nextel, and Matra Marconi, with Motorola
being the prime contractor. Teledesic will use 288 satellites
flying at an altitude of less than 800 miles. This means that
Teledesic will have five times the satellites as the other systems
and will fly lower than the other systems. This translates into
more bandwidth per region (smaller coverage area per satellite)
and smaller cellular phone devices (lower power since satellites
are closer). Teledesic also plans to provide high-speed data
access, starting at 64 megabits per second. Teledesic has begun
development of their satellites, and expects to start service
in 2003. |
| |
| Other Future
Systems |
| |
| There are several other future cellular
systems planned. One of these systems involves sending unmanned
blimps high above the city at 70,000 feet. These blimps would
provide cellular voice and data service to a city, with service
similar to what LEO’s provide. Another plan is to have high-flying
(flying at a 52,000 foot altitude) unmanned solar-powered aircraft
that would continually fly west to east across the US and provide
LEO like cellular voice and data service. Though these other
ideas appear farfetched, if they are implemented, they will
provide more competition and thus lower prices. |
| |
| Conclusions |
| |
| Cellular phone customers clearly
have many different service choices that they did not have several
years ago. Furthermore, it is inevitable that as the technology
evolves, the quality of service will increase and the equipment
cost will decrease. Older technologies will become cheaper as
newer technologies are introduced to the global market. |
| |
| The analysis in the "Cost Factors"
section has demonstrated on a theoretical level how newer technologies
such as CDMA can give finer control over the cost per user of
providing service by regulating user capacity as a function
of signal noise. Although limiting factors still exist, the
number of frequency bands allocated in the total frequency range
no longer fixes the user capacity. This allows service providers
to target a more optimum quality of service and user load, which
benefits both user and service provider. |
| |
| By eliminating the requirement that
towers transmit at constant power, CDMA systems can better optimize
their utilization. Other technologies such as satellite networks
and GSM improve their utilization by similarly exploiting their
advances over standard FDMA and TDMA networks. The result is
that newer cellular networks will be intelligent enough to improve
their utilization and quality of service, which in turn benefits
the user. |
| |
| Source: |
| |
| http://www.airtouch.com/ |
| |
| http://www.attws.com/ |
| |
| http://www.cdg.org/a_ross/ |
| |
| http://www.cdg.org/cdma_tech.html |
| |
| http://www.ee.surrey.ac.uk/Personal/L.Wood/constellations/ |
| |
| http://www.ericsson.com/ |
| |
| http://www.globalstar.com/ |
| |
| http://www.iridium.com/
|
| |
| |
|
|
|
