15-10-2012, 02:17 PM
Wireless Mobile Communications at the Start of the 21st Century
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ABSTRACT
At the start of the 21st century, the wireless
mobile markets are witnessing unprecedented
growth fueled by an information explosion and
a technology revolution. In the radio frequency
arena, the trend is to move from narrowband
to wideband with a family of standards tailored
to a variety of application needs. Many
enabling technologies including wideband
code-division multiple access, software-defined
radio, intelligent antennas, and digital processing
devices are greatly improving the spectral
efficiency of third-generation systems. In the
mobile network area, the trend is to move from
traditional circuit-switched systems to packetswitched
programmable networks that integrate
both voice and packet services, and
eventually evolve toward an all-IP network.
Furthermore, accompanied by wireless mobile
location technology, wireless mobile Internet is
expected to revolutionize the services that can
be provided to consumers in the right place
and at the right time. Wireless mobile communications
may not only complement the wellestablished
wireline network; it may also
become a serious competitor in years to come.
In this article we briefly review the history of
the wireless mobile communications, examine
the current progress in standards and technologies,
and discuss possible trends for wireless
mobile solutions.
THE HISTORY OF
WIRELESS MOBILE COMMUNICATIONS
Halfway through the 20th century, after the
invention of two-way radio communications at
the beginning of the century, the key foundations
of wireless mobile systems were invented in
Bell Laboratories. The concept is to reuse the
same limited radio frequency (RF) in a group of
cells arranged in a cellular structure to serve an
unlimited number of users. Furthermore, calls
are systematically handed off from one cell to
another to accommodate vehicle mobility from
cell to cell. It is interesting to observe that these
seemingly simple ideas have since revolutionized
wireless communications and led to a multibillion
dollar industry
FREQUENCY SPECTRUM
As discussed above, both the subscriber base and
subscriber penetration rate are expected to grow
significantly in time. The combined growth rate is
expected to be much faster than can be achieved
only through the spectral efficiency improvement
of various new technologies. Consequently, new
spectrum in addition to that of the traditional cellular
band (800–900 MHz) needs to be allocated
for 3G systems to accommodate the exponential
growth. To make the global roaming of wireless
communications easier, frequencies in similar
bands worldwide have been proposed by the
International Telecommunication Union (ITU).
Unfortunately, the allocated frequency bands are
not necessarily aligned from country to country.
SOFTWARE-DEFINED RADIO
From the discussions above, it is clearly desirable
to have a system that can operate on different
standards and in different frequency bands.
This approach becomes possible because of the
rapid progress in semiconductor and digital technologies.
More specifically:
• The advances in analog-to-digital (A/D) and
D/A conversions have made it possible to
directly convert signals closer to the antenna
at high speed with adequate dynamic
range. This reduces the radio components
needed and greatly facilitates digital implementation,
as shown in Fig. 6.
• With the signals converted to digital in the
very early stages of the system at high
speed, a wideband radio approach can be
adopted that provides inherent flexibility to
support different standards which operate
with different frequency bandwidths.
• The rapid growth in the use of general-purpose
digital signal processing (DSP) and
field programmable gate array (FPGA)
chips makes it commercially viable to manufacture
general-purpose programmable
devices at low cost. Furthermore, the speed
of these devices using software or firmware
implementation is now adequate, and its
performance can be comparable to that of a
hardware implementation.
INTELLIGENT ANTENNAS
Due to the continued rapid growth of wireless
mobile applications, more system capacity is
needed as well as the identification of new frequency
bands. Intelligent antennas are a promising
technique that can significantly increase
system capacity. Current narrow beam proposals
include two techniques: the steered-beam and
switched-beam approaches.
The steered-beam approach utilizes phasedarray
antennas, with multiple columns of antenna
elements in pairs or equally spaced. The
phased-array elements are used in the antenna
to create a narrower beam directed only to the
intended mobile on the forward link. This beam
is then steered with the mobile as it moves. Forward
link transmit diversity can be provided as
part of the intelligent antenna solution. In this
case, the total gain will be the combination of
the gains from transmit diversity and the aperture
due to reduction of the antenna beam relative
to the corresponding sector antenna beam.
CONCLUSIONS
With less than 50 years of commercial history,
wireless mobile communications have already
changed the way people communicate with each
other. While the progress has been impressive,
much more is yet to come that will revolutionize
communications as we know it, leading eventually
to communicating with anyone or any device
at anyplace and any time.