World Journal Of Engineering
ENHANCING MODERN MOBILE COMMUNICATION SYSTEMS USING
TUNABLE COMPONENTS MADE OF MATERIAL UTILIZED IN NEW FIELDS OF
APPLICATION
Kira Kastell
Department of Computer Science and Engineering, Frankfurt University of Applied Sciences,
Nibelungenplatz 1, 60318 Frankfurt am Main, Germany. kastell@fb2.fh-frankfurt.de
Introduction
Modern mobile communication systems face several
challenges, some of the most challenging of them resulting
from customer expectations. These challenges only can be
coped with by an integrated approach modifying network
planning and protocols and enhancing hardware
components. As there will be further yet undiscovered
challenges in the future, flexible hardware structures
would be preferable, especially because the exchange of
mobile devices owned by the users is nearly not possible
without causing enormous costs.
What are the current challenges and where do they come
from? The users nowadays have a very positive experience
with existing mobile communication systems which they
got to know as reliable and always available. Besides that,
they are also used to ever increasing data rates in the fixed
line networks. In the user’s perception the increasing data
rate can be directly transferred to mobile communication.
This is because from the average user’s point of view there
is no difference between the high data rate but short range
Wi-Fi transmission and other mobile communication
systems. This experience is strengthened by the fact that
the mobile phone also works with Wi-Fi and users often
do not recognize the network they are using for a certain
service. Therefore the users expect increasing data rates
for every mobile communication independent from the
network used and in addition independent from their
mobility pattern or speed.
In wide area mobile communication systems these user
expectations pose severe challenges to system and
components design. The high mobility of the users
combined with comparatively high speed resulting in a
real mobile use of the equipment rather than a nomadic
use needs to be handled. Therefore mobility management
has to be very fast especially with ongoing (streaming)
data transmission resulting in handover or relocation. This
sophisticated mobility management needs to be sped up as
the first implementations haven’t been designed for high
speed; or to be more precise the interdependence of speed
and cell size/cell overlap has not been taken into account
at initial network planning.
Other challenges are the increasing number of users, the
coexistence of different communication systems within a
small frequency range and the simultaneous use of the
same frequency band by different operators. The
increasing number of customers and the scarcity of
spectrum make capacity planning without degradation of
the quality of service (QoS) a hard task.
Improvements in different network levels
The above mentioned expectations can be dealt with in
different ways. The building of new networks needs to
fulfill harder constraints, especially if there is no change in
the design of the hardware or the protocol structure (see
below). The cell overlap area has to be increased to deal
with the higher speed of the users. This results in longer
time intervals available for the processing of the handover,
which is the most time critical part in the communication.
Besides that, the composition of location areas has to take
into consideration the smaller cell size. (The cell size in
future networks will be smaller as the new systems work
at higher frequencies. Decreased cell sizes are also the
result of higher numbers of users, and thus demand for
more network capacity, which can be dealt with by
substituting one large cell with several smaller ones each
providing the same capacity as the large cell in total.) If
the network overhead for location update shall remain the
same, the location areas have to be planned more carefully
according to user group mobility patterns and also they
may need to consist of more cells. Drawback of larger
location areas is the increased (backbone) overhead for
paging, but this is not time critical.
Depending on the multiple access scheme used in the
network the discrimination of the users has to be improved
to avoid interference. Especially in networks with
frequency
division
multiple
access
(FDMA)
intermodulation is an increasing problem. This problem
can be best dealt with during network planning. But the
planning only avoids interference if all operators of one
area work together as all adjacent frequencies will
interfere independent of the operator. A second way to
reduce intermodulation is a more proper space division
scheme in parallel to FDMA. This can be achieved by use
of antennas with specially designed antenna patterns. To
simultaneously deal with the mobility, reconfigurable
antenna patterns will be the optimum solution to achieve
adjustable space division multiple access (SDMA). This
will result in fewer interferences (as the propagation area
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World Journal Of Engineering
is customized to the actual user distribution), higher
capacity (as the power can be focused to areas were it is
needed), and thus higher data rates and better QoS will be
experienced by the user.
Reconfigurability will be of advantage for many other
purposes in (mobile) communication. Here the software
defined radio (SDR) has gained attention over the past
years. The SDR concept describes the ambition to
implement the complete signal processing of a radio
frequency transceiver using reconfigurable hardware or
software to flexibly deal with changing environments and
user or service demands. The reconfigurable hardware
mostly consists of digital signal processors (DSP) or field
programmable gate arrays (FPGA). However not only
conventional components play a role but also devices as
reconfigurable antennas, phase shifters, tunable filters and
oscillators, and adaptive matching networks. Here new
materials come into play as well as well-known materials
in new application areas. An example for the latter case is
the use of liquid crystals (LC) for tuning components in
the microwave frequency range [1].
Reconfigurable elements
The following reconfigurable elements will help to
overcome the challenges. The best solution, of course, will
consist of an integrated approach of matched network
planning and network configuration.
Reconfigurable antennas are needed if flexible SDMA
shall be applied. Flexible SDMA allows at least the
network access point (NAP) to track single users or user
groups. Depending on the size of the antenna and the size
of the mobile device, it may also possible for the mobile
device to track the NAP. This tracking helps focusing the
available power to the users. It may also extend the range
of the NAP in a certain direction if there are a lot of users,
e.g. close to a cell border. This may prevent them from
performing (frequent) handover(s) or may force a
handover to balance the user distribution between the
neighboring cells. Reconfigurable antennas may be built
from different material. Among the new concepts there are
reflect arrays with liquid crystal [2], barium strontium
titanate (BST) bulk ceramics [3, 4], and others. The
reconfigurability can be achieved by tuning the antenna
elements themselves or by tuning via an adaptive
matching network [5, 6].
Tunable phase shifters [3] will help to cope with group
delay problems that will arise more often as modern
communication systems use broader frequency bands.
They also can be used to design complex antennas and
help with user separation by polarization, which may be an
additional distinction combined with SDMA. Tunable
filters may help to implement integrated SDR receiver
structures that can be adjusted to a broad range of
frequency bands. In this way new communication systems
with new spectrum assignments may be easily integrated
in the communication (interface) profile of and existing
mobile device.
Results and Discussion
In this paper a brief overview of challenges for mobile
communication systems is given. Besides network
planning and protocol enhancements new hardware will
play an important role in solving them. Different concepts
and applications for the use of modern material have been
given. All concepts given here aim for reconfigurability.
This makes the components flexible for the use in different
communication systems and reduces the amount of
transceiver chains needed in every single device. The SDR
concept will not only be helpful in mobile communication
but it will simplify the modification of all transceivers and
therefore also help to conserve resources of rare materials,
because components just may be reconfigured instead of
exchanged.
These different new approaches still have some
drawbacks, mostly in terms of switching times that are not
fast enough to implement real time solutions. Therefore
measurements need to be taken to calculate predictions of
user behavior instead of real time measurements with real
time adjustments. Here is still room to improve the
system’s components.
References
1. Weil, C., Müller, S., Scheele, P., Best, P., Lüssem, G. and
Jakoby, R. Highly-anisotropic liquid-crystal mixtures for
tunable microwave devices. Electronics Letters, vol. 39,
(2003), pp. 1732-1734.
2. Moessinger, A., Marin, R., Freese, J., Müller, S., Manabe,
A., and Jakoby, R. Investigations on Tunable Reflectarray
Unit Cells with Liquid Crystal. European Conference on
Antennas Propagation, (2006).
3. Zimmermann, F., Voigts, M., Weil, C., Jakoby, R., Wang,
P., Menesklou, and W., Ivers-Tiffée, E. Investigation of
barium strontium titanate thick films for tunable phase
shifters. Journal of the European Ceramic Society (2001)
vol. 21, issue: 10-11, pp. 2019-2023.
4. Giere, A., Zheng, Y., Gieser, H., Marquardt, K., Wolf, H.,
Scheele, P., and Jakoby, R. Coating of Planar BariumStrontium-Titanate Thick-Film Varactors to Increase
Tunability. Proc. 37th European Microwave Conf., (2007).
5. Maune, H., Zheng, Y., Sazegar, M., Giere, A., and Jakoby,
R. Compact Devices for Complex Wave Monitoring for
Tunable Impedance Matching Networks. Frequenz, vol. 3/4,
(2009), pp. 63-65.
6. Damm, C., Schüßler, M., and Jakoby, R. Antenna matching
using a tunable artificial line. Proc. IEEE Antennas and
Propagation International Symposium, (2007). pp. 3464-3467.
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