- In the field of communication systems the following tendencies can be recognized:
n the one hand the data rates of the offered and planned services are increasing and on the other
hand the demand for mobile services is increasing. These tendencies motivate the following research
For mobile communication systems, higher data rates can be achieved through adaptive modulation and
channel coding schemes: In order to increase bandwidth efficiency, these schemes optimally adapt
to the time-variant characteristic of the mobile radio channel. The capacity of a cellular mobile
radio system can be increased exploiting the spatial aspect of wave propagation by smart antennas.
Because of the high demand for mobile services, current mobile communication systems suffer
from capacity problems. The conventional way to increase the capacity of a cellular mobile radio system
is the acquisition of new licences for enlarging the bandwidth or to reduce the cell size. Concerning
new licences it must be realised that the available bandwidth is strictly limited and that licence fees
(which may be rather high) must be paid. For reducing the cell size, many new base stations have
to be built, with the consequence of additional costs. Furthermore, to find locations for new base
stations is becoming more and more difficult for network operators because of acceptance problems
in parts of the population.
Principle of a mobile radio system with smart antennas
An elegant way to solve these problems is to increase the capacity of mobile communication systems
with the help of smart antennas. The idea of smart antennas is to exploit the spatial properties of
wave propagation. Because of a limitation in space, smart antennas can only be employed at base
stations. Smart antennas consist of an array of antenna elements and a smart processing of antenna
signals. With the help of signal processing, the beam pattern of the antenna array is optimally
adapted to the mobile radio channel. The goal is to receive in the uplink (transmission direction:
mobile station ® base station) as much power as possible from the desired mobile station and
to attenuate as much as possible the signals from any undesired mobile station. In the downlink
(transmission direction: base station ® mobile station) as much power as possible has to be transmitted
into the direction of the desired mobile station and as little power as possible to undesired mobiles.
By this method, cochannel interference in a cellular system can be reduced, with the consequence that
the capacity increases. This principle is illustrated in the figure above. It has to be mentioned that
for each desired mobile station an individual beam pattern must be optimized.
Future mobile radio systems: Adaption to the radio channel
Considering future mobile radio systems, there is clearly the tendency towards larger bandwidths. Since
the available bandwidth is in any case strictly limited for future generation mobile radio systems,
new transmission techniques with higher bandwidth efficiency must be developed. Bandwidth efficiency
can be increased by using transmission techniques which better adapt to the mobile radio channel.
Possible methods for adaptation to the channel are: smart antennas, adaptive modulation and adaptive
channel coding techniques.
Adaptive modulation and channel coding techniques seem to be particularly promising for multi-carrier
systems. For these systems it is possible to adapt to the frequency-selectivity of the mobile radio
channel in an optimum way. Similar methods are used for the transmission on twisted pair lines (ADSL).
Mobile radio communication systems face the additional problem of the time-variance of the channel.
Using antenna arrays on both the transmitter and the receiver side, it is possible to achieve
a considerable increase of data rate. Such communication systems are called multiple input multiple
output systems (MIMO).