Mobile radio communications are evolving from pure telephony systems to multimedia
platforms offering a variety of services ranging from simple file transfers and audio and
video streaming, to interactive applications and positioning tasks. Naturally, these services
have different constraints concerning data rate, delay, and reliability (quality-of-service
(QoS)). Hence, future mobile radio systems have to provide a large flexibility and scalability
to match these heterogeneous requirements. Additionally, bandwidth has become
an extremely valuable resource emphasizing the need for transmission schemes with high
spectral efficiency. To cope with these challenges, three key areas have been the focus of
research in the last decade and are addressed in this book: Code division multiple access
(CDMA), multiple antenna systems, and strong error control coding.
CDMA was chosen as a multiple access scheme in third generation mobile radio systems
such as the universal mobile telecommunication system (UMTS) and CDMA 2000.
The main ingredient of CDMA systems is the inherent spectral spreading that allows a
certain coexistence with narrow band systems. Owing to the large bandwidth, it generally
provides a higher diversity degree and thus a better link reliability. Compared to second
generation mobile radio systems, the third generation offers increased flexibility like different
and much higher data rates as required for the large variety of services. The frequency
reuse factor in such cellular networks allows neighboring cells to operate at the same frequency,
leading to a more efficient use of the resource frequency. Moreover, this allows
simpler soft handover compared to the ‘break before make’ strategy in global system for
mobile communication (GSM) when mobile subscribers change the serving cell. The main
drawback of CDMA systems is the multiuser interference requiring appropriate detection
algorithms at the receiver.
Multiple antenna systems represent the second major research area. Owing to their high
potential in improving the system efficiency they have already found their way into several
standards. On one hand, multiple antennas at the receiver and transmitter allow the transmission
of several spatially separated data streams. For point-to-point communications, this
is termed space division multiplexing (SDM), and in multiuser scenarios, it is called space
division multiple access (SDMA). In both the scenarios, the system’s spectral efficiency
can be remarkably increased compared to the single antenna case. On the other hand, the
link reliability can be improved by beamforming and diverse techniques.