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Wireless Communications over MIMO Channels
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.
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