Multiple Antenna Techniques for Wireless Communications
The use of multiple antennas for wireless communication systems has gained overwhelming interest during the last decade – both in academia and industry. Multiple antennas can be utilized in order to accomplish a multiplexing gain, a diversity gain, or an antenna gain, thus enhancing the bit rate, the error performance, or the signal-to-noise-plus-interference ratio of wireless systems, respectively. With an enormous amount of yearly publications, the ﬁeld of multiple-antenna systems, often called multiple-input multiple-output (MIMO) systems, has evolved rapidly. To date, there are numerous papers on the performance limits of MIMO systems, and an abundance of transmitter and receiver concepts has been proposed. The objective of this literature survey is to provide non-specialists working in the general area of digital communications with a comprehensive overview of this exciting research ﬁeld. To this end, the last ten years of research efforts are recapitulated, with focus on spatial multiplexing and spatial diversity techniques. In particular, topics such as transmitter and receiver structures, channel coding, MIMO techniques for frequency-selective fading channels, diversity reception and space-time coding techniques, differential and non-coherent schemes, beamforming techniques and closedloop MIMO techniques, cooperative diversity schemes, as well as practical aspects inﬂuencing the performance of multiple-antenna systems are addressed. Although the list of references is certainly not intended to be exhaustive, the publications cited will serve as a good starting point for further reading.
HOW IS IT possible to design reliable high-speed wireless communication systems? Wireless communication is based on radio signals. Traditionally, wireless applications were voice-centric and demanded only moderate data rates, while most high-rate applications such as ﬁle transfer or video streaming were wireline applications. In recent years, however, there has been a shift to wireless multimedia applications, Manuscript received 20 February 2007; revised 29 October 2007. This work was partly supported by a postdoctoral fellowship from the German Academic Exchange Service (DAAD). which is reﬂected in the convergence of digital wireless networks and the Internet. For example, cell phones with integrated digital cameras are ubiquitous already today. One can take a photo, email it to a friend – and make a phone call, of course.
In order to guarantee a certain quality of service, not only high bit rates are required, but also a good error performance. However, the disruptive characteristics of wireless channels, mainly caused by multipath signal propagation (due to reﬂections and diffraction) and fading effects, make it challenging to accomplish both of these goals at the same time. In particular, given a ﬁxed bandwidth, there is always a fundamental tradeoff between bandwidth efﬁciency (high bit rates) and power efﬁciency (small error rates). Conventional single-antenna transmission techniques aiming at an optimal wireless system performance operate in the time domain and/or in the frequency domain. In particular, channel coding is typically employed, so as to overcome the detrimental effects of multipath fading. However, with regard to the ever-growing demands of wireless services, the time is now ripe for evolving the antenna part of the radio system. In fact, when utilizing multiple antennas, the previously unused spatial domain can be exploited. The great potential of using multiple antennas for wireless communications has only become apparent during the last decade. In particular, at the end of the 1990s multiple-antenna techniques were shown to provide a novel means to achieve both higher bit rates and smaller error rates. In addition to this, multiple antennas can also be utilized in order to mitigate co-channel interference, which is another major source of disruption in (cellular) wireless communication systems. Altogether, multiple-antenna techniques thus constitute a key technology for modern wireless communications. The beneﬁts of multiple antennas for wireless communication systems are summarized in Fig. 1. In the sequel, they are characterized in more detail.