Introduction to the Issue on Managing Complexity in Multiuser MIMO Systems

A DECADE has passed since the pioneering works in which multiple-input multiple-output (MIMO) wireless systems have been introduced. By using multiple antennas at both link ends, MIMO in theory offers tremendous rate and reliability gains. These promises have spurred an extraordinary amount of research activities. By now, MIMO has become a well-established research topic, with thousands of papers published on the topic each year (a search for “MIMO” on IEEEXplore reports 264 hits for the year 2000 and 2880 hits for 2008), addressing signal processing topics like data detection, space-time code design, precoding, beamforming, channel estimation, and synchronization (list not complete). So far, the largest part of MIMO research has been devoted to point-to-point (single-user) channels. It is only fairly recently that MIMO started to be considered in the context of multiuser (multipoint) systems, where additional opportunities like spatial multiple access, interference suppression, and improved resource allocation exist. In this context, the recent survey “Shifting the MIMO paradigm” by Gesbert et al. in the Sept. 2007 issue of the IEEE Signal Processing Magazine is recommended reading. Soon after its introduction, MIMO technology also started to attract the attention of standardization bodies. By now, MIMO techniques have been incorporated into the mainstream of wireless standards like IEEE 802.11n (high-throughput WiFi), IEEE 802.16x (802.16e, a.k.a. mobile WiMAX, and upcoming 802. 16m), and 3GPP’s Long Term Evolution (LTE). While WiFi and 802.16e build on single-user MIMO techniques like spatial multiplexing and space-time coding, LTE Advanced and 802.16m also add multiuser MIMO features. In spite of the plethora of MIMO research and the fact that MIMO is now part of several standards, industry still faces problems with integrating MIMO into their products. The principal barrier to widespread industry adoption is implementation complexity and the sensitivity of receiver signal processing to departures from the standard propagation assumption of Rayleigh scattering. Management of signal processing complexity is thus essential to realizing the theoretical promise of MIMO technology. Here, the goal is to cleverly design tunable algorithms for the terminal and infrastructure side that entail a graceful trade-off between performance and implementation complexity. Substantial advances in this area would certainly have a big practical impact, e.g., with chip manufacturers and terminal and infrastructure vendors. There are 17 papers in this special issue that deal with various aspects of this challenging new thread in MIMO research. We have grouped these papers according to the following three major research themes: efficiently decodable space–time codes,