Guest editorial special issue on mimo wireless communications

HE major challenges in future wireless communications system design are increased spectral efficiency and improved link reliability. The radio channel constitutes a hostile propagation medium, which suffers from fading (signal level fluctuations due to destructive addition of multipaths) and interference from other users. Diversity provides the receiver with several ideally independent replicas of the transmitted signal and is therefore a powerful technique to mitigate fading and interference from other users, thereby improving link reliability. The use of spatial or antenna diversity has recently become very popular since it can be provided without loss in spectral efficiency. The corresponding signal processing and coding techniques are known as space-time coding and require that multiple antennas at the transmitter and one or more antennas at the receiver are employed. The use of multiple antennas at both the transmitter and the receiver of a wireless link [multiple-input multiple-output (MIMO) technology] has recently been demonstrated, both theoretically and practically, to achieve an order of magnitude improvement in terms of spectral efficiency when compared with existing systems. MIMO wireless is evolving into one of the most active research segments in wireless communications, and the first standards based on MIMO technology are being defined. Despite the significant research efforts in the MIMO area, numerous problems related to ultimate performance limits, code design, and receiver design remain open. The present special issue is an attempt to cover a wide range of topics in MIMO wireless by publishing 26 high-quality papers reporting on recent results in the above mentioned areas. As an indication of the level of activity in this research area, there were a significant number of high-quality papers we could not include in this issue due to space constraints. The papers in this special issue correspond to advances in the understanding of six different areas of MIMO communication. The first two are analytical and include understanding the ultimate capacity that MIMO wireless links can provide as well as understanding the performance of MIMO systems. Performance and capacity are being assessed with ever increasingly sophisticated models to reflect the complex environment that MIMO communication represents. The second two relate