Guest Editorial: Virtual MIMO

MULTIPLE antenna (also known as MIMO: multiple input and multiple output) techniques have been widely accepted as a promising scheme to improve the spectral efficiency of mobile communication systems. MIMO becomes increasingly essential as the demand for broadband wireless data transmission increases. It has been supported by various international standards such as 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), LTE-A (LTEAdvanced), and IEEE 802.16e/m. However, since the size of a mobile transmitter or receiver is very small, implementing conventional MIMO antenna technologies becomes challenging. Fortunately, spatial diversity can be obtained through cooperation among mobile terminals, and multiple singleantenna mobile terminals can cooperate to form a virtual MIMO (VMIMO) system through relaying. In the VMIMO system, antennas at the terminals are virtually joined together to form a so-called virtual antenna array. The impetus for this special issue has been spurred by the strong desire to understand VMIMO, which is a rapidly growing research area. VMIMO is believed to be a key technology for beyond 4th generation mobile communications technologies (B4G). It enables one to make use of all the neighboring terminals and amortize the cost of multiple antennas; hence, a large MIMO channel can be created to increase capacity significantly as well as improve error rate performance. Nevertheless, fundamental roadblocks need to be addressed in order to take full advantage of VMIMO. First of all, due to the limited processing capability of base stations, only a small number of mobile terminals can be supported to form a VMIMO system in a cellular network. So it becomes a huge challenge to optimally select mobile terminals to form a useful VMIMO system. Secondly, although mobile terminals can be grouped together to form a VMIMO system, information exchange among the mobile terminals is needed and should be minimized. Moreover, channel state information (CSI) of each mobile terminal is required at the BSs to carry out precoding, which results in a significant pilot and signaling overhead, especially in