Multiple-Antenna Techniques for Wireless Communications

multiple-input multiple-output (MIMO) techniques to improve the robustness and performance of wireless links. Here, the term multiple-input multiple-output refers to the use of an array of antennas for both transmitting and receiving. MIMO approaches show promise of enabling better wireless communications because they mitigate problems inherent in ground-to-ground links, which are the most common links used by wireless devices, including cell phones and WiFi. Typically, ground-to-ground links are not line of sight. The electromagnetic waves transmitted from the antennas bounce around the environment in a complicated fashion and end up at the receiver coming from multiple directions and with varying delays. The effect produced by the direction/delay interactions is referred to as multipath, a condition that must be accommodated by ground-to-ground systems. With the use of MIMO communication techniques [1], multipath need not be a hindrance and can be exploited to increase potential data rates and simultaneously improve the robustness of the links (Figure 1). Increased military and commercial dependence on wireless communication has intensified the need for more robust links. For example, the lack of adequately robust, reliable links has limited the usefulness of remotely controlled robots in military environments. More is also being asked of the links in terms of flexibility of use and higher data rates. Furthermore, ad hoc wireless networks, which are inevitably becoming integrated into military and commercial applications, require more robust, flexible, and higher-data-rate links. Lincoln Laboratory is pushing the limits of MIMO technology, developing record-setting space-time codes. Spacetime codes describe what is transmitted by the array of transmitters in a MIMO communication link [2]. These Tech Notes www.ll.mit.edu September 2008