Multiple Antenna Technology: When is it MIMO? When is it Diversity?

Many academic and commercial technical presentations have been made at conferences worldwide. Despite all this attention, many engineers hoping to find an introduction to MIMO are confused by some of the statements made in those papers. It appears that some of the papers discuss multiple antennas in systems that do not have the signal content that defines MIMO. Attendees at the conference, however, hear those papers in the same sessions with MIMO papers and become confused. Worse, they may believe that all multiple antenna systems can be referred to as “MIMO,” which is, of course, not correct. This note points out the difference between the long-standing technique of diversity reception using two or more receiving antennas, and true MIMO, which uses two or more antennas at both the transmitter and receiver locations.