History of Adaptive Array Antenna

The adaptive array antennas appeared as the array antenna systems that flexibly receive the desired
wave while rejecting the unwanted waves. The term, “adaptive” is used to describe the ability of thesystems to change the characteristics by radio environments under which the systems are operating forcommunications and radars. The adaptive array antennas are composed of array antennas that detect
the incident signals and real-time signal processor which automatically adjusts the array pattern. The
advent of highly compact, inexpensive and high-speed digital computers has made it possible toemploy the adaptive array antennas in wider applications such as mobile communications andadvanced driver assistant systems. The functions of the adaptive array antennas are classified into
adaptive beamforming and adaptive null steering. The former is to automatically form the mainbeamof the array antenna in the desired direction. The latter is to produce the pattern nulls in the directionsof unwanted waves (interference).
In 1964, the first “special issue on active and adaptive antennas” was published by IEEE
Transactions on Antennas and Propagation. At that time, it was a fledgling field and characterized byretrodirective and self-steering or self-focusing array systems. It can be said that many researchers
focused on automatic mainbeam steering. In 1976, we had the second publication of “special issue on
adaptive antennas” by the same IEEE Transactions. It included the adaptive interference nulling as thekey capability. After another decade, we reached the third special issue on adaptive antennas in 1986.
Just as the difference between the first and second special issues, the third issue differs markedly fromthe second one due to the inclusion of the key capability for high-resolution spatial spectrum
estimation or DOA estimation.
The high-resolution algorithms for DOA estimation have been developed by Pisarenko, Capon, andSchmidt, et al. Among them, Schmidt presented a much-referenced algorithm named MUSIC(Multiple Signal Classification), which provides DOS estimates of multiple signals with highresolution or super-resolution. Currently, MUSIC and other high-resolution algorithms are used inradar systems to detect targets.
Nowadays, the most effective and most attractive technology for a high transmission rate is thought
to be multiple-input multiple-output (MIMO). MIMO uses multiple antennas at the transmitter and thereceiver both, and enables us to send and receive more than one data signal simultaneously over thesame radio channel by exploiting multipath propagation. In MIMO transmission, adaptive array
techniques are utilized for signal separation. Moreover, multi-user MIMO (MU-MIMO) systems haverecently attracted much attention as a technology that enhances the total system capacity between abase station and multiple user terminals. In MU-MIMO, adaptive array techniques also are used forsuppressing inter-user interference. In this way, we can find that adaptive array antennas continue tolive in modern wireless systems, and therefore a history of adaptive array antennas changing theirperformance arenas will be presented in this talk.