  In past 802.11 systems there is a single Radio Frequency (RF) chain on the
Wi-Fi device. Multiple antennas use the same hardware to process the radio
signal. So only one antenna can transmit or receive at a time as all radio
signals need to go through the single RF chain. In MIMO there can be a separate
RF chain for each antenna allowing multiple RF chains to coexist. MIMO
technology has attracted attention in wireless communications, because it
offers significant increases in data throughput and link range without
additional bandwidth or increased transmit power. It achieves this goal by
spreading the same total transmit power over the antennas to achieve an array
gain that improves the spectral efficiency (more bits per second per hertz of
bandwidth) or to achieve a diversity gain that improves the link reliability.
Multiple Input/Multiple Output (MIMO) is an area of intense development in the
wireless industry because it delivers profound gains in range, throughput and
reliability. As a result, manufacturers of wireless local area network (WLAN),
wireless metropolitan area network (WMAN), and mobile phone equipment are
embracing MIMO technology. In this paper we are interested to compare the MIMO
Antenna functions with traditional Antenna functions. And we take an example of
IRT for illustration.
