A technique for increasing wireless bandwidth by spatial antenna diversity, MIMO is incorporated into IEEE 802.11n specifications for wireless LAN (WLAN) and 802.16 specifications for broadband wireless access (BWA), more commonly known as WiMAX. As radio signals travel from transmitter to receiver in an enclosed space, they propagate along multiple paths. The signal elements traveling a direct path along a line of sight (LOS) arrive first and strongest.Those that travel the least direct paths, having reflected off walls, floors, ceiling, potted plants, people, and other obstructions, not only arrive last, but also suffer the greatest attenuation due to absorption, diffusion, and other contributing factors. MIMO technology employs multiple spatially diverse transmit antennas to actually encourage the signals to traverse multiple paths and multiple receive antennas to extract additional information from the signals that do so. MIMO algorithms in the receive device correlate and recombine the signals, realizing diversity gain, i.e., an increase in signal strength, in the process. MIMO technology doubles the spectral efficiency.The 802.11n MIMO technology, for example, is expected to yield a theoretical maximum signaling rate of 108 Mbps, compared to the 54 Mbps yielded by the earlier 802.11g technology. See also 802.11g, 802.11n, 802.16, absorption, attenuation, bandwidth, BWA, diffusion, gain, IEEE, LOS, spatial diversity, spectral efficiency, WiMAX, and WLAN.
(Multiple Input/Multiple Output) Pronounced "my-mo," it is the use of multiple transmitters and receivers (multiple antennas) on wireless devices for improved performance. When two transmitters and two or more receivers are used, two simultaneous data streams can be sent, which double the data rate. Multiple receivers alone allow greater distances between devices. For example, the IEEE 802.11n (Wi-Fi) wireless standard uses MIMO to increase speed to 100 Mbps and beyond, doubling at minimum the 802.11a and 11g rates. MIMO antennas are also used in WiMAX and LTE. MIMO, MISO and SIMO The M, S, I and O relate to the air, not the device. For example, multiple inputs (MI) means multiple transmitters send multiple data streams "into" the air. Multiple outputs (MO) means multiple receivers acquire multiple data streams "out of" the air (see illustration below). See 802.11n, antenna diversity, beamforming and HSPA. ARCHITECTURE AND ADVANTAGES OF MULTIPLE ANTENNA TECHNOLOGIES Compared to Single Antenna (SISO) Technologies -------------- Transmit Receive Data Type Antennas Antennas Rate Distance MIMO Multiple Multiple > > MISO Multiple Single = = SIMO Single Multiple = > M = Multiple S = Single I = Input O = Output