Multiple-Input Multiple-Output (MIMO) technology is a wireless technology that uses multiple transmitters and receivers to transfer more data at the same time (Figure 1). Wireless products with 802.11n support MIMO. This is part of the technology that allows 802.11n to reach much higher speeds than products without 802.11n.

MIMO technology takes advantage of a radio-wave phenomenon called multipath where transmitted information bounces off walls, ceilings, and other objects, reaching the receiving antenna multiple times via different angles and at slightly different times. Multipath is a natural occurrence for all radio sources. Radio signals bounce off objects and move at different speeds towards the receiver. In the past multipath caused interference and slowed down wireless signals. MIMO takes advantage of multipath to combine the information from multiple signals improving both speed and data integrity.

Figure 1. MIMO technology uses multiple radios to transfer more data at the same time

MIMO technology leverages multipath behavior by using multiple, “smart” transmitters and receivers with an added “spatial” dimension to dramatically increase performance and range. MIMO allows multiple antennas to send and receive multiple spatial streams at the same time. Smart transmitters and receivers are used with all 802.11n devices. Using multiple antennas the data can be sent and received through multiple signals. More antennas usually equates to higher speeds. A wireless adapter with 3 antennas may have a speed of 600mbps while an adapter with 2 antennas has a speed of 300mbps. The router also needs to have multiple antennas and fully support all of the features of 802.11n to gain the highest speed possible.

MIMO makes antennas work smarter by enabling them to combine data streams arriving from different paths and at different times to effectively increase receiver signal-capturing power. Smart antennas use spatial diversity technology, which puts surplus antennas to good use. If there are more antennas than spatial streams, the additional antennas can add receiver diversity and increase range.

In order to implement MIMO, either the station (mobile device) or the access point (AP) need to support MIMO. Optimal performance and range can only be obtained when both the station and the AP support MIMO.

Legacy wireless devices can’t take advantage of multipath because they use a Single Input, Single Output (SISO) technology. Systems that use SISO can only send or receive a single spatial stream at one time.

Related Links
Video: SISO vs. MIMO - All 802.11n is NOT Equal*
Video: Buyer Beware: Not all consumer 802.11n Wi-Fi Products are the same*
How does MIMO use multiple antennas to improve performance?
How does wireless diversity work?
Intel® Centrino® 802.11n Wi-Fi Gets Down to Business
MIMO architecture for wireless communication
Helping Define 802.11n and other Wireless LAN Standards

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