MIMO is an innovative advancement in wireless data transmission. It turns the long-time nemesis of wireless “multipath” into a friend. Multipath is a common occurrence indoors where a wireless signal reflects from surfaces thus creating multiple signals that add together in the air. While today’s 802.11a,b,g radios struggle with multipath, MIMO radios actually take advantage of multiple paths to send multiple data streams and thereby increase the rate of transmission. Early MIMO chipsets from Broadcom, Airgo (now Qualcomm) and Marvell feature 2 transmitters and 2 to 3 receivers, but the standard specifies up to a 4×4 configuration. An NxM MIMO system has N transmitters and M receivers.
The standard incorporates two MIMO techniques “Spatial Multiplexing” and Beamforming. Spatial Multiplexing divides a data stream into multiple streams and sends these streams simultaneously via multiple paths in a multi-path channel. These streams are re-combined in the receiver to recover the transmitted data. Beamforming sends multiple versions of same data stream to improve reception and thereby maximize throughput.
802.11n devices can operate in 3 modes: Legacy (802.11a,b,g), Mixed mode (802.11n and 802.11a,b,g) or Green Field (802.11n only). The highest throughput is achievable in the Green Field mode. In mixed mode, a single legacy station can slow down the entire 802.11n network.
For handhelds, 802.11n has the potential of improving battery life by minimizing the time required to send and receive data packets and through the use of improved power saving techniques.
802.11n uses the traditional unlicensed bands at 2.4GHz and 5GHz and will also use the newly available 3650 MHz to 3700 MHz band, as will the legacy 802.11 a,b,g networks once operation in this band is standardized toward the end of 2007. While the legacy 802.11 networks use 20 and 25MHz channels, 802.11n networks use 20 or 40MHz channels.
The IEEE 802.11n standard is expected to be ratified in mid-2008, but the draft should be stabilized in the first half of 2007, which gives the green light to pre-standard implementations. Currently the most contentious issues are whether Green Field mode is mandatory and whether 40 MHz channels are allowed in the 2.4 GHz band.
About the author: Fanny Mlinarsky runs a Boston area consulting firm, octoScope (www.octoscope.com), specializing in wireless and RF product development and advocacy. Fanny is an active contributor to the IEEE standards, a prolific author and presenter.