[11] The use of MIMO-OFDM (orthogonal frequency division multiplexing) to increase the data rate while maintaining the same spectrum as 802.11a was first demonstrated by Airgo Networks.
[12] The purpose of the standard is to improve network throughput over the two previous standards—802.11a and 802.11g—with a significant increase in the maximum net data rate from 54 Mbit/s to 72 Mbit/s with a single spatial stream in a 20 MHz channel, and 600 Mbit/s (slightly higher gross bit rate including for example error-correction codes, and slightly lower maximum throughput) with the use of four spatial streams at a channel width of 40 MHz.
802.11 is a set of IEEE standards that govern wireless networking transmission methods.
There were older proprietary implementations of MIMO and 40MHz channels such as Xpress, Super G and Nitro which were based upon 802.11g and 802.11a technology, but this was the first time it was standardized across all radio manufacturers.
In addition, MIMO technology requires a separate radio-frequency chain and analog-to-digital converter for each antenna, making it more expensive to implement than non-MIMO systems.
[16] The transmitter and receiver use precoding and postcoding techniques, respectively, to achieve the capacity of a MIMO link.
However, the individual radios often further limit the number of spatial streams that may carry unique data.
All three configurations have the same maximum throughputs and features, and differ only in the amount of diversity the antenna systems provide.
Data rates up to 600 Mbit/s are achieved only with the maximum of four spatial streams using one 40 MHz-wide channel.
The main media access control (MAC) feature that provides a performance improvement is aggregation.
Two types of aggregation are defined: Frame aggregation is a process of packing multiple MSDUs or MPDUs together to reduce the overheads and average them over multiple frames, thereby increasing the user level data rate.
A-MPDU aggregation requires the use of block acknowledgement or BlockAck, which was introduced in 802.11e and has been optimized in 802.11n.
802.11n extends the coexistence management to protect its transmissions from legacy devices, which include 802.11g, 802.11b and 802.11a.
There are MAC and PHY level protection mechanisms as listed below: To achieve maximum output, a pure 802.11n 5 GHz network is recommended.
[23] The Wi-Fi Alliance began certifying products based on IEEE 802.11n draft 2.0 mid-2007.
A number of vendors in both the consumer and enterprise spaces have built products that have achieved this certification.