IEEE 802.11n specifies enhancements to the IEEE 802.11 standard to provide higher data rates in the 2.4 and 5 GHz band.
The main improvements that make these high data rates possible are:
Changes to OFDM implementation
Changes are made in the implementation of the OFDM modulation scheme. This increases the data rate from 54 Mbit/s to 65 Mbit/s.
Channel bonding combines two standard 20 MHz channels to a wideband 40 MHz channel, which effectively doubles the data rate. However, this reduces the number of non-overlapping channels, which increases the interference from other networks. This problem is especially severe in the 2.4 GHz which has a much smaller channel spacing of only 5 MHz compared to 10 MHz in the 5 GHz band, and also has a much smaller number of available channels.
MIMO (Multiple Input Multiple Output) is a technology whereby the data is split over a number of data streams which are transmitted through seperate antennas to corresponding antennas at the receiver. doubling the number of streams will also double the total data rate. IEEE 802.11n allows for up to 4 data streams.
Most smartphones only have one transmitter to save battery power. This reduces the maximum available data rate from 600 Mbit/s to 150 Mbit/s.
The maximum performance of IEEE 802.11n will only be possible in a IEEE 802.11n only network. If the network is mixed with IEEE 802.11a/b/g devices the performance will be significantly lower.