The 802.11n standard introduced Multi-Input-Multi-Output (MIMO) into WLAN. This enabled Wi-Fi devices to increase their data rate. Further enhancements were also introduced in the 802.11n standard significant of which was MAC layer data aggregation, Channel bonding, transmit beam-forming and higher QAM modulation schemes. The current article delves into the basics of MIMO.
MIMO stands for Multiple-Input and Multiple-output. In the 80s and the early 90s – significant research was conducted in the area of multi-channel transmission techniques in exploiting multi-path propagation to send multiple information streams via multiple antennas simultaneously.
It was based on the concept of Multi-path propagation. That is – each signal that is transmitted from an antenna collides and bounces off of opaque hard objects on its way to the receiver. The received signal would be a mix of the transmitted signal arriving at
- Different instances of time and also
- At different angles of arrival
The theory was that if the multiple streams were configured such that the signals transmitted by them were sufficiently apart such that each of the received signals can be independently decoded at the receiver – that would lead to an increase in the throughput of the system.
The above paragraph indicates that a MIMO system would require multiple antennas on the transmitter and receiver. The different transmitted streams are multiplexed on the different antennas and are placed spatially apart such that the different streams can be decoded at the receiver efficiently. The number of spatial streams that can be achieved is limited to the lesser of number of transmitter and receiver.
For a more in depth understanding of the MIMO system – refer the below Wiki link
https://en.wikipedia.org/wiki/MIMO
MIMO can be divided into three groups
- Spatial Multiplexing
- sending different data on multiple antennas simultaneously using more than one spatial stream
- Pre-coding
- knowing the channel characteristics, modulating the signal and transmitting the signal in a beam (transmit beam-forming) so that the signal can be more appropriately targeted at the receiver
- Diversity Coding
- sending the same signal on different antennas but encoding the signals differently using coding techniques such as Space Time Block Coding (STBC). Same signal is sent on different antennas but the signal is differently encoded and hence Diversity Coding does not increase the throughput