A key factor to the performance of Multiple Input Multiple Output (MIMO) is the number of “Spatial Layers” of the wireless channel which determines the ability to improve the spectral efficiency.
Spatial layers are born out of the multipath and scattering environment between transmitters and receivers.
The increase in the data rate of a MIMO system is linearly proportional with the minimum number of transmit and receive antennas subject to the limit of the “rank” of the propogation channel estimate.
The “rank” is a measure of the number of independent spatial layers.
Hence, a 4 Tx x 2 Rx antenna MIMO system provides double the data rate provided that there are two spatial layers (rank=2) in the wireless channel. IN line of site conditions, the channel matrix rank is one, hence even with 4 antenna we cannot increase the spectral efficiency of the channel.
Leave a reply to What is the key factor performance of MIMO