106 Keysight WLAN Measurement Guide
Concepts
MIMO (Multi Input Multi Output) Concepts
many receivers as there are transmit streams. The number of transmit streams
should not be confused with the number of transmit antennas. Consider the Tx
diversity (MISO) case in which two transmitters are present but only one data
stream. Adding receive diversity (SIMO) does not turn this into MIMO, even
though there are now two Tx and two Rx antennas involved. SIMO + MISO
≠
MIMO. It is always possible to have more transmitters than data streams but
not the other way around. If N data streams are transmitted from fewer than N
antennas, the data cannot be fully descrambled by any number of receivers
since overlapping streams without the addition of spatial diversity just creates
interference. However, by spatially separating N streams across at least N
antennas, N receivers will be able to fully reconstruct the original data streams
provided the crosstalk and noise in the radio channel are low enough.
One other crucial factor for MIMO operation is that the transmissions from
each antenna must be uniquely identifiable so that each receiver can
determine what combination of transmissions has been received. This
identification is usually done with pilot signals, which use orthogonal patterns
for each antenna.
The theoretical gains from MIMO are a function of the number of transmit and
receive antennas, the radio propagation conditions, the ability of the
transmitter to adapt to the changing conditions, and the SNR. The ideal case is
one in which the paths in the radio channel are completely uncorrelated,
almost as if separate, physically cabled connections with no crosstalk existed
between the transmitters and receivers. Such conditions are almost impossible
to achieve in free space, and with the potential for so many variables, it is
neither helpful nor possible to quote MIMO gains without stating the
conditions. The upper limit of MIMO gain in ideal conditions is more easily
defined, and for a 2x2 system with two simultaneous data streams a doubling
of capacity and data rate is possible. MIMO works best in high SNR conditions
with minimal line of sight. Line of sight equates to channel crosstalk and
seriously diminishes the potential for gains. As a result, MIMO is particularly
suited to indoor environments, which can exhibit a high degree of multi-path
and limited line of sight.