In MIMO with 4 transmit antennas and 2 receive antennas, which is the upper limit on the number of independent data flows (streams) that spatial multiplexing can send at the same frequency and time?

Answer: A. 2

In MIMO spatial multiplexing, different data are sent simultaneously on the same frequency from several transmit antennas, and the receiver separates them using the differences in the propagation channels between each pair of transmit and receive antennas. The number of separable streams is limited by the smaller of the numbers of transmit and receive antennas, here min(4, 2) = 2. In practice, when the propagation channels are similar (highly correlated), as in places with clear line of sight, fewer streams than this limit can be used.

• A. Correct. min(4, 2) = 2 is the limit.
• B. This is the number of transmit antennas. With 2 receive antennas, 4 streams cannot be separated.
• C. This is the product of the numbers of transmit and receive antennas, which is the number of propagation channels (transmit-receive antenna pairs).