The number of columns of the signal input must be equal to the number of transmit antennas.

This statement is true in the context of MIMO (multiple input multiple output) systems. In MIMO, multiple antennas are used at both the transmitter and receiver ends to improve the capacity and reliability of wireless communication. The number of columns of the signal input matrix at the transmitter must match the number of transmit antennas to enable independent transmission of data streams to each antenna. This is known as spatial multiplexing, where multiple data streams are transmitted simultaneously through different antennas to increase the data rate. At the receiver, the number of columns of the received signal matrix must also match the number of receive antennas to enable the decoding of the transmitted data streams.