According to your last question, I agree with second explanation given by Damian. The pilot signal is transmitted during the connection session. At the same time, this signal is known at the receiver as well and used for future comparison. Therefore, Multiple Input Multiple Output (MIMO) is allowed in these systems. For further details, I can refer you to a recommendation of ITU-T. Please, take a look at the following ITU-T recommendation: G Suppl.RoF.