A method for communication includes receiving in a receiver signals, which include one or more dedicated reference signals and are transmitted from a transmitter over a communication channel in multiple blocks. The signals in each block, including the dedicated reference signals, are transmitted on a respective group of subcarriers over a respective time interval and are precoded using a respective precoding scheme that maps the signals onto multiple antenna ports of the transmitter. Two or more of the blocks whose respective precoding schemes differ from one another by no more than a predefined distance are identified. One or more parameters of the communication channel are estimated over the dedicated reference signals included in the identified blocks. The signals are decoded based on the estimated parameters.