A method for communication includes, in a transmitter having a first number of transmit antenna ports, setting an upper limit on a second number of spatial layers to be used by the transmitter to be less than the first number. An actual number of the spatial layers, which does not exceed the upper limit, is allocated for transmission to a given receiver. One or more streams of modulated symbols are mapped onto the allocated actual number of the spatial layers. The actual number of the spatial layers are transmitted from the transmitter to the given receiver. A precoding operation maps the spatial layers onto the transmit antenna ports. A first upper limit is set when the precoding operation depends on feedback from the given receiver. A second upper limit, which does not exceed the first upper limit, is set when the precoding operation is not dependent on the feedback.