For example, a transmitter may be configured to assign a stream of encoded data bits of a Physical layer (PHY) Protocol Data Unit (PPDU) to one or more sequences of encoded bit blocks for one or more spatial streams based on an Unequal Modulation and Coding Scheme (MCS) (UEM) assignment including an assignment of a plurality of MCSs to a plurality of frequency sub-blocks for the one or more spatial streams. For example, the transmitter may be configured to assign the one or more sequences of encoded bit blocks to the plurality of frequency sub-blocks for the one or more spatial streams.