This disclosure provides methods, devices and systems of wireless communication in accordance with hybrid automatic repeat request (HARQ) techniques. Some implementations more specifically relate to aligning aggregate medium access control (MAC) protocol data unit (A-MPDU) subframes with codeword boundaries in a physical-layer service data unit (PSDU). The alignment may be performed by selectively adding padding bits to the PSDU based on the size of each A-MPDU subframe and the lengths of the codewords. In some aspects, an A-MPDU subframe may be aligned with a first codeword of the PSDU so that the first A-MPDU subframe is encoded exclusively within the first codeword. In some other aspects, an A-MPDU subframe may be aligned with two or more contiguous codewords of the PSDU so that the two or more contiguous codewords include the A-MPDU subframe and no portions of any other A-MPDU subframe.