heme must, by bits than the previous transmission. The receiver com- definition, represent the same set of information bits as bines the retransmission with previous transmission at- the original transmission. However, the set of coded bits tempts of the same packet. As the retransmission may transmitted in each retransmission may be selected dif- contain additional parity bits, not included in the pre- ferently as long as they represent the same set of in- vious transmission attempts, the resulting code rate is formation bits. Hybrid ARQ with soft combining is generally lowered by a retransmission. Furthermore, each therefore usually categorized into Chase combining and retransmission does not necessarily have to consist of the Incremental Redundancy, depending on whether the same number of coded bits as the original and, in general, retransmitted bits are required to be identical to the also the modulation scheme can be different for different original transmission or not. retransmissions. Hence, incremental redundancy can be In Chase combining, the retransmissions consist of the seen as a generalization of Chase combining or, stated same set of coded bits as the original transmission. After differently, Chase combining is a special case of in- each retransmission, the receiver uses maximum-ratio cremental redundancy. combining to combine each received channel bit with any Typically, incremental redundancy is based on a low- previous transmissions of the same bit and the combined rate code and the different redundancy versions are signal is fed to the decoder. As each retransmission is generated by puncturing the output of the encoder. In an identical copy of the original transmission, retrans- the first transmission only a limited number of the coded missions with Chase combining can be seen as additional bits are transmitted, effectively leading to a high-rate repetition coding. Therefore, as no new redundancy is code. In the retransmissions, additional coded bits are transmitted, Chase combining does not giv