ission carries additional parity bits, the effective code rate is lowered by each re-transmission, as shown in Figure 7-6. The incremental redundancy is based on low-rate code and the different redundancy versions are generated by puncturing the channel coder output. In the example shown in Figure 7-6, the mother code rate is R, and 1/3 of the coded bits are transmitted in each re-transmission. Aside from increasing the received signal Eb/No ratio by each re-transmission due to combining, there is a coding gain as a result of each re-transmission. It must be noted that chase combining is a special case of incremental redundancy HARQ where the re-transmissions are identical copies of the original coded bits. The coding gain is defined as the difference between Eb/No required to achieve a given Bit Error Rate (BER) in a coded system, and the Eb/No required to achieve the same BER in an uncoded system. CHAPTER 7 The IEEE 802.16m MAC CPS (Part II) The incremental redundancy HARQ was an optional feature in Release 1.0 of mobile WiMAX systems [8]; however, it has been made the mandatory HARQ scheme for IEEE 802.16m systems. It is assumed that the receiver has received all previously transmitted redundancy and that each re-transmission provides some amount of information about the data packet. The puncturing pattern to be used for a given HARQ transmission is indicated by the Sub-Packet Identifier (SPID). By default, the SPID of the first transmission is always zero where all, for instance, turbo code systematic bits and some parity bits are sent. Note that only the parity bits are punctured, and each transmission by itself decodable. The SPID of the subsequent transmissions can be arbitrarily chosen by the system [7]. It must be noted that if the initial transmission with SPID of 0 is not properly received or lost, the re-transmission of only additional parity bits will not typically help decoding of the data packet, and a fresh transmission of the systematic bits might be necessary. 3. Type III HARQ: in a type III HAR