e any addi- transmitted. As an example, assume a basic rate-1/4 tional coding gain but only increases the accumulated code. In the first transmission, only every third coded bit received Eb/No for each retransmission (Figure 4.5-5). is transmitted, effectively giving a rate-3/4 code as il- Several variants of Chase combining exist. For exam- lustrated in Figure 4.5-6. In case of a decoding error and ple, only a subset of the bits transmitted in the original a subsequent request for a retransmission, additional bits transmission might be retransmitted, so-called partial are transmitted, effectively leading to a rate-3/8 code. Chase combining. Furthermore, although combining is After a second retransmission the code rate is 1/4. In case often done after demodulation but before channel of more than two retransmissions, already transmitted decoding, combining can also be done at the modulation coded bits would be repeated. In addition to a gain in symbol level before demodulation, as long as the accumulated received Eb/No, incremental redundancy Info bits CRC insertion, rate-3/4 error-correcting coding Coded bits Transmitted Initial transmission First retransmission Second retransmission Third retransmission Bits input to decoder Accumulated energy Resulting R=3/4 R=3/4 R 3/4 R=3/4 code rate Figure 4.5-5 Example of chase combining. CHAPTER 4.5 Scheduling, link adaptation and hybrid ARQ Info bits CRC insertion, rate-1/4 error-correcting coding Coded bits Puncturing to generate different redundancy versions Transmitted Redundancy version 1 Redundancy version 2 Redundancy version 3 Redundancy version 1 Initial transmission First retransmission Second retransmission Third retransmission Bits input to decoder Accumulated energy Resulting R=3/4 R=3/8 R 1/4 R=1/4 code rate Figure 4.5-6 Example of incremental redundancy. also results in a coding gain for each retransmission. The transmission was received with poor quality or not at all, gain with IR compared to Chase is larger for high initial a retransmission with only pari