the information bits prior or Turbo codes for error correction, but in principle any to transmission (alternatively, the transmission could error-detecting and error-correcting code can be used. consists of parity bits alone, depending on the coding scheme used). The parity bits are computed from the 4.5.4 Hybrid ARQ with soft information bits using a method given by the coding structure used. Thus, the number of bits transmitted over combining the channel is larger then the number of original in- formation bits and a certain amount of redundancy has The hybrid ARQ operation described above discards er- been introduced in the transmitted signal. roneously received packets and requests retransmission. Another approach to handle transmissions errors is to However, despite the fact that the packet was not pos- use Automatic Repeat Request (ARQ). In an ARQ sible to decode, the received signal contains information, scheme, the receiver uses an error-detecting code, typi- which is lost by discarding erroneously received packets. cally a Cyclic Redundancy Check (CRC), to detect if the This shortcoming is addressed by hybrid ARQ with soft Scheduling, link adaptation and hybrid ARQ CHAPTER 4.5 combining. In hybrid ARQ with soft combining, the er- modulation scheme is unchanged between transmission roneously received packet is stored in a buffer memory and retransmission. and later combined with the retransmission to obtain With Incremental Redundancy (IR), each retrans- a single, combined packet which is more reliable than its mission does not have to be identical to the original constituents. Decoding of the error-correcting code transmission. Instead, multiple sets of coded bits are operates on the combined signal. If the decoding fails generated, each of them representing the same set of (typically a CRC code is used to detect this event), information bits. Whenever a retransmission is required, a retransmission is requested. the retransmission typically uses a different set of coded Retransmission in any hybrid ARQ sc