 achieves a better performance than ordinary ARQ techniques. In addition to the error-detection information added to the data, forward error correction (FEC) bits are appended before transmission. Whenever an error is detected, hybrid ARQ is requested to perform a retransmission. Typically, two types of HARQ retransmission strategies are implemented. In the first one, known as chase combining or type I HARQ, each retransmission is identical to the original transmission. Once a transport block is received, its integ- rity is checked by calculating its CRC and comparing it with the CRC sequence. A decoding error raises a retransmission request that is answered by the transmitter by sending again the same transport block. An incorrectly received transport block is stored at the receiver in order to be combined with the retransmitted transport block to increase the probability of successful decoding (see Figure 3.15). Different criteria, such as the link quality associated with each transport block, may be applied to decide how the transport blocks are combined. © 2009 by Taylor & Francis Group, LLC Long Term Evolution: 3GPP LTE Radio and Cellular Technology Original TB, CRC error Retransmitted TB, CRC error Combined TB (HARQ I) CRC OK Error-free bits Erroneous bits Figure 3.15 Example of chase combining HARQ. In the second one, known as incremental redundancy or type II HARQ, a coded transport block (i.e., a code word) is sent to the receiver. This code word is com- pounded by most of the systematic bits and a few parity bits for error correction. If an error occurs in the decoding process, a retransmission request is sent to the transmitter. Additional parity bits are transmitted and combined upon arrival with the previous received version of the coded transport block, resulting in a lower coding rate. The process is repeated until a successful decoding is performed or until the transport block is discarded when the retransmission limit is reached (see Figure 3.16). Incremental redundancy has the potential of achie