Section: 6.9 Physical hybrid ARQ indicator channel
Content: The PHICH carries the hybrid-ARQ ACK/NACK. Multiple PHICHs mapped to the same set of resource elements constitute a PHICH group, where PHICHs within the same PHICH group are separated through different orthogonal sequences. A PHICH resource is identified by the index pair , where is the PHICH group number and is the orthogonal sequence index within the group. For frame structure type 1 and type 3, the number of PHICH groups is constant in all subframes and given by where is provided by higher layers. The index ranges from to . For frame structure type 2, the number of PHICH groups may vary between subframes and is given by where is given by the expression above and is given by Table 6.9-1 with the uplink-downlink configuration provided by the higher-layer parameter subframeAssignment. The index in a subframe with non-zero PHICH resources ranges from to . Table 6.9-1: The factor for frame structure type 2