either on PUCCH or multiplexed within uplink data transmission on PUSCH. Up to eight HARQ concurrent processes are supported in the downlink. The ACK/NACK transmission in FDD mode refers to the downlink packet that was received four subframes earlier. In TDD mode, the uplink ACK/NACK timing depends on the uplink/downlink configuration. For TDD, the use of a single ACK/NACK response for multiple PDSCH transmissions is possible; this is referred to as ACK/NACK bundling. A synchronous HARQ re-transmission protocol is supported in the uplink. The eNB can request re- transmissions of incorrectly received data packets on PUSCH. The ACK/NACK feedback in the downlink is sent on PHICH. After a PUSCH transmission, the UE will monitor the corresponding PHICH resource four subframes later in FDD mode. In TDD mode, the PHICH subframe to monitor is derived from the uplink/downlink configuration and from the PUSCH subframe number. The PHICH resource is determined from the lowest index physical resource block of the uplink resource allocation and the uplink demodulation reference symbol cyclic shift associated with the PUSCH transmission, both indicated in the PDCCH with DCI format 0 granting the PUSCH transmission. A PHICH group consists of multiple PHICHs that are mapped to the same set of resource elements and are separated through different orthogonal sequences. The UE derives the PHICH group number and a specific PHICH inside that group from the information on the lowest resource block number in the PUSCH allocation and the cyclic shift of the demodulation reference signal. The UE can derive the redundancy version to use on PUSCH from the uplink scheduling grant in DCI format 0. The 3GPP LTE supports up to eight HARQ concurrent processes in the uplink for FDD, while for TDD the number of HARQ processes depends on the uplink-downlink configuration. 9.15.9 Physical Random Access Channel The physical layer random access preamble, illustrated in Figure 9-79 consists of a cyclic prefix of length TCP and a sequence part of length