urce-element groups not assigned to PCFICH or PHICH is NREG. The CCEs available in the system are numbered from 0 to NCCE 1, where NCCE = floor(NREG/9). The PDCCH supports multiple formats as listed in Table 9-24. The PDCCH format should not be confused with the DCI format. A PDCCH consisting of n consecutive CCEs may only start on a CCE satisfying mod = 0, where i is the CCE number. Multiple PDCCHs can be transmitted in a subframe. The Physical HARQ Indicator Channel (PHICH) is used to acknowledge uplink data transmission. The resources used for the HARQ acknowledgment channel are configured on a semi-static basis and are defined independently of the grant channel (i.e., a set of resource elements are semi-statically allocated for CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) Control Region Data Region PCFICH Mini-CCE Mini-CCE Boundary RB Boundary User Mini-CCE and CCE Mini-CCE Boundary Reference Signals Slot 2n Slot 2n+1 FIGURE 9-75 Structure of 3GPP LTE downlink control channels [92] 9.15 3GPP LTE physical layer protocols Radio Frame = 10 ms PDSCH Reference Signals PDCCH S-SCH P-SCH Slot 1 Slot 0 Subframe = 1 ms FIGURE 9-76 Structure of P-SCH, S-SCH, and PBCH in time and frequency Table 9-23 PDCCH Candidates Monitored by a UE [109] Search Space Number of Size in Number of PDCCH DCI Format Aggregation candidates Level 0, 1, 1A, 1B, 2 UE-specific search space 0, 1A, 1C, 3/3A Common search space CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) Table 9-24 PDCCH Formats [107] Number of Resource- PDCCH Format Number of CCEs Element Groups Number of PDCCH Bits this purpose). Since only one information bit is to be transmitted, a combination of CDM/FDM multi- plexing among acknowledgments is used. Hybrid CDM/FDM allows for power control between acknowledgments for different users and provides good interference averaging. In addition, it can provide frequency diversity for different users. The ACK/NACK resource assignment is based on an implicit relationship based on the resource block assignment. With BPSK mo