 and uplink is required to allow channel estimation, measurements of channel quality indicators such as the SINR, frequency offset esti- mation, etc. To optimize the system performance in different propagation environments and applications, IEEE 802.16m supports both common (to all or group of users) and dedicated (to a single user) pilot structures. The common pilot structures can be used by all users and the pilots can be precoded (using a non-adaptive fixed precoding) similar to the data sub-carriers within the same resource block. The dedicated pilots can be used with both localized and distributed allo- cation, and are associated with a specific resource allocation which can be used by the users that are assigned to the specific resource block and therefore are precoded or beam-formed (adaptive pre- coding) in the same way as the data sub-carriers of the resource block. The pilot structure is defined for up to eight streams in the downlink and up to four streams in the uplink, using a unified structure for common and dedicated pilots. The pilot density scales with the number of transmit streams; however, there is not necessarily equal pilot density per OFDM symbol on the downlink/ 9.9 Pilot structure and channel estimation PUSC 0 LRU34 PUSC 1 LRU33 PUSC 2 PUSC 3 LRU25 LRU10 LRU24 PUSC 31 PUSC 32 PUSC 33 LRU33 PUSC 34 LRU31 Legacy UL-PUSC Legacy UL-PUSC Concatenation of Contiguous UL-PUSC Subchannels to Form IEEE 802.16m IEEE 802.16m Distributed LRU Subchannels PUSC Tile LRU Index Renumbering 4 Sub-carries x 3 OFDM Symbols 4 Sub-carries x 6 OFDM Symbols UL-PUSC Tile New PUSC Tile 1 Subchannel = 6 UL-PUSC Tiles FIGURE 9-31 An illustration of an FDM-based PUSC subchannelization procedure [2] uplink subframes. Furthermore, within the same subframe, there is an equal number of pilots for each PRU of a data burst assigned to an MS. 9.9.1 Pilot Structure Design Criteria In this section we briefly discuss the theoretical criteria for efficient pilot pattern design for OFDM systems. Pilot-based channel estimation is 