source unit are dedicated to each user and can be precoded or beam- formed similar to data sub-carriers of that logical resource unit. The uplink pilot structure is defined for up to four data streams. The pilot pattern may support variable pilot boosting factors. When pilots are boosted, each data sub-carrier has the same transmission power across all OFDM symbols in a resource block. Figure 9-38 shows the pilot structure for distributed logical resource units for single and dual data streams. Note that the pilot patterns for UL contiguous LRUs are the same as those in the downlink. For three data stream MIMO transmissions, the first three of the four pilot streams will be used and the unused pilot stream is allocated for data transmission. The pilot pattern of type-4 subframes (i.e., uplink subframes with 9 OFDM symbols that are used when supporting legacy 8.75 MHz bandwidth) is derived from the type-2 subframe pattern. The first seven symbols of the type-4 subframe pilot patterns are identical to the type-2 subframe patterns. The last two symbols of the type- 4 subframe pilot patterns are generated by appending the first two symbols of the type-2 subframe pilot patterns. 6 OFDM Symbols 6 OFDM Symbols Uplink Tile Pilot Patterns for Single and Dual Data Streams 6 OFDM Symbols -6 OFDM Symbols Uplink PUSC Tile Pilot Patterns for Single and Dual Data Streams FIGURE 9-38 Uplink Pilot Patterns for Basic Resource Units [2] CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) 9.10 MIMO MIDAMBLE The pilot structures that have been discussed thus far were all narrowband type, i.e., the reference tones only provide channel information for a fraction of the transmission bandwidth due to the fact that the pilot sub-carriers are contained in physical or logical resource blocks. Some closed-loop MIMO modes require channel information over the entire transmission bandwidth. This information can only be provided through non-precoded periodic wideband signals. The MIMO midamble is used for pre- coding matrix index selection in cl