ms are shown in Figure 9-35 with the sub-carrier index increasing from top to bottom and the OFDM symbol index increasing from left to right. The demodulation pilots or reference signals in a downlink PRU for a given antenna port are pre- coded in the same way that data tones on the same stream in that PRU are precoded. In a distributed LRU, the data transmitted in a PRU on a given stream may be sent to several mobile stations, but in different tones using the same precoder. Two pilot streams are transmitted in the distributed LRUs inside outside the open-loop regioni iii regardless of what data is transmitted by the BS in all distributed LRUs. The precoder may be adaptive (user-specific) or non-adaptive (non-user-specific) depending on 'An open-loop region with k < min(N, Nr) MIMO streams is defined as a time-frequency resource using the k stream's pilot pattern and a given open-loop MIMO mode without rank adaptation. The open-loop region allows base stations to coordinate their open-loop MIMO transmissions in order to create a stable interference environment where the precoders and number of MIMO streams are not time-varying. The LRUs used for the open-loop region are identified in AAI_SCD MAC control message. These LRUs are aligned across cells. A limited set of open-loop MIMO modes are allowed for transmission in the open-loop region. CHAPTER 9 The IEEE 802. .16m Physical Layer (Part I) -6 OFDM Symbols -6 OFDM Symbols FIGURE 9-35 Pilot structure for 4 and 8 spatial streams [2] the downlink MIMO mode. Non-adaptive precoders are determined according to the MIMO mode, the number of data streams, the type of LRU, operation inside or outside the open-loop region, and the physical index of the sub-band or mini-band where the precoder is applied. In multi-user MIMO transmissions using a contiguous LRU, each pilot stream is dedicated to one MS. The MS uses its dedicated pilot stream for channel estimation within the allocated resources. Other pilot streams may be used for inter-stream interference estimation. The tot