a wide range of mobility and channel conditions. The time/frequency characteristics of a number of typical radio channel models, mobility classes, and the associated time-frequency sampling rate requirements are summarized in Table 9-5. 9.9.2 Downlink Pilot Structure The IEEE 802. .16m downlink pilot patterns have been designed based on the principles that were described in the previous section. The pilot patterns are specified within a physical resource unit. The downlink pilot patterns used for data transmission for one and two spatial streams (dedicated and common types) are shown in Figure 9-34 with the sub-carrier index increasing from top to bottom and the OFDM symbol index increasing from left to right. In Figure 9-34, the pilot tones are marked with Pi where index i denotes the antenna port number. Note that there is at least one pilot tone on every OFDM symbol. The pilot density for one and two data streams is 5.5% and 11.11%, respectively. It must be further noted that the pilot structures in Figure 9-34 are distinguished based on the number of spatial streams and the number of transmit antenna ports. When a single data stream is transmitted from two antennas, the pilot tones for each antenna port are located in non-overlapping positions in the physical resource unit and the tones corresponding to pilots of the other antenna port are used for data transmission; therefore, the pilot density per antenna port remains the same and equal to 5.5%. However, if dual data streams are trans- mitted from two antennas, the tones corresponding to pilots of the other antenna port are blank and not used for data transmission, therefore reducing the interference of data sub-carriers with pilot tones of the other antenna port. The null sub-carriers are marked with "X" in Figure 9-34. The pilot patterns for four and eight data streams are shown in Figure 9-35. The pilot pattern of the type-3 subframes (five OFDM symbols) is obtained by deleting the last OFDM symbol of the type-1 subframe (six OFDM symbols). The pilot pat