s used by another base station/sector are made null and not used. The difference of this pilot pattern with the cyclically-shifted interlaced pilot pattern is that in the latter the overlapping locations of pilot sub-carriers used by other base stations/cells can be used for data transmission. To overcome the effects of pilot interference among the neighboring sectors or base stations, an interlaced pilot structure is utilized in the downlink by cyclically shifting the base pilot pattern such that the pilots of neighboring cells do not overlap. The base pilot patterns used for two downlink data streams are shown in Figure 9-37 with the sub-carrier index increasing from top to bottom and the OFDM symbol index increasing from left to right. The index of each pilot sub-carrier indicates the corresponding pilot stream (or the antenna port). The interlaced pilot patterns are used by different BSs for one and two data streams. Each BS selects one of the three pilot patterns. The index of the pilot pattern used by a particular BS with cell identifier IDcell is denoted by Pk = [IDcell/256]. For a single data stream, each BS has the choice of selecting one of the two possible locations for the pilot tone, depending whether the value of its cell identifier is even or odd (i.e., IDcellmod2). For three-stream MIMO transmissions, the first three of the four pilot streams will be used and the unused pilot tones are allocated for data transmission. Once again, the pilot pattern of the type-3 subframe is obtained by deleting the third OFDM symbol of the type-1 subframe. The pilot pattern of the type-2 subframe is obtained by adding the third OFDM symbol of the type-1 subframe to the end of the type-1 9.9 Pilot structure and channel estimation ---6 OFDM Symbols 6 OFDM Symbols --6 OFDM Symbols ---6 OFDM Symbols Antenna Port 1 Antenna Port 2 Antenna Port 1 Antenna Port 2 Single Data Stream Dual Data Stream FIGURE 9-34 Pilot structures for single and dual spatial streams [2] subframe. The pilot patterns for eight downlink data strea