shifting. To avoid cross-symbol power fluctuation when using pilot boosting, it is desirable to place the pilot sub-carriers regularly on every symbol or to employ power adjustment for data sub-carriers over a symbol. CHAPTER 9 The IEEE 802. .16m Physical Layer (Part I) Per antenna power balance: when using multiple transmit antennas, it is important to maintain balanced power distribution across antennas. For this purpose, if there are two transmit antennas, each OFDM symbol should contain the same number of pilot tones associated with different antenna ports. As an example and by taking the above additional criteria into consideration, the structure and link-level performance (i.e., bit error rate versus SNR curves) of several hypothetical common pilot patterns for two transmit antennas and two different multi-antenna transmission schemes are shown in Figure 9-33. In this example, the pilot overhead of type A and type B patterns is 11.11% (i.e., 5.5% per antenna port) and the transmit power per antenna is balanced. The type C pilot pattern has unbalanced power across antennas and symbols. The pilot tone powers have been boosted by 3 dB relative to data sub-carriers. The accuracy of channel estimation using common pilots affects the performance of downlink/ uplink data and control channel decoding. Figure 9-33 shows the link-level performance of spatial multiplexing with the example pilot patterns, where type B provides the best performance under low and high mobility conditions. The ideal channel estimation is used as a reference. Also shown in Figure 9-33 is the link-level performance of space-frequency block codes with the example pilot patterns. In this case, type A outperforms the other structures. It can be concluded that type A is the best among the three example patterns. Note that the example pilot patterns satisfy the sampling theorem constraint of Equation (9-19). This example underlines the criteria for the design of robust and high performance pilot structures for OFDMA cellular system operation in 