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Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

Multi-Carrier and Spread Spectrum Systems: From OFDM and MC ...

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Wireless Local Area Networks 285frequency8.125 MHz0−8.125 MHz800 ns 4 µstimedatapilotsFigure 5-51 <strong>OFDM</strong> frame of IEEE 802.11a5.5.3.1 Frame StructureThe <strong>OFDM</strong> frame structure specified by IEEE 802.11a is shown in Figure 5-51. Theframe starts with up to 10 short pilot symbols, depending on the frame type. These pilotsymbols are used for coarse frequency synchronization, frame detection, <strong>and</strong> automaticgain control (AGC). The following two <strong>OFDM</strong> symbols contain pilots used for finefrequency synchronization <strong>and</strong> channel estimation. The <strong>OFDM</strong> frame has four pilot subcarriers,which are the sub-carriers −21, −7, 7, <strong>and</strong> 21. These pilot sub-carriers are usedfor compensation of frequency offsets. The sub-carrier 0 is not used to avoid problemswith DC offsets.5.5.3.2 FEC Coding <strong>and</strong> ModulationThe IEEE 802.11a st<strong>and</strong>ard supports the modulation schemes BPSK, QPSK, 16-QAM,<strong>and</strong> 64-QAM in combination with punctured convolutional codes (CC) with rates in the

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