In 802.11n, theHigh Throughput Long Training Field (HT-LTF)is utilized by the receiver to estimate the MIMO channel characteristics necessary for demodulating spatially multiplexed signals. The number of HT-LTF symbols corresponds to the number of spatial streams being transmitted. These fields are present in bothMixed ModeandGreenfield Modepreambles.
HT-LTF in Mixed Mode:The preamble includes legacy fields (L-STF, L-LTF, L-SIG) for backward compatibility, followed by HT-specific fields including HT-LTFs for MIMO channel estimation.
HT-LTF in Greenfield Mode:The preamble omits legacy fields and consists solely of HT-specific fields, including HT-LTFs, as there is no need for backward compatibility.
[Reference:CWDP-305 Study Guide, Chapter on ProtocolandSpectrum Analysis – 802.11n Preamble Structure., , ]
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