preamble --> Purpose:
1. Signal detection
2. Time synchronization
3. Frequency offset estimation
4. Automatic gain control
5. Channel estimation
Signal detection, that is, to detect the presence or absence of a signal. Method: autocorrelation. Because the autocorrelation of noise is still noise, but the autocorrelation of the signal is different.
Time synchronization, that is, determine the start and end time of each symbol. Method: The cross-correlation method can be used to cross-correlate the local signal and the received signal.
Frequency offset estimation.
Automatic gain control.
Channel estimation. In fact, this is channel equalization. On different channels, the channel response is different, and it is necessary to equalize them.
preamble -->Connotation:
1. The length of the symbol. The longer the symbol, the narrower the bandwidth and the more detailed channel cutting, which can be used for frequency and channel estimation. The symbol is short, the channel is cut short, and signal detection can be performed.
2. Modulation method. A simple modulation method can obviously reduce the complexity of implementation.
3. The number of symbols. Need enough symbols to realize time synchronization. Usually about 10. It is also used to estimate the carrier frequency and requires 20 symbols.
4.structure. -->Impact: Time synchronization.