The PHY layer of a Wi-Fi transmitter sends a frame (formally called PPDU) in a specific format as shown in the picture. The PSDU portion in the PPDU is basically the PHY layer payload. The PPDU starts with a 8us long Legacy Short Training Field (L-STF) which comprises a periodic signal with 0.8us periodicity. This is a followed by a 8us long Legacy Long Training Field (L-LTF). The 0.8us periodicity property of L-STF can be used by a receiver to perform Automatic Gain Control (AGC) and detect a Wi-Fi signal (perform self correlation and apply a detection threshold for example) and . By the end of L-STF, the receiver AGC would have settled on the right Rx gain and the L-LTF is used for frequency/timing offset estimation and channel estimation. The L-STF and L-LTF are known waveforms which don’t contain any data and are together referred as PHY preamble. The Legacy Signal (L-SIG) field that follows is a 4us OFDM symbol with data subcarriers modulated with BPSK and rate 1/2 convolutional code. The L-SIG contains information on the payload duration and also has a parity check to confirm integrity of the contents. So a Wi-Fi receiver can rely on L-SIG parity check to confirm that the received signal is a potential Wi-Fi frame.
If a receiver finds the signature of Wi-Fi (i.e., 0.8us periodicity during L-STF and L-SIG parity check passing), it will start the demodulation and decoding operations for the PSDU portion. Else, it will conclude it is non Wi-Fi signal and apply the Clear Channel Assessment (CCA) threshold to declare whether the medium is busy or idle. The CCA threshold is supposed to be set to Rx sensitivity + 20 dB. The Rx sensitivity parameter here is defined as the lowest Rx power level at which a receiver can decode a Wi-Fi frame at lowest data rate with a 10% packet error rate. The 802.11 standard requires that the Rx sensitivity of a Wi-Fi device be -82 dBm or lower.
Please note that while a Wi-Fi device is doing reception, it cannot transmit due to half duplex constraint. Any time spent on receiving is therefore time taken away from transmit. Therefore, it is necessary to ensure there are minimal false alarms for Wi-Fi PHY preamble detection. Too many false alarms for PHY preamble detection can result in high access times for transmission. It is therefore a challenge to detect weak Wi-Fi signals while at the same time be robust to interference. Note that any signal that resembles the 0.8us periodic pattern of L-STF can potentially trip a Wi-Fi receiver to think of it as Wi-Fi and waste airtime. For example, even on board noise coupling onto a Wi-Fi receiver antenna can cause problems.
Any Wi-Fi device should therefore continuously monitor its own false alarm rate for PHY preamble detection. and keep the false alarm rate under control.
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