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The core idea of the auto-correlation packet detection block is to search for the large auto-correlation in the periodic STF. The STF is composed of 10 copies of a 16-sample sequence (10 STS). This periodicity defines the autocorrelation length. The Schmidl-Cox technique normalizes the raw auto-correlation value with the magnitude of the same samples, in effect measuring how "correlate-y" the samples are independent of the Rx power. This comparison is implemented as the correlation threshold in our designs. Our logic also includes a minimum magnitude threshold to reject the (many) false positives that can occur with very small signals in the time between actual transmissions. You could use a longer correlation window, but this would delay the detection decision which reduces the time AGC has to settle. We aim for a quick packet detection, then refine the timing using the LTF correlator (or reject the detection event if there is no subsequent LTF correlation event).