An auto-detection scheme may be applied to a physical layer (PHY) preamble of a communications packet, such as an 802.11 packet, to identify which generation of a communication standard was used to generate the packet. A packet of a wireless transmission may be received by a wireless device. The packet may include a PHY preamble, including a first field, such as a legacy long training field (L-LTF), and a second, subsequent field, such as a non-legacy Signal field. The wireless device may determine that the first field is encoded using a Fast-Fourier Transform (FFT) of a first size, and that the second field is encoded using a FFT of a second, different size. This determining may identify a generation of the communication standard used to generate the packet. In response to the determining, the wireless device may decode the packet according to the identified generation of the communication standard.