Systems and methods of expanding indication capabilities of existent PPDU preamble fields. An existing field “Doppler mode” in a preamble is redefined into a dual-use field for indicating channel variation for a short packet as well as for indicating presence of mid-amble(s) in a long packet. Particularly, for a long packet (longer than a predefined data transmission time or a longer than a predefined number of data symbols), the two possible values of the one-bit “Doppler mode” field indicate whether there is any mid-amble carrying training symbols in the payload. On the other hand, for a short packet, the two possible values in the same field indicate whether the channel for transmitting the instant packet is currently time-variant as determined by the transmitting device.