which modulation scheme is followed by ieee 802.15.4 standard
IEEE 802.15.4 The 2006 revision improves the maximum data rates of the 868/915 MHz bands, bringing them up to support 100 and 250 kbit/s as well. Moreover, it goes on to define four physical layers depending on the modulation method used. Three of them preserve the DSSS approach: in the 868/915 MHz bands, using either binary or offset quadrature phase shift keying (the second of which is optional); in the 2450 MHz band, using the latter. An alternative, optional 868/915 MHz layer is defined using a combination of binary keying and amplitude shift keying (thus based on parallel, not sequential spread spectrum, PSSS). Dynamic switching between supported 868/915 MHz PHYs is possible.