Hi, 

If my understanding is correct, most of the 802.15.4 receiver implementations perform frame synchronization i.e. find the start of the frame using the preambles which are nothing but 8 zeros(each of which is further converted to 32 bit chip). 

Moreover this frame sync is performed once the complex data is already converted to bits i.e after OQPSK demodulation has happened. 

I was wondering why 802.15.4 does not do frame sync using the complex samples itself like WiFi do using STS ? I mean, it already has a pattern of 8 repeated zeros in the beginning of every frame. 

What I am missing here ? 

Regards

Sumit