Revisiting the So-called Constructive Interference in Concurrent Transmission

Many works attributed the successful reception of concurrent transmission (CT) to the constructive interference. However, due to the inevitable carrier frequency offset (CFO) and the resulted beating effect, the claim of constructive interference is actually not valid. To clarify the reason behind the successful receptions under CT, we conduct extensive evaluations and identify the following findings. 1) We show that the IEEE 802.15.4 receivers survive the beating effect mainly because of the direct sequence spread spectrum (DSSS), while systems without the protection of DSSS are not applicable to CT. 2) We identify a counterintuitive phenomenon that the IEEE 802.15.4 receiver could survive only the beating results from large CFO, while performing poorly when CFO is small. 3) We demonstrate that, even if the receivers survive, CT links lead to little performance improvements compared to conventional signal transmission links from the SNR point of view.