I sent the iphone to apple service centre in Singapore and detected that it was a hardware issue.
I regret that their quality control over this device is very low.
However, their service is good, they find no solution to fix my iphone and replaced it with a new one
within minutes. So for all who experienced the same problem, just bring the device to apple and
they will replace it with a new one.
I hope they will be more careful on the launch of the next device and pay more attention to
better quality control.