Qualcomm DSP Firmware Engineer interview questions

I was referred by an employee and got email from the HR to set up the phone interview. After about 45 min I got the on site interview recommendation. During the phone interview I was asked a lot questions about what I was doing and and some theory algorithm associated with them. For example: How to do the channel estimation? How to implement the power control algorithm. all the technique related questions are based on the interviewer's working experience. I don't have on site interview scheduled yet. But my experience is you have to really know what your were doing and make the interviewer believes that you really did the work as you claimed.