Qualcomm interview questions

Submit my resume online, and after one week get a phone interview about one hour. Most of the phone interview are signal processing related basic questions such as how to detect the voice pitch, how to do adaptive noise reduction, filter designs, etc. After two weeks, get an invitation for on-site interview in San Diego. I met total 7 people including the recruiter at the first. One hour for each person. The rest of them are engineers including the manager in the group. Basic signal processing questions are asked and I feel I am doing well. One of them asked some coding problems and asked me to write the code on the blackboard. The problems are very basic such as reversing a linked list, and matrix operation, etc. However, I think I am not doing well in the coding questions, which is probably the main reason of the rejection.