1. Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695‐7911
2. The Royal Institute of Technology, Electrum, Solid State Electronics, S‐16428, Kista, Sweden
3. ELLEMTEL Telecommunications Systems Laboratories, S‐12525, Älvsjö, Sweden
4. MCNC, Research Triangle Park, North Carolina 27709
5. Department of Materials Science, North Carolina State University, Raleigh, North Carolina 27695