1. AT&T Bell Laboratories, Reading, Pennsylvania 19603‐0856; Institute of Electronics, National Chiao‐Tung University, Hsin‐Chu, Taiwan, 300, China; Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695‐7911
2. IBM T.J. Watson Research Center, Yorktown Heights, New York 10598
3. Institute of Electronics, National Chiao‐Tung University, Hsin‐Chu, Taiwan 300, China