1. School of Instrumentation and Optoelectronic Engineering Beihang University Beijing 100191 P. R. China
2. Department of Electronic Engineering The Chinese University of Hong Kong Hong Kong 999077 P. R. China
3. University of Reading Whiteknights Reading Berkshire RG6 6AH UK
4. College of Communication and Art Design University of Shanghai for Science and Technology Shanghai 200093 P. R. China
5. Department of Rehabilitation Medicine School of Clinical Medicine Beijing Tsinghua Changgung Hospital Tsinghua University Beijing 102218 P. R. China
6. Darwin College University of Cambridge Cambridge CB3 9EU UK