1 Dept. of Electron. Eng., Jinan University, Guangzhou 510632, CHN
2 Dept. of Phys., China University of Science and Technology, Hefei 230026, CHN
3 Guangzhou Telecom, Guangzhou 510600, CHN
4 Opitcom, San Jose, CA, 95112, USA
5 School of Inform. and Optoelectron. Eng., South China Normal University, Guangzhou 510631, CHN