Funding for this research was provided by:
National Science Foundation of China (61272213)
Cuiying Grant of China Telecom, Gansu Branch (lzudxcy- 2013-3)
Cuiying Grant of China Telecom, Gansu Branch (2013-3-1)
Cuiying Grant of China Telecom, Gansu Branch (44th)
National Natural Science Foundation of China (61370219)