**5. Acknowledgment**

This work was supported by National Key Basic Research Program of China (973 Program) under Grant No. 2012CB315802, National Natural Science Foundation of China under Grant No. 60802034, No. 61171102 and No. 61132001, Beijing Nova Program under Grant No. 2008B50 and New generation broadband wireless mobile communication network Key Projects for Science and Technology Development under Grant No. 2011ZX03002-002-01. We thank Huawei Technologies Co., Ltd. for cooperation in promotion of this work. Thanks also to Dr. Anna Fensel, a Senior Researcher at FTW – Telecommunications Research Center Vienna and STI Innsbruck, University of Innsbruck, Austria, for her valuable comments and suggestions.
