1. State Key Laboratory of Rail Traffic Control and Safety, Beijing Jiaotong University, Beijing, China
2. Wireless Communications and Networks Department, Fraunhofer Heinrich Hertz Institute, Berlin, Germany
3. Mobile Application Research Department, Electronics and Telecommunications Research Institute, Daejeon, South Korea
4. Future Mobile Communication Research Division, Electronics and Telecommunications Research Institute, Daejeon, South Korea
5. School of Electronic Engineering and Computer Science, Queen Mary University of London, London, U.K.