1. Beijing Jiaotong University,State Key Laboratory of Rail Traffic Control and Safety,Beijing,China,100044
2. State Key Laboratory of Mobile Network and Mobile Multimedia Technology,Shenzhen,China,518055
3. Instituto de Telecomunicações,Aveiro,Portugal,1049-001
4. School of Aerospace, Transport and Manufacturing, Cranfield University, Central Bedfordshire,Cranfield,UK,2717