Tuyển tập báo cáo các nghiên cứu khoa học quốc tế ngành hóa học dành cho các bạn yêu hóa học tham khảo đề tài: Research Article Secure Collision-Free Frequency Hopping for OFDMA-Based Wireless Networks | Hindawi Publishing Corporation EURASIP Journal on Advances in Signal Processing Volume 2009 Article ID 361063 11 pages doi 2009 361063 Research Article Secure Collision-Free Frequency Hopping for OFDMA-Based Wireless Networks Leonard Lightfoot Lei Zhang Jian Ren and Tongtong Li Department of Electrical Computer Engineering Michigan State University East Lansing MI 48824 USA Correspondence should be addressed to Tongtong Li tongli@ Received 16 February 2009 Accepted 2 July 2009 Recommended by K. Subbalakshmi This paper considers highly efficient antijamming system design using secure dynamic spectrum access control. First we propose a collision-free frequency hopping CFFH system based on the OFDMA framework and an innovative secure subcarrier assignment scheme. The CFFH system is designed to ensure that each user hops to a new set of subcarriers in a pseudorandom manner at the beginning of each hopping period and different users always transmit on nonoverlapping sets of subcarriers. The CFFH scheme can effectively mitigate the jamming interference including both random jamming and follower jamming. Moreover it has the same high spectral efficiency as that of the OFDM system and can relax the complex frequency synchronization problem suffered by conventional FH. Second we enhance the antijamming property of CFFH by incorporating the space-time coding STC scheme. The enhanced system is referred to as STC-CFFH. Our analysis indicates that the combination of space-time coding and CFFH is particularly powerful in eliminating channel interference and hostile jamming interference especially random jamming. Simulation examples are provided to illustrate the performance of the proposed schemes. The proposed scheme provides a promising solution for secure and efficient spectrum sharing among different users and services in cognitive networks. Copyright 2009 Leonard Lightfoot et al. This is an open access article distributed under the Creative Commons .