Research On Detection And Capacity Of DSTBC System With Multiple Carrier Frequency Offsets

The next generation mobile communication systems are supposed to have better quality and coverage, be more power and bandwidth efficient, and be deployed in diverse environments. The key techniques in the area of current wireless communication include Orthogonal Frequency Division Multiplexing (OFDM), Multiple Input Multiple Output (MIMO), and Space-Time Block Coding (STBC). A recently proposed concept, Cooperative Diversity or Relay Diversity, has inspired researchers'great interests. It could take advantage of the antennas of several remotes in a certain neighborhood, rather than multiple antennas of a single base station, to form virtual MIMO and improve diversity gain.This paper introduced cooperative diversity into traditional STBC system and STBC-OFDM system, to set up new Distributed STBC and Distributed STBC-OFDM system, respectively. In the distributed system, due to mismatch among oscillators, there are multiple carrier frequency offsets (CFO) between relay nodes and the receiver. The receiver cannot compensate those offsets at the same time, thus multiple CFOs will always exist, which will destroy the orthogonality of STBC and introduce inter-carrier interference (ICI) to OFDM. We establish the mathematical models for DSTBC and DSTBC-OFDM systems, give the analysis of symbol detection, and proposed new equalization technique in frequency domain to combat performance degradation introduced by multiple CFOs. We also deducted the ergodic capacity expression and outage capacity expression for DSTBC-OFDM system.Both the theoretical analysis and simulation results show that, in the DSTBC system, no matter which detector is implemented, multiple CFOs will cause the decrease of BER performance. From the tradeoff view of computational complex and BER performance, generalized simplified ML detector (GSMLD) would be chosen as the best candidate. For DSTBC-OFDM system, analysis of BER performance is consistent with that of system capacity, i.e., in the case of normalized carrier frequency offsets (with respect to OFDM sub-carrier spacing) difference smaller than 0.2, DSTBC-OFDM system performs well. However when the difference of normalized frequency offsets is relatively large, i.e. around 0.4, large frequency offsets will destroy the orthogonality of STBC, and the DSTBC-OFDM system reaches the worst performance and lowest capacity, even poorer than single-in single-out OFDM system without space diversity. Therefore, in the real environment with large frequency offset, frequency synchronization mechanism and related protocol must be provided at the transmitter nodes to ensure that the difference of normalized frequency offsets is below about 0.2.