On The Synchronization For Cooperative Transmissions

In cooperative transmissions, multiple devices with single antenna are organized together to behave like one unit with multiple antennas, so that the same diversity gain can be obtained as that in multiple-antenna systems. In many distributed communication systems, such as sensor networks, the nodes are sparse and the number of nodes is very large, so central control or communication between nodes for synchronized cooperation will decrease the transmit efficiency. How to design a low complex synchronization method without central control and information exchange between nodes is a key problem in the realization of efficient cooperative transmissions. For such requirement, this dissertation studied the synchronization problem of multiple timing offsets (TOs), multiple carrier frequency offsets (CFOs) and multiple sampling frequency offsets (SFOs) in the OFDM based cooperative transmissions, and establish a cooperative transmission system on the SDR platform.First, this dissertation proposes a common pre-synchronization scheme for both TOs and CFOs, establish a common error model, and analyze the statistics properties of the error model. In this scheme, the offset (TO or CFO) in each branch is estimated individually, and the value for the pre-synchronization is a power-weighted combination of the multiple estimates. The theoretical analysis shows that, the expectation and variance of the sample variance of the pre-synch errors converges after several hops, and the pre-synch error can be controlled within the tolerance of the receiver under some conditions, including the number of relays, the SNR of single branch, and so on. Simulation and experiment results prove our conclusions.Second, making use of the structure of the STBC-OFDM based cooperative transmissions, this dissertation proposes a residual CFOs estimation method based on the pilot symbols. This method improves dramatically the performance of the CFO pre-synchronization in the STBC-OFDM based cooperative transmission systems. Third, making use of the symmetric property of the phase shifts in the frequency domain caused by SFO, this dissertation proposed a SFO self-cancellation scheme for the conventional OFDM systems; then a self-cancellation scheme for two SFOs in the Alamouti coded OFDM based cooperative transmissions is proposed by combining the SFO self-cancellation scheme for conventional OFDM systems and the interpolation/decimation method. Simulation results show that this self-cancellation scheme not only removes the effect of the two SFOs effectively, but also achieves the frequency diversity gain over frequency selective fading channels.Based on above research, we get new understanding for the synchronization problem in cooperative transmissions. These understanding also provides new prospective for the solutions of other synchronization problems in cooperative transmissions.In addition, the dissertation gives a complete introduction of the GNU Radio + USRP platform, and describes the details of the implementation of a STBC-OFDM based cooperative transmission system on the platform. Current problems, potential reasons and possible solutions are also discussed.