A Study Of Synchronization And Channel Estimation Algorithms In Cooperative OFDM Systems

To avoid the inter-antenna interference in multi-input multi-output (MIMO) systems, the distance between antennas should be rigidly restricted, which causes that MIMO technique cannot be directly applied to the networks with small-size terminals, e.g. cell mobile communication systems, wireless local area networks (WLAN), and wireless Ad hoc networks. By sharing the antennas of partners in the same network to construct a virtual antenna array, cooperative communication technique makes it possible for networks with small-size terminals to attain space multiplexing and diversity gain. Meanwhile, time offsets exist among received signals due to inconsistent time instant and paths of signals transmission, which may cause severe performance degradation. Since orthogonal frequency division multiplexing (OFDM) is robust to the delay which is less than the length of cyclic prefix (CP), the combination of cooperative communication and OFDM, i.e. cooperative OFDM (CO-OFDM) systems, could not only extend the channel capacity, increase the spectrum efficiency, but also be robust to multi-path delay and narrowband interference.Due to the multiple CFOs among destination and cooperative relays in CO-OFDM systems, the estimation of multiple CFOs as well as the cancellation of inter-carrier interference (ICI) become much complicated than conventional OFDM systems. To mitigate the ICI affection on the received signals, we have to solve the synchronization and channel estimation problems first. The main contributions of this dissertation are summarized as follows:Firstly, a CO-OFDM system model is introduced. Based on the system model, the inter-carrier interference (ICI) caused by CFO and the inter-symbol interference (ISI) caused by multi-path effect are analyzed theoretically and investigated by simulations.Secondly, by utilizing the repetitive structure of OFDM symbols, an improved exponentially weighted moving average (EWMA), i.e. IEWMA timing synchronization method is proposed. Simulation shows that the cooperative system becomes more robust to timing errors by IEWMA method.Thirdly, after the investigation of multiple CFOs estimation methods in OFDMA uplink systems, the principle of sub-carrier allocation scheme is used to attain CFO estimation with high accuracy in CO-OFDM systems. Moreover, based on the accurate frequency synchronization in broadcasting phase, all the frequencies of relays can be adjusted towards the source, then the computation complexity is greatly decreased with required BER performance by frequency asynchronous scheme in cooperative phase.Fourthly, the data-aided channel estimation methods adopted in MIMO-OFDM systems are proved to be also available to CO-OFDM systems; meanwhile, the phase tracing technique could be utilized to trace the phase rotation of received signals caused by multiple CFOs. Simulation results show that the phase tracing technique can be applied to CO-OFDM systems based on 1-2-1 and 1-3-1 models.Finally, the simulation platforms for CO-OFDM systems, with two different data detection schemes designed at the receivers, e.g. frequency asynchronous scheme and ordered-successive parallel interference cancellation (OSPIC) scheme, are constructed in this dissertation. The frequency asynchronous scheme, with lower computation complexity as well as acceptable BER performance, is more proper under practical circumstances, which is supported by simulations.