Research On Techniques For Non-Data-Aided Synchronizations And PAPR Reduction In OFDM Systems

Orthogonal Frequency-Division Multiplexing (OFDM) is now regarded as a main alternative to the conventional single carrier transmission techniques for wideband wireless communication systems, for its high spectrum efficiency, inherent equalization simplicity and immunity to narrowband interference. Hence OFDM has been employed widely. At the same time, there're some major drawbacks of OFDM, such as sensitivity to synchronization errors and high peak-to-average power ratio (PAPR).In this dissertation, we focus on Non-Data-Aided (NDA) system synchronizations and PAPR reduction techniques in OFDM systems, including the analysis of OFDM system symbol error ratio (SER) caused by synchronization offset, research on NDA system synchronizations and PAPR reduction techniques.Firstly, SER performance of OFDM systems with symbol timing offset (STO) is presented, and a close-form SER expression is derived for QPSK-OFDM systems with sampling frequency offset (SFO). We model the received signals of OFDM systems in additive white gaussian noise (AWGN), rayleigh flat fading (RFF), rayleigh frequency selective fading (RFSF) channels, respectively. Regarding the inter-symbol-interference (ISI) and inter-carrier-interference (ICI) caused by STO as Gaussian noise equivalently, an expression of SINR of received signals is derived. Based on the relationship between signal-to-interference-and-noise ratio (SINR) and SER, an SER expression of OFDM systems with STO is obtained, which is verified by simulation results. Then, the ICI of OFDM subcarrier brought by SFO is analyzed. The analysis results show that the ICI is not only have relation to the space of each subcarrier, but also have relation to the index of the analyzed subcarrier, so it can not be equivalent to the ICI brought by CFO. We obtained the characteristic function (CHF) and probability density functions (PDF) of the received signals in different channels, and get the exact probability of error formula of QPSK OFDM systems in such three kinds of channel.Secondly, NDA synchronization algorithms for OFDM systems are studied. Based on the time-delay cross correlation characteristic of OFDM signals, a NDA sampling frequency synchronization algorithm is proposed for OFDM systems with redundancy CP. We also analyze and discuss the estimation time, sensitivity, estimation variance, estimation scope and complexity of the algorithm. Combining with existing joint symbol timing and carrier frequency synchronization algorithm, we propose a joint sampling frequency, symbol timing and carrier frequency synchronization algorithm with low complexity. And we study the practical implementation of the algorithm. In the OFDM systems without redundancy CP, a cost function, which is based on the power difference between adjacent subcarriers of OFDM system with constant modulus signaling, is proposed on the assumption that the adjacent subcarriers have the same channel frequency responses. Combining with the existing carrier frequency synchronization algorithm, we propose a NDA joint symbol timing and carrier frequency synchronization algorithm, and present the implementation method of the algorithm. Note that there's a close-form solution to the frequency synchronization in the algorithm and that an iterative algorithm can be applied. So, the complexity of this algorithm is very low.Finally, PAPR reduction algorithms for OFDM system are studied. We analyze the PAPR suppression algorithms based on virtual subcarriers, and present an improved iterative scheme, in which the complexity can be controlled by setting the iterative parameter. Moreover, side information and extra complexity at the receiver are not needed for the algorithm, and only some extra transmission power may be consumed. In the systems with some anti-spy ability, the extra transmission power could increase the difficulty to the enemy's scout greatly. Form the fact that the PAPR is different with common information using different modulation type, a PAPR reduction algorithm based on adaptive modulation is proposed. And an OFDM system with the PAPR reduction algorithm is designed. The algorithm firstly makes a grouping to the subcarriers, and chooses the modulation type adaptively by the PAPR of OFDM signal so that the transmitted OFDM signal has smallest PAPR. It needn't transmit side information for the algorithm too, but need recognize the modulation type of each subcarrier group at receiver. Compared with the same kind PAPR reduction algorithms, the algorithm has the same performance of PAPR reduction with the same complexity and its BER performance is generally better than the OFDM systems which use other algorithms in multi-path time delay channel.