Research On Preamble-aided OFDM Synchronization Algorithm

Orthogonal Frequency Division Multiplexing (OFDM) is one of the most potentialcommunication technologies due to its high utilization of frequency spectrum and robustnessagainst frequency-selectivity in multipath channel as well as the easiness of channelequalization. Since synchronization error would severely influence the system performance,synchronization is a key technology of OFDM. Only when the synchronization problem hasbeen solved properly can the advantage of OFDM technology be achieved.This article studies the preamble-aided OFDM synchronization algorithm, and the maincontents and results are listed as follows:1. A lot of preamble-aided OFDM synchronization algorithms have been analyzed indetail. According to their characteristics, these algorithms are divided into two categories: thetiming synchronization algorithm based on the Received Preamble Only (RPO) and that basedon the Combination of Received Preamble and Local Preamble (CRPLP).2. Four traditional timing algorithms have been improved. SC timing method has anuncertainty plateau which causes a high timing offset. This article proposes a new method toreduce or eliminate the plateau, which significantly improves the performance without morecomputation complexity. In Min1algorithm, timing offset does not get smaller but larger asthe number of sub-carriers and guard interval increase. This thesis proposes a solution toovercome Min1algorithm’s shortage, in which an adjustable sliding window is introduced. InMin2algorithm, timing performance is influenced by the ratio of the length of cyclic prefix tothe number of sub-carriers. The cause of Min2algorithm’s shortage is analyzed, and animproved method is proposed in this paper. Ren algorithm has an impulse like timingenvelope, and shows good stability under large frequency offset conditions. This article pointsout that good performance in Ren’s method is attributed to the combined use ofself-correlation and cross-correlation operations. In this paper, Ren’s idea is applied to a casewhere multi-segment repeated preamble structure is used, and good performance is achieved.3. To avoid sideband in timing envelope, a general method is proposed. The blunt timingenvelope is caused by the redundancy introduced by cyclic prefix or generated by timingmetric. The sideband introduced by the cyclic prefix can be eliminated through replacing thecyclic prefix with other type of guard interval, while the other case can be solved by usingscrambling vector and decoding vector.4. A novel timing synchronization method is proposed. This method is not sensitive tocarrier frequency offset, and shows excellent performance even in low signal-to-noise ratio situation. This paper generalizes the ideal used in HM algorithm to more general conditions.The new method can be applied to various standardized system for it does not depend on thestructure of the preamble.5. A new method for frequency estimation is proposed. SC algorithm exploits correlationof two repetition segments of the preamble to estimate fractional frequency offset. In case ofmulti-segment repeated preamble, SC algorithm simply treated it as a special case oftwo-segment repeated preamble. Obviously, the algorithm does not take full advantage of theinformation provided by multiple times repeated structure. To fully exploit the repetitivepreamble, a new method is proposed in this paper, and the performance is improvedremarkably.6. Based on aforementioned improved algorithms and classic algorithms, this paperproposes a new stable and efficient synchronization scheme, in which advantages of variousalgorithms are exploited fully, and overall performance is noticeably improved.