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Resolving Training Symbol Redundancy and Timing Metric Plateau for OFDM System

Sarita Verma, Vikash Gupta

Abstract


A number of approaches to estimation timing and frequency offset in OFDM systems have been presented in the literature. Many of these operate in the time domain (before the FFT) and use the repeating pattern of the preamble or the cycle prefix, or both, to gain information about the symbol timing and frequency offset. The timing is determined by noticing that the correlation of the signal with a delayed version of itself will reach a peak when the repeated pattern is located. The frequency offset can be estimated by, for example, calculating the phase offset between one occurrence of a pattern and the next. Moose presented a simple method using the cycle prefix, as did Van de Beek et al. This approach is useful in systems in which a continuous stream of OFDM symbols are

Transmitted (e.g. DTB-T). The algorithm proposed by Schmidl and Cox uses the repetition in the preamble, which proves more robust compared to methods that use the cycle prefix when this is short. Other, more Timing and frequency s synchronization algorithm is derived for orthogonal frequency division multiplexing (OFDM) signals transmitting over multipath fading channels. Due to its high spectrum efficiency and good ability in coping with multi-path fading, (OFDM) technology has attracted considerable attention from researchers. However, OFDM system is very sensitive to symbol timing offset and carrier frequency offset, so accurate timing synchronization and frequency synchronization are necessary to OFDM system. This paper discusses on OFDM synchronization technology based on training symbol. OFDM synchronization algorithms based on training symbol are investigated, and two algorithms are proposed. An improved algorithm is proposed aimed at resolving training symbol redundancy and timing metric plateau in Schmidl algorithm. The training symbol in this algorithm is conjugated symmetrical and repeated. The timing metric has a sharp peak and eliminates the plateau. Both integral and fractional frequency offset estimation employ the same training symbol, and therefore the system overhead is saved and the complexity is degraded. The second algorithm proposed in this paper does not just improve symbol timing method or frequency offset estimation method. A new training symbol with special structure is designed, and a new joint symbol timing and frequency synchronization algorithm is presented based on this symbol. New frequency offset estimation algorithms are characteristic of large estimation range and high accuracy. At the same time, timing synchronization can produce as harp peak, through which we can find where the OFDM symbol starts. The simulation results show that the performance of the synchronization algorithms proposed here is better than that of conventional methods. Simulation results show that the use of virtual subcarriers has negligible performance loss

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