# Timing and Frequency Synchronization Using CAZAC Sequences for OFDM Systems
## 2 Related Works  
In recent decades, related signal processing techniques have been extensively researched, and many pioneering results have been obtained. In 1997, [PERSON] and [PERSON] [8] proposed a method of timing and frequency synchronization by using a training sequence of two symbols; however, the timing metric function curve of the estimator had a platform, which made the timing synchronization estimation ambiguous. In 2000, [PERSON] et al. [9] proposed a new structure of the training sequences, involving a sharp synchronous autocorrelation peak. This eliminated the plateau effect of [PERSON]'s method. However, [PERSON]'s method still suffered from the problem of time ambiguity due to the existence of many sidelobe peaks on both sides of the main synchronous correlation peak of the timing metric function curve. In 2003, [PERSON] et al. [10] proposed a new preamble structure and correlation method, which obtained an impulse-like shaped timing metric function curve and, thus, further improved the timing accuracy. However, under the conditions of a low signal-to-noise ratio (SNR), [PERSON]'s method still had two large sidelobes, which resulted in an unsatisfactory timing performance and a large estimation error. In 2008, [PERSON] et al. [11] designed a new training symbol and proposed a novel time and frequency synchronization scheme, in which the timing metric function curve had a distinct peak and which also eliminated the sidelobes. In 2016, [PERSON] et al. [12] surveyed the literature from 2010 to 2014 and presented a comprehensive literature review and classification of the latest research progress in the timing and frequency synchronization techniques that had been proposedfor different communication systems. They focused on the system model conceptions for synchronization, the synchronization challenges, and the most advanced synchronization methods and their limitations. In 2018, [PERSON] et al. [13] proposed a novel preamble-assisted approach for the joint estimation of timing, carrier frequency offset, and channels in OFDM systems that operate under high mobility conditions. In addition, an efficient algorithm was proposed to reduce the computational complexity of joint estimation. In 2019, [PERSON] et al. [14] considered a consensus problem of high-order multiagent systems with antagonistic interactions and communication noises. In the same year, [PERSON] et al. [15] proposed a new timing synchronization scheme for OFDM communication systems, which exploited the phase shifts caused by timing errors and the power differences among the subcarriers. In the same year, [PERSON] et al. [16] proposed a new timing estimation algorithm by designing a new structure of training symbols. The simulation results showed that the algorithm could obtain accurate STO estimations due to its timing metric, which had a sharp and clear peak. In 2020, [PERSON] et al. [17] presented a robust timing synchronization method based on a fast timing search window and a dual threshold decision. The fast timing search window effectively reduced the probability of false alarms, and the dual threshold judgment reduced the probability of acquisition loss. Furthermore, simulations were performed to verify the effectiveness of the proposed strategy. In 2021, [PERSON] et al. [18] proposed a novel 5G-based centralized switch fault current limiter framework as well as a method to allocate such flexible fault current limiters optimally. In the same year, [PERSON] et al. [19] proposed a simple method for estimating STO and fractional CFO in cyclic prefix (CP) OFDM-based systems. A CP-based correlation function was proposed in the pre-fast Fourier transform (FFT) synchronization to avoid the effects of ISI, which are caused by multipath fading channels, and the effects of ICI, which are caused by the performance differences in local oscillators between the transmitter and the receiver. In 2022, [PERSON] et al.