# Robust Symbol and Frequency Synchronization Method for Burst OFDM Systems in UAV Communication
###### Abstract
Considering the energy limitations and processing ability constraints of UAVs, reducing the computational complexity of theses synchronization algorithms is crucial. [PERSON] reduced the computational complexity to a certain extent by designing a training sequence with symmetric characteristics and reducing the number of multiplication operations [22]. To achieve fast synchronization, preamble-based algorithms based on autocorrelation, cross-correlation, and their combination are proposed [23; 24; 25]. In [26], the local autocorrelation sequence was mapped into an elaborately simplified version, effectively halving the computational complexity. In [27], the hardware complexity was reduced by replacing the multipliers in a symmetric correlator with adders that compute the difference in magnitude between pairs of received samples. In [28], the elements of the training sequence were quantized to the nearest integer power of two, allowing the multiplication operation to be converted into a shift operation. In [29; 30], the authors proposed a synchronization method based on 1-bit quantization. This method converts the multiplication operations in the correlation process into 1-bit addition operations, significantly reducing implementation complexity. However, this approach disregards the amplitude information of the training sequence, leading to a certain loss in synchronization performance. Most existing studies on OFDM synchronization assume that AGC adjustment is completed before the preamble of the transmission frame. While this assumption works well in continuous communication modes, in burst communication modes, a longer AGC settling time can lead to significant reductions in frame efficiency. This work aims to address the aforementioned issues by developing a low-overhead frame structure, leveraging AGC control aided by symbol timing synchronization, and introducing robust synchronization methods for OFDM. These advancements collectively contribute to enhancing the frame efficiency and reliability of OFDM synchronization in UAV communication channels. The main contributions of this paper are summarized as follows. (1) A more realistic frame structure with high transmission efficiency is designed. A novel AGC control circuit is developed based on the proposed model. (2) By introducing a 2-bit non-uniform quantization approach, the preamble for symbol timing synchronization can be quantized into a format represented by one sign bit and one amplitude bit. This allows synchronization correlation operations to be implemented with bitwise XOR and addition operations, resulting in low complexity. (3) To mitigate the impact of AGC-induced amplitude distortion on frequency synchronization performance, a novel frequency synchronization method based on AGC gain compensation is developed. The rest of the paper is organized as follows. In Section 2, the system model and OFDM synchronization algorithms are introduced. In Section 3, a low-overhead frame for a burst OFDM system is designed. Based on the new frame, robust symbol and frequency synchronization algorithms are presented. In Section 4, the performances of the synchronization methods are analyzed. Finally, conclusions are drawn in Section 5.