# Robust Symbol and Frequency Synchronization Method for Burst OFDM Systems in UAV Communication
###### Abstract  
This paper introduces a robust synchronization method for orthogonal frequency division multiplexing (OFDM) in multi-unmanned aerial vehicle (UAV) communication systems, focusing on minimizing overhead while achieving reliable synchronization. The proposed synchronization scheme enhances both frame efficiency and implementation simplicity. Initially, a high-efficiency frame structure is designed without a guard time interval, utilizing a preamble sequence to simultaneously achieve both symbol synchronization and automatic gain control (AGC) before demodulation. Subsequently, a novel 2-bit non-uniform quantization method for the Zadoff-Chu sequences is developed, enabling the correlation operations in the traditional symbol synchronization algorithm to be implemented via bitwise exclusive OR (XOR) and addition operations. The complexity of hardware implementation and the energy consumption for symbol synchronization can be reduced significantly. Furthermore, the impact of AGC on frequency synchronization performance is examined, and an improved frequency synchronization method based on AGC gain compensation is proposed. Finally, the performance of the proposed method is rigorously analyzed and compared with that of the traditional method through computer simulations, demonstrating the effectiveness and superiority of the proposed approach. AGC compensation; synchronization; burst OFDM 2024 14 July 2024 16  
significant inter-symbol and inter-carrier interference, adversely affecting bit error rate (BER) performance at the receiver. Therefore, accurate symbol and frequency synchronizations are essential for achieving reliable OFDM demodulation and optimal communication quality. When the OFDM is combined with frequency hopping technology [12; 13] or used for time-division multiple access (TDMA) networking [14], a single communication process is short and bursty. In such scenarios, achieving low-complexity, fast, and reliable OFDM synchronization can be particularly challenging. OFDM systems are significantly affected by multi-path propagation and the Doppler effect in UAV communication channels [15]. The received signal can vary over a large dynamic range because of path loss and small-scale fading. Consequently, an automatic gain control (AGC) circuit is generally required to adjust the received signal power before coherent demodulation. In the traditional burst OFDM frame design, an AGC guard interval is often introduced for power estimation and the adjustment of variable gain amplifier (VGA). Generally, the duration of the AGC guard interval should be longer than the setting time of the AGC, allowing the gain adjustment to be completed before the fast Fourier transform (FFT) demodulation. Otherwise, the subcarrier orthogonality would be compromised, leading to a degraded system performance [16; 17]. Undoubtedly, the introduction of the AGC guard interval increases the overall time overhead, thus reducing the effective data transmission time within each frame. This inefficiency becomes more pronounced in bursty communication scenarios typical of UAV operations, where maintaining high data throughput is crucial. Therefore, optimizing the frame structure to minimize time overhead while ensuring reliable AGC adjustment is essential for improving the overall efficiency and performance of UAV communication systems [18]. Synchronization methods based on the Zadoff-Chu (ZC) sequences are widely used in real-world systems because of their ideal correlation properties [19]. Symbol timing synchronization is achieved through correlation operation and correlation peak searches between the received signal and a locally generated ZC sequence [20]. Additionally, frequency synchronization is accomplished by deriving the phase difference between adjacent training symbols [21]. Both synchronization algorithms require a number of complex multiplication operations that are linearly proportional to the length of the ZC sequence.