基于FPGA的大频偏扩频信号快速捕获算法设计

Design of fast capture algorithm for large frequency offset spread spectrum signal based on FPGA

星载扩频应答机的数据比特与扩频码是异步的,由于传输系统噪声及多普勒频移的影响,会引起接收扩频码与发送扩频码相关峰值的衰减,导致捕获性能下降。传统的捕获技术通常存在算法复杂度高,捕获速度慢,难以适应上百千赫兹大频偏的要求等问题。本文提出了一种将扩频序列截成两段分别作相关运算的扩频序列搜索法,并结合信号平方和FFT环进行大频偏锁频的捕获方案,有效抑制了相关峰的衰减,提高了伪码捕获性能。MATLAB仿真及FPGA板级测试表明,本文所提出的扩频信号捕获方案能够对抗高达±300 kHz的多普勒频移,平均捕获时间约为95 ms。另外,该算法的FPGA实现与传统结构相比节省了约47%的LUT、43%的Register以及一半以上的DSP和BRAM资源,在资源受限的实时通信系统中具有很大的应用价值。

The data bits and spread spectrum codes of the spaceborne spread-spectrum transponder are asynchronous.Due to the influence of transmission system noise and Doppler frequency shift,it can cause attenuation of peak values related to receiving and transmitting spread spectrum codes,leading to a decrease in capture performance.Traditional capture techniques often have problems such as high algorithm complexity,slow capture speed,and difficulty adapting to the requirements of large frequency offsets of hundreds of kilohertz.This article proposes a spread spectrum sequence search method that truncates the spread spectrum sequence into two segments for correlation operations,and combines the signal squared sum FFT loop for a large frequency offset locking,effectively suppressing the attenuation of correlation peaks and improving pseudocode capture performance.MATLAB simulation and FPGA board level testing show that the proposed spread spectrum signal capture scheme can resist Doppler frequency shifts of up to±300 kHz,with an average capture time of about 95 ms.In addition,the FPGA implementation of this algorithm saves about 47%of LUT,43%of Register,and more than half of DSP and BRAM resources compared to traditional structures,making it of great application value in resource limited real-time communication systems.