﻿ 一种基于PSS序列的OFDM系统低复杂高精度帧同步方法及其FPGA实现

# 一种基于PSS序列的OFDM系统低复杂高精度帧同步方法及其FPGA实现A Low Complex and High Precision Frame Synchronization Method for OFDM System Based on PSS Sequence and Its FPGA Implementation

Abstract: Wireless frame synchronization is one of the key technologies in OFDM system. Only through frame synchronization between the base station and the user and the starting point of the wireless frame is found, communication signal can be successfully processed. Synchronization precision directly affects the performance of signal processing in the system. At the same time, how to reduce the complexity of the algorithm and reduce the consumption of hardware resources is also a key problem that must be considered in the algorithm design. In view of the above problems, a low complex and high precision frame synchronization algorithm is proposed in this paper. On the basis of traditional wireless frame synchronization, the algorithm only adds a plural multiplier, two accumulators and related logic modules. It can implement low complex and high precision wireless frame synchronization based on PSS sequence. Meanwhile, this paper implements the algorithm in hardware using LabVIEW FPGA language and the algorithm is deployed in the actual MIMO-OFDM system, operating stably and effectively.

1. 引言

2. 低复杂高精度帧同步算法

2.1. LTE中同步序列PSS

PSS序列是由长度为63点的ZC [3] 序列(Zadoff-Chu)组成，本身为频域信号，具有自相关性极高互相关性极低的特点，通过IFFT (Inverse Fast Fourier Transformation)转换到时域，所得时域信号也具有同样的特点，适用于加入到无线帧的头部，可用于无线帧同步使用。

${a}_{q}=\mathrm{exp}\left[-j2\text{π}q\frac{n\left(n+1\right)/2+l*n}{{N}_{ZC}}\right]$ (1)

ZC序列有以下特性：

1) ZC序列具有恒定振幅，经过IFFT也是恒定振幅的。这限制了峰均值比，降低了对数据信号的干扰。

2) 任何长度的ZC序列具有极好的循环自相关性。

$Z{C}_{q}^{63}\left(n\right)=\mathrm{exp}\left[-j\frac{\text{π}qn\left(n+1\right)}{63}\right]$ (2)

Figure 1. Wireless frame format

Figure 2. Frequency domain signal of PSS

Figure 3. PSS sequence autocorrelation

2.2. 粗同步算法

2.3. 高精度帧同步算法

Figure 4. Block diagram of rough synchronization algorithm for OFDM system

Figure 5. The flow of high precision frame synchronization algorithm

3. 高精度无线帧同步的LabVIEW FPGA实现

LabVIEW FPGA是美国国家仪器公司推出的一款图形化界面FPGA开发语言 [5] [6] ，具有开发简单、可读性强等特点，被广泛应用于该公司的FPGA系列芯片的开发中，本论文采用该语言进行帧同步算法的实现。粗同步模块是高精度无线帧同步算法的必须步骤，首先简要介绍粗同步模块的实现 [5] [6] [7] 。

Figure 6. 2048 point correlator input data range

Figure 7. Low pass sampling, cross correlation and other processes

Figure 8. Low pass filter module

Figure 9. 128 point cross-correlation

$\begin{array}{l}{C}_{n}=\underset{i=1}{\overset{128}{\sum }}\left(r{e}_{local,i}+i\ast i{m}_{local,i}\right){\left(r{e}_{time,n+i}+i\ast i{m}_{time,n+i}\right)}^{\ast }\\ \text{}=\underset{i=1}{\overset{128}{\sum }}\left[\left(r{e}_{local,i}\ast r{e}_{time,n+i}+i{m}_{local,i}\ast i{m}_{time,n+i}\right)+i\left(-r{e}_{local,i}\ast i{m}_{time,n+i}+i{m}_{local,i}\ast r{e}_{time,n+i}\right)\right]\\ \text{}=\underset{i=1}{\overset{128}{\sum }}r{e}_{local,i}\ast r{e}_{time,n+i}-i\ast \underset{i=1}{\overset{128}{\sum }}r{e}_{local,i}\ast i{m}_{time,n+i}+i\ast \underset{i=1}{\overset{128}{\sum }}i{m}_{local,i}\ast r{e}_{time,n+i}+\underset{i=1}{\overset{128}{\sum }}i{m}_{local,i}\ast i{m}_{time,n+i}\end{array}$ (3)

Figure 10. The realization of 2048 cross-correlation

Figure 11. 50 times peak search module

Figure 12. Peak search and validity determination of synchronization results

Figure 13. The successful operation of this algorithm in the MIMO-OFDM system

4. 应用案例

5. 结论

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