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OFDM（Orthogonal Frequency Division Multiplexing，正交频分复用）是一种高效的数据传输技术，广泛应用于现代无线通信系统，如Wi-Fi、4G、5G和有线宽带网络等。该技术通过将高速数据流分割成多个较低速率的子数据流，在多个正交子载波上进行调制，从而实现高速、大容量的数据传输。

标题中的”4QAM”指的是4-Quadrature Amplitude Modulation，即四象限幅度调制，是数字调制的一种方式。在4QAM中，信号可以在两个幅度和两个相位上变化，总共提供四个可能的状态，因此每个符号可以携带2比特的信息。这种调制方式在OFDM系统中常用于平衡传输效率和信号质量。

“子载波”是OFDM技术的核心组成部分，每个子载波承载一部分数据。在OFDM系统中，信号被分配到一系列正交的子载波上，这些子载波在频域上是相互独立的，减少了多径传播和频率选择性衰落的影响。子载波的数量和带宽决定了系统的总数据传输速率。

“载波恢复”是OFDM系统中的重要环节，目的是在接收端重新同步信号的载波频率和相位，确保解调的正确性。在实际通信环境中，由于各种因素如信道衰落、多普勒效应等，载波可能会发生偏移，需要通过特定算法进行校正。

源码文件可能包含了实现OFDM系统的关键部分，例如：

1. **预处理**：包括插入循环前缀（CP）以克服符号间干扰（ISI），以及预编码来改善信号质量。

2. **IFFT/FFT**：在发送端，通过快速傅里叶变换（IFFT）将数据从时域转换为频域，而在接收端，使用快速傅里叶逆变换（FFT）进行反向转换。

3. **子载波映射**：根据调制方式（如4QAM）将数据分配到各个子载波上。

4. **载波同步**：实现载波频率和相位的校正，可能涉及诸如 Costas环、 Gardner 算法或基于导频的同步方法。

5. **解映射与解码**：从接收到的子载波信号中提取原始数据，并进行错误检测和纠正。

此源码可能涵盖了以上步骤的实现，对于理解OFDM系统的工作原理和开发相关应用非常有价值。开发者可以通过阅读和分析代码来学习如何在实际系统中应用这些概念。

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