1. Introduction

2. Simulation of the Low-frequency Radio Spectra
2.1. Redshifted 21 cm Signal from the Epoch of Reionization
2.2. Foreground Emission
2.3. Instrumental Noise

3. Continuous Wavelet Transform
3.1. From Short-time Fourier Transform to Continuous Wavelet Transform
3.2. Boundary Effects

4. Subtraction of Foreground Signal
4.1. Wavelet Coefficients of Different Kinds of Sources
4.2. Filtering Out the Foreground Signal
4.3. Results

5. Discussion
5.1. Comparison with the Polynomial Fitting Based Method
5.2. Comparison with Wp Smoothing Method
5.3. Risk of Instrumental Calibration Uncertainty
5.4. Application to an Extremely Sharp Turnover Condition
5.5. Handling Higher and More Complex Noise
5.6. Testing Other Signal Extension Methods
5.7. What Kind of Conditions are Different Methods Suitable to?

6. Conclusion