sum frequency的用法和样例：
例句
- It realizes graphics protraction of time and frequency domain in every time period.
它实现了各个信号的时域和频域的图形的绘制。 - The simulation for QPSK shows that the algorithm can estimate the carrier phase and frequency offset accurately and reduce the system BER effectively.
结果表明，用该算法可以精确估计载波相位和频差，有效地降低系统的误码率。
