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Explain what sdr (signal-to-data-ratio) and SNR (signal-to-noise-ratio) mean and how they can be used for evaluation.解释什么是sdr（信号数据比）和SNR（信号噪声比）?他们是被如何用于评估的？Answer:sdr(signal-to-data ratio): a ratio of the number of signal elements to the number of data elements. If sdr > 1, a signal may contain self-synchronization information. SNR (Signal-to-noise ratio): a ratio of the average of signal power to the average of noise power. It is a measure to limit the theoretical bit rate.

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Compare the capability of high-frequency signals and low-frequency signals in straight-line propagation, reflection, refraction, and diffraction.比较高频信号和低频信号分别在直线传播、反射、折射和衍射的功能？Answer:Low-frequency signals can travel around the lower part of the atmosphere by ground propagation. They can travel with diffraction, refraction, reflection, or straight-line propagation.Higher-frequency signals can travel up to the ionosphere and reflect down to the earth by sky propagation. Most of them can also travel with refraction, diffraction, reflection, and straight-line propagation.Very high-frequency signals which are very unidirectional are transmitted from the source to the destination directly only by straight-line propagation.

1. Among unipolar NRZ-L, Polar NRZ-L, NRZ-I, and RZ, Manchester, differential Manchester, AMI, and MLT-3, which schemes have no issues on synchronization, baseline wandering, and DC components, respectively?Answer:With self-synchronization problem: unipolar NRZ-L, polar NRZ-L, polar NRZ-I, AMI, MLT-3With baseline problem: unipolar NRZ-L, polar NRZ-L, polar NRZ-IWith DC components problem: unipolar NRZ-L, polar NRZ-L, polar NRZ-IWithout self-synchronization problem: polar RZ, Manchester, differential ManchesterWithout baseline wandering problem: Manchester, differential ManchesterWithout or with very little DC components problem: polar RZ, Manchester, differential Manchester, AMI

8．使用单极性NRZ-L，极性NRZ-L，NRZ-I和RZ分别绘制以下数据流的波形。计算sdr值（信号-数据比率）和平均波特率。a. 101010101010b. 111111000000c. 111000111000d. 000000000000e. 111111111111

9．使用曼彻斯特和差分曼彻斯特方案绘制以下数据流的波形。计算sdr值（信号到数据的比例）和平均波特率。a. 101010101010b. 111111000000c. 111000111000d. 000000000000e. 111111111111