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What is the difference between fading and attenuation?3．衰落和衰减之间的差异是什么？Answer:Fading: Fading is the fluctuation in the strength of radio signals that varied in time, frequency, and phase in the transmission medium. It is often modeled as a random process. In wireless communications, a modulated waveform traveling over a certain medium could experience fading. Fading is a time-varying deviation of attenuation since it varies with time, geographical position, or radio frequency. There are two types of fading: multipath fading if caused by multipath propagation and shadow fading if shadowed by obstacles. A channel experiencing fading is called a fading channel.Attenuation : Attenuation is the gradual loss in intensity of flux such as radio waves or electrical signals. Attenuation affects the propagation of waves and signals. Amplification is a means of countering the attenuation impairment.

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What is the difference between noise and interference?噪声和干扰之间的差异是什么？Answer:Interference: Interference disrupts a signal that travels over a channel and usually adds unwanted signals to the desired signal. Several famous interference examples include co-channel interference (CCI), also known as crosstalk, inter-symbol interference (ISI), and intercarrier interference (ICI).Noise: Noise is a random fluctuation of an analog signal, such as electronic noise,thermal noise, induced noise, impulse noise, and quantization noise. Noise interferes with receivers in retrieving data from the received signal. To compensate for the impact of noise on the transmitted data, we may either raise the signal power or lower the transmission bit rate. In addition, modulation techniques can be used because they are more robust against noise.

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.

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