數據通信畢業論文外文文獻英文翻譯(編輯修改稿)

　

【文章內容簡介】 the Frequency ponents. If all the Fourier ponents are equivalent attenuation, then the signal reception while Receive termination at an amplitude up Attenuation, but the distortion did not happen. However, all the transmission channel and equipment for different frequency ponents of the degree of attenuation is differentSome frequency ponents almost no attenuation, and attenuation of some frequency ponents by a 6 number, that is to say, channel also has a certain amount of vibration Increase the frequency characteristics, resulting in output signal distortion. Usually are frequency of 0 Hz to fcwide channel at Chuan harmonic lost during the attenuation does not occur (or are a very small attenuation constant), whereas in the fc frequency harmonics at all above the transmission cross Decay process a lot, we put the signal in the transmission channel of the amplitude attenuation of a ponent to the original (that is, the output signal Reduce by half the power) when the frequency of the corresponding channel known as the cutoff frequency (cut off frequency). Cutoff frequency transmission medium reflects the inherent physical properties. Other cases, it is because people interested in Line filter is installed to limit the bandwidth used by each user. In some cases, because of the add channel Twopass filter, which corresponds to twochannel cutoff frequency f1 and f2, they were called up under the cutoff frequency and the cutoff frequency. This difference between the two cutoff frequency f2f1 is called the channel bandwidth. If the input signal bandwidth is less than the bandwidth of channel, the n the entire input signal Frequency ponents can be adopted by the Department of channels, which the letter Road to be the output of the output waveform will be true yet. However, if the input signal bandwidth greater than the channel bandwidth, the signal of a Frequency ponents can not be more on the channel, so that the signal output will be sent with the sending end of the signal is somewhat different, that is produced Distortion. In order to ensure the accuracy of data transmission, we must limit the signal bandwidth. 4 Data transfer rate Channel maximum data transfer rate Unit time to be able to transfer binary data transfer rate as the median. Improve data transfer rate means that the space occupied by each Reduce the time that the sequence of binary digital pulse will reduce the cycle time, of course, will also reduce the pulse width. The previous section we already know, even if the binary digital pulse signal through a limited bandwidth channel will also be the ideal generated wave Shape distortion, and when must the input signal bandwidth, the smaller channel bandwidth, output waveform 7 distortion will be greater. Another angle Degree that when a certain channel bandwidth, the greater the bandwidth of the input signal, the output signal the greater the distortion, so when the data transmission Rate to a certain degree (signal bandwidth increases to a certain extent), in the onchannel output signal from the receiver could not have been Distortion of the output signal sent to recover a number of sequences. That is to say, even for an ideal channel, the limited bandwidth limit System of channel data transfer rate. At early 1924, H. Nyquist (Nyquist) to recognize the basic limitations of this existence, and deduced that the noisefree Limited bandwidth channel maximum data transfer rate formula. In 1948, C. Shannon (Shannon) put into the work of Nyquist 1 Stepbystep expansion of the channel by the random noise interference. Here we do not add on to prove to those now seen as the result of a classic. Nyquist proved that any continuous signal f (t) through a noisefree bandwidth for channel B, its output signal as a Time bandwidth of B continuous signal g (t). If you want to output digital signal, it must be the rate of g (t) for interval Sample. 2B samples per second times faster than are meaningless, because the signal bandwidth B is higher than the highfrequency ponent other than a letter has been Road decay away. If g (t) by V of discrete levels, namely, the likely oute of each sample for the V level of a discrete one, The biggest channel data rate Rm ax as follows: Rmax = 2Blog 2 V (bit / s) For example, a 3000 Hz noise bandwidth of the channel should not transmit rate of more than 6,000 bits / second binary digital signal. In front of us considered only the ideal noisefree channel. There is noise in the channel, the situation will rapidly deteriorate. Channel Thermal noise with signal power and noise power ratio to measure the signal power and noise power as the signaltonoise ratio (S ignal to Noise Ratio). If we express the signal power S, and N express the noise power, while signal to noise ratio should be expressed as S / N. However, people Usually do not use the absolute value of signal to noise ratio, but the use of 10 lo g1 0S / N to indicate the units are decibels (d B). For the S / N equal 10 Channel, said its signal to noise ratio for the 1 0 d B。 the same token, if the channel S / N equal to one hundred, then the sign