【正文】 g expression set up: 0NEb = WNSTb/ = WNRS b// () sbN/0 data rate is one of the most monly used indicator of the digital munications to simplify the description, the book will remember the bit rate R is R. 畢業(yè)設(shè)計 (專業(yè)外文翻譯 ) 3 To underline the 0NEb , NS normalized bandwidth and bit rate form of equation () into ??????? RWNSNEb0 () Digital munication systems, the most important performance measure is one of the bit error rate BP and 0NEb curve (see Figure ), the 00 xNEb ? , of 0PPB? . Dimensionless ratio 0NEb is a standard digital munications system performance indicators that can be 0NEb as a measure of parative performance advantages and disadvantages of the two munication systems: the conditions for a given error probability, the required 0NEb is smaller, the higher accuracy of detection. Figure General shape of the PB versus Eb /No curve Why Eb /No is a Natural Figure of Merit Beginner digital munication, the reader may doubt the usefulness of the parameter 0NEb . In analog munication, NS is a very useful indicator of the molecular expect to maintain the transmission of power, the denominator represents the size of the noise. But why you want to use in digital munication with the different indicators (per bit energy to noise power spectral density ratio)? The explanation is given below . In section , we define the power signal average power is limited, while the infinite energy of the large signal, and the energy signal is defined as the average power is E b /N x0 P0 BP ??????? RWNSNEb0 For 00 xNEb ? , 0PPB? 畢業(yè)設(shè)計 (專業(yè)外文翻譯 ) 4 equal to zero while the energylimited signal. This classification is very useful to pare the analog and digital signals. The analog signal is classified as a power signal. What is the significance? Usually the duration of the simulation waveform of infinite length, do not need to partition or increase the time window. Infinite time domain signal waveform, its energy is infinite and therefore can not use the energy to describe the signal. For analog signals, the power (or energy transfer rate) is a more useful parameter. However, in the digital munication system using the length of time for the waveform of the symbol interval sT to send and receive code. The average power for each symbol (average) is equal to zero throughout the timeline, so the power can not be used to describe a digital signal. Therefore, the digital signal should be used within the time window to measure the signal measure. In other words, the symbol energy (power points) in sT is a more suitable description of the digital signal waveform parameters. Received energy can be well described by the digital signal, but it did not say why the NS is a good indicator of the digital system. The digital waveform is a medium for representing digital information, the information may contain a bit (binary) bit (quaternary), ..., 10bit (1024 hex). This discrete information structure is pletely different source of information of the analog munication system of unlimited quantified continuous wave. The digital system metrics must be more than the premium on the performance of both systems. Because the digital signal waveform may contain only 1 bit, 2 bits ... 10 bits, etc., so the NS of the digital signal can not be described. For example, if a given error probability, a binary digital signal required for the NS is 20. Note that the meaning of the figures contained in its equivalent of the digital signal waveform. Binary waveform contains a bit of information per bit required for the NS is 20. If the signal is 1024 hex, the required NS is still 20. The waveform contains 10 bits of information per bit required for the NS 2. Thus raises the question of why is it not more suitable parameters the bit level on the energy parameters 0NEb to describe this indicator? With the same NS , 0NEb is a dimensionless ratio, the following expression to prove this point: 畢業(yè)設(shè)計 (專業(yè)外文翻譯 ) 5 0NEb = Hzper Watt Joule = sWatt sWatt 2 INTERSYMBOL INTERGERENCE Figure a describes the filtering problem of a typical digital munication system. The entire system (transmitter, receiver and channel) is different types of filters (as well as inert circuit ponents such as inductors and capacitors). In the transmitter, the message symbols of the form of pulses or level debugging filtered into pulses that meet the bandwidth requirements. The baseband has system, the channel (cable) distribution reactance pulse signal distortion. Bandpass system (such as wireless systems) is usually fading channel (see Chapter 15), this type of channel is equivalent to do not expect the filter resulting in signal distortion. To pensate for the transmitter and channel is distortion caused by the acceptance filter equalization filter or accept / equalization filter (receiving / equalizing filter). Figure b shows a simple model of such systems, it will be all the filtering effect is equivalent to a system transfer function: ? ? ? ? ? ? ? ?fHfHfHfH rct? () ? ?fHt transmit filter ? ?fHc channel filter ? ?fHr acceptance / equalization filter. ??fH on behalf of the entire system transfer function, a bination of all of the filtering effect of the transmitter, channel and receiver link. In a binary PCM munication system (such as NRZL), the detector according to the received signal sampling and threshold decision. For example, in Figure a of the detector, if accepted signal is greater than zero, then the judgment sent bit is 1。畢業(yè)設(shè)計 (專業(yè)外文翻譯 ) 1 Baseba