1、Baseband Demodulation/DetectionIn the case of baseband signaling, the received waveforms are already in a pulse-like form. One might ask , why then ,is a demodulator needed to recover the pulse waveforms? The answer is that the arriving baseband pulses are not in the form of ideal pulse shapes, each
2、 one occupying its own symbol interval. The filtering to suffer from inter-symbol interference (ISI) and thus appear as an amorphous “smeared” signal, not quite ready for sampling and detection .The goal of the demodulator (receiving filter) is to receiver a baseband pulse with the best possible sig
3、nal-to-noise ration (SNR),free of any ISI. Equalization, covered in this chapter, is a technique used to help accomplish this goal. The equalization process is not required for every type of communication channel. However, since equalization embodies a sophisticated set of signal-processing techniqu
4、es, making it possible to compensate for channel-induced interference, it is an important area for many systems. The band-pass model of the fourth chapter deals with the detection process, and described in this chapter essentially the same as the baseband model. Before testing, you must first bandpa
5、ss conversion to the baseband signal. For signal detection in linear systems mathematical expressions do not accept the frequency move, and have the following equivalence theorem (equivalence theorem states): the current processing is first bandpass signal is converted to the base, and then using th
6、e heterodyne method band signal; the results with the first heterodyne signal is converted to the baseband signal, the corresponding linear processing on the baseband signal. Heterodyne (heterodying) refers to a technique called frequency converter (frequency conversion), or mixing of the frequency
7、a shame that software mixing is signal processing, he realized the spectrum of the signal move. A corollary of the equivalence theorem, and linear simulation process the Cheng Duiji band signal (generally) the result is the same as the result of bandpass role, This indicates that most of digital com
8、munication systems as a baseband system to describe and analyze.1 SIGNALS AND NOISE1.1 Error-performance Degradation in Communication SystemsThe purpose of the detector is based on as few errors as possible to restore the original signal stream from the received waveform distortion. There are two ma
9、in reasons for the error performance degradation. The first reason is that 3.3 will introduce a sender channel and receiver filter, non-ideal system transfer function will cause the symbol tail arising from inter-symbol interference (ISI).Another reason is that the electronic noise and other noise s
10、ources, such as atmospheric noise of the universe, the switching transient noise, inter-modulation noise and interference signals from other noise sources (these will be discussed in Chapter 5). Appropriate preventive measures can effectively reduce or even eliminate the number of receiver noise and
11、 interference. However, there is a noise can not be eliminated, is the electronic thermal motion of the conductor thermal noise, the noise is an additive noise, exist in the amplifier circuit. The use of quantum mechanics has been informed of the statistical properties of the thermal noise.1.2 The B
12、asic SNR Parameter for Digital Communication SystemsLearned analog communication readers are familiar with an indicator that the signal average power and noise average power ratio, referred to as the signal to noise ratio (or SNR) of In digital communication systems, usually using the normalized for
13、m of signal to noise ratio as the performance index. is the energy per bit is equal to the product of the signal energy with each bit duration, is the noise power spectral density equal to the ratio of the noise power and bandwidth ; reciprocal and because each bit duration and the bit rate can be u
14、sed instead of , therefore the following expression set up:= (1.1) data rate is one of the most commonly used indicator of the digital communications to simplify the description, the book will remember the bit rate R is R. To underline the , normalized bandwidth and bit rate form of equation (1.1) i
15、nto (1.2)Digital communication systems, the most important performance measure is one of the bit error rate and curve (see Figure 1.1), the , of . Dimensionless ratio is a standard digital communications system performance indicators that can be as a measure of comparative performance advantages and
16、 disadvantages of the two communication systems: the conditions for a given error probability, the required is smaller, the higher accuracy of detection.E/NxPFor ,Figure 1.1 General shape of the Pversus E/Ncurve1.3 Why E/Nis a Natural Figure of MeritBeginner digital communication, the reader may dou
17、bt the usefulness of the parameter . In analog communication, 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 communication with the different indicators (per bit energy to
18、noise power spectral density ratio)? The explanation is given below .In section 1.2.4, 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 equal to zero while the energy-limited signal. This clas
19、sification is very useful to compare 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 wave
20、form, 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 communication system using the length of time for the waveform of the symbol interval to send and recei
21、ve 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
22、) in 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 is a good indicator of the digital system. The digital waveform is a medium for representing digital information, the information may
23、contain a bit (binary) bit (quaternary), ., 10-bit (1024 hex). This discrete information structure is completely different source of information of the analog communication system of unlimited quantified continuous wave. The digital system metrics must be more than the premium on the performance of
24、both systems. Because the digital signal waveform may contain only 1 bit, 2 bits . 10 bits, etc., so the of the digital signal can not be described. For example, if a given error probability, a binary digital signal required for the is 20. Note that the meaning of the figures contained in its equiva
25、lent of the digital signal waveform. Binary waveform contains a bit of information per bit required for the is 20. If the signal is 1024 hex, the required is still 20. The waveform contains 10 bits of information per bit required for the 2. Thus raises the question of why is it not more suitable par
26、ameters - the bit level on the energy parameters to describe this indicator? With the same , is a dimensionless ratio, the following expression to prove this point: =2 INTERSYMBOL INTERGERENCEFigure a describes the filtering problem of a typical digital communication system. The entire system (trans
27、mitter, receiver and channel) is different types of filters (as well as inert circuit components 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
28、 channel (cable) distribution reactance pulse signal distortion. Band-pass 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 compensate for the transmitter and channel is dis
29、tortion 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: (2.1) transmit filter channel filter acceptance
30、 / equalization filter. on behalf of the entire system transfer function, a combination of all of the filtering effect of the transmitter, channel and receiver link. In a binary PCM communication system (such as NRZ-L), the detector according to the received signal sampling and threshold decision. F
31、or example, in Figure a of the detector, if accepted signal is greater than zero, then the judgment sent bit is 1; if less than zero, then the judgment bit is 0. Due to the filtering effect of the system to receive the pulse overlap, shown in Figure b. Trailing pulse appears to occupy the adjacent s
32、ymbol interval, thus interfering with the signal detection process, thereby resulting in the reduction of the error performance; this type of interference called ISI (inter-symbol interference, ISI). Even without the distortion caused by the noise, filtering, and channel will also result in intersym
33、bol interference. In some cases, is fixed, the problem becomes how to determineand, so that the output signal of to obtain the minimum ISI.The Nyquist study the receiving end does not produce the ISI received pulse shape. He proved: make the code rate code / s signal is not present the ISI, the theo
34、retically required minimum system bandwidth /2Hz. Minimum conditions set up by the system bandwidth, the system transfer function is the rectangular function as shown in Figure a. Baseband system, The unilateral bandwidth 1/2T rectangular function (ideal Nyquist filter), the systems impulse response
35、 the inverse Fourier transform ( called the ideal Nyquist pulse) = sinc(t / T), sinc(t / T), multiple lobe, including a main lobe and side lobe, side lobe, also known as the main valve before and after the trailing pulse to both sides of the infinite extension. Nyquist proved that if the received se
36、quence of each pulse is sinc(t / T) shape, the pulse sequence free from the effects of intersymbol interference is detected. Figure b illustrates the reasons to avoid intersymbol interference. There are two adjacent pulses and , has a long trailing pulse, but the sampling points in t = T-time = 0, S
37、imilarly, in the pulse (k=1，2，. ), the sampling of h (t) sidelobe values are zero. Therefore, the sampling time accurate and not exist inter-symbol interference. Baseband system ISI detect the symbol rate 1 / T pulse (symbol) the required bandwidth of 1/2T; In other words, the bandwidth /2Hz. The sy
38、stem in ensuring no inter crosstalk conditions can support the maximum transmission rate of code / s (Nyquist limit). Thus, the ideal Nyquist filter system (to ensure no inter crosstalk) every Hertz the maximum possible transmission rate (called the code rate compression) for 2 symbol / s / Hz. Idea
39、l Nyquist filter transfer function of a rectangular shape, the corresponding impulse response for an infinitely long, it is clear that the filter can not be achieved, can only approximate to achieve.2.1 Pulse Shaping to Reduce ISIMore narrow spectrum of the signal allows the data rate higher, while
40、the number of users receiving services and more. Communications service provider, which has great significance, because the higher the income the more you can use the bandwidth utilization. Most of the communication system (Chapter 12, except for Spread Spectrum Systems) goal is to minimize the requ
41、ired bandwidth. Nyquist reduce system bandwidth limit. If the system bandwidth is what will happen? Pulses in time domain will be extended, the resulting code will reduce the crosstalk between the error performance of the system. Therefore a reasonable target, the compressed data pulse so that it ha
42、s Nyquist minimum bandwidth slightly larger bandwidth. This can be a Nyquist filter pulse formation. If the band edge of the filter is relatively steep, close to the rectangle in Figure 3.16b, the signal spectrum of the narrowest. However, the duration of the impulse response of this filter is close
43、 to infinity (see Figure b), the entire sequence of pulse overlap. Time domain width of the impulse response corresponding to the characteristics in the frequency domain is the amplitude of each near the main lobe, side lobe. These sidelobe is undesirable, because from Figure b sampling only in the
44、correct sampling time does not exist in ISI; large sidelobe level, a small sampling of the timing deviation will lead to inter-symbol crosstalk. So, while the narrow-band spectrum signals can provide the best bandwidth utilization, but its timing errors caused by inter-symbol interference is very se
45、nsitive.2.2 Two Types of Error-Performance DegradationReduce the error performance of digital communication systems, there are two cases. The first, due to increase in the energy of the received signal energy to reduce or noise (interference signal) the signal to noise ratio reduction, making the pe
46、rformance degradation; the second is attributed to the signal distortion, such as by the inter-symbol crosstalk caused by distortion. The following discuss the difference of these two cases.Assume that the need to design the comme that the constmunication system, Wu-bit rate (BER) of curve in Figure
47、 a practice. Assuructed system, test system performance can be found in Wu bit rate of curve is not a theoretical curve, but the dotted line in Figure Because the signal is less bad and noise (interference) to improve the level, causing the signal to noise ratio of is less bad. Assume that expectati
48、ons of the bit error rate of 10, then in theory, the 10dB. The bit error rate of the same case, due to a decline in system performance required for rose to 12dB (known by the dashed lines). If you can not solve less bad, need to achieve the same bit error rate of the number of ? The answer is 2dB. T
49、his is a very serious problem, especially in the system is a power limit, multi-2dB signal to noise ratio is very difficult. However, the signal distortion caused by performance degradation compared to the reduction of signal to noise ratio is not too terrible.Still assume that the system does not meet expectations o
陕公网安备 61072602000132号 违法和不良信息举报：0916-4228922