OFDM基础中英文对照外文翻译文献.docx

1、OFDM基础中英文对照外文翻译文献中英文对照外文翻译文献(文档含英文原文和中文翻译)外文:OFDM BasicsINTRODUCTION The basic principle of OFDM is to split a high-rate data stream into a number of lowerrate streams that are transmitted simultaneously over a number of subcarriers. Because the symbol duration increases for the lower rate parallel

2、subcarriers, the relative amount of dispersion in time caused by multipath delay spread is decreased. Inter symbol interference is eliminated almost completely by introducing a guard time in every OFDM symbol. In the guard time, the OFDM symbol is cyclically extended to avoid inter carrier interfere

3、nce In OFDM system design, a number of parameters are up for consideration, such as the number of subcarriers, guard time, symbol duration, subcarrier spacing,modulation type per subcarrier, and the type of forward error correction coding. The choice of parameters is influenced by system requirement

4、s such as available bandwidth,required bit rate, tolerable delay spread, and Doppler values. Some requirements are conflicting. For instance, to get a good delay spread tolerance, a large number of subcarriers with a small subcarrier spacing is desirable, but the opposite is true for a good toleranc

5、e against Doppler spread and phase noiseGENERATION OF SUBCARRIERS USING THE IFFT An OFDM signal consists of a sum of subcarriers that are modulated by using phase shift keying PSK or quadrature amplitude modulation QAM.If di are the complex QAM symbols, N is the number of subcarriers, T is the symbo

6、l duration, and f is the carrier frequency, then one OFDM symbol starting at t t, can be written as2.1 In the literature, often the equivalent complex baseband notation is used, which is given by 2.2. In this representation, the real and imaginary parts correspond to the in-phase and quadrature part

7、s of the OFDM signal, which have to be multiplied by a cosine and sine of the desired carrier frequency to produce the final OFDM signal.Figure 2.1 shows the operation of the OFDM modulator in a block diagram. 2.2 Figure 2.1 OFDM modulator As an example,Figure2.2 shows four subcarriers from one OFDM

8、 signal. In this example, all subcarriers have the same phase and amplitude, but in practice the amplitudes and phases may be modulated differently for each subcarrier. Note that each subcarrier has exactly an integer number of cycles in the interval T, and the number of cycles between adjacent subc

9、arriers differs by exactly one. This property accounts forthe orthogonality between the subcarriers. For instance, if the jth subcarrier from 2.2 is demodulated by down converting the signal with a frequency of j/T and then integrating the signal over T seconds, the result is as written in 2.3. By l

10、ooking at the intermediate result, it can be seen that a complex carrier is integrated over T seconds.For the demodulated subcarrier j, this integration gives the desired output multiplied by a constant factor T, which is the QAM value for that particular subcarrier. For all other subcarriers, the i

11、ntegration is zero, because the frequency difference produces an integer number of cycles within the integration interval T,such that the integration result is always zero. 2.3 The orthogonality of the different OFDM subcarriers can also be demonstrated in another way. According to 2.1, each OFDM sy

12、mbol contains subcarriers that are nonzero over a T-second interval. Hence, the spectrum of a single symbol is a convolution of a group of Dirac pulses located at the subcarrier frequencies with the spectrum of a square pulse that is one for a T-second period and zero otherwise. The amplitude spectr

13、um of the square pulse is equal to sincnJT, which has zeros for all frequencies f that are an integer multiple of 1IT. This effect is shown in Figure 2.2,which shows the overlapping sinc spectra of individual subcarriers. At the imum of each subcarrier spectrum, all other subcarrier spectra are zero

14、. Because an OFDM receiver essentially calculates the spectrum values at those points that correspond to the ima of individual subcarriers, it can demodulate each subcarrier free from any interference from the other subcarriers. Basically, Figure 2.3 shows that the OFDM spectrum fulfills Nyquists cr

15、iterium for an intersymbol interference free pulse shape.Notice that the pulse shape is present in the frequency domain and not in the time domain, for which the Nyquist criterium usually is applied. Therefore, instead of intersymbol interference ISI, it is intercarrier interference ICI that is avoi

16、ded by havingthe imum of one subcarrier spectrum correspond to zero crossings of all the others. Figure 2.2 Example of four subcarriers within one OFDM symbol The complex baseband OFDM signal as defined by 2.2 is in fact nothing more than the inverse Fourier transform of N, QAM input symbols. The ti

17、me discrete equivalent is the inverse discrete Fourier transform IDFT, which is given by 2.4,where the time t is replaced by a sample number n. In practice, this transform can be implemented very efficiently by the inverse fast Fourier transform IFFT. An N point IDFT requires a total of N complex mu

18、ltiplications-which are actually only phase rotations. Of course, there are also additions necessary to do an IDFT, but since the hardware complexity of an adder is significantly lower than that of a multiplier or phase rotator, only the multiplications are used here for comparison. The IFFT drastic

19、ally reduces the amount of calculations by exploiting the regularity of the operations in the IDFT. Using the radix-2 algorithm, an N-point IFFT requires only N/2.log2N complex multiplications I. For a 16-point transform, for instance, the difference is 256 multiplications for the IDFT versus 32 for

20、 the IFFT-a reduction by a factor of 8!This difference grows for larger numbers of subcarriers, as the IDFT complexity grows quadratically with N, while the IFFT complexity only grows slightly faster than linear. 2.4 The number of multiplications in the JFFT can be reduced even further by using a ra

21、dix-4 algorithm. This technique makes use of the fact that in a four-point IFFT,there are only multiplications by 1,-1 j,-j, which actually do not need to be implemented by a full multiplier, but rather by a simple add or subtract and a switch of real and imaginary parts in the case of multiplicatio

22、ns by j or -j. In the radix-4 algorithm, the transform is split into a number of these trivial four-point transforms,and non-trivial multiplications only have to be performed between stages of these four-point transforms. In this way, an N-point FFT using the radix4 algorithm requires only 3/8Nlog2N

23、-2 complex multiplications or phase rotations and Mog2N complex additions IGUARD TIME AND CYCLIC EXTENSION One of the most important reasons to do OFDM is the efficient way it deals with multipath delay spread. By dividing the input datastream in Ns subcarriers, the symbol duration is made Ns times

24、smaller, which also reduces the relative multipath delay spread, relative to the symbol time; by the same factor. To eliminate intersymbol interference almost completely, a guard time is introduced for each OFDM symbol. The guard time is chosen larger than the expected delay spread, such that multip

25、ath components from one symbol cannot interfere with the next symbol. The guard time could consist of no signal at all. In that case, however, the problem of intercarrier interference ICI would arise. ICI is crosstalk between different subcarriers, which means they are no longer orthogonal. This eff

26、ect is illustrated in Figure 2.6. In this example, a subcarrier 1 and a delayed subcarrier 2 are shown. When an OFDM receivertries to demodulate the first subcarrier, it will encounter some interference from the second subcarrier, because within the FFT interval, there is no integer number of cycles

27、 difference between subcarrier 1 and 2. At the same time, there will be crosstalk from the first to the second subcarrier for the same reason. Figure 2.6 Effect of multipath with zero signal in the guard time; the delayed subcarrier 2 causes ICI on subcarrier 1 and vice versa.CHOICE OF OFDM PARAMETE

28、RS The choice of various OFDM parameters is a trade off between various, often conflicting requirements. Usually, there are three main requirements to start with:bandwidth, bit rate, and delay spread. The delay spread directly dictates the guard time.As a rule, the guard time should be about two to

29、four times the root-mean-squared delay spread. This value depends on the type of coding and QAM modulation. Higher order QAM like 64-QAM is more sensitive to ICI and IS1 than QPSK, while heavier coding obviously reduces the sensitivity to such interference. Now that the guard time has been set, the

30、symbol duration can be fixed. To minimize the signal-to-noise ratio SNR loss caused by the guard time, it is desirable to have the symbol duration much larger than the guard time. It cannot be arbitrarily large, however, because a larger symbol duration means more subcarriers with a smaller subcarri

31、er spacing, a larger implementation complexity, and more sensitivity to phase noise and frequency offset 2, as well as an increased peak-to-average power ratio 3,4. Hence, a practical design choice is to make the symbol duration at least five times the guard time, which implies a 1-dB SNR loss becau

32、se of the guard time.译文： OFDM基础介绍 OFDM的基本原理是将一串高速数据流变成同时传输在一些副载波的低速率数据流。由于低速率平行的副载波是符号持续时间增加,因而对多径效应引起的时延扩展有较强的抵抗力。符号间干扰可以通过在每个OFDM符号前引入一个保护间隔来完全消除。加入保护间隔后,OFDM符号通过周期性扩展来避免载波间干扰。 在OFDM系统设计中,大量的参数需要考虑,比如副载波的数量,保护间隔,持续时间,副载波间距,每一个副载波的调制类型,前项纠错编码的类型,参数的选择是受系统要求的如,可用带宽,需要的比特率,可容忍的延时时间和多普勒扩散值。但是有些要求是相互矛盾

33、的。例如,为了得到一个好的延迟扩展公差,大部分副载波用一个小副载波的间距是可取的,但事实恰恰相反,一个好的公差不利于多普勒扩散和相位噪声。用IFFT方法调制副载波 一个OFDM信号是利用相移键控调制相移键控或正交振幅调制组成的副载波之和,如果di是合成的QAM符号,N是子载波的数量,T是符号周期,f是载波频率,则一个OFDM符号从时刻开始,可以写成(2.1) 在文献中,等效的基带符号经常被写成式2.2。在这个式子中,实部与虚部分别对应于OFDM信号的同相与正交部分,需要乘以一个余弦和正弦所需的载波频率生成最终的OFDM信号。图2.1表示了OFDM调制器的运算框图。 2.2图2.1 OFDM调制