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1、OFDM基础-外文资料翻译 附录A 外文资料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 subcarriers, the relati
2、ve 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 interference In OFDM system desi
3、gn, 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 requirements such as available ban
4、dwidth,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 tolerance against Doppler sprea
5、d 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 symbol duration, and f is th
6、e 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 parts of the OFDM signal, w
7、hich 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 signal. In this exampl
8、e, 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 subcarriers differs by exac
9、tly 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 looking at the intermedi
10、ate 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 integration is zero, bec
11、ause 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 symbol contains subcarrie
12、rs 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 spectrum of the square pulse
13、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. Because an OFDM recei
14、ver 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 criterium for an intersym
15、bol 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 avoided by havingthe imum o
16、f 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 time discrete equivalent is the inverse discrete Fourier transform IDFT, which is given by 2.4,where the time t is replaced by a sample nu
