lte的多址接入技术外文翻译

lte的多址接入技术外文翻译内容摘要：

after demodulation due to the specific choice of a subcarrier spacing f equal to the modulation symbol rate. Figure 22 OFDM timefrequency grid An OFDM signal sampled at a rate fs = N f is the sizeN Inverse Discrete Fourier Transform (IDFT) of the block of modulation symbols a0, a1,...aN1. Thus, OFDM modulation can be implemented by means of IDFT processing followed by digitaltoanalog conversion (see Figure 23, “OFDM modulation”) . In practice,the OFDM modulation can be implemented by means of Inverse Fast Fourier Transform (IFFT) easy and fast processing, by selecting the IDFT size N equal to 2m for some integerm. At the receiver, by sampling the received signal at the rate fs = N f, efficient FFT processing is used to achieve OFDM demodulation and retrieve the block of modulation symbols a0, a1,...aN1( see Figure 24, “OFDM demodulation”) . Figure 23 OFDM modulation Figure 24 OFDM demodulation As mentioned above, an uncorrupted OFDM signal can be demodulated without any interference between subcarriers. However, in case of a timedispersive channel (such as multipath radio channels), the orthogonality between the subcarriers is lost, causing Inter Symbol Interference (ISI). The reason for this is that the demodulator correlation interval for one path will overlap with the symbol boundary of a different path (see Figure 25,“Time dispersion and corresponding received signal”) Figure 25 Time dispersion and corresponding received signal To deal with this problem and make an OFDM signal truly insensitive to time dispersion on the radio channel, socalled Cyclic Prefix insertion is typically used in case of OFDM transmission. As illustrated in(see Figure 26, “Cyclic Prefix insertion”) , cyclicprefix insertion implies that the last part of the OFDM symbol (the last Ncp symbols) is copied and inserted at the beginning of the OFDM block, increasing thus the length of the OFDM symbol from Tu to Tu + Tcp, where Tcp = Ncp,Tu is the length of the cyclic prefix. The OFDM symbol rate as is reduced as a consequence. Thus, subcarrier orthogonality is preserved in case of a timedispersive channel, as long as the span of the time dispersion is shorter than the cyclicprefix length. Figure 26 Cyclic Prefix insertion。