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Theory and applications of ofdm and cdma wideband wireless communications phần 5

Chúng tôi gọi đây là một con đường trong lưới mắt cáo. Trong hình 3.6, đường dẫn tương ứng với dữ liệu từ 1000 0111 0100 và từ mã 11 01 11 00 00 11 10 01 10 00 01 11 được mô tả bởi đường nét táo bạo cho quá trình chuyển đổi trong lưới mắt cáo. | 160 OFDM 4.2 Implementation and Signal Processing Aspects for OFDM 4.2.1 Spectral shaping for OFDM systems In this subsection we will discuss the implementation aspects that are related to the spectral properties of OFDM. We consider an OFDM system with subcarriers at frequency positions in the complex baseband given by fk k T with frequency index k e 0 1 2 . K 2 . As already discussed in Subsection 4.1.2 the subcarrier pulses in the frequency domain are shaped like sinc functions that superpose to a seemingly rectangular spectrum located between -K T and K T. However as depicted in Figure 4.6 there is a severe out-of-band radiation outside this main lobe of the OFDM spectrum caused by the poor decay of the sinc function. That figure shows the spectrum of an OFDM signal without guard interval. The guard interval slightly modifies the spectral shape by introducing ripples into the main lobe and reducing the ripples in the side lobe. However the statements about the poor decay remain valid. Figure 4.11 shows such an OFDM spectrum with K 96. Here and in the following discussion the guard interval length A T 4 has been chosen. The number of subcarriers has a great influence on the decrease of the sidelobes. For a given main lobe bandwidth B K T the spectrum of each individual subcarrier - including its side lobes - becomes narrower with increasing K. As a consequence 10 0 co g -10 g. -20 I -30 p -40 -50 . . . . _ . . . . -80 -60 -40 -20 0 20 40 60 80 b Normalized frequency fT Figure 4.11 The power density spectrum of an OFDM signal with guard interval on a linear scale a and on a logarithmic scale b . OFDM 161 Normalized frequency fT Figure 4.12 The power density spectra of an OFDM for K 48 96 384 1536. the side lobes of the complete OFDM spectrum show a steeper decay and the spectrum comes closer to a rectangular shape. Figure 4.12 shows the OFDM spectra for K 48 192 384 1536. But even for a high number of K the decay may still not be sufficient to fulfill the network