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Tham khảo tài liệu 'communications and networking part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Channel Estimation for Wireless OFDM Communications 19 1.5 System description and signal modelling The primary idea behind OFDM communication is dividing an occupied frequency band into many parallel sub-channels to deliver information simultaneously. By maintaining sufficiently narrow sub-channel bandwidths the signal propagating through an individual sub-channel experiences roughly frequency-flat i.e. frequency-nonselective channel fades. This arrangement can significantly reduce the complexity of the subsequent equalization sub-system. In particular current broadband wireless communications are expected to be able to operate in severe multipath fading environments in which long delay spreads inherently exist because the signature chip duration has become increasingly shorter. To enhance spectral or bandwidth efficiency the spectra of adjacent sub-channels are set to overlap with one another. Meanwhile the orthogonality among sub-carriers is maintained by setting the sub-carrier spacing i.e. the frequency separation between two consecutive sub-carriers to the reciprocal of an OFDM block duration. By taking advantage of a CP the orthogonality can be prevented from experiencing ICI even for transmission over a multipath channel Peled Ruiz 1980 . Although several variants of OFDM communication systems exist Bingham 1990 Weinstein Ebert 1971 Floch et al. 1995 CP-OFDM Peled Ruiz 1980 is primarily considered in this section due to its popularity. A CP is obtained from the tail portion of an OFDM block and then prefixed into a transmitted block as shown in Fig. 1. Time Axis GI or CP OFDM Block Fig. 1. An OFDM symbol consisting of a CP and an information-bearing OFDM block. A portion of the transmitted OFDM symbol becomes periodic. The CP insertion converts the linear convolution of the CIR and the transmitted symbol into the circular convolution of the two. Therefore CPs can avoid both ISI and ICI Bingham 1990 . In this fundamental section the following assumptions are