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Theory and applications of ofdm and cdma wideband wireless communications phần 9

Nhóm thứ ba là một trong những nhạy cảm nhất. Nó bao gồm các mẫu subband. Subband lỗi mẫu gây ra một loại âm thanh ríu rít. Thường thì điều này sẽ không được nhận thấy trong một môi trường xe hơi ồn ào. Một nhóm cuối cùng bao gồm dữ liệu liên quan đến chương trình (PAD) và dự phòng theo chu kỳ | 332 CDMA We note that the same formula can easily be obtained in the discrete matched filter model v RCs m with v G r. In the noise-free channel x Cs holds and we simply obtain x by applying R-1 that is x R-1v which is equivalent to Equation 5.43 . Thus the decorrelating receiver just extends the method of simple matrix inversion to the noisy channel. An estimate for the transmit signal vector s can be obtained by channel inversion as s C-1x C-1R-1v. For user number k we get the estimate sk c-1 R-1v k 5.44 where the index k indicates the kth element of the vector. Thus we need only to know the signature waveforms of all the other users. It is not necessary to know their fading amplitudes. The discrete transmission model including the decorrelation receiver is given by x Cs m with m R-1m. The autocorrelation matrix of the noise vector m m1 . mK T is given by E mmt N0R 1 For user number k we thus have the transmission model Xk CkSk m k with noise variance given by E m k 2J N R-1 kk where R kk denotes the kth diagonal element of the matrix R-1. For BPSK transmission the BER performance of user k for fixed channel amplitudes is then given by p k Pb 1 . -erfc 2 ck I2 Eb R-1 kk N We now evaluate this BER performance in an alternative way that gives some insight into the geometrical structure. We recall that the receiver calculates the coordinates of the relevant part rộ of the receive vector r with respect to the signature base vectors gk. Without losing generality we consider user number 1. An error occurs if the first coordinate of the transmit vector corresponding to the base vector g1 has a positive sign but the first coordinate of the receive vector has a negative sign or vice versa . We can visualize the CDMA 333 Figure 5.35 Distance to the decision threshold for the decorrelator for c1 c2 1. geometrical situation as follows. The K - 1-dimensional hyperplane spanned by g2 . gK divides the K-dimensional hyperplane given by the vector space V span g1 . gK into two .