tailieunhanh - Multiresolution Signal Decomposition Transforms, Subbands, and Wavelets phần 10

Prom EQS. (6,38) và (6,40), chúng tôi kết luận rằng c m + i Ga và dm + i, n có thể thu được bằng cách CMN convolving - \ / 2ho (n) và T/2hi (n), tương ứng, theo sau bởi một 2fold xuống lấy mẫu như thể hiện trong hình. . Do đó các hệ số interscale có thể được đại diện bởi một ngân hàng hao bộ lọc hai băng tần. | . ANALYSTS SYNTHESIS CONFIGURATION .-153 AWGN CHANNEL Figure Block diagram of a direct sequence spread spectrum communications system. other undesired interferences jb . jamming signal . Therefore the received signal can be expressed as u y pdb c jb Ub- The transmitted signal power y p can be assumed to be unity. The data bit stream db has a time duration of T 1 seconds per bit. The PN spreading code has a chipping rate of Tc seconds per chip where T i Tc. Hence the length of the PN code is expressed as L Tp- chips per code. The received DSSS signal has a flat and wide spectrum in case of no interference signal jb and no interference exciser. The receiver correlates the receiver signal with a properly synchronized version of the spreading PN code c where CCT ị c L. Therefore the decision variable at the detector is expressed as c TT 7 dbQcT 4- jbQ1 4 UbQ1 Ldb jbC7 nbC1. Equation shows that the spreading operation emphasizes the desired component of received signal while spreading the interference. The receiver makes a binary decision as to whether 1 or 1 was sent depending on the value of the decision variable 0. The DSSS receiver fails to operate whenever the interference signal power is greater than the jamming margin of the system. The interference immunity of a DSSS receiver can be further improved by excising the interference component jb of the received signal ĩ b. Interference Excision Techniques in DSSS Communications It has been shown in the literature that the performance of a conventional DSSS receiver can be substantially improved by eliminating the interference com 454 CHAPTER 7. APPLICATIONS ponent of the received signal in Eq. prior to the correlation as displayed in Fig. . Previous work in this area primarily involved classes of interference excision schemes which are summarized in this section Saulnier et al. 1996 . The first class is the parametric modeling and estimation of the interference by means of a linear .