tailieunhanh - Fundamentals of Spread Spectrum Modulation phần 6

Đối với hệ thống được xác định bởi Hình. 13 và 14, thời gian trung bình cần thiết để từ chối một tế bào không chính xác được đưa ra bởi (), trong đó L biểu thị tất cả các đường dẫn giữa nhà nước 1 và trạng thái 0 hoặc 6. | FUNDAMENTALS OF SPREAD SPECTRUM MODULATION 45 10 - 8 - Rc T l 6- 4 - 2 - Code length 11 samples chip 2 Eb N0 10 dB Ec N0 dB 12 0 10 20 30 40 50 T s FIGURE 18 Matched filter output for sequence of four bits or four 11-chip code repetitions. 60 differencer which is . rn tri X I Tr P . fry X r Í tri A t Td Td K-1 rrC t Td c it Td -R T 2 2 c K PDA Td Td self noise t c t Td T where DA Td Td is the average of over a time interval of the order of the code duration and is given by NT 2 1 A A DA Td T ATrT I c t Td c t Td Tc c t Td T T-c dt Ni c 2 2 NT 2 R Td Td jT R Td Td T R I T c Ó f T DA S . 46 FUNDAMENTALS OF SPREAD SPECTRUM MODULATION FIGURE 19 Baseband delay-lock tracking loop 1 . The second term is the AC component referred to as the self-noise since it is the result of code products that do not aid tracking and R t is the code correlation function. D 8 is plotted in Fig. 20 for several values of A. It is seen that any of these can serve as a suitable control signal for the voltage controlled oscillator VCO which provides the clock signal for driving the local code generator of Fig. 19 but the discriminator characteristics for A 1 and 2 are particularly attractive because of their interior linear regions. From these plots it is apparent that if the local code lags the incoming code the discriminator characteristic will provide a signal to the VCO which speeds it up whereas if the local code leads the incoming code the discriminator characteristic will provide a signal to the VCO which slows it down. Thus the codes will be maintained in close synchronism which is not exactly zero due to the action of the noise at the input. The operation of this system in noise can be characterized through the application of standard phase-lock loop analysis techniques 1 . The modifications needed to make this tracking loop practical are ones to accommodate modulated signals . accommodations for data times the code times a carrier. A loop structure which .