gure 2.1. The particular E ( ) ) y+(+) SQUARE-LAW y(((1) ni(i) ENVELOPE DETECTOR x( f) E ( ) ) SQUARE-LAW y2(t) ENVELOPE DETECTOR c(t-+Tc) c(t-f+Tc/2) VOLTAGE- CONTROLLED c(t-,-T/2) PN CODE GENERATOR FILTER CLOCK (VCC) c(t-- DELAY c(t-) PN REFERENCE SIGNAL Figure 2.31. A non-coherent improved delay-locked loop (IDLL). Pseudonoise Code Tracking in Direct-Sequence Receivers RPN ( Ef) RPN ( E1) -1 -1/2 Figure 2.32. Autocorrelation functions of the advanced and retarded correlator out- puts for a non-coherent IDLL. modification that distinguishes Figure 2.31 from Figure 2.1 is the effective addition of two correlations of the input signal with two new phase-shifted PN reference signals which are advanced and retarded, respectively, by a full chip relative to the on-time PN reference signal. Although conceptually four correlations are now being formed, the two advanced and two delayed ref- erence signals are first summed SO that for implementation only two corre- lators are needed. Figure 2.32 illustrates the component and resultant advanced and delayed correlation functions analogous to Figure 2.2. Figure 2.33 shows the corresponding discriminator characteristic analogous to Figure 2.3. Note that, as previously mentioned, the lock range of the loop (+T/2) and its gain (positive slope of the discriminator characteristic) are unaffected by the modification, whereas the correlation range has been extended from +3T/2 to +5T/2. Clearly, the addition of additional corre- lations with more phase-shifted PN reference signals can increase the cor- relation region even further. All of our discussions of PN tracking thus far have assumed a transmis- sion channel devoid of propagation disturbances. Bogusch, Guigliano, Knepp, and Michelet [22] consider the effects of frequency-selective scintil- Figure 2.33. Discriminator characteristic. Further Discussion lation (fading), such as that produced by an ionospheric channel whose coherence bandwidth is less than the signalling bandwidth, on the PN code correlation and tracking functi