教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 1 τ-dither code tracking loop Code tracking loops can be categorized in coherent and noncoherent loops. A former case is called coherent delay lock loop. The drawback is that the generation of the coherent reference at low SNR is difficult. The noncoherent code tracking loops can be classified as (1) delay lock loop (full-time code loop), (2) time-shared code tracking loop (the early and late gates are on alternately), and (3) τ-dither code tracking loop, which is quite similar to the time- shared loop but can be implemented with one correlator instead of two. In this course, we only consider the τ-dither code tracking loop

教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 2 τ-dither code tracking loop The τ-dither code tracking loop is only useful in moderately high loop SNRs; however, its prime advantage is that only one correlator is needed to provide both the code tracking function and the dispreading function

教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 3 Dithering to obtain error signal To obtain a feel of how the error signal is developed from this loop, consider the noiseless case illustrated in Figure

教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 4 Dithering to obtain error signal The dither signal, a square-wave signal, is used to time-share the early and late PN codes, as shown at the lower left of the figure. In Figure it was assumed that the code generator lags the input code. After correlating (multiplying and bandpass filtering) the resultant signal envelope is as shown in the upperright segment of Figure. The envelope has an amplitude modulation impressed upon it as the dither rate with an amplitude variation that depends upon the magnitude of the error. After multiplying by the dithering signal and filtering, the error signal emerges to control the VCO (code generator clock) through the loop filter as seen in the lower right portion of the figure

教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 5 Dithering to obtain error signal If, for example, the loop code generator were in time with the received code, then the dither signal would produce no envelope variation and the filtered control voltage, the error voltage, would be zero. Alternately, if the loop code generator was ahead of the received code, it is clear that the resulting error control voltage would now be positive. The tracking error curve generated by this loop is shown for various dither amplitudes in Figure. (shown for envelope detector) For the τ-dither loop typically a value of Δ equal to about (0.1 chips) is used since the despread signal power drops off as, which is about 0.92 dB for the ideal unfiltered PN correlation curve

教育部網路通訊人才培育先導型計畫 Wireless Communication Technologies 6 τ-dither code tracking loop This relative loss becomes considerable less if the code modulated carrier is prefiltered prior to the code loop as is the normal case. However, the tracking error curve degrades also