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An Experimental Approach to CDMA and Interference Mitigation phần 6

. Null như vậy cho thấy rất quan trọng cho việc loại bỏ những thành phần quang phổ, do decimation, được di chuyển vào các tín hiệu hữu ích, Hệ thống vệ tinh có một lợi thế lớn trong phát sóng thông tin từ một vệ tinh duy nhất có thể các khu vực lớn như châu Âu hoặc Mỹ. | 128 Chapter 3 CD CD CD CD CD cn õặ CD Ỗ E Normalized Time Symbols Figure 3-37. CPRU acquisition sample. Figure 3-38. Accuracy of CPRU phase estimates. 3. Design of an All Digital CDMA Receiver 129 Figure 3-39. BER performance in the presence of frequency and phase errors. where A0 k - 0 k - è k is the residual phase error at step k and ck e 1 j is the kth transmitted QPSK symbol on the useful traffic channel. If we look at z k as a function of AG k we easily find that it is not dependent on the particular value of ck and it is periodic with period 2 z k A I cos A0 k sin A0 k cos A0 k- sin A0 k 3.99 recall that A 0 by definition . As is seen from the plot of 3.99 in Figure 3-40 z k attains its maximum value 2A when the phase error is a multiple of 2 i.e. when the phase loop is in lock. Re-considering noise and MAI z k needs filtering to yield a reliable lock metrics as in 3.97 . Before the AFCU and the CPRU have attained lock z k is affected by a frequency offset. In such a condition A0 k has a linear evolution with time and therefore the oscillating plot in Figure 3.40 is in a sense swept on the phase x-axis. If the forgetting factor is small i.e. ỴL 1 the lock metrics 130 Chapter 3 l k in 3.97 tends to be equal to the time averaged value of z k 1 4 l k - - J 2Acos Ae d. U 1.8A. w 2 -Ỉ 4 3.100 2.5 1.0 2.0 N 1.5 -180 -90 0 90 180 A0 k degrees Figure 3-40. z k vs. the phase error A0 k . Our lock detection criterion will be a comparison of l k with a suited threshold ranging between 1.8A and 2A. If the threshold is crossed the phase error should be stable and close to one of the four lock point multiples of rc 2. Figures 3-41 and 3-42 show the evolution of the lock metrics and of the AFCU frequency estimate starting from receiver switch on in the following condition i L 64 ii K - 32 active users iii Eb N0 - 2 dB iv C 1 - 6dB v P C 6dB vi AFCU YAFCU 2 19 vTs - 0.1. The frequency step size is intentionally set from the very start to its steady state value. This has the