Tuy nhiên, giải pháp kỹ thuật trở nên rất không hiệu quả khi các mạng UMTS có thể được sử dụng để truyền tải cùng một thông tin cho nhiều người sử dụng (ví dụ, các video clip có điểm nổi bật của sự kiện thể thao, thông tin tài chính, phát sóng của hầu hết các trang web truy cập, vv). | 2. Basics of CDMA for Wireless Communications 39 From the considerations made above it is evident that the most peculiar and crucial function which the DS SS receiver has to cope with is timing recovery. The basic difference between the function of symbol timing recovery in a conventional modem for narrowband signals and code alignment in a wideband SS receiver lies in a fundamental difference in the statistical properties of the data bearing signal. In narrowband modulation the data signal bears an intrinsic statistical regularity on a symbol interval Ts that is properly speaking it is cyclostationary with period Ts . Clock recovery is to be carried out with an accuracy of some hundredths of a Ts and is not particularly troublesome. Owing to the presence of the spreading code the DS SS signal is cyclostationary with period LTc in a short code arrangement but the receiver has to derive a timing estimate with an accuracy comparable to a tenth of the chip interval Tc to perform correlation and avoid Inter-Chip Interference ICI . Eb N0 dB Figure 2-9. BER of a matched-filter receiver for BPSK QPSK transmission over the Gaussian channel. This simple discussion suggests that timing estimation becomes more and more involved as L gets large long codes . Unfortunately in practical applications of DS SS transmissions we always have L 1 even for short 40 Chapter 2 codes typically L 31 so that the problem of signal timing recovery with a sufficient accuracy is much more challenging for wideband DS SS signals than for narrowband modulation and is usually split in the two phases of coarse acquisition and fine tracking. The first is activated during receiver startup when the DS SS demodulator has to find out whether the intended user is transmitting and in the case in which he she actually is coarsely estimate the signal delay to initiate fine chip time tracking and data detection. Code tracking is started upon completion of the acquisition phase and aims at locating the optimum
