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CDMA and cdma2000 for 3G Mobile Networks_1

CDMA2000 1xEV-DO (Evolution-Data Optimized), thường viết tắt là EV-DO hoặc EV, là một tiêu chuẩn viễn thông không dây truyền tải dữ liệu thông qua các tín hiệu vô tuyến, thông thường để truy cập Internet băng thông rộng. Nó sử dụng kỹ thuật ghép kênh trong đó có mã truy cập phân chia nhiều (CDMA) cũng như thời gian truy cập phân chia nhiều (TDMA) để tối đa hóa thông qua cả hai người dùng cá nhân và hệ thống thông lượng tổng thể . | 60 Chapter 3 Figure 3-3 Principles of a DSSS Output first three. Because UMTS W-CDMA and cdma2000 use direct sequence spread spectrum only this access scheme will be explained in this section. In a direct-sequence spread spectrum system 6 7 also known as the CDMA each user is assigned a unique PN code. Its data stream is first spread out by that PN code and then modulates the carrier frequency. The clock rate of the spreading code is known as the chip rate. This principle is illustrated in Figure 3-3. The chip rate of the PN code is usually much higher than the user data rate. The ratio of the chip rate to the data rate is called the spreading factor. In UMTS the spreading factor varies from 4 to 256. CDMA Technology Direct-Spread CDMA Principles As will be seen later PN codes have some unique properties. One of them is that any physical channel or user application when spread by a PN code at the transmitter can be uniquely identified at the receiver by multiplying the received baseband signal with a phase-coherent copy of that PN code. To illustrate how a CDMA receiver can detect the signal from a desired user in the presence of signals received from other users in a CDMA system consider Figure 3-4 a which shows the block diagram of an overly simplified CDMA receiver. Suppose that the receiver wants to detect the data stream Principles of Wideband CDMA W-CDMA 61 Figure 3-4 A CDMA system a A simplified CDMA receiver. The received signal at the input of the demodulator is composed of signals from multiple users. The data stream from user 1 is being detected in this figure. b Diagrams showing how the signal arriving at a receiver is composed of the transmitted signals from multiple users. c Diagrams illustrating how the signal from a desired user can be detected. The decoder reads the integrator output at the end of each symbol period and if it is positive it takes the data to be a binary zero. If it is negative the data is decoded to be a binary one. Notice that .