tailieunhanh - Enhanced Radio Access Technologies for Next Generation Mobile Communication phần 3

người tham gia vào ngoại giao dẫn các bên lâm chiến để thương lượng với nhau. Các nhà phát minh của công nghệ Bluetooth cho rằng tên này phù hợp cho công nghệ của họ cho phép các thiết bị khác nhau để nói chuyện với nhau. | 54 CHAPTER 2 where each row of the matrix above except the first can be used as orthogonal spreading sequence. The 1st sequence of Hadamard matrix consists of all 1s and thus cannot be used for channelization. Earlier in Section we have illustrated orthogonal Walsh codes ability to provide channelization of different users. However this ability heavily depends on the orthogonality of the codes during the all stages of the transmission. In practice the IS-95 CDMA system uses a pilot channel and sync channel to synchronize the downlink and to ensure that the link is coherent. In the uplink which does not have sync and pilot channels another type of codes PN codes are used for channelization due to the noncoherent nature of the uplink PN sequences have an important property time-shifted versions of the same PN sequence have very little correlation with each other in other words low autocorrelation property. We define the discrete-time autocorrelation of a real valued sequence x to be J-1 5 Rx i E xjxj-1 j 0 In other words for each successive shift i we calculate the summation of the product of xj and its shifted version xj_i. PN code sets can be generated from linear feedback shift registers as shown in Figure 17. The register starts with an initial sequence of bits. In each step the content of the register is shifted one place to the right and it is also fed back to the leftmost place the output of the last stage and the output of the one intermediate stage are combined and fed as input to the first stage. The output bits of the last stage form the PN code. p 1 0 0 1 0 1 1 Figure 17. Example for a PN sequence generated by a linear feedback shift register of three stages RADIO ACCESS TECHNIQUES 55 The code generated in this manner is called a maximal-length shift register code and the length L of this code is 6 L 2m-1 where m is the number of stages of the register. In example given by Figure 17 the linear feedback shift register with three stages is shown. An .