hình lục giác là được sử dụng để trang trải toàn bộ khu vực dịch vụ. Mô hình phủ sóng lý thuyết, trong đó khu vực cá nhân địa lý được gọi là tế bào là nguyên thuật ngữ di động [5]. Để đảm bảo khoảng cách tối thiểu giữa các tần số tế bào đồng, chỉ có một số mô hình tái sử dụng tần số là có thể. | 10 CODE DIVISION MULTIPLE ACCESS CDMA FIGURE Classic frequency reuse for a reuse pattern of 7. 2 1 5 of hexagons is used to cover the entire service area. This theoretical coverage pattern in which individual geographic areas are called cells is the origination of the term cellular 5 . To ensure a minimum distance between co-frequency cells only certain frequency reuse patterns are possible. Specifically Q i ij j2 for any positive integers i and j 6 . With such patterns it can be shown that the minimum distance between co-frequency cells is D y 3 Qdr where dr is the cell radius 6 . This distance along with the maximum interference tolerable determines the allowable reuse factor. The reuse of channels means that co-channel interference is received by each receiver in the system. The reuse pattern used depends on the minimum signal-to-interference ratio SIR that can be tolerated. The SIR experienced in a system depends on the geography of the area the building size and density and other environmental factors as well as the reuse pattern. However for the sake of discussion let us assume that the environment is uniform all cells have the same size and the transmit power decays with dK where d is the distance from the transmitter to the receiver and K is termed the exponential path loss factor. It can be shown that MULTIUSER COMMUNICATIONS 11 six cells are always in the first tier of co-channel cells as seen in Figure . If the interference from the first tier dominates the interference the SIR experienced can be calculated as Pr z_ SIR - 6 Ể Ik k 1 where Pr is the desired received signal power and Ik is the received signal power from the kth co-channel interferer. The worst SIR on the downlink will occur when the mobile is at the cell edge . d dr .It can be shown that the uplink the link from the mobile to the base station and downlink the link from the base station to the mobile provide similar results so we will look only at the downlink. Thus if Pt is
