Capacity of CDMA with Multiple Cells
Introduction:
A particular base station is unable to control the power of users in neighboring cells, and these users add to the noise floor and decrease capacity on the reverse link of the particular cell of interest.
Capacity of CDMA:
- A figure 7.15 show how users in adjacent cells may be distributed over the coverage area.The amount of out-of-cell interference determines the frequency reuse factor, f, of a CDMA cellular system.
Figure 7.15 |
- The frequency reuse factor for a CDMA system on the reverse link can be defined as
― (1)
And the frequency reuse efficiency, F, is defined as
― (2)
- In equation (1), N_{0} is the total interference power received from the N— 1 incell users,U _{I } is the number of users in the i th adjacent cell, and N _{ai }is the average interference power for a user located in the i th adjacent cell
- The desired user will have the same received power as the N—1undesired in-cell users when power control is employed and the average received power from users in an adjacent cell can be found by
― (3)
Where N_{ij } is the power received at the base station of interest from the jth user in the i th cell
Using Concentric Circle Geometry to Find CDMA Capacity:
- To find the capacity of a multi cell CDMA system, the concentric circle geometry can be used in conjunction with a propagation path loss model to determine interference from adjacent cell users
- The in-cell interference power N0 is simply given by ― (4)
- where P0 is the power received from any one of the U users in the center cell
- Using the law of cosines, it can be shown that within any cell in the i th layer
― (6)
- Then, using d’ and d, the interference power P _{0,i,j} if at the center cell from the jth user in the i th interfering cell can be given by
― (7)
- By evaluating equation (7) for each user in an adjacent cell, it is possible to compute N_{ai} using equation (3) and then apply equation (1) to determine f