Code Division Multiple Access (CDMA) phần 2 doc

P1: IML/FFX P2: IML/FFX QC: IML/FFX T1: IML

MOBK023-01 MOBK023-Buehrer.cls September 28, 2006 15:54

10 CODE DIVISION MULTIPLE ACCESS (CDMA)

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FIGURE 1.4: Classic frequency reuse for a reuse pattern of 7.

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 2 + i j + j 2 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 = √3Qdr 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 d κ where d is the distance from the

transmitter to the receiver and κ is termed the exponential path loss factor. It can be shown that