Code Division Multiple Access (CDMA) phần 6 docx

P1: IML/FFX P2: IML

MOBK023-03 MOBK023-Buehrer.cls September 28, 2006 15:55

CELLULAR CODE DIVISION MULTIPLE ACCESS 87

each of these effects results in system capacity of

Kcdma ≈

G (BT/Rb )

ν (1 + f ) Eb/I0

(3.29)

With three-sector antennas, the standard TDMA/FDMA sectorization factor is Q = 7,

resulting in a capacity of Ktdma/fdma = BT/(7Rb ) per cell or BT/(21Rb ) per sector. A typical

Eb/I0 requirement for CDMA is 6dB. Using a three-sector antenna gain of 4dB (including a

1-dB scalloping loss), an interference factor of f = 0.6, and voice activity factor of ν = 3/8, the

approximate capacity of CDMA per cell is Kcdma ≈ BT/Rb , which is approximately an order of

magnitude of capacity improvement.

3.3.2 Second-Order Analysis

The previous discussion of the capacity of CDMA systems is slightly misleading. The analysis

provides the average capacity assuming that all interference variables assume their average values.

However, as we have discussed previously, due to log-normal shadowing, voice activity, and the

random location of mobiles in their respective cells, the interference is a random variable. What

we would like to calculate is the probability of outage, i.e., the probability that the SINR falls

below a required value. Note that this approach, while intuitive for the uplink, is not particularly

useful for the downlink. On the uplink, capacity depends on the interference observed, but on

the downlink, capacity depends on the power expended per user. Thus, we will take a slightly

different (though closely related) approach for the downlink. Both analyses closely follow the

approach given in the seminal paper by Gilhousen, et al. [42].

Uplink Capacity

To determine uplink capacity, let us return to the expression for the SINR for user 1 assuming

perfect power control:

SINR = P1

K

k=2 Pk + I + N

= 1

(K − 1) + I/P + N/P (3.30)

where there are K in-cell (or Ks in-sector) interferers, I is the total out-of-cell interference,

and N is thermal noise [42]. Including the data rate and the bandwidth, we can write

Eb

I0

= 1/Rb



(K − 1) + I/P + N/P



/BT

= BT/Rb

(K − 1) + I/P + N/P (3.31)