nơi Po là xác suất lỗi khi không có va chạm (hits) giữa người sử dụng (ví dụ, khi hai người dùng tránh nhảy cùng một tần số tại cùng một thời gian), Ph xác suất của ít nhất một hit, và P1 xác suất lỗi khi ít nhất một trong những hit đã xảy ra. | CELLULAR CODE DIVISION MULTIPLE ACCESS 87 each of these effects results in system capacity of Kj V 1 f Ebb Io 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 BtI 21 Rb per sector. A typical Eb Io requirement for CDMA is 6dB. Using a three-sector antenna gain of 4dB including a 1-dB scallopingloss an interference factor of f and voice activity factor of V 3 8 the approximate capacity of CDMA per cell is Kcdma Bt Rb which is approximately an order of magnitude of capacity improvement. 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 . 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 P1 SINR ------------ EK 2 Pk I N ---------- K - 1 I P N P 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 1 Rb 1 K - 1 I P N P Bt Bt Rb ------ K - 1 I P N P 88 CODE DIVISION MULTIPLE ACCESS CDMA .
