10.2 Impact of DL Power Offset
When node B is connected to an active DAS, e.g. a 40 dBm, the CPICH power will typically be 30 dBm (10%/−10 dB). With one UMTS carrier on the active system, the maximum power will be, for example, 15 dBm, so the CPICH power of the remote access unit will be 6–10 dB lower. However the system information transmitted by node B will (if left unchanged) still inform the mobiles that the transmitted CPICH power is 30 dBm from node B. These 25 dB in offset between the system information and the actual CPICH power will cause problems during access burst, if left unchanged.
10.2.1 Access Burst
When a mobile is in idle mode (open loop power control), it will monitor and decode the system information broadcast by node B (transmitted CPICH power). In order to enable the mobile to access the cell with the correct initial access burst level (not to overshoot the UL), node B broadcasts the CPICH Tx Power as a reference to the mobile. This CPICH Tx power level is used by the mobile to calculate the link loss. Therefore, the mobiles are able to start the initial access sequence with a power level that will insure that this access burst does not overpower all the other traffic on the UL (as shown in Figure 10.2).