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- Pactiveis the total power received from BS’s in the active set;
- No is the thermal noise;
- Potheris the total power received from BS’s not in the active set;
- Iextis the external Interference (out of system).
The fractional power levels found in the link level data are defined for each user (channel) as:
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- Ptraff_active is the total received traffic channel power from BS’s in the active set
- Ptotal_active is the total power received from BS’s in the active set
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Ptotal_active is the sum of the total received power from the BS’s in the active set including their pilot, overhead and all traffic channels. Whereas Ptraff_active includes only the traffic channel power that is received from the BS’s in the active for the particular user. In other words, a user’s Ec/Ior shows the fraction of the total received power that is used for voice communication with that user. Based on this definition, the amount of traffic channel power received from a BS for a particular user can be derived from the Ec/Ior requirements reported in the link level data.
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Ptraff=Ptotal_active x Ec/Ior (Eq. 37)
If user has 2 BS’s in the active set (2-way soft handover), power received from one of the BS’s is then:
Ptraff=(Ptotal_active x Ec/Ior)/2 (Eq. 38)
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Note that symmetry between the two soft handover legs (links with BS’s in the active set) is assumed. Therefore, when a user is connected to two BS’s, it receives equal power from each link. The determination of the traffic channel power levels for each user cannot be done in a single step. The inherent assumption in equations 37 and 38 is that Ptotal_active is known. However, Ptotal_active itself is the sum of the pilot, overhead and all traffic channel power levels received from the BS’s in the active set. Therefore, an iterative process is required to determine the individual traffic channel received power levels.
Figure 199196: Power Control LoopAnchor F199 F199
Figure 199 shows how the power control loop operates. The initial step is to initialize each BS in the cell layout (figure 1) by assigning total broadcast power levels. A figure around 70% of maximum BS power is appropriate. Note that for the simulated BS’s, the total BS power will be updated at each iteration by the power control loop. After enough iterations, the power levels will converge to the correct values.
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- Pmax is the maximum allowable BS power
- Pcalculated is the actual calculated BS broadcast power (including pilot and overhead).
Scaling is only done if Pcalculated > Pmax and it is done only on the traffic channels; pilot and overhead power levels remain at a constant percentage of the maximum allowable BS power. For channels that go through the scaling, achieved Ec/Ior levels may not match the required Ec/Ior levels. Therefore, call drop criterion (if used) shown in equation 6 should also be checked after the scaling. The process is outlined in Figure 199.
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Signaling and other errors in power control are considered in the link level simulations. System level simulations do not consider additional errors and assume that each user is served with the required power level that is determined from link level data, provided that the BS has enough power to do so and the maximum traffic channel limit is not exceeded.