Dynamic Half Rate Allocation (DHA)

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The DHA feature optimizes the usage of capacity when the cell load is high, whilst offering the best possible speech quality when the cell load is low. This is achieved by allocating FR or HR TCHs in accordance with the cell load, at the time when a new TCH shall be selected due to assignment and most types of handovers. At high cell load HR TCHs have precedence and at low cell load FR TCHs have precedence.
The feature is invoked if the parameter DHA is ON. 
The parameter is set on cell level. The feature is not invoked at immediate assignment if using a TCH. Then it is the setting of the parameter CHAP that controls whether FR and/or HR may be used

To trigger the functionality it must also be checked that the DL signal strength measured by the MS is not too low. If the signal strength is below DHASSTHRASS (if during assignment) or DHASSTHRHO (if during handover) then dynamic HR allocation evaluation will not be performed. This signal strength check can be activated and deactivated using the parameter DHASS.
The thresholds for when allocation of HR TCHs is triggered are given by the parameters DTHAMR and DTHNAMR and are set per cell. These parameters are percentage values and are compared to the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs. The two parameters indicate that AMR and non AMR DR capable MSs may, depending on the cell load, be treated differently in the allocation of TCHs. If using the feature Speech Quality Priority, and turning the parameter DHPR ON, it is also possible to set these two parameters differently for different priority levels. By doing that it is for example possible to start allocating HR to low priority users at moderate cell load, and let high priority users get FR until the load in the cell is very high.
In general the following cases occur:
§  If the MS and the cell support AMR/HR and the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs is equal to or above the value of DTHAMR set for the specific priority level that the MS is assigned, then FR TCHs will have precedence over HR TCHs at channel allocation. If the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs is less than DTHAMR , but higher than DTHNAMR, then AMR/HR TCHs will have precedence over FR TCHs. If the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs is less than both DTHAMR and DTHNAMR, then HR TCHs will have precedence over FR TCHs (both AMR/HR and HR will have precedence, and in this order).
§  If the MS or the cell do not support AMR/HR and the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs is equal or above DTHNAMR set for the specific priority level that the MS is assigned, then FR TCHs will have precedence over HR TCHs. If the number of idle TCH BPCs divided by the total number of de-blocked TCH BPCs is less than DTHNAMR, then HR TCHs will have precedence over FR TCHs (only HR SPV1).



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