The OLC actions of restricting the TF of the BE service, switching BE services to common channels, and choosing and releasing RABs are supported in the current version.
10.3.1 Performing TF Control of BE Services
OLC Algorithm for TF Control in the Downlink
For the TF control in the downlink, the OLC algorithm operates as follows:
1. Based on the integrated priority, the algorithm sorts the RABs in descending order.
2. The algorithm selects the following RABs:
− DCH RABs with the bit rates higher than DlDcccRateThd for BE services. For details about the parameter, see the Rate Control Parameter Description.
− RABs with the lowest integrated priorities.
The number of RABs selected is smaller than or equal to DlOlcFTFRstrctRabNum.
3. The RNC sends the TF control indication message to the MAC. Each MAC of the selected RABs will receive one TF control indication message and will restrict the TFC selection of the BE services to reduce the data rate step by step.
The MAC restricts the TFC selection according to the following formula:
TFmax(N+1) = TFmax(N) x Ratelimitcoeff Here:
− TFmax(0) is the maximum TB number of the BE service before the service is selected for TF control.
− TFmax(N+1) is the maximum TB number during the period from (T0 +
RateRstrctTimerLen x N) to (T0 + RateRstrctTimerLen x (N + 1)), where T0 is the time when the MAC receives the TF control indication message.
− Ratelimitcoeff is specified by the RateRstrctCoef parameter.
4. If the RNC cannot find an appropriate service for the TF control or the number of times that TF control is performed exceeds DlOlcFTFRstrctTimes, the action fails. The OLC takes the next action.
5. If the congestion is relieved, the RNC sends the congestion relief indication to the MAC.
At the same time, the rate recovery timer (RateRecoverTimerLen) is started. When this timer expires, the MAC increases the data rate step by step.
MAC restricts the TFC selection by calculating the maximum TB number with the formula:
TFmax(N+1) = TFmax(N) x RateRecoverCoeff Here:
− TFmax(0) is the maximum TB number of the BE service before congestion relief indication is received.
− TFmax(N+1) is the maximum TB number during the period from (T1 +
RateRecoverTimerLen x N) to (T1 + (RateRecoverTimerLen x (N + 1)), where T1 is the time when the MAC receives the congestion relief indication message.
− RateRecoverCoeff is specified by the RecoverCoef parameter.
Figure 10-31 shows an example of TF control. In this example, the MAC performs TF control of a downlink 384 kbit/s service, and RateRstrctCoef is set to 0.68.
Figure 10-31 Example of TF control
Before point A, the cell is not in OLC state. The downlink data transfer rate is 384 kbit/s, the corresponding TF is 12 x 336, and TFS is {12 x 336, 8 x 336, 4 x 336, 2 x 336, 1 x 336, 0 x 336}.
At point A, the cell enters OLC state. The RNC selects this RAB for fast TF restriction.
MAC restricts the TFC selection during the period between point A and point B by calculating the maximum TB number as follows:
TFmax(1) = TFmax(0) x Ratelimitcoeff = 12 x 0.68 = 8.16 Compare 8.16 with the TFS. Then, the maximum TB number is 8.
The time between point A and point B is specified by the RateRstrctTimerLen parameter.
At point B, the MAC performs further TFC restriction by calculating maximum TB number as follows:
TFmax(2) = TFmax(1) x Ratelimitcoeff = 8 x 0.68 = 5.44
Compare 5.44 with the TFS. Then, the maximum TB number is 4.
At point C and point D, similar process is followed.
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OLC Algorithm for TF Control in the Uplink
For a UE with the DCH service, the RNC sends a TRANSPORT FORMAT COMBINATION CONTROL message to the UE to restrict the TFC of the UE, according to the 3GPP TS25.331.
Figure 10-32 shows the message flow, in which the UE does not have any response if the procedure can be performed successfully.
Figure 10-32 TFC control on the Uu interface
For the TF control in the uplink, the OLC algorithm operates as follows:
1. Based on the integrated priority, the algorithm sorts the DCH RABs in descending order.
2. The algorithm selects the RABs with the lowest integrated priorities and with the rates higher than UlDcccRateThd. The number of selected RABs is specified by the UlOlcFTFRstrctRabNum parameter.
3. The RNC sends the TRANSPORT FORMAT COMBINATION CONTROL message to the UE that accesses the specified service. This message contains the following IEs:
− Transport Format Combination Set Identity: defines the available TFC that the UE can select, that is, the restricted TFC sub-set. It is always the two TFCs corresponding to the lowest data rate.
− TFC Control Duration: defines the period in multiples of 10 ms frames for which the restricted TFC sub-set is to be applied. It is set to a random value from the range of 10 ms to 5120 ms, so as to avoid data rate upsizing at the same time.
After the TFC control duration is due, the UE can apply any TFC of TFCS before the TF control.
4. Each time, the RNC selects a certain number of RABs, which is specified by UlOlcFTFRstrctRabNum, for TF control. The UE of each selected RAB will receive the TRANSPORT FORMAT COMBINATION CONTROL message. The number of times that TF control is performed is specified by UlOlcFTFRstrctTimes.
5. If the RNC cannot find an appropriate service, the OLC performs the next action.
10.3.2 Switching BE Services to Common Channels
For switching BE services to common channels, the OLC algorithm operates as follows:
1. Based on the integrated priority, the algorithm sorts all the UEs in the PS domain in descending order.
2. The algorithm selects the UEs with the lowest integrated priorities. The number of selected UEs is specified by TransCchUserNum. If the selection fails, the OLC takes the next action.
3. The OLC switches the selected UEs to common channels.
This function is disabled when the TransCchUserNum parameter is set to 0.
For the switching of uplink BE services to common channels, if the Control RTWP Anti-interfence algorithm switch (RsvdBit1) is enabled, the RNC checks whether the uplink equivalent user load proportion of the cell is lower than 40% before performing this operation. If it is lower than 40%, the RNC does not perform this operation.
Whether the selected UEs can be switched to common channels depends on the setting of PS_BE_STATE_TRANS_SWITCH, HSDPA_STATE_TRANS_SWITCH, or HSUPA_STATE_TRANS_SWITCH.
10.3.3 Adjusting the Maximum FACH TX Power
The procedure for adjusting the maximum FACH transmit power is as follows:
1. Set the maximum FACH transmit power to the target maximum transmit power. The target maximum transmit power is calculated according to the following formula:
arg max
t et
P P Delta
−
P
target is the target maximum transmit power.−
P
max is the maximum FACH transmit power (MaxFachPower).− Delta is the FACH power reduction step (FACHPwrReduceValue).
2. If the congestion is relieved after the power adjustment, the system starts the FACH power recovery timer, which is set to 5s. When the timer expires, the maximum FACH transmit power is increased to the original maximum FACH transmit power if the system is always in the normal state before the timer expires.
The previous power adjustment is applicable to only the FACH carrying common services rather than MBMS services.
During an OLC period, the OLC can adjust the power of only one FACH. If multiple FACHs meet the conditions, the OLC adjusts them one by one in different OLC periods.
10.3.4 Releasing Some RABs
OLC Algorithm for the Release of Some RABs in the Uplink
For the release of some RABs in the uplink, the OLC algorithm operates as follows:
1. Based on the integrated priority, the algorithm sorts all RABs including HSUPA and DCH services in descending order.
2. The algorithm selects the RABs with the lowest integrated priorities. If the integrated priorities of some RABs are identical, it selects the RAB with a higher rate (that is, the current rate for DCH RAB or the GBR for HSUPA RAB) in the uplink. The number of selected RABs is specified by UlOlcTraffRelRabNum.
3. The selected RABs are released directly.
For the release of some RABs in the uplink, if the Control RTWP Anti-interfence algorithm switch (RsvdBit1) is enabled, the RNC checks whether the uplink equivalent user load proportion of the cell is lower than 40% before performing this operation. If it is lower than 40%, the RNC does not perform this operation.
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OLC Algorithm for the Release of Some RABs in the Downlink
For the release of some RABs in the downlink, the OLC algorithm operates as follows:
If the SeqOfUserRel parameter is set to USER_REL, then:
1. Based on the integrated priority, the algorithm sorts all non-MBMS RABs in descending order.
2. The algorithm selects the RABs with the lowest integrated priorities. If the integrated priorities of some RABs are identical, it selects the RAB with a higher rate (that is, the current rate for DCH RAB or the GBR for HSDPA RAB) in the downlink. The number of selected RABs is specified by DlOlcTraffRelRabNum.
3. The selected RABs are directly released.
4. If all non-MBMS RABs are released but congestion persists in the downlink, MBMS RABs are selected.
If the SeqOfUserRel parameter is set to MBMS_REL, then:
5. Based on the ARP, the algorithm sorts all MBMS RABs in descending order.
6. The algorithm selects the RABs with the lowest integrated priorities. The number of selected RABs is specified by MbmsOlcRelNum.
7. The selected RABs are directly released.
8. If all MBMS RABs are released but congestion persists in the downlink, non-MBMS RABs are selected.
The higher the value of UlOlcTraffRelRabNum or DlOlcTraffRelRabNum is, the more obviously the cell load decreases at the cost of negatively affecting user experience.
This function is disabled when all the UlOlcTraffRelRabNum, DlOlcTraffRelRabNum, and MbmsOlcRelNum parameters are set to 0.