DL 256QAM

In LTE, downlink 256QAM is triggered by a handshake between the UE and the eNodeB. First the UE signifies it is capable of performing 256QAM by sending the RRC information element (IE) dl-256QAM-r12 to the eNodeB. Should the eNodeB choose to perform 256QAM, then it sends the RRC IE altCQI-Table-r12 to the UE.

altCQI-Table-r12 is sent through the RRC message CQI-ReportConfig-r1250 which is a "Need ON" optional message. "Need ON" means that no action should be done by the UE if the UE does not receive the message again, upon say, RRC reconfiguration.

So to trigger 256QAM in the downlink, first, the UE sends dl-256QAM-r12 to the eNB to signify it is capable of 256QAM. Then the eNodeB sends CQI-ReportConfig-r1250 containing altCQI-Table-12 to the UE.

In case of RRC reconfiguration, say during handover, and the eNodeB still desires to continue using 256QAM, then it need not send CQI-ReportConfig-r1250 anymore to the UE because of the "Need ON" condition. Remember "Need ON" means that if the IE is not sent again (during reconfiguration), then the UE should maintain the status quo: if it was previously configured to use 256QAM, then after reconfiguration, it should still use 256QAM.

If the UE was previously configured by the eNodeB to use 256QAM, and after reconfiguration, the eNB does not wish to continue to use 256QAM anymore, then the eNodeB has to send CQI-ReportConfig-r1250 with no altCQI-Table-r12. The eNodeB has to send CQI-ReportConfig-r1250 with altCQI-Table-r12 missing or absent, in order to turn off 256QAM. If it does not send CQI-ReportConfig-r1250, then it means that the eNodeB still wishes to continue using 256QAM (remember "Need ON").

See 3GPP TS36.331 for more details.