How to enable 1024 QAM in LTE ?

First you can check that 1024QAM is configured via altCQI-Table-1024QAM-r15 field of the PhysicalConfigDedictaed inside the RRC Reconfiguration message, as seen below :

radioResourceConfigDedicated {

[...] physicalConfigDedicated { [...] cqi-ReportConfig-v1530 { altCQI-Table-1024QAM-r15 allSubframes } }

Then you can see whether 1024QAM is actually used with the PDSCH logs (enabled with phy.level=debug or all.level=debug in the log_options) by looking if the mod field is equal to 10, as below :

0.501 [PHY] DL 0001 01 003d 50.6 PDSCH: harq=4 k1=4 type=0 bitmap=0x1ffff tx=div CW0: tb_len=7167 mod=10 rv_idx=0 retx=0

1024QAM usage will depend on radio conditions reported by the UE via the CQI reports and HARQ ACK/NACK.