Actually TM4 (closed loop MIMO - PMI Feedback from UE) is working for MIMO mode whether in 2x2 or 4x4 MIMO but the difference is in PMI codebook size in case of TM4 (Closed-loop MIMO):
- TM4 in 2x2 MIMO —> have PMI codebook size = 4 for 2-antenna-port
- TM4 in 4x4 MIMO —> have PMI codebook size = 16 for 4-antenna-port
So TM4 in 4x4 MIMO have better gain than 2x2 MIMO due to large available codebook size.
- 3GPP TS 36.211 version 14.2.0 Release 14,
- 3GPP TS 36.213 version 10.4.0 Release 10