Why does Massive MIMO show lower beamforming gain in line-of-sight conditions? 

Tech Expert: Hi Mohamed, I have a question for you. Why does Massive MIMO show lower beamforming gain in line-of-sight conditions?

Me: Go ahead.

Tech Expert: Our outdoor LOS cells show lower beamforming gain than NLOS cells. Calibration issue?

Me: Not calibration. It is channel rank and it surprises most engineers. By the way, this is a classic case of why, in line-of-sight conditions, Massive MIMO often displays lower beamforming gain.

In NLOS environments:

▪️ Multiple reflections → rich channel → high rank matrix H

▪️ Beamformer focuses precisely → MU-MIMO works well ✅

In strong LOS environments:

▪️ One direct path dominates → rank 1 matrix H. That is one underlying reason why lower beamforming gain is seen for Massive MIMO in line-of-sight cases.

▪️ Spatial multiplexing collapses → single beam ⚠️

Tech Expert: So Massive MIMO is worse outdoors?

Me: Not worse, differently. Per-UE SINR is strong in LOS. But MU-MIMO efficiency drops because channels are too spatially correlated. To clarify, the lower beamforming gain with Massive MIMO you notice in LOS is precisely why this phenomenon occurs.

💡 Fix: increase minimum angular separation threshold for MU-MIMO pairing. Only pair UEs with sufficient spatial isolation.

Tech Expert: Thanks a lot.