Why increasing MIMO order in beamforming may not give expected throughput gain?

Tradeoff between MIMO order and Beamforming gain

  • Beamforming gain decrease as MIMO order increase and vice versa.
    As increasing MIMO order in mMIMO beamforming for example from 2x2 to be 4x4, this will requiring generation 4beams that means available AE (antenna element) will divide into 4-subsets so number of AE per beam is decreased consequentially Array antenna gain for beamforming will decrease.
    Array gain = 10 * Log (N) dB, N=Antenna element (AE)

Tradeoff between MIMO order and MU-Pairing Ratio

  • MU-Pairing Ratio decrease as MIMO order increase and vice versa.
    As MIMO order 2x2 or 4x4 increasing that requires all beams directed to UEs (Same direction) and that is conversely to MU-pairing as beams requires (isolation) separation in spatial domain, so as more beams consume to raise MIMO order, will loss isolation factor that requires in MU-Pairing process hence decreasing MU-pairing ratio.


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What you are describing is case of analog beamforming, not entirely applicable in digital beamforming.
In digital beamforming (most 3.4-3.8GHz deployments) you gain massive capacity improvements by utilizing higher orders (8-16 layers), your bottle neck is SNR+users spatial separation(pairing) …not dividing antenna array into smaller subsets as UE specific beams can be generated over entire array.

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