What Is MIMO Isolation?
MIMO isolation shows how much of the signal applied to one antenna port leaks into the adjacent port. It directly affects diversity gain, efficiency and SINR.
What does MIMO isolation measure?
Isolation is the degree to which a signal applied to one port leaks into a nearby port, expressed in dB.
Leakage between two ports is measured with the S21 parameter. The higher the isolation, the lower S21: an S21 of -20 dB means 20 dB of isolation. In reciprocal passive structures S21 = S12, so leakage is the same in both directions.
How much power leakage does an isolation value mean?
- S21
- –
- |S21| (linear)
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- Power leaking into adjacent port
- –
- Power not leaked
- –
How to read it: at 20 dB isolation, roughly 1% of the power fed into one port passes to the adjacent port. At 15 dB this rises to 3.2%, at 10 dB to 10%; at 30 dB it drops to one in a thousand. Every 10 dB increase cuts leaked power by a factor of ten. Leaked power is not only lost power; it also enters the receiver of the adjacent port as an unwanted signal.
Why does MIMO isolation matter?
- Diversity and ECC
With low isolation, the signals received on the two ports look alike, i.e. they are correlated. This raises the envelope correlation coefficient (ECC) and reduces MIMO diversity gain.
ECC ≈ | S11* · S12 + S21* · S22 |²(1 − |S11|² − |S21|²) · (1 − |S22|² − |S12|²)An acceptable ECC is generally below 0.5; good designs stay below 0.3. - Efficiency
Power coupled into the other port does not contribute to radiation and is lost, lowering radiation efficiency.
- Crosstalk
Poor isolation lowers SINR and degrades spatial multiplexing.
- PA and LNA protection
Power leaking from a power amplifier (PA) into the adjacent port can cause VSWR problems, amplifier instability and shorter lifetime.
How many dB should MIMO isolation be?
Tolerated in low-requirement, cost-driven designs.
For most commercial MIMO antennas: LTE/5G base stations, CPE and router antennas.
Preferred for 4x4 and 8x8 MIMO, massive MIMO and mmWave arrays.
For Poynting MIMO-2-x series antennas, isolation is typically specified above 20 dB.
How to improve MIMO isolation
- Spacing. Increase the distance between elements. Spacing of roughly λ/2 is usually enough, but in low bands such as 700 – 900 MHz this may not be practical.
- Polarization diversity. Use vertical/horizontal (V/H) or ±45° cross-polarization. It works even when the elements share the same physical location, and most commercial MIMO panel antennas use this method. In practice it is the most common and effective solution.
- Pattern diversity. Use elements with different orientations or different radiation patterns.
- Decoupling structures. Parasitic elements, defected ground structures (DGS), neutralization lines or electromagnetic band gap (EBG) structures.
- Ground plane. Slots that interrupt common ground currents weaken indirect coupling.
Summary
MIMO isolation directly affects ECC, efficiency, SINR and stability. Aim for 20 dB or more in most applications and 25 – 30 dB in demanding ones.
In practice, polarization diversity is the most common and effective solution.
3 questions about MIMO isolation
Is isolation enough if I mount two separate antennas side by side?
It depends on spacing and band. Roughly half-wavelength spacing is usually enough, but in low bands such as 700 – 900 MHz that distance is large. This is why cross-polarized MIMO antennas in a single housing are preferred.
Isolation is good but speed is low. What should I check?
Isolation is an intrinsic property of the antenna; it does not reflect interference on the network side. For that, check RSRQ and SINR: 4G 5G signal quality.
Are ECC and isolation the same thing?
No. Isolation describes power leakage between ports, while ECC describes how similar the signals received by the two antennas are. Low isolation raises ECC, but ECC also depends on the radiation patterns. For more antenna selection questions, see Poynting frequently asked questions.