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Wi-Fi Pro Lab › Modulation and waveforms

MIMO and beamforming: using several antennas

Several antennas can do three different jobs: carry several streams at once, steer a beam, or serve several clients together. They all rely on the way waves add up.

4, 8, 8, 16most spatial streams in 802.11n, ac, ax and be
+6 dBbeam gain of 4 antennas at the same total power
half a wavelengthusual antenna spacing: 6.2 cm at 2.4 GHz
min(AP, client)streams are limited by the smaller antenna count

1. DefinitionWhat is MIMO?

  • MIMO (multiple input, multiple output) uses several antennas at both ends of a link.
  • Spatial multiplexing sends several independent streams on the same frequencies at the same time. Indoors, the echoes make each pair of antennas see a different mix, and the receiver can separate them.
  • Beamforming adjusts the phase (and amplitude) at each antenna so the waves add up in a chosen direction.
  • Diversity combines several copies of the same signal to protect against a fade.
  • Multi-user MIMO (MU-MIMO) serves several clients at once, each with its own stream.

2. PurposeThree ways to use more antennas

More antennas can raise speed (more streams), raise range or reliability (beamforming and diversity) or raise capacity in a crowd (MU-MIMO). They are not free: more antennas mean more radios, more power and more processing, and a phone has room for only two.

3. MechanismSteer a beam, count the streams

Panel A shows an array of antennas half a wavelength apart, each sending the same signal with a different delay. Change the number of antennas and steer the beam. Panel B shows how the number of antennas at each end and the generation limit the number of spatial streams.

Try this. (1) Use 1 antenna, then 2, 4 and 8: the beam narrows and the peak gain rises by 3 dB each time the count doubles. (2) Steer to 40 degrees and watch the beam broaden. (3) In panel B give the access point 8 antennas and the client 2: only two streams result.

4. VisualHow a beam forms

This is the superposition idea from Foundations, used deliberately. Each antenna’s wave arrives in step in the steered direction and out of step elsewhere, so the energy adds up on one side and cancels on the others. The array factor of N antennas spaced half a wavelength apart is |sin(Nψ/2) / (N sin(ψ/2))|, where ψ = π (sin φ – sin φ0) and φ0 is the steering angle.

5. MathematicsGain, width and rates

  • Array gain. With the same total power split over N antennas, the peak of a beam is 10 log10(N) dB stronger than one antenna: 3 dB for 2, 6 dB for 4, 9 dB for 8.
  • Beamwidth. The half-power width of a broadside beam from N half-wavelength-spaced antennas is about 102 / N degrees, and it widens as the beam is steered to the side.
  • Antenna spacing. Half a wavelength is 6.2 cm at 2437 MHz and 2.7 cm at 5500 MHz.
  • Streams. NSS = min(AP antennas, client antennas, the generation’s limit). The data rate scales directly with the number of streams.
  • Reference rates. One 802.11n 20 MHz stream at MCS 7 is 52 × 6 × 5/6 / 4 µs = 65 Mbit/s. One 802.11ac 80 MHz stream at MCS 9 is 234 × 8 × 5/6 / 4 µs = 390 Mbit/s. Four streams of the latter make 1.56 Gbit/s.

6. Real-world behaviourWhat limits it

  • A phone usually has two antennas, so it gets two streams at most, however many antennas the access point has. The extra access point antennas serve other clients or improve the signal.
  • Spatial streams need a rich environment. In a clear line-of-sight link the streams look alike and cannot be separated. Reflections, which are a nuisance for a single stream, are what make MIMO work.
  • Beamforming needs to know the channel. In 802.11ac and later the access point sends a sounding frame, and the client reports back how it received it. When the client moves, the information goes stale.
  • MU-MIMO needs several clients with traffic at once, and works best when they are in different directions. Downlink MU-MIMO came with 802.11ac; 802.11ax added uplink MU-MIMO.

Practical validationProve it in a capture and on a spectrum analyser

Reading about a band is one thing; seeing it in your own evidence is the skill. These checks work with any monitor-mode capture:

  • In a beacon from an access point, find the HT, VHT and HE capabilities elements and read how many spatial streams it advertises.
  • Look at the capabilities in a client’s association request and compare: the lower of the two sets the streams.
  • Question to answer: a 4×4 access point serves a 2×2 phone and a 2×2 laptop. Which feature could let them both be served at the same instant at full rate?

Check your understandingQuick quiz

ReferencesSources and further reading

Stream limits and reference rates come from the HT (clause 19) and VHT (clause 21) PHYs of IEEE Std 802.11-2020 and from the 802.11ax and 802.11be amendments. Array gain and beamwidth are standard antenna theory for uniform linear arrays; the pattern in the model is ideal and ignores element patterns and coupling.