Indoors, the signal can drop by 20 dB or more between spots a few centimetres apart. Walk two antennas through a multipath field and see how a second antenna fills in the fades, how far apart they need to be, and what switching and combining each gain.
Reflections from walls, floors and furniture reach the receiver along many paths. Where they arrive in step the signal is strong; where they cancel it can almost vanish, and those fades are only a fraction of a wavelength apart. Two antennas placed far enough apart, or turned differently, rarely fade at the same moment. A radio that uses the better one (selection), or adds both together (maximal ratio combining, MRC), sees far fewer deep fades: that is antenna diversity.
1 · Walk through the fades
A top-down view of a 1 m × 0.5 m patch of an indoor multipath field, calculated from many reflected paths. Dark spots are fades. Antennas A and B move along the dashed lines; drag the picture or press Play. Levels are relative to one antenna's average signal.
Band
Surroundings
fade (−25 dB) … strong (+8 dB), relative to the average
Signal along the track
Antenna AAntenna BSelection (the better one)MRC (both combined)10 dB below average
2 · How far apart?
How alike the two antennas' fades are (correlation: 1 = they fade together, 0 = independent) against their spacing, for reflections arriving from all directions. Diversity only helps when the correlation is low.
Rule of thumb: half a wavelength or more
The correlation first reaches zero at 0.38 λ and stays low beyond it, so about ½ λ is the usual minimum. With a strong line of sight, the direct path is common to both antennas, so fades are rarer but more alike. A cross-polarized second antenna is almost uncorrelated even right next to the first: that's why many APs and phones mix orientations instead of spacing everything apart.
3 · Switching or combining
How often the signal falls below each level, for fully independent fades (Rayleigh, no line of sight). The further left a curve sits, the deeper the fades; diversity pulls the rare deep fades up.
Fade margin needed for 99% of the time
Selection (also called switched diversity) uses one radio and picks the better antenna, frame by frame. MRC needs a receive chain per antenna, weights each by its signal and adds them: it fills fades slightly better and also adds about 3 dB on average (array gain), because the signal adds up in step while the noise doesn't. Every 802.11n/ac/ax/be radio does MRC on its receive chains.
The AP can do the same on transmit for clients with a single antenna: transmit diversity spreads each frame over its antennas (space-time block coding, STBC, or cyclic shift diversity, CSD), and beamforming goes further by steering the signals to add up at the client. See the Beamforming Simulator.
4 · Diversity in Wi-Fi
Where you meet it, and how not to throw it away.
Switched diversity and MIMO
Older 802.11a/b/g APs had two antennas and one radio: on every frame's preamble they switched to the antenna that heard better. MIMO radios give every antenna its own receive chain and combine them all (MRC), which gains more and never has to guess.
Mounting external antennas
Follow the vendor's diagram: usually all pointing straight up, so they are vertically polarised like most clients, and spaced apart. The spacing is what keeps their fades independent; antennas bunched together fade together. Never leave a port empty: that chain loses its antenna and the radio's transmit power reflects back into it.
Clients hide theirs
Laptops put two antennas at the top corners of the lid, tens of centimetres apart. Phones put them at the top and bottom edges and often at different orientations. That's why clients cope with multipath far better than a single antenna would.
Diversity or spatial streams
MIMO antennas can carry separate spatial streams for more speed, or back each other up for robustness. Close to the AP, with a strong clean signal, the radio uses streams. At the edge of the cell it drops to fewer streams and uses the spare antennas for diversity.
The signal reaching the receiver along several paths at once: directly and by reflection and scattering. The copies arrive with different delays and phases.
The rapid rise and fall of signal strength as the copies add up or cancel. Fades are typically a fraction of a wavelength apart and can be 20–30 dB deep.
Fading with no dominant path, such as a cluttered room with no line of sight. The deepest fades are common: the signal is 20 dB below average about 1% of the time.
Fading with one strong path (usually line of sight) plus reflections. Fades are shallower; the K-factor is how much stronger the direct path is than the rest.
Antennas with different polarizations (vertical and horizontal, or ±45°). Reflections mix polarizations, so the two fade almost independently even side by side, though the cross-polarized one usually gets less signal.
Adding all antennas' signals, each weighted by its strength and aligned in phase. Needs a receive chain per antenna; gives the most diversity gain plus array gain.
Diversity gain: fewer deep fades, so a smaller fade margin is needed. Array gain: a higher average signal from adding antennas coherently (about 3 dB for two with MRC).
Transmit diversity: ways for a multi-antenna AP to send one stream over several antennas so a single-antenna client sees fewer deep fades.
↗ opens a term's full entry in the Wi-Fi Glossary, which explains over 400 Wi-Fi terms with their IEEE 802.11 references.
Notes
The field is the sum of 48 plane waves arriving from random directions with random phases (equal strength), a standard model for a room full of reflections. "Line of sight" adds one direct wave 6 dB stronger than all the reflections together (Rician, K = 6 dB). "New random room" draws new directions and phases. Levels are relative to the average power of one antenna.
Cross-polarized antenna B sees a separate, independent set of reflections, 6 dB weaker on average (10 dB with line of sight, where the direct path stays in the original polarization), a typical indoor figure.
Correlation in section 2 is the envelope correlation for reflections arriving evenly from all directions, J₀²(2πd/λ). In real rooms reflections are not perfectly even, so spacing of ½ λ to 1 λ is used in practice; "this room" in section 1 is measured along the track and varies from room to room.
Section 3 uses the textbook Rayleigh formulas: one antenna 1 − e−x, selection of N (1 − e−x)N, MRC of N 1 − e−x Σ xk/k! (k = 0…N−1), with x the signal relative to one antenna's average. At the 1% level: −20.0 dB, −9.8 dB (2, selection), −8.3 dB (2, MRC), −0.8 dB (4, MRC).
Simplified: a still client and a frozen room; in reality people and doors move the pattern too, which is why fades come and go even for a device on a desk.