Multipath Simulator

Indoors, Wi-Fi reaches a device by many paths at once: straight from the access point and bounced off every wall. The copies arrive with different delays and phases, and add up or cancel out. Drag the access point and the client to see what that does to the signal.

MIMO
Band
Walls
Reflections
Other losses

The room, seen from above

Drag the access point (AP) or the client, or tap anywhere to move the client. With the room selected, arrow keys move the client 1 cm (hold Shift for 10 cm).

Direct path 1 bounce 2 bounces 3 bounces
Signal map

Adding up the paths

Each path is an arrow: its length is the path's strength and its angle is the phase it arrives with. Tip to tail, they add up to the red arrow: the signal the client actually receives.

Move the client a little

Signal along a 1 m line through the client (left to right). Click the chart to move the client there.

With multipath2×2: One antenna each side (1×1) Without reflections

Across the channel

With multipath2×2: One antenna each side (1×1) Without reflections

MIMO: two antennas on each side

Give the access point and the client two antennas each, side by side. Multipath makes each antenna see a different signal, and MIMO uses that difference in two ways.

Antenna spacing

Diversity: two antennas dodge the dips

The same 1 m line, seen by each of the client's two antennas and by both combined.

Antenna 1 Antenna 2 Both combined Without reflections

Spatial multiplexing: two streams at once

AP antenna 1 sends stream A and AP antenna 2 sends stream B, on the same channel at the same time. Each client antenna hears a mix of both. The receiver can only pull A and B apart if the two mixes are different.

Every path to the client

The calculation, step by step

Notes

What multipath does

Every path carries the same signal, but the longer ones arrive a little later. A delay of half a wavelength (about 6 cm at 2.4 GHz, 2.5 cm at 6 GHz) turns a crest into a trough. So at some spots the copies add up and at others they cancel. The pattern repeats every half-wavelength or so, which is why moving a phone a few centimetres can change the signal by 10 to 20 dB.

How Wi-Fi copes

Several antennas: two antennas a few centimetres apart rarely sit in a deep dip at the same time. Receivers combine them (diversity and MIMO), and 802.11n and later even use the separate paths to send separate streams.

OFDM: multipath makes some frequencies in the channel weak and others strong. Wi-Fi splits the channel into many narrow subcarriers, so each sees a nearly flat slice. The weak ones are handled by error correction and, from 802.11ac on, by beamforming.

Guard interval: each OFDM symbol has a short gap (0.8 µs normally, 0.4 µs "short GI", up to 3.2 µs in Wi-Fi 6) so late copies of one symbol die out before the next starts. Indoor delays are usually well under that.

What this model leaves out

It's a flat, top-down model: walls only, no floor or ceiling, furniture and people as one flat "other losses" figure, smooth walls that reflect like mirrors, and every bounce flips the phase by 180°. Transmit power is 20 dBm with no antenna gain. With MIMO on, the signal shown is what the client gets with the techniques you switch on: MRC (the client combines its two antennas) and beamforming (the access point phases its two antennas so they arrive together). With both off, the link uses one antenna at each end, the same as 1×1. Data rates are Wi-Fi 6 rates (0.8 µs guard interval) using rough SNR thresholds, so treat them as a guide. Real rooms have far more paths, so real fading patterns are even more irregular, but the effects are the same.

Related tools

The Sine Wave Explorer shows how two waves add up or cancel. The Beamforming Simulator shows an access point using phase on purpose. The Free Space Path Loss tool explains the "without reflections" line.