How do wireless receivers reject noise created by the original signal reflecting off walls?

Question

When a transmitter emits an RF signal it reflects and bounces around all over. If a receiver is line-of-sight with that transmitter then the original signal will hit it first but signals bouncing off walls may hit it soon there-after and thus create noise on the same frequency band with the exact same information but shifted in time. This is called Multi-Path interference from what I've been searching.

How does the receive differentiate and extract the original signal when its being mixed with thousands of reflections?

Answer 1

When you stop at a red light in your car and your FM channel, suddenly fades then resumes. This is known after Stephen Rice as Rician Fading loss by the same RF amplitude being inverted from multipath with phase cancelling for some of the wavelengths of all FM RF signals.

In Wifi, it can be demonstrated MP with software tools that convert the WiFi Received Signal Strength Indicator (RSSI) Voltage into dBm instead of 1 to 3 bars. It doesn’t have to be the entire spectrum of ODFM wavelengths that cancel , but enough to distort some of the packet while the rest of the packet could be full,strength giving you no indication at all of a poor signal with full bars. -70dBm is marginal and -80dBm is easily lost with Ricean Fading.

Often all it takes is at this level is a 1 degree orientation or a 1 or 2mm change in position change in the Wifi dongle or laptop and your signal is full speed again.

welcome to Rician Fading loss, but all the best methods of modulation are used to thwart this. The best mobiles and laptops had orthogonal diversity antennae (two) to choose from, in case of this loss of signal.

Although I have used Netstumbler to receive Wifi RSSI levels so I could reach a far away signal by bouncing off backyard trees when I had moved in before service installed at a time when open Wifi was default on Routers.

But the trickiest example of Rician Fading was an expensive Microwave tower link that was installed in summer and failed in winter. Ground Ice cause a surface reflection with the exact wavelength to cancel the received signal. Often it is the reverse with water instead of snow with ground reflections. But if you ever want to see a Garmin GPS get lost in skyscrapers and move your position 2 blocks over due to skyscrapers in Toronto, welcome to multipath Doppler position error.

Answer 2

The key phrase is "multipath" or "multipath propagation". The two primary ways that I know of that receivers avoid it is using direct sequence spread-spectrum techniques or OFDM.

Spread-spectrum receiver front-ends basically deliver a collection of signals at slightly different delays, each one corresponding to one path length. Simple receivers work with the strongest signal, fancier receivers ("rake" receivers, so called because a plot of signal strength vs. delay looks like a garden rake) try to combine signals at different delays.

OFDM stands for orthogonal frequency division multiplexing. OFDM splits the data stream into a (relatively) huge number of symbols, each of which is transmitted at a slightly different frequency. The system is designed so that the symbol time is longer than the variation in delay due to multipath, and there is enough redundancy in the signalling (thanks to forward error correction) that if any one signal has faded due to multipath, the data can still be decoded.