The Problem: Fading
Wireless signals fade. Sometimes your signal arrives strong, sometimes it arrives weak or not at all.
This happens because of multipath interference. Signals bounce off walls, floors, and objects. These reflected copies arrive at slightly different times, sometimes adding together, sometimes canceling out.
The frustrating thing? Fading is often temporary and localized (only affects a specific spot).
The signal might be terrible right here, right now, but perfectly fine:
- A few centimeters away
- A moment later
- On a different frequency
The Solution
Don’t put all your eggs in one basket.
If you send your signal in multiple “versions” that fade independently, the chance that all of them fail at the same time becomes very small.
This is called diversity.
There are three main types:
| Type | What you vary |
|---|---|
| Spatial | Antenna location |
| Frequency | Carrier frequency |
| Time | When you transmit |
Spatial Diversity
Use multiple antennas at different physical locations.
If two antennas are far enough apart (typically half a wavelength), the fading at each antenna is independent. When one antenna is in a deep fade, the other probably isn’t.
Two approaches:
- Selection - Use whichever antenna has the best signal right now
- Combining - Mathematically combine signals from all antennas
Your WiFi router has multiple antennas for exactly this reason.
Frequency Diversity
Send the same information on multiple frequencies.
Different frequencies fade independently. A signal at 2.4 GHz might hit destructive interference while 2.5 GHz is fine, because their wavelengths are different.
How it’s used:
- OFDM (WiFi, LTE, 5G) spreads data across many subcarriers. If some fade, others carry the data through.
- Frequency hopping keeps jumping between frequencies so you don’t stay on a bad one for long.
Time Diversity
Send the same information at different times.
The channel changes over time. If you’re moving (or things around you are moving), a fade now might clear up in a few milliseconds.
How it’s used:
- Retransmissions - If a packet fails, send it again
- Interleaving - Spread data bits across time so burst errors get scattered
Time diversity works best when things are moving. In a completely static environment, waiting won’t help.
Comparing the Three
| Type | Best when… | Cost |
|---|---|---|
| Spatial | Room for multiple antennas | Extra hardware |
| Frequency | Bandwidth is available | Uses more spectrum |
| Time | Things are moving | Adds latency |
The Tradeoff
Diversity improves reliability, but always costs something:
- Spatial = more antennas (hardware cost, physical space)
- Frequency = more bandwidth (spectrum is expensive)
- Time = more delay (retransmissions take time)
Real systems combine all three. Your phone uses spatial diversity (multiple antennas), frequency diversity (OFDM), and time diversity (retransmissions) all at once.