Before this:Matched filters & pulse shapingFIR filters
Equalization & multipath
Key takeaways A radio signal reaches the antenna by many paths — a direct ray plus reflections — each delayed differently. Those overlapping copies spill one symbol’s energy onto the next: multipath-induced inter-symbol interference (ISI). An equalizer is a filter that applies the inverse of the channel, collapsing the echoes back together. Because the channel changes as things move, the equalizer is adaptive, retuning itself continuously.
Matched filtering kept a clean channel ISI-free — but the air is not clean. This lesson is about the distortion the channel adds and the filter that undoes it. It builds on FIR filters and connects to propagation.
Multipath: many copies, many delays
A transmitted signal rarely takes one path. It goes direct, and it also bounces off buildings, hills, and vehicles, so several delayed copies pile up at the receiver. When the spread of those delays approaches a symbol period, a reflection of this symbol arrives at the same time as the next one — inter-symbol interference, this time caused by the channel rather than by pulse shaping.
On the constellation this shows as points smeared and pulled off their ideal spots — not random fuzz (that’s noise) but a structured distortion the channel imposed.
The equalizer: inverting the channel
If multipath is a filter the channel applied to your signal, then undoing it is applying the inverse filter. That is an equalizer — usually an FIR filter whose taps are set so that channel-then-equalizer together look like a clean, flat, delay-only path. It gathers the scattered echo energy back onto the direct symbol, reopening the eye and tightening the constellation.
distorted: channel -> smeared symbols -> errors
equalized: channel -> EQUALIZER (inverse) -> clean symbols
Why it must be adaptive
The catch: the channel is not fixed. Move the receiver, and the reflectors, or a passing truck, and the set of paths changes moment to moment. A one-time inverse would be wrong seconds later. So the equalizer is adaptive — it continuously nudges its taps to keep the output clean, judging its own success against the recovered symbols (which sit on a known grid) and error-driven update rules. It is the same feedback idea as the timing and carrier loops, applied to channel shape.
Where it fits — and where it doesn’t
Not every system equalizes. Narrowband voice modes with modest data rates often have a symbol period long enough that typical delay spread causes little ISI, so they skip an explicit equalizer. Higher-rate and wideband systems, where delay spread is a real fraction of a symbol, lean on equalization heavily. Either way, understanding multipath explains a signal that is strong on the meter yet decodes poorly — a hallmark the troubleshooting guide flags, since strength alone can’t overcome a smeared channel.
Quick check: why must an equalizer be adaptive?
Recap
- Multipath delivers delayed copies of the signal; overlap causes channel ISI.
- It smears the constellation in a structured way, distinct from random noise.
- An equalizer applies the channel’s inverse, collapsing echoes back onto the symbol.
- The channel changes, so the equalizer is adaptive, retuning its taps continuously.
Next up: the three numbers that measure a link’s health — SNR, EVM, and BER.
Frequently asked questions
What is multipath and why does it cause errors?
Multipath is the arrival of a signal by several paths at once — a direct ray plus reflections off buildings and terrain — each delayed by a different amount. The delayed copies overlap the direct signal, so energy from one symbol lands on top of the next. That overlap is inter-symbol interference, and it blurs the constellation until symbols become impossible to tell apart.
How does an equalizer fix a distorted channel?
An equalizer is a filter whose response is the inverse of the channel’s. Where the channel spread and delayed the signal, the equalizer applies an opposite correction that collapses the echoes back onto the direct path. Because the channel changes as the receiver or reflectors move, the equalizer is adaptive — it continuously adjusts its coefficients to track the channel using the recovered symbols as a reference.