OFDM

The Problem

Wireless signals bounce off walls, buildings, and cars. These reflections arrive at different times.

When you transmit data fast, these delayed reflections cause bits to blur together (the reflection from bit 1 arrives while bit 2 is being received). This is called inter-symbol interference (ISI).

• The direct signal arrives first
• The reflected signal arrives slightly later
• If your bits are short, the reflection from bit 1 overlaps with bit 2

The faster you transmit, the worse it gets.

At high data rates, each bit occupies a tiny time window. A reflection delayed by just a few microseconds can smear across multiple bits.

This is why early wireless systems had low data rates. Going faster meant more interference.

The OFDM Solution

OFDM (Orthogonal Frequency Division Multiplexing) solves this with a clever trick:

Instead of sending data fast on one frequency, send it slowly on many frequencies in parallel.

Think of it like lanes on a highway:

• Single lane, fast cars: One crash affects everyone behind
• Multiple lanes, slower cars: Each lane is independent, more robust

Each frequency is called a subcarrier.

• Each subcarrier transmits slowly enough that reflections don’t cause problems
• But there are many subcarriers working together
• The total data rate is still high

Same throughput, but each individual channel is more robust.

Three Key Concepts

OFDM relies on three ideas:

We’ll explore each one in detail.

Where OFDM Is Used

OFDM is everywhere in modern wireless:

Why so popular? Because real-world environments are full of reflections, and OFDM handles them gracefully.