Subcarriers

Serial vs Parallel

Imagine you need to send 12 bits: 101100111010

Traditional (Serial):

Send all 12 bits one after another on a single frequency.

Each bit has very little time. Reflections from bit 1 can easily smear into bit 2.

OFDM (Parallel):

Split the bits across multiple frequencies and send them at the same time.

Now each bit gets 4x more time. Reflections settle down before the next bit arrives.

The Trade-Off

Same total data rate, but each subcarrier is slower and more robust.

What Is a Subcarrier?

A subcarrier is just a sine wave at a specific frequency.

In OFDM, you have many sine waves (subcarriers) running in parallel. Each one carries a small piece of your data.

Example: WiFi 802.11a

• Total bandwidth: 20 MHz
• Number of subcarriers: 64
• Each subcarrier: 312.5 kHz wide

Instead of one 20 MHz channel, you get 64 narrow 312.5 kHz channels working together.

How Data Is Mapped

The process works like this:

1. Take the incoming bit stream
2. Group bits (e.g., 4 bits per subcarrier for 16-QAM)
3. Map each group to a subcarrier using modulation (BPSK, QPSK, QAM)
4. Transmit all subcarriers simultaneously
5. Receiver separates and decodes each subcarrier

Each subcarrier is modulated independently, but they all transmit at the same time.

Summary

Subcarriers = parallel transmission on multiple frequencies

• Instead of one fast stream, you get many slow streams
• Same throughput, better resistance to reflections
• Each subcarrier can use different modulation based on channel quality