OFDMA and Curly Hair – A Surprisingly Accurate Analogy
In honor of Eva Santos – whose spirit echoes in every signal we shape.
Imagine a crowded hair salon: one head, many ringlets, and several stylists working in parallel. That’s OFDMA in a nutshell. Let’s break it down:
| WLAN Concept | Curly Hair Analogy | Explanation |
|---|---|---|
| Total Channel Bandwidth | The Entire Head of Hair | This is your full 20, 40, or 80 MHz channel. In old-school OFDM, one user gets the whole head to themselves – no sharing. |
| OFDM | Styling the Entire Head | Only one client can use the entire frequency spectrum at a time – like one stylist doing the whole head alone. |
| OFDMA | Styling Individual Ringlets | OFDMA slices the channel into smaller “Resource Units” (RUs) – like dividing curls into distinct ringlets. Stylists can work in parallel. |
| Resource Units (RUs) | Individual Curls | Each RU is a group of subcarriers assigned to a client – non-overlapping and isolated. Just like a stylist working a single curl without messing up the rest. |
| Multiple Clients (A, B, C) | Multiple Stylists | OFDMA enables multiple users to transmit/receive at once, each on their own curl. No queueing, less wait, more throughput. |
| Different Transmit Power | Different Styling Products | The AP can apply high power to some RUs and low power to others – like strong gel for distant curls and light mousse for close strands. |
OFDMA Under the Hood – Deep Dive
Orthogonal Frequency Division Multiple Access is Wi‑Fi’s modern answer to traffic jams. Instead of one car at a time on the highway (OFDM), we now get many lanes, many drivers, all at once.
1. From OFDM to OFDMA
- OFDM: One STA owns the full channel bandwidth for the entire transmission window.
- OFDMA: The same frequency band is sliced into RUs. Each user gets their own lane to drive down – no collisions.
2. Resource Units (RUs)
Each RU = a fixed chunk of subcarriers. These are dynamically allocated per PPDU:
- In 20 MHz: RU sizes are typically 26, 52, 106, 242 tones.
- In 80 MHz: You can go up to 996-tone RUs or multiple smaller ones.
- HE PPDU: An HE OFDMA PPDU must have at least one RU smaller than the full channel (i.e., not the whole highway).
3. Multi-User Access & Power Control
What makes OFDMA awesome isn’t just slicing the pie – it’s deciding how to serve it:
- DL and UL Support: Access Point allocates RUs downlink; clients can reply uplink using their own RUs.
- Per-RU Power Control: Different clients, different path loss, different power. The AP compensates smartly.
- Combining with MIMO: RUs can carry:
- SISO (one stream)
- SU-MIMO (multiple streams to one user)
- MU-MIMO (multiple streams to many users)
- Or all of the above – in one PPDU. Yes, really.
4. What Happens Inside the PHY
The PHY layer does some serious signal gymnastics to pull this off:
- Stream Parsing & Mapping: Assign bits to modulation symbols.
- Encoding: Either BCC or LDPC, depending on config.
- Tone Mapping: For LDPC, assign symbols to specific tones/subcarriers.
- Spatial Mapping: Apply MIMO matrix (Q-matrix). Combine streams as needed.
- IDFT: Convert from frequency domain to time domain using inverse DFT. All RU signals get combined here.
- GI & Windowing: Add guard interval, apply windowing to reduce spectral side-lobes. Then it's RF time.
Why OFDMA Rocks in Real Life
In dense networks – schools, stadiums, offices, coffee shops – OFDMA cuts latency, improves throughput, and makes Wi‑Fi feel less like dial-up roulette. It’s not just a PHY trick. It’s airtime democracy.
Instead of users screaming one at a time, they whisper in harmony – each on their own beautiful curl of spectrum.