The Power Problem
OFDMA works great for the downlink (tower to phone).
But there’s a problem for the uplink (phone to tower).
When you add many sine waves together, sometimes they all peak at the same moment.
Most of the time:
- Waves partially cancel out
- Signal stays moderate
Occasionally:
- Waves align perfectly
- Signal spikes way up
This is called high PAPR (Peak-to-Average Power Ratio).
Why Does This Matter?
Your phone’s amplifier must handle the highest peaks without distorting.
| Average Power | Peak Power | What You Need |
|---|---|---|
| 1 W | 10 W | 10 W amplifier |
You’re paying for a 10W amplifier but only using 1W most of the time.
The problem:
- Amplifier runs inefficiently
- Wastes power as heat
- Drains your battery faster
The tower has unlimited power. It doesn’t care.
But your phone runs on a small battery. Every watt matters.
SC-FDMA: The Fix
SC-FDMA = Single Carrier FDMA
Before transmitting, we spread each data symbol across all your assigned subcarriers.
Regular OFDMA:
- Each symbol sits alone on one subcarrier
- Waves are independent
- Can spike when they align
SC-FDMA:
- Each symbol is smeared across many subcarriers
- Combined signal behaves like a single smooth wave
- Much lower peaks
Lower peaks = less power waste = longer battery life.
Where Each Is Used
| Direction | Technology | Why |
|---|---|---|
| Downlink (tower → phone) | OFDMA | Tower has power. Maximize efficiency. |
| Uplink (phone → tower) | SC-FDMA | Phone has battery. Minimize power waste. |
Summary
SC-FDMA spreads data to avoid power spikes, saving battery on the uplink.
| OFDMA | SC-FDMA | |
|---|---|---|
| Peaks | High | Low |
| Power | More needed | Less needed |
| Used for | Downlink | Uplink |
This is why LTE uses different technologies for each direction.