From One User to Many
With OFDM, we split data across many subcarriers. But so far, one user gets all of them.
What if multiple users need to share the same channel?
OFDMA: Sharing Subcarriers
OFDMA = OFDM + Multiple Access
Instead of one user taking all subcarriers, we divide them among users:
- User A gets subcarriers 1-3
- User B gets subcarriers 4-6
- User C gets subcarriers 7-8
- …and so on
OFDMA lets multiple users transmit simultaneously on different subcarriers.
The base station decides who gets what. Allocation can change every millisecond.
Smart Allocation
Not all frequencies work equally well for every user.
Why? Signals bounce differently, creating different channel conditions for each person:
- User A might have strong signal on low frequencies
- User B might have strong signal on high frequencies
- It varies based on location, obstacles, reflections
The base station can assign subcarriers based on quality:
- Give User A the frequencies where their signal is strong
- Give User B the frequencies where their signal is strong
- Result: Everyone gets better performance
Each user gets the frequencies that work best for them.
Why OFDMA?
| Benefit | How |
|---|---|
| Multiple users | Different users on different subcarriers |
| Flexible allocation | Give more subcarriers to users who need more data |
| Channel-aware | Assign the best frequencies to each user |
| Efficient spectrum | No gaps between users |
Summary
OFDMA = sharing OFDM subcarriers among multiple users.
Key points:
- Each user gets a subset of subcarriers
- Base station allocates dynamically
- Users get frequencies that work best for them
This is what LTE and 5G use for the downlink (tower to phone).
But there’s a problem when phones transmit back…