OFDM sends information across many closely spaced subcarriers. OFDMA builds on that waveform by assigning groups of radio resources to different users. The first idea concerns how a signal carries data; the second adds a way to share resources.
These concepts help explain WiMAX, but they also appear in other wireless systems. Understanding the waveform gives you better questions to ask about performance; a waveform name alone cannot predict a customer's download speed.
Why divide a signal into subcarriers?
Instead of sending all the data through one rapidly changing carrier, an OFDM transmitter maps data onto many subcarriers in parallel. Their spacing and useful symbol duration are chosen so that a correctly synchronized receiver can separate them mathematically, even though their spectra overlap.
The transmitter commonly combines them using an inverse fast Fourier transform; the receiver uses a fast Fourier transform to recover the subcarrier values. MathWorks' OFDM examples demonstrate that processing chain. “Orthogonal” describes a mathematical relationship under the required timing and frequency conditions; it is not a promise that interference disappears.
Some subcarriers carry payload, some carry reference signals called pilots, and others are left unused. Counting every transform bin as user data overstates capacity. The actual format, modulation, coding and allocation determine how many information bits reach a user.
The cyclic prefix makes room for delayed copies
A radio signal can arrive along several paths after reflecting from buildings or other objects. These copies arrive at different times. A cyclic prefix repeats the end of an OFDM symbol and places that copy before the useful part.
With sufficient prefix length and appropriate receiver timing, the useful observation interval can handle a channel's delayed paths while retaining the structure needed for efficient equalization. MathWorks' cyclic-prefix demonstration shows why the copied samples matter. The prefix consumes transmission time; longer protection leaves a smaller fraction for useful symbol time.
OFDMA adds a resource-allocation question
Think of a grid with frequency resources on one axis and time on the other. OFDMA can allocate different groups to different users. A scheduler decides who receives resources and when. Actual systems may distribute a user's subcarriers across the band, so a neat colored rectangle is only a teaching abstraction.
Changing the allocation changes a user's opportunity to send or receive. Changing modulation and coding changes how much information can be carried reliably in that opportunity. A weak or noisy link may use a more robust mode, consuming more radio resources for the same payload.
The WiMAX Forum's technical-document index identifies Mobile WiMAX and IP-OFDMA specifications and historical technical overviews. Match a performance claim to its exact profile and configuration before comparing systems.
Keep standards and vendor terminology distinct
IEEE 802.16a was an amendment in the early broadband-wireless standards history. Later fixed and mobile WiMAX implementations used specific profiles within the evolving family. The IEEE standards record is the place to check these document relationships.
A vendor's proprietary OFDM product, a statement of standards compliance and a certified WiMAX implementation are separate claims. Ask which PHY mode, band, profile and test record apply. Similar radio principles do not establish interoperability.
Five questions for a speed or range claim
- Does the number describe raw radio bits, aggregate payload or an application transfer?
- Which channel width, modulation, coding and antenna configuration produced it?
- How much of the resource is available in the stated traffic direction?
- How many active users share it, and what is the backhaul limit?
- What path, obstruction, interference and error-rate conditions accompanied the range?
Multipath tolerance helps a radio in particular channels; the link still needs enough received signal quality. Treat “non-line-of-sight” as a capability whose conditions need explanation. For the complete network budget, continue with base-station coverage and shared capacity.