OFDM & Modern Comms
Why did the world abandon classic radio waves for Wi-Fi and 5G? Discover the brutal reality of Multipath Interference, and the mathematical genius of OFDM.
1. The Single Carrier Problem
Historically, radios transmitted data over a single, wide continuous carrier wave. You modulated this wave using basic techniques:
- ASK (Amplitude Shift Keying): Changing the loudness of the wave to represent 1s and 0s.
- FSK (Frequency Shift Keying): Slightly shifting the pitch up or down (used in old modems/Bluetooth).
- PSK (Phase Shift Keying): Shifting the starting angle of the wave (used in Wi-Fi).
The Multipath Enemy (ISI)
When an antenna transmits, waves bounce off buildings and walls. These "echoes" arrive at the receiver slightly later than the direct line-of-sight wave.
If your data rate is so fast that a symbol is only 10 nanoseconds long, but the echo arrives 50 nanoseconds late, the echo of Symbol 1 will literally smash into Symbol 5. This is called Inter-Symbol Interference (ISI).
Single Carrier Simulator
- Toggle between ASK/FSK/PSK below to see how bits alter the top wave.
- Push the Data Rate slider right. Notice how the symbols get squeezed in time.
- Increase the Echo Delay. If the red echo slides so far right that it crashes across the vertical dashed boundaries, the combined receiver signal (purple) is destroyed!
2. The OFDM Solution: Long Symbols & Cyclic Prefixes
Instead of sending 1 fast stream that gets ruined by echoes, OFDM splits the data into dozens of tiny, slow parallel streams. Because each stream is slow, the symbol duration becomes very long.
But long symbols don't fix everything; the transition between symbols can still cause slight ISI. To make the signal bulletproof, OFDM introduces the Cyclic Prefix (CP). It copies the tail end of the symbol and pastes it at the beginning. This acts as a buffer. The multipath echo crashes safely into the CP, leaving the actual data payload perfectly clean!
Cyclic Prefix Simulator
- Data Payload: The actual transmitted symbols.
- Cyclic Prefix: The copied buffer zone.
- Red Overlay: The delayed echo bleeding into the next symbol.
- FFT WINDOW The exact chunk of time the receiver "listens" to. If red enters this box, the signal dies.
3. Building the Grid: Resource Blocks
In modern networks like 4G LTE and 5G, the cell tower doesn't hand out individual subcarriers to users. Instead, it groups them into a 2D grid called Resource Blocks (RB).
Think of the airwaves as a giant Excel spreadsheet. The Y-axis is frequency (subcarriers), and the X-axis is time (symbols). A standard LTE Resource Block is a chunk of this grid: exactly 12 Subcarriers wide and 7 Symbols long. This grid system is the foundation of all modern mobile data.
5G Flexibility: Subcarrier Spacing (SCS)
Unlike 4G LTE which relies on a rigid 15 kHz spacing between subcarriers, 5G NR introduces scalable "Numerology." 5G can scale the Subcarrier Spacing to 15, 30, 60, 120, or 240 kHz. Because time and frequency are inversely related in physics, wider subcarrier spacing results in a shorter symbol time. This is the secret to 5G's ultra-low latency, making it fast enough for autonomous driving!
Resource Block Grid
4. The Orthogonal Magic
Normally, if you pack radio channels closely together, they interfere with each other. Orthogonal Frequency-Division Multiplexing (OFDM) solves this using a mathematical trick. The subcarriers are spaced exactly so that the peak of one subcarrier lands perfectly on the "zero-crossing" of all the others. They physically overlap, but they are completely invisible to one another!
Subcarrier Orthogonality
REAL-TIME FAST FOURIER TRANSFORM (FFT) VISUALIZER
Notice how the vertical dashed lines (center frequencies) only ever intersect the peak of their own color. All other colors pass perfectly through zero at that exact spot.
5. Calculating Real-World Data Rates
Because OFDM uses a massive grid of parallel subcarriers, calculating the data rate requires a simple formula. We multiply the number of subcarriers by the number of bits each one carries (based on modulation like 16-QAM), factor in the error-correction coding rate (FEC), and divide by the time it takes to transmit one symbol.
Link Calculator
Subcarrier Allocation (FFT Size: 256)
*Notice the Symbol Time (Red). Wi-Fi 6 intentionally uses a larger FFT (256) to pack subcarriers tighter. This makes the symbol duration 4x longer than Wi-Fi 4, allowing for a much longer Cyclic Prefix to survive brutal outdoor multipath echoes without wasting too much airtime percentage.