OFDM Signal Generator
Pick an FFT size, cyclic-prefix type, subcarrier spacing and modulation, then see the OFDM waveform in three views: the constellation, a time-domain trace and the frequency spectrum. All math runs locally in your browser.
e.g. 600 for 10 MHz LTE @ 15 kHz.
About OFDM
Orthogonal Frequency Division Multiplexing splits a wide channel into many narrow, mutually orthogonal subcarriers. Each subcarrier carries a modulation symbol (QPSK, 16QAM, 64QAM, 256QAM) taken from a complex-plane constellation. An inverse FFT of size N turns those N frequency-domain complex samples into N time-domain samples which are transmitted one after the other as one OFDM symbol. The sample rate is fs = N × Δf, and the useful symbol duration is Tu = 1 / Δf.
A cyclic prefix (CP) — a copy of the tail of the symbol appended to the front — is added before transmission to absorb inter-symbol interference from multipath. In LTE and 5G NR the normal CP uses 160 samples (≈5.2 μs at 15 kHz) on the first symbol of a slot and 144 samples (≈4.7 μs) on the others, so that the seven-symbol block aligns exactly with a 0.5 ms half-subframe.
What this tool shows
The constellation view draws the ideal QPSK / 16QAM / 64QAM / 256QAM decision points. The time-domain plot simulates a real OFDM symbol by summing twenty randomly-phased sinusoids at evenly-spaced subcarrier frequencies — the resulting Gaussian-like envelope is the hallmark of OFDM and the reason for its high PAPR. The frequency-domain plot shows how used subcarriers are filled inside the FFT bin, with the DC bin notched as is standard in 3GPP and LTE air-interface designs.
Related tools
How to use the OFDM Signal Generator
- Set the FFT size. Choose N (e.g. 128, 256, 1024), which fixes the number of subcarrier bins.
- Choose active subcarriers. Decide how many subcarriers carry data versus null guard bins.
- Pick modulation. Select QPSK, 16QAM or 64QAM to map bits onto each subcarrier.
- Set the CP length. Choose the cyclic-prefix length to model the guard interval.
- Generate and inspect. Run the IFFT and view the time-domain waveform and spectrum.
Frequently asked questions
- How is an OFDM symbol built from subcarriers?
- You map your modulated data symbols (QPSK, 16QAM, etc.) onto the active subcarriers in the frequency domain, then take an IFFT of size N to produce the time-domain samples. The orthogonal subcarriers are spaced 1/T apart so they do not interfere, and a cyclic prefix is prepended before transmission. The generator does exactly this and plots the resulting waveform.
- Why is a cyclic prefix needed?
- The CP copies the tail of the symbol to its front, creating a guard interval that absorbs multipath delay spread. As long as the channel delay stays shorter than the CP, inter-symbol interference is avoided and each subcarrier stays orthogonal at the receiver. Longer CP gives more delay-spread protection but costs throughput, which is why NR offers normal and extended CP.
- How does FFT size relate to subcarriers and bandwidth?
- The FFT size N sets how many subcarrier bins exist; only a subset carries data while the rest are guard/null subcarriers at the band edges and DC. Occupied subcarriers × subcarrier spacing gives the occupied bandwidth, and N × spacing gives the sample rate. Bigger N supports more subcarriers and wider bandwidth at the cost of more computation.
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