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.

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