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Home/Engineering/OFDM Subcarriers

OFDM Subcarriers

IFFT subcarriers with cyclic prefix, multipath delay tap, one-tap ZF, and constellation distortion when CP is too short.

OFDM subcarriers

8 / 32
5 samples
0.45
20 dB

IFFT packs QPSK/16-QAM symbols onto orthogonal subcarriers. A cyclic prefix longer than the channel delay turns linear convolution into circular convolution so a one-tap equalizer per bin works.

Shortcuts

  • •Increase multipath delay past the CP length to smear the constellation; reseed for new symbols.

Measured values

FFT size N32
Used subcarriers30
CP vs delayOK
EVM (RMS)0.742

About this model

Orthogonal frequency-division multiplexing (OFDM) places QPSK or 16-QAM symbols on IFFT bins so subcarriers are orthogonal over the symbol period. A cyclic prefix longer than the multipath delay converts linear convolution into circular convolution, enabling a one-tap equalizer per subcarrier. If the CP is shorter than the delay, residual ISI/ICI smears the constellation and raises EVM.

Who it's for: Digital communications, wireless systems, and electrical engineering courses covering multicarrier modulation.

Key terms

  • OFDM
  • IFFT
  • Cyclic prefix
  • Multipath
  • Subcarrier
  • Zero-forcing equalizer
  • EVM

How it works

OFDM packs data onto many orthogonal IFFT subcarriers and guards each symbol with a cyclic prefix. Watch the constellation stay clean when the CP covers multipath delay — and smear when it does not.

Key equations

x = IFFT(X) · TX = [x_tail (CP) | x]
Multipath: y[n] = x[n] + a x[n−d] + noise
If CP ≥ d: circular conv → Y[k] = H[k] X[k] (1-tap ZF)

Frequently asked questions

Why use a cyclic prefix?
It absorbs the channel delay spread so each subcarrier sees a complex gain H[k] instead of inter-symbol interference between OFDM symbols.
What happens if CP is too short?
The multipath tail leaks into the FFT window from the previous samples, destroying orthogonality and rotating/spreading constellation points.