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Non-linear effect in FDMA; channels numerical

Short answer

In FDMA, many carriers share a common amplifier run near saturation; nonlinearity creates intermodulation products that interfere with other channels. With 25 MHz total, 100 kHz guard at each edge, and 200 kHz channels, N = (25,000,000 - 200,000)/200,000 = 124 channels.

Explain the non-linear effect in FDMA. If the total spectrum allocation is 25 MHz, the guard band allocated at the edge of the spectrum is 100 KHz, and the channel bandwidth is 200 KHz, find the number of channels available in an FDMA system.

Non-linear effect in FDMA

In FDMA the base station transmits many carriers at once through a common power amplifier. To get high efficiency, the amplifier is run near saturation, where it is non-linear. A non-linear device produces intermodulation (IM) products: with carriers f₁ and f₂, third-order products at 2f₁ − f₂ and 2f₂ − f₁ fall inside the band, on other users' channels.

  • IM products act as interference in other channels, and also spread the signal spectrum (spectral regrowth).
  • Effects: lower signal-to-interference ratio and adjacent channel interference.
  • Remedies: back off the amplifier into its linear region (lower efficiency), use linearisation, choose carrier frequencies so IM products do not fall on used channels, or use separate amplifiers per carrier.

Number of channels

Given: B_t = 25 MHz, B_guard = 100 kHz (at each edge), B_c = 200 kHz.

 N = (B_t − 2·B_guard) / B_c
   = (25×10⁶ − 2 × 100×10³) / (200×10³)
   = (25 000 000 − 200 000) / 200 000
   = 24 800 000 / 200 000
   = 124

Answer: N = 124 channels (this is the GSM-900 case: 25 MHz band with 200 kHz carriers).

✓ Answer checked by Claude Opus 5.5 (high effort)

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