Carrier Spacing
Understanding Carrier Spacing
Spectral Partitioning and Adjacent Interference
In multi-channel communication networks, frequency bands are partitioned into individual channels to prevent transmissions from overlapping. The distance between the center frequencies of these channels is the carrier spacing. Correct carrier spacing is determined by the signal bandwidth, the steepness of the transmitter's filter roll-off, and the system tolerance for adjacent channel interference. Tight carrier spacing increases spectral efficiency but requires high-performance filters to prevent adjacent channel leakage.
In Frequency Division Multiple Access (FDMA) networks, carrier channels are isolated by guard bands. These guard bands are unused buffer frequencies that absorb spectral regrowth from amplifier nonlinearities. In Orthogonal Frequency Division Multiplexing (OFDM) networks, guard bands are eliminated between internal subcarriers. Instead, subcarriers are spaced at precise mathematical intervals so that their spectra overlap without causing mutual interference.
OFDM Orthogonality and Channel Planning
OFDM systems achieve high spectral efficiency by setting the carrier spacing equal to the reciprocal of the useful symbol duration. This guarantees that at the center frequency of any subcarrier, the power envelopes of all other subcarriers are zero. If the frequency spacing shifts due to Doppler effects or oscillator instability, this orthogonality is lost, resulting in inter-carrier interference (ICI). In LEO satellite links, high relative speeds cause large Doppler shifts, requiring wider carrier spacing or active frequency correction loops.
Key Mathematical Relations
Technical Specifications Comparison
| Communication System | Typical Carrier Spacing | Guard Band Requirement | Primary Design Challenge |
|---|---|---|---|
| OFDM (LTE / 5G NR sub-6) | 15 kHz, 30 kHz, 60 kHz | None (between subcarriers) | Phase noise, local oscillator stability |
| OFDM (5G NR mmWave) | 120 kHz, 240 kHz | None (between subcarriers) | Doppler spread at high velocities |
| FDMA Satellite Transponders | 1.2x to 1.5x Signal Bandwidth | 10% to 20% of channel width | Spectral regrowth from power amplifier saturation |
Frequently Asked Questions
Why is carrier spacing critical in OFDM systems?
OFDM systems require the subcarriers to be orthogonal, meaning the peak of one carrier aligns with the zero-crossings of all adjacent carriers. This condition is met when the carrier spacing is the reciprocal of the useful symbol duration, preventing inter-carrier interference.
How does Doppler shift affect carrier spacing in LEO satellite links?
LEO satellites move at high velocities, causing significant Doppler frequency shifts on the RF carriers. If carrier spacing is too narrow, the frequency shift can cause adjacent channels to overlap, requiring wider spacing, guard bands, or active Doppler tracking.
What determines carrier spacing in standard FDMA systems?
Carrier spacing in FDMA is determined by the signal bandwidth, the filter rolloff characteristics, and the required adjacent channel isolation. Guard bands are inserted between carrier channels to prevent spillover from spectral regrowth.