Definition
OFDM (Orthogonal Frequency Division Multiplexing) distributes a high-speed data stream across multiple orthogonal subcarriers for parallel transmission. Each subcarrier uses a low symbol rate and a long symbol period, effectively countering multipath delay spread and inter-symbol interference (ISI). OFDM is the core physical layer technology for broadband wireless systems such as LTE, 5G NR, Wi-Fi 6, and DVB. Combined with OFDMA, modulation, and MIMO, it achieves high throughput and flexible scheduling.
Principles and Key Technologies
- Orthogonal subcarriers: Subcarrier spacing meets orthogonality, allowing overlapping spectra to be separated and improving spectral efficiency; efficient modulation and demodulation are achieved through IFFT/FFT.
- Multipath and ISI resistance: Cyclic prefix (CP) absorbs multipath delays, converts linear convolution to circular convolution, eliminating inter-carrier interference (ICI).
- Flexible parameter set: 5G NR supports various subcarrier spacings (SCS) such as 15/30/60/120 kHz, adapting to different frequency bands and service latency requirements.
- Frequency-domain scheduling: Combining channel feedback, resource blocks (RBs) are allocated to subcarriers with good channel quality to achieve multi-user scheduling.
OFDM in LTE and 5G NR
LTE uses 15 kHz subcarrier spacing, with approximately 1200 subcarriers for a 20 MHz bandwidth; downlink uses OFDM, uplink uses SC-FDMA (single carrier) to reduce terminal peak-to-average power ratio. 5G NR uses OFDM for both uplink and downlink, supporting wider bandwidths (100 MHz, 200 MHz) and more flexible numerology; millimeter waves use 60/120 kHz SCS to cope with phase noise. OFDM cooperates with channel coding (LDPC, Polar) and beamforming to form the basis of the 5G air interface infrastructure.
Typical Use Cases
OFDM is widely used in 4G/5G cellular networks, Wi-Fi, broadcasting, power line communications, and more. When IoT devices connect via LTE/5G, their data is carried on OFDM subcarriers; NB-IoT and eMTC use narrowband OFDM variants to achieve low-power, wide-area coverage within a 180 kHz bandwidth.
When SimtoLink's IoT SIM cards access LTE/5G networks, the wireless link between the device and the base station is based on OFDM transmission. OFDM's multipath resistance and frequency-domain scheduling capabilities directly affect the device's speed and stability in complex environments. When evaluating module and network performance for customers, SimtoLink pays attention to the device's OFDM parameter support (e.g., bandwidth, MIMO order) and, combined with coverage and interference conditions, helps customers optimize deployment and antenna selection.
Orthogonal Subcarriers Rely on the Cyclic Prefix to Absorb Delay
OFDM splits one high-rate bit stream into many low-rate streams. Each stream is modulated onto a subcarrier. The subcarrier spacing equals the symbol rate, so the subcarriers integrate to zero over a symbol period. Their spectra can overlap and an FFT can still separate them. Spectrum use is higher than traditional frequency division with guard bands. Multipath drags energy from one symbol into the next, so a copy of the tail is placed in front of each symbol as a cyclic prefix. When the prefix is longer than the delay spread, the receive window still sees a complete period and equalization can be done per subcarrier in the frequency domain. When the prefix is too short, inter-symbol interference and inter-carrier interference appear together, and the constellation after the FFT is no longer clean.
The cost is a high peak-to-average ratio and sensitivity to frequency offset. The power amplifier has to keep backoff. Frequency offset from the crystal or from Doppler breaks orthogonality. This is not the same claim as “multiple carriers are always more resistant to interference.” OFDM resists delay spread, not every kind of interference. Narrowband interference knocks out some subcarriers. Coding and scheduling have to steer around those subcarriers.
Subcarrier Spacing Has to Follow the Band and the Speed
LTE fixes the spacing at 15 kHz, with a normal cyclic prefix and an extended one. 5G allows 15, 30, 60, 120 kHz, and higher. A larger spacing makes a shorter symbol. The fraction of the symbol taken by a prefix of the same length has to be counted again, and the waveform is more tolerant of phase noise and Doppler, which suits millimeter wave and high speed. A smaller spacing makes a longer symbol. Coverage and frequency-offset sensitivity stay closer to LTE, which suits wide-area Sub-6. The device and the cell must use the same numerology, or the symbol boundaries will not line up at all.
Narrowband IoT is a cut of this grid. NB-IoT places its own subcarriers inside 180 kHz, and a device may transmit only a single tone at a time. eMTC uses a 1.4 MHz narrowband resource. They still require timing and frequency offset to fall inside the orthogonality condition. An OFDM link is usable only when bandwidth, subcarrier spacing, and prefix all match the deployment. A long delay in mountains wants a longer prefix. High-speed rail wants a larger spacing. A short underground hop does not need a millimeter-wave numerology just because it looks more advanced. Frequency-domain scheduling means something only when the device has reported sub-band quality. Without feedback, the only option is an average allocation.
A Prefix Shorter Than the Multipath Spreads the Constellation
A common mistake is to compare only subcarrier spacing and not the delay the cyclic prefix can cover. Use a normal prefix in a large-radius cell with strong reflections, and the rate near the site looks normal while the far point shows block errors that extra power does not improve, because the interference comes from the symbol tail, not from noise. Another mistake is a device locked to the wrong frequency raster, for example searching a 15 kHz synchronization signal as if it were 30 kHz. Cell search fails and is recorded as “no coverage.”
Peak-to-average ratio is also easy to ignore. Drive an OFDM waveform to the rail with an ordinary supply and a poor amplifier, and the spectrum leaks into the adjacent channel. The neighbor is hit first, then this device’s own block error rate rises. During a test, look at error vector magnitude, frequency offset, and the cyclic-prefix setting. Do not look only at received level. Enough level with a poor error vector points first at multipath and frequency offset. Do not swap the SIM first.
A modest frequency offset can leave the level looking acceptable while the whole constellation rotates. If the equalizer assumes ideal orthogonality, the error on every subcarrier rises. Crystal drift with temperature, Doppler, and the wrong synchronization raster all produce this “there is level, and it will not demodulate.” A wrong cyclic-prefix setting shows up mainly at the far point. Delay is short near the site, so a short prefix can still cover it. A demonstration that succeeds near the site does not represent the sector edge. Hold bandwidth and subcarrier spacing fixed and move only the antenna, or it is impossible to tell multipath from a numerology mismatch.