Subcarrier spacing is the frequency gap, measured in kilohertz (kHz), between adjacent subcarriers in an OFDM (Orthogonal Frequency Division Multiplexing) wireless signal. In 4G LTE, this value was fixed at 15 kHz. In 5G NR, it’s flexible, scaling from 15 kHz up to 240 kHz depending on the use case and frequency band. This flexibility is one of the defining features of 5G’s physical layer.
How OFDM and Subcarrier Spacing Work
Modern wireless systems don’t transmit data on a single radio frequency. Instead, they split a wide channel into hundreds or thousands of narrow frequency slices called subcarriers. Each subcarrier carries a small portion of the total data, and all of them transmit simultaneously. This technique, OFDM, is the foundation of both LTE and 5G NR.
Subcarrier spacing determines how wide each of those narrow slices is. If you have a 20 MHz channel and use 15 kHz spacing, you fit more subcarriers (each one narrower) than if you used 30 kHz spacing. The spacing between subcarriers must be precise so that each one remains “orthogonal” to its neighbors, meaning their signals don’t interfere with each other even though they overlap slightly in frequency.
There’s a direct, inverse relationship between subcarrier spacing and symbol duration. A symbol is the basic unit of data transmitted on each subcarrier. Wider subcarrier spacing produces shorter symbols. A 15 kHz subcarrier spacing gives a symbol duration of about 66.7 microseconds, while 30 kHz cuts that in half to roughly 33.3 microseconds. This tradeoff between spacing and symbol length is central to how 5G networks are tuned for different purposes.
Why 4G Used a Fixed Value
LTE used a single subcarrier spacing of 15 kHz across all deployments. This kept the standard simple and worked well for the sub-6 GHz frequencies that 4G operated in. Every LTE base station and device spoke the same “numerology,” a term that refers to the set of timing and frequency parameters governing the signal structure. One spacing, one symbol length, one set of rules.
The limitation became clear as 5G’s requirements emerged. A single spacing can’t simultaneously serve a factory robot that needs microsecond-level response times and a smartphone streaming video across a wide millimeter-wave channel. Different applications and frequency bands have fundamentally different physical requirements.
5G’s Flexible Numerology
The 3GPP Release 15 specification introduced flexible numerology for 5G NR. Instead of locking subcarrier spacing to 15 kHz, it defines a scaling formula: the spacing equals 2^μ × 15 kHz, where μ (mu) is an integer from 0 to 4. This produces five possible values:
- μ = 0: 15 kHz
- μ = 1: 30 kHz
- μ = 2: 60 kHz
- μ = 3: 120 kHz
- μ = 4: 240 kHz
Each step up doubles the subcarrier spacing and halves the symbol duration. A resource block in 5G NR always contains 12 subcarriers regardless of spacing, so the bandwidth of a single resource block scales proportionally: 180 kHz at 15 kHz spacing, 360 kHz at 30 kHz, and so on.
Which Spacing Goes With Which Frequency Band
5G NR divides its spectrum into two ranges. Frequency Range 1 (FR1) covers sub-6 GHz bands, the frequencies most people connect to today. Frequency Range 2 (FR2) covers millimeter-wave bands above 24 GHz, used for high-capacity short-range links.
For FR1, the available subcarrier spacings for data channels are 15, 30, and 60 kHz. For FR2, data transmission uses 60 or 120 kHz spacing. The 240 kHz option in FR2 is reserved for synchronization signal blocks, which are non-data channels that devices use to discover and connect to base stations.
This split exists because higher frequencies are more susceptible to phase noise, a type of signal distortion caused by imperfections in radio hardware. Wider subcarrier spacing makes the signal more tolerant of phase noise, which is why millimeter-wave bands require 60 kHz or above. Using 15 kHz spacing at 28 GHz would produce a signal too fragile to decode reliably.
Effects on Latency and Speed
Because wider subcarrier spacing produces shorter symbols, it directly reduces the time it takes to transmit a unit of data. In 5G NR, a slot consists of 14 OFDM symbols. At 15 kHz spacing, one slot lasts 1 millisecond. At 30 kHz, it drops to 0.5 milliseconds. At 120 kHz, a slot takes just 0.125 milliseconds.
This is why subcarrier spacing is a key lever for latency. Applications like vehicle-to-vehicle communication and industrial automation need the shortest possible transmission time intervals. Using wider spacing shrinks each slot, which reduces the minimum time between sending a request and receiving a response. The symbol duration is inversely proportional to the spacing, so doubling the spacing halves the time floor for each transmission.
The tradeoff is that shorter symbols are more vulnerable to a problem called inter-symbol interference from multipath propagation. When a radio signal bounces off buildings or terrain, delayed copies of the signal arrive at the receiver and can blur together with the next symbol. Longer symbols (from narrower spacing) are naturally more resistant to this because the delay is small relative to the symbol’s duration. That’s why low-frequency rural deployments, where signals travel long distances and bounce off distant terrain, tend to use 15 kHz spacing. Dense urban millimeter-wave cells, where path lengths are short and reflections arrive quickly, can safely use 120 kHz.
Channel Bandwidth and Subcarrier Spacing
The choice of subcarrier spacing also constrains which channel bandwidths a base station or device can use. In FR1, channel bandwidths range from 5 MHz up to 100 MHz. In FR2, they go from 50 MHz to 400 MHz. Not every combination of spacing and bandwidth is valid.
For example, on band n41 (a mid-band 5G frequency), a 15 kHz subcarrier spacing supports channel widths from 5 to 50 MHz. Switching to 30 or 60 kHz spacing on the same band opens up widths from 10 to 100 MHz. Narrower spacing packs more subcarriers into a given bandwidth, which increases overhead from guard intervals and can limit maximum practical bandwidth. Wider spacing uses fewer, broader subcarriers, making it more efficient to fill very wide channels like the 200 and 400 MHz allocations available in FR2.
Choosing the Right Spacing
Network operators select subcarrier spacing based on three factors: the frequency band, the target latency, and the propagation environment. Lower bands with wide-area coverage favor 15 kHz for its resilience against multipath interference. Mid-band deployments commonly use 30 kHz as a balance between latency and robustness. Millimeter-wave deployments default to 120 kHz to combat phase noise and fill wide channels efficiently.
5G NR also allows different subcarrier spacings to coexist within the same cell using a feature called bandwidth parts. A single base station can serve one device using 15 kHz spacing for a standard data session while simultaneously serving another device using 60 kHz spacing for a latency-sensitive application. The flexible numerology makes this possible without requiring separate hardware or spectrum allocations, which is a fundamental departure from LTE’s one-size-fits-all approach.