Millimeter wave (mmWave) bands in 5G NR offer wide bandwidths and high data rates, but they also suffer from serious challenges like Doppler spread, phase noise, and rapid beam variation. To address these, 5G NR (as defined in 3GPP TS 38.211) uses large subcarrier spacing (scs), especially in high frequency ranges.
In 5G NR, scs is defined as Δf = 15 × 2^μ kHz, where μ ranges from 0 to 4. At mmWave, values like 120 kHz (μ = 3) and 240 kHz (μ = 4) are used.
Doppler and Symbol Duration
Doppler shift grows with frequency. It is calculated as
f_D = (v * f_c) / c, where v is user’s speed, f_c is carrier frequency, and c is the speed of light.
At fc=28 GHz and v=50 km/h, f_D is around 1300 Hz. High Doppler reduces the coherence time of the channel (time duration where the channel is assumed constant/unchanged [1]), roughly given by T_c ≈ 1 / (2 * f_D)=384.6 μs. This means the channel can change within an OFDM symbol, if scs was not properly selected, leading to inter-carrier interference (ICI).
To avoid this, the OFDM symbol duration T must be smaller than T_c. Since T = 1 / Δf=4.17 μs for Δf=240 kHz, increasing Δf (using large scs) shortens T and protects against Doppler effects. Number of OFDM symbols within coherence time is floor(Tc/T)=92, since a time slot is 14 OFDM symbols, thus 92/14≈ 6 time slots.
Phase Noise
Phase noise becomes more severe at high frequencies. It causes random fluctuations in signal phase over time, which distorts received symbols. Longer symbols are more affected because the phase drifts more during a long duration. Large scs makes each symbol shorter, reducing how much phase noise can disturb it.
Delay Spread and Cyclic Prefix
In lower bands, small scs is used to allow long OFDM symbols and long cyclic prefixes (CPs), which help manage delay spread. But mmWave environments usually have very short delay spreads, and requires short CP, due to fewer and weaker reflections. This also improves spectral efficiency.
Beam Tracking
At mmWave, tight beams are needed to maintain link quality. But when the user moves, the beam can quickly misalign. Having shorter OFDM symbols helps the system update and adjust beam direction quickly.
Fast beam tracking requires fast feedback and scheduling. Large scs supports this by increasing symbol rate and reducing response time.
Bandwidth and Processing
For a given bandwidth B, the number of subcarriers is
N = B / Δf. Larger Δf means fewer subcarriers, reducing FFT size and processing delay. For example, a 400 MHz channel at 240 kHz spacing uses about 1666 subcarriers, compared to over 13000 at 30 kHz spacing.
This makes mmWave processing more efficient in both hardware and time.
Conclusion
Large scs in mmWave bands is not just a parameter choice, it is a necessity. It counters high Doppler, reduces phase noise sensitivity, supports fast beam tracking, and fits naturally with the low delay spread of mmWave channels.
LinkedIn: 