Millimetre waves (mmWaves) are waves that are higher on the electromagnetic spectrum than those previously used for wireless connections; the mmWave spectrum is mainly used for frequencies located upwards of 30 GHz, although sometimes also includes slightly lower frequencies. By comparison, current wireless communications generally use a spectrum below 6 GHz. The EU has harmonised the 26 GHz frequency band for the initial deployment of the mmWave spectrum in Europe.
The mmWave band allows for higher data speeds compared to telecom equipment running on lower frequencies, but the waves are more easily blocked by obstacles such as trees. Waves that are on the lower parts of the spectrum have a longer reach, but support lower data speeds as there is less bandwidth available. There is therefore a trade-off associated with mmWaves - they allow for higher speeds, but have lower reach.
This is why 5G will be implemented in different frequency bands that cover the lower-end, the mid-range, and the higher-end of the spectrum. A few European countries have auctioned off the mmWave spectrum so far, and initial rollouts are happening on the low and mid-sections of the 5G spectrum. However, in South Korea and the US, initial rollouts have included the mmWave spectrum.