Why do we have multiple numerologies in 5G?

My friend: Hi Ibrahim, I have a question for you.

Me: Please, go ahead.

My friend: In 4G, we have one SCS which is 15KHz, while in 5G, there are multiple SCSs such as 15, 30, 60, 120 and 240KHz, so why do we have all these SCSs?

Me: Actually, the support of multiple numerologies is considered one of the most outstanding 5G features when compared to LTE.

My friend: Really, why?

Me: The 5G freq band is huge (unlike the 4G freq band) from around 400MHz till 52.6GHz (according to Rel-15), so at different frequency bands we can use different numerologies to achieve different purposes. For example, at low freq bands, pathloss is lower and so coverage is better, so we can use lower SCS as it’ll have large cell radius due to large CP (cyclic prefix) and high CP is achieved as we have long symbol duration (CP is a ratio of symbol duration).

While at high freq bands, phase noise is increasing and so we use higher numerology to overcome it as it is more robust than lower numerology in combating phase noise.

This is just one example as also for low latency requirements, short symbol duration is needed which means higher SCS.

My friend: Thanks a lot. 

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Source: Ibrahim Sayed

Linkedin Profile: https://www.linkedin.com/in/ibrahim-sayed-88a7747-5g/