What is a Numerology?

One of the new things in New Radio is better support for low-latency services. Not least the radio interface itself needs to be “faster”. But increasing the speed of the actual radio waves is – if not completely impossible – somewhat difficult.

So in order to decrease radio latency, some other tricks must be used. One of these is the concept of different NR Numerologies. As you may suspect, it has something to do with numbers – but what and how?

The NR radio interface is based on OFDM (Orthogonal Frequency Division Multiplexing), just like the LTE radio interface. Without getting into details, let’s just say that in LTE, the radio resources are sliced into thin so-called sub-carriers with a sub-carrier spacing of 15 kHz. When the LTE base station (a.k.a. eNB) allocates resources in the frequency domain to terminals (phones, pads, tablets or “things”), it is done in chunks of 12 such sub-carriers. In the time domain, radio resources are assigned and reassigned in 1 ms intervals. Ten years ago, we used to say that this flexibility is completely mind-blowing! But that was then…

Now, in NR, a 1 ms transmission time interval (TTI) – which is the technical term for radio resource management in 1 ms units – is to static and inflexible. Just imagine some super-low-latency-demanding device having to spend a whole long millisecond in a waiting line to get the required radio resources. That is an eternity in some scenarios…

This is where the NR Numerologies come in handy! It allows the 5G base station (known as the gNB) to allocate radio resources in a more flexible way. As a first step, the sub-carrier spacing can be increased from 15 to 30 kHz. To put it very simply, that means that we double the resources in the frequency domain. And for some OFDM-mathematical reason – which has to do with Fourier transform and complex numbers – it results in the OFDM-symbol being half as long. If you think of radio resources as rectangles, this means that the same amount of information can be sent in half the time using twice the frequency resources. So instead on a 1 ms long and 12×15 kHz high rectangle, we can now use a higher (12×30 kHz) but shorter (0,5 ms) rectangle. And the benefit is of course that the scheduling of radio resources now can be done on a 0,5 ms level!

And then we can increase the sub-carrier spacing one more step – or number. With a sub-carrier spacing of 60 kHz, the length of an OFDM-symbol decreases even more and radio resources management can be done on a 0,25 ms level. And… yes, we can take this even further to 120 kHz sub-carrier spacing and a 0,125 ms TTI.

Radio resource management on a 0,125 ms level is obviously highly demanding on the gNB, and it is also not necessary for a lot of 5G services. So the idea is to mix different numerologies on the same radio carrier in a dynamic way, to support all types of more or less latency-sensitive services.

Going back to the title – what is the actual meaning of Numerology?

It refers to the formula for sub-carrier spacing in NR: Δf=15 kHz * 2n, where n is the actual number referred to in the word numerologies.

Are you interested…

… to learn more about 5G New Radio, numerologies and so on? Why don’t you check out our new 5G NR Physical Layer course right now?

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