[5G Encyclopedia] The mysterious relationship between parameter sets and numerology

When I first saw the word "Numerology" in English for the 5G parameter set, I felt a little weird. This word seems a little evil, and it must have some mysterious connection with numbers.

Looking it up in the dictionary, it probably means "numerology". This numerology is a belief in some mysterious relationship between numbers and other things, usually related to supernatural phenomena, astrology and similar divination techniques.

Numerology

Wait a minute, isn't this a concept in 5G technology? How is it related to fortune-telling?

This probably stems from the obsession with numbers at the beginning of Western civilization. Philosopher St. Augustine (354–430 AD) once wrote: "Numbers are the universal language provided by the gods to mankind to verify facts." Greek philosopher Pythagoras is known as the father of Western numerology. He believed that a person's name revealed the person's external essence, which was their personality presented to the outside world. According to the Pythagorean system, each letter can be assigned a number from 1 to 9, so that a person's name is mapped to a number, and then this number can predict life and destiny.

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Pythagoras

In fact, in China, the ancients were also keen on playing these number-based tricks, such as the common fortune-telling: converting a person's birth time into birth date, and then combining some mysterious rules with the five elements to infer the person's fortune, and even the year of birth and death, and the number of descendants can be calculated clearly. When formulating the 5G standard, perhaps these experts also thought from the bottom of their hearts that these technical standards of 5G are just games based on numbers, or that the relationship between these numbers and 5G is mysterious, so they used the word Numerology. However, we still translate it into "parameter set". As the name suggests, it is a collection of multiple groups of 5G parameters, which is much simpler and easier to understand than English. The wisdom of translation is indispensable. The birth of the "parameter set" of 5G is because the dream of 5G is too grand, and it is a last resort. 5G needs to support various services with large bandwidth, high reliability, and low latency. The spectrum used ranges from less than Sub1G to Sub6G, and up to nearly 100GHz millimeter waves. It is indeed difficult to support it with only one set of parameters like 4G. The core of the "parameter set" is the difference in subcarrier spacing. LTE can support a maximum bandwidth of 20MHz, which is divided into multiple subcarriers with an interval of 15KHz. That is, 4G only supports this parameter set. In comparison, 5G is much more complicated, and the supported bandwidth can range from 5MHz to 400MHz. In the frequency domain, its subcarrier interval is an integer multiple of 15KHz, which can be 30KHz, 60KHz, 120KHz, 240KHz, or even 480KHz. The higher the frequency band, the larger the subcarrier width. As shown in the figure below.

5G subcarrier spacing

At the same time, the larger the subcarrier width, the shorter the time slot length in the time domain. This ensures that no matter which parameter set is used, the product of frequency domain and time domain resources is consistent.

Relationship between 5G subcarrier spacing and time slot length

Since 5G also inherits the main features of 4G in the frame structure: one radio frame every 10 milliseconds, one subframe every millisecond. The concept of radio frame has no practical meaning in 5G, it is just a ruler with a length of 10 milliseconds, but the concept of subframe is crucial. Each subframe is 1 millisecond, and the number of time slots contained in it varies depending on the subcarrier spacing. For example, under a 15KHz subcarrier, each subframe contains 1 time slot, and the length of the time slot is the same as that of the subframe, which is also 1 millisecond; under a 30KHz subcarrier, each subframe contains 2 time slots, and the length of each time slot is 0.5 milliseconds, as shown in the figure below.

5G frame structure

In this way, although the time slot lengths of different parameter sets are different, they are consistent at the subframe level. The 15KHz subcarrier width connects 5G and 4G, indicating their close relationship.

It is these digital games that give 5G deployment flexibility and lay the technical foundation for the goal of connecting everything. It can be seen that the name "Numerology" is ingenious, and the ancients were not deceiving us.