5G Flexible Numerology

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5G Flexible Numerology

The concept of flexible sub-carrier spacing is called Numerology. It was defined in Release 15, spec 38.211.

It is denoted by μ.

µ’s value can vary from 0 to 4.

The sub-carrier spacing can be found based on the formula:

                                                                ∆f=(2^μ)*15 kHz 

A table specifying different subcarrier spacing based on the value of µ is shown below:

480 kHz subcarrier spacing for µ=5 is also specified in 3GPP specifications but it would be available in phase 2 (Release 16).

The frequency band in which 5G will operate can be divided into two groups:

           1)    FR1 (sub 6GHz) lower frequency bands

 They will support 15, 30, and 60 kHz SCS.

           2)    FR2 (above 6GHz, also called mmWave) higher frequency bands

 They will support 60, 120 and 240 kHz SCS.

Another reason that higher frequencies support larger SCS is because they experience greater Doppler shift.

Need for different SCS in 5G

As frequency increases, Inter Carrier Interference (ICI) also increases. If default Sub Carrier Spacing (SCS) of 15 kHz is used, then there is a high chance of ICI. So, in order to mitigate this problem, different SCS was introduced in 5G NR.

Radio Frame Structure

The support of multiple subcarrier spacing has a huge impact on physical layer parameters as well as UL/DL frames.

As in LTE, the duration of 1 frame (10 ms) and 1 subframe (1 ms) is fixed.

What varies is the number of slots per subframe based on the selected numerology (μ).

As the Numerology increases, the number of slots within a subframe also increases.

  

See table below:

Another parameter that can vary is the number of OFDM symbols within a slot. This varies according to the Slot Configuration.

For Slot Configuration = 0, there are 14 Symbols per Slot.

For Slot Configuration = 1, there are 7 Symbols per Slot.

The number of Symbols per Slot is 12 in case of Extended Cyclic Prefix.

From the above information, it is clear that with more SCS, more number of Symbols can be accommodated within 1 TT1 leading to higher data rate.

Slot Format

In LTE, the assignment of UL and DL data happens at the subframe level i.e. if a subframe is configured for DL/UL, then all the symbols within that subframe can be used only to carry DL/UL data.

This is not the case in NR. The type of data being carried (UL/DL) can vary within the symbols of a subframe. This is called as Slot Format.

The symbols within the slots can be DL, UL, or flexible. 

Since NR slot structure allows for dynamic assignment of the link direction in each OFDM symbol within the slot, the network can dynamically balance UL and DL traffic and can thus optimize traffic for different service types.

Multiplexing of different numerologies

It is possible to transmit different numerologies on the same carrier frequency using a new feature called Bandwidth Parts. 

The different numerologies will be multiplexed in the frequency domain. 

The multiplexing of numerologies gives us the flexibility to give diverse services to be transmitted on the same carrier frequency; hence the possibility to introduce new challenges with interference between the different services increases like eMBB, mMTC and URLLC.

Mini Slots

A standard NR slot has 14 OFDM symbols. But 5G also supports mini-slots which can contain 7, 4, or 2 OFDM symbols. They can also start immediately without needing to wait for slot boundaries, thus enabling quick delivery of low-latency payloads. Apart from being useful for low-latency applications, they also play an important role in LTE-NR coexistence and beam-forming.

reference: spec 38.211

- Pinal Dobariya