Unified Air Interface- 5G NR is the foundation of 5G, which enables new services like massive IoT, mission-critical control such as autonomous driving, remote surgery, health monitoring etc.,
To offer this kind of services the new radio utilizes OFDM (Orthogonal frequency-division multiplexing), a waveform modulation technique. This technique is also used by LTE and IEEE 802.11(Wi-Fi).
TR 38.913 describes the key performance indicators for next-generation access technologies. Based upon the 5G new radio scenarios and KPIs, there was a need for a common numerology framework. This framework helps to cover the complete frequency ranges up to 100GHz and support for all the use case scenario deployments.
The utmost critical use case of 5G is the Low-Latency services. To support Low-Latency services, not only fast radio interface but also increase in the speed of radio waves is required. To reduce the radio latency, the NR uses the scalable numerologies.
Let’s get a little deeper into what is scalable numerology???…. “The term numerology refers to the configuration of waveform parameters” … based on the term in 3GPP specification “It is a subcarrier spacing type”. This subcarrier spacing is capable of scaling with the channel width with variable guard interval (Cyclic Prefix).
In all mobile communication technologies, we don’t need any specific terminology to mention subcarrier spacing since these technologies use only one subcarrier spacing. Whereas in 5G NR there are several types of subcarrier spacing is used.
Let us consider a scenario where the LTE radio resources are sliced into so-called thin sub-carriers with a subcarrier spacing of 15KHz. The LTE Base Station, i.e., eNB allocates resources to terminals by splitting 12 such sub-carriers in the frequency domain.
In the time domain, radio resources are allocated and reallocated in 1ms intervals. This kind of provision is considered as a flexible allocation before ten years, but in NR case, the 1ms Transmission Time Interval (TTI) is static and inflexible.
But the question here is “when this kind of allocations is made inflexible?” … Consider the scenario of ultra-low latency in which it has to spend 1ms in a waiting line for the required radio resources.
So, the scalable numerology has paved the way for the implementation of diverse use case.
The formula for subcarrier spacing (SCS) in general is,
Where µ- actual number referred to numerologies. This means that depending on µ, the SCS will take values which are multiples of 15KHz. The combination of and cyclic prefix together defines the numerology.
The LTE resource structure is given in the below figures,
|Radio frames||10ms length|
|Subframes||1ms length. One frame consists of 10 subframes|
|Slot||0.5ms length. One subframe consists of two slots|
|OFDM symbol||Approx. 71.4µs length. One slot consists of 7 OFDM symbols.|
The NR is also having the frame duration of 10ms which is split into ten 1ms subframes. Each subframe consists of number of slots depends on µ, which is represented as . OFDM symbols per slot is not altered by µ, it is a constant value which as 14 symbols per slot. The number of slots per frame simply multiple of 10.
|µ||No. of OFDM symbols in slot||No. of Slots in a frame||No. of slots in a subframe|
The number of OFDM symbol is constant for different numerologies ‘µ’. LTE 4G has OFDM symbols 12 or 14 depends on length of cyclic prefix. Each symbol carries cyclic prefix and OFDM symbol duration.
When µ= 0 it is a baseline structure which suits for earlier generation system, it has one slot and number of symbols per subframe is 14.
When µ=1 it has two slots with the 28 OFDM symbols in all subframes. So, the Ts, TCP is 33.33µs and 2.34µs which is half when compared to the previous case.
OFDM employs a large number of subcarriers. Each subcarrier is placed orthogonally in nature. Subcarrier spacing is equal to the subcarrier bandwidth, which is 15KHz as given in the below figure.
The resource unit is the combination of the smallest time domain unit, one OFDM symbol and the smallest frequency domain unit, one subcarrier. This unit is known as the Resource Element (RE).
Several REs are grouped into a Physical Resource Block (RB). RB is 7 OFDM symbol times in the time domain and 12 consecutive subcarriers in the frequency domain. The scheduling block consists of two RBs adjacent in time, and it is the smallest unit that can be scheduled to UE.
Each OFDM symbol is consists of several subcarriers depending upon the size of bandwidth that is available. A Group of 12 subcarriers forms a resource block, which constitutes of resource elements and each resource element is one subcarrier of one OFDM symbol for an antenna port.
As seen before depends on µ value the Bandwidth is allocated for the transmission. So µ=1 will make the resource element longer and narrower in bandwidth whereas the resource grid will remain the same.
From this we can conclude that depends on µ various uses cases of 5G can be designed flexibly. This provides a flexible Physical layer which was much needed for a long time in the telecom industries.