5G has so many promises it’s difficult to tell how they all can be achieved. From extreme data download speeds, to self-driving automobiles, to IoT devices monitoring over many years. One of the key enablers for these to happen is the flexible numerology recently defined in 3GPP Release 15. I find this part of 5G fascinating and think it will be a key part of 5G to support a wide range of frequencies and scheduling for many diverse services.
The top five key features of 5G flexible numerology are:
1. Subcarrier spacing is no longer fixed to 15 kHz. Instead, the subcarrier spacing scales by 2µ x 15 kHz to cover different services: QoS, latency requirements and frequency ranges. 15, 30, and 60 kHz subcarrier spacing are used for the lower frequency bands, and 60, 120, and 240 kHz subcarrier spacing are used for the higher frequency bands.
2. Number of slots increases as numerology (µ) increases. Same as LTE, each frame is 10 ms, each subframe is 1 ms. Ten subframes to a frame. In normal CP, each slot has 14 symbols. As, numerology increases, the number of slots in a subframe increase, therefore increasing the number of symbols sent in a given time. As shown in figure 1, more slots as the frequency increases results in shorter slot duration.
Slot length scales with the subcarrier spacing: slot length = 1 ms/2 μ
# slots per subframe
Figure 1. Slots within a subframe and the associated slot duration time.
3. Mini-slots for low latency applications. A standard slot has 14 OFDM symbols. In contrast, mini-slots can contain 7, 4, or 2 OFDM symbols. Mini-slots can also start immediately without needing to wait for slot boundaries, enabling quick delivery of low-latency payloads. Mini-slots are not only useful for low-latency applications, but they also play an important role in LTE-NR coexistence and beamforming.
4. Slots can be DL, UL, or flexible. NR slot structure allows for dynamic assignment of the link direction in each OFDM symbol within the slot. With this, the network can dynamically balance UL and DL traffic. This can be used to optimize traffic for different service types.
Figure 2. Link direction can be dynamically assigned.
5. Multiplexing of different numerologies. Different numerologies can be transmitted on the same carrier frequency with a new feature called bandwidth parts. These can be multiplexed in the frequency domain. Mixing different numerologies on a carrier can cause interference with subcarriers of another numerology. While this provides the flexibility for diverse services to be sent on the same carrier frequency, it also introduces new challenges with interference between the different services.
Why should you care? I see it like a multi-lane super highway with lots of control. These lanes represent the different types of services offered in 5G. You have the fast lanes that are very speedy and can handle a lot of cars. You have the slow lanes where traffic may be at turtle’s pace. And now, throw in a motorbike that can speed in and out of lanes at any time. Now you need to be concerned about traffic and possible collisions.
Flexible numerology in 5G is much different from numerology found in 4G. It enables a lot of flexibility, but it also introduces new challenges with the way waveforms are built and managed. Now you need to consider subcarrier spacing, UL, DL configurations, and bandwidth parts. The number of test cases explodes, and device designers will need to create and analyze waveforms in the frequency-, time-, and modulation domains, as well as verify the device’s performance on the network with many different numerologies.
If you are interested in learning more about 5G Numerology, I’d highly recommend watching the webinar: Understanding the 5G NR Physical Layer by Javier Campos. It provides lots of information on the new standards and goes into details on 5G numerology, waveforms, and new access procedures.