5G aims to support a wide variety of new and enhanced services, such as holograms, automated factories, and self-driving cars. The New Radio (NR)-based air interface is one of the key components of 5G. Here, we will discuss Orthogonal Frequency Division Multiplexing (OFDM) numerologies, which represent one of the key aspects of 5G NR radio characteristics. OFDM numerologies share close relationships with operating bands, subcarrier spacing, and frame structure. We will also discuss how OFDM numerologies compare with the way OFDM is used in 4G LTE.
5G NR is defined to support deployment across a wide range of frequencies. 3GPP has defined two operating frequency ranges for 5G [1] as follows:
- FR1 (Frequency Range 1) corresponds to 450 MHz – 6,000 MHz
- FR2 (Frequency Range 2) corresponds to 24,250 MHz – 52,600 MHz
The two frequency ranges vary in characteristics due to the large gap in the frequencies; hence, the requirements for each FR are defined separately within the standards as needed. 4G LTE standards were defined for operating band FR1 under 6,000 MHz frequencies only.
5G NR defines five distinct OFDM numerologies (i.e., configurations) to support radio operations in both FR1 and FR2 in Phase 1. More specifically, the parameter SubcarrierSpacing index (µ) can have five possible values in 5G Phase 1 as specified in 3GPP TS 38.211 [2]. Each value of µ maps to a specific subcarrier spacing value using the formula 2µ * 15 kHz. For the range of µ values 0 through 4, this translates into inter-subcarrier spacing of 15, 30, 60, 120 and 240 kHz, respectively. As a point of comparison, 4G LTE supports inter-subcarrier spacing of 15 kHz only. 5G NR supports normal cyclic prefix for all subcarrier spacing options mentioned above. However, unlike LTE, extended cyclic prefix is supported only for µ value of 2 (i.e., for subcarrier spacing of 60 kHz). The subcarrier spacings of 15 kHz to 120 kHz are used for shared channels, such as those carrying user traffic, while the subcarrier spacing of 240 kHz is available for synchronization signals. The specific set of subcarrier spacing values depends on the exact frequency band. For example, 28 GHz and 39 GHz mmW spectrum bands can use the subcarrier spacing of 60 kHz and 120 kHz.
In OFDM, useful symbol time Tu and subcarrier spacing Δf are related by the equation: Tu = 1/Δf. Because 5G NR defines five Δf values (15, 30, 60, 120 and 240 kHz) in Phase 1, OFDM useful symbol times Tu’s will be halved when Δf value is doubled. In other words, the corresponding Tu values for the supported Δf values in 5G NR are Tu, 0.5Tu, 0.25Tu , 0.125Tu, 0.0625Tu, where Tu = 66.67 microseconds. 5G NR defines frame and slot structure to support the different Tu values, which differ from 4G LTE.
5G NR defines a frame to be 10 milliseconds (ms) in duration, like 4G LTE. Each frame is divided into 10 subframes of 1 ms each. The 1 ms subframe is then divided into one or more slots in 5G, whereas LTE has exactly two slots in a subframe. The slot size is defined based on the Tu value. The number of OFDM symbols per slot is 14 for a configuration using normal cyclic prefix. For extended cyclic prefix, the number of OFDM symbols per slot is 12. In 5G NR, the slot can be viewed as the basis for scheduling, although different scheduling intervals are supported.
5G NR supports a wide variety of use cases, and the OFDM numerologies help with supporting these use cases effectively. When higher frequency bands are used, the Doppler shift increases. An increased Doppler shift means increased Inter Carrier Interference (ICI). Wider subcarrier spacing would provide better resistance to such increased Doppler shifts at higher frequency bands. Furthermore, 5G NR provides a flexible solution to support various applications by choosing an appropriate configuration. For example, when higher Δf is used, it results in a shorter slot duration. This would allow for faster scheduling and delivery of packets over the air interface. If we want to support an Ultra-Reliable and Low-Latency Communications (URLLC) application, we can use a relatively higher subcarrier spacing at a given frequency band (e.g., 60 kHz instead of 30 kHz). Additionally, 5G NR provides for the flexibility to use multiple subcarrier spacing values on the same carrier to help address the needs of different applications and different users. Key benefits from this approach include reduced implementation complexity and support for the different use cases for which 5G NR has been designed.
OFDM numerologies define the subcarrier spacing and the cyclic prefix value, and associated transmission parameters, such as slot length. OFDM numerologies enable 5G to support different frequency bands and diverse services cost-effectively and efficiently.
References
[1] 3GPP, TS 38.101-1
[2] 3GPP, TS 38.211
[3] 3GPP, TS 38.300
About the Author
Ramki Rajagopalan is Award Solutions co-founder and vice president. With 27 years of industry experience, he has worked on a wide variety of projects, ranging from system reliability and performance modeling and analysis, to the design and implementation of wireless and data systems. Ramki’s presentation skills, coupled with his dedication to teaching have contributed to his success in the training arena.
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