The Fifth Generation of New Radio technology introduces a revolutionary concept known as numerology, which serves as the foundational structure for flexible and efficient wireless communication. This framework, defined by the 3rd Generation Partnership Project (3GPP), allows the network to dynamically adapt to various frequencies, bandwidths, and latency requirements—a capability that previous generations like LTE struggled to provide effectively. At its core, 5G NR numerology defines the parameters that organize Orthogonal Frequency Division Multiplexing (OFDM) signals, specifically focusing on subcarrier spacing, symbol duration, and cyclic prefix. By utilizing a scalable approach, 5G NR can support a wide range of applications, from enhanced mobile broadband to ultra-reliable low-latency communication and massive machine-type communications.
The concept of numerology in 5G NR is essentially a set of configurations that dictate how subcarriers are packed within a channel. Each configuration, referred to as a numerology index (μ), determines the spacing between subcarriers. The relationship is mathematically expressed as Δf = 15 × 2^μ kHz, where Δf represents the subcarrier spacing and μ is the index ranging from 0 to 4. This formula results in specific spacing values: μ = 0 yields 15 kHz, μ = 1 yields 30 kHz, μ = 2 yields 60 kHz, μ = 3 yields 120 kHz, and μ = 4 yields 240 kHz. Each of these numerologies is designed for specific use cases and frequency ranges, balancing factors such as coverage, latency, and bandwidth to meet diverse operational needs.
Understanding Subcarrier Spacing and Resource Elements
Subcarrier spacing (SCS) is a critical parameter in 5G NR numerology, defining the frequency gap between adjacent OFDM subcarriers. The 15 kHz spacing, which is the baseline derived from LTE, is utilized for low-band frequencies below 3 GHz, often referred to as Frequency Range 1 (FR1). However, for higher frequency bands and more demanding applications, larger spacings are necessary. The introduction of multiple larger SCS values, specifically 2^μ × 15 kHz, supports mobility requirements up to 500 km/h, addressing the need for robust performance in high-speed scenarios.
A fundamental unit in the 5G resource grid is the Resource Element (RE), which is defined by one subcarrier in the frequency domain and one OFDM symbol in the time domain. The subcarrier spacing directly influences the bandwidth of the Resource Element, while the duration of the symbol (excluding the cyclic prefix) is inversely proportional to the subcarrier spacing. As the subcarrier spacing increases, the symbol duration decreases, which has significant implications for latency and throughput. Multiple Resource Elements form a grid that serves as the basis for mapping physical channels and signals. Notably, resource block boundaries are aligned across different numerologies. This alignment means that two resource blocks at a subcarrier spacing of Δf occupy the same frequency range as one resource block at a spacing of 2Δf, ensuring efficient spectrum utilization and compatibility across diverse configurations.
Frame Structure and Scheduling Intervals
The 5G NR frame structure is designed to be flexible, adapting to the chosen numerology. A frame in 5G NR has a duration of 10 ms, similar to LTE, and consists of 10 subframes, each lasting 1 ms. However, the structure within a subframe varies based on the numerology. Each subframe is composed of slots, and the number of slots per subframe depends on the subcarrier spacing. For instance, with 15 kHz spacing (μ=0), there is 1 slot per subframe. As the spacing increases, the number of slots per subframe doubles: 30 kHz (μ=1) has 2 slots per subframe, 60 kHz (μ=2) has 4 slots, 120 kHz (μ=3) has 8 slots, and 240 kHz (μ=4) has 16 slots per subframe. Consequently, the number of slots per frame is ten times the number of slots per subframe.
Each slot consists of OFDM symbols. For normal cyclic prefix (CP), a slot contains 14 OFDM symbols. For extended CP, which is supported specifically for 60 kHz spacing, a slot contains 12 OFDM symbols. The duration of these symbols and the CP changes with the subcarrier spacing. As the spacing increases, the symbol duration shortens, and the CP duration is proportionally reduced to maintain a consistent overhead ratio (approximately 7.03% for normal CP, similar to LTE). This reduction in symbol duration directly contributes to lower latency.
Scheduling intervals are also governed by the numerology. The minimum scheduling interval can be measured in symbols, slots, or milliseconds. For all specified numerologies (15 kHz to 120 kHz), the minimum scheduling interval in symbols is 14. In terms of slots, the minimum is 1 for all. However, the duration in milliseconds decreases as the numerology index increases: 1.0 ms for 15 kHz, 0.5 ms for 30 kHz, 0.25 ms for 60 kHz, and 0.125 ms for 120 kHz. This scalability allows the network to schedule transmissions more frequently in time-critical applications.
Impact on Network Performance and Use Cases
The choice of numerology has a profound impact on various aspects of network performance, including latency, coverage, mobility, and throughput.
- Latency: Higher subcarrier spacing results in shorter symbol durations, which in turn reduces transmission latency. For example, a 120 kHz spacing yields a symbol duration of 8.3 μs, compared to 66.7 μs for 15 kHz. This enables ultra-low latency communications, essential for applications like autonomous driving and industrial automation.
- Coverage: Lower subcarrier spacing allows for longer symbols, which improves tolerance to delay spread and enhances coverage. This makes lower numerologies ideal for wide-area coverage in rural or suburban environments.
- Mobility: Smaller subcarrier spacing provides better tolerance to Doppler shifts caused by high-speed movement. This is crucial for maintaining stable connections for users in fast-moving vehicles.
- Throughput: Larger spacing supports wider bandwidths and faster data rates, which is beneficial for high-data-rate applications like high-definition video streaming and virtual reality.
5G NR supports multiple numerologies on the same carrier, allowing the network to allocate different resource sets for different services simultaneously. This multi-numerology support is a key enabler for the diverse service requirements of 5G, such as Enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communication (URLLC), and Massive Machine-Type Communication (mMTC).
Practical Deployment Scenarios
The flexibility of 5G NR numerology is best illustrated through practical deployment examples.
In an urban eMBB deployment using the 3.5 GHz frequency band, a numerology of 30 kHz is typically employed. This configuration supports a bandwidth of up to 100 MHz and achieves a latency of 0.5 ms, providing high data speeds for dense urban populations.
For mmWave deployments, such as at 28 GHz, a higher numerology of 120 kHz is used. This supports a bandwidth of up to 400 MHz and reduces latency to 0.125 ms, enabling extreme throughput and responsiveness for applications in concentrated areas like stadiums or business districts.
These examples demonstrate how operators can efficiently deliver a range of 5G applications by selecting the appropriate numerology based on frequency band and service requirements. The ability to dynamically adapt the waveform structure is what sets 5G apart from previous generations, ushering in an era of network intelligence, speed, and scalability.
Comparison with LTE
A comparison between LTE and 5G NR highlights the advancements brought by scalable numerology.
- Subcarrier Spacing: LTE uses a fixed 15 kHz spacing, whereas 5G NR offers variable spacing from 15 kHz to 240 kHz.
- Symbol Duration: LTE has a fixed symbol duration, while 5G NR features scalable symbol duration based on the chosen numerology.
- Frame Structure: LTE employs a fixed frame structure, while 5G NR utilizes a flexible frame structure that adapts to the numerology.
- Latency: LTE latency is generally 1 ms or higher, while 5G NR can achieve latencies as low as 0.125 ms.
- Frequency Range: LTE is limited to frequencies below 6 GHz, whereas 5G NR supports frequencies up to 100 GHz.
This comparison underscores the suitability of 5G NR's scalable OFDM numerology for a broad spectrum of scenarios, from low-frequency rural coverage to high-frequency urban and indoor deployments, contrasting sharply with the rigid structure of LTE.
Conclusion
5G NR numerology is the cornerstone of the flexibility and efficiency that define the 5G standard. By defining a set of parameters—subcarrier spacing, symbol duration, cyclic prefix, and scheduling intervals—that can be scaled according to need, 5G NR supports a diverse array of services and deployment environments. The mathematical foundation of Δf = 15 × 2^μ kHz provides a systematic way to balance coverage, latency, and throughput. Whether it is providing broad coverage with 15 kHz spacing or enabling ultra-low latency with 120 kHz or 240 kHz spacing, numerology allows the network to optimize performance for every user scenario. This adaptability ensures that 5G can meet the demands of future applications, from massive IoT deployments to mission-critical communications, making it a transformative technology in the world of mobile communications.