Frequency Division Multiplexing of Numerologies for 5G New Radio (NR)

The evolution of wireless communication has led to increasingly complex and diverse requirements, especially in the 5G era. One of the key innovations in 5G New Radio (NR) is its flexible and scalable waveforms based on numerologies, which enable the system to support varied applications ranging from enhanced mobile broadband to ultra-reliable low latency communications and massive Internet-of-Things (IoT). However, these different numerologies coexist on the same spectrum, resulting in challenges in multiplexing and interference management.

Understanding Numerologies in 5G NR

In traditional wireless systems like LTE, a fixed subcarrier spacing (SCS) and symbol duration were applied. The 5G NR standard introduces the concept of multiple numerologies, where each numerology is characterized mainly by its subcarrier spacing and cyclic prefix length. The primary motivation behind this is to support multiple service types with different requirements in one unified framework.

Numerology Parameters

• Subcarrier Spacing (SCS): The spacing between subcarriers in the OFDM waveform, typically multiples of 15 kHz (e.g., 15, 30, 60, 120, or 240 kHz).
• Symbol Duration and Slot Length: Inversely proportional to SCS; higher SCS means shorter symbol duration.
• Cyclic Prefix (CP): Length of the CP can also vary depending on deployment and subcarrier spacing, ensuring robustness against multipath.

These parameters are standardized in 3GPP Release 15 and beyond. For example, a numerology with 15 kHz SCS is suited for wide-area coverage and low mobility, whereas 120 kHz SCS is relevant for high-frequency millimeter-wave bands and low latency.

Why Use Multiple Numerologies?

5G is designed to meet diverse use cases:

• Enhanced Mobile Broadband (eMBB): Requires high throughput and wide bandwidth, typically served by numerologies with smaller subcarrier spacing (e.g., 15 kHz) for better spectral efficiency.
• Ultra-Reliable Low Latency Communications (URLLC): Demands very low latency and reliable transmissions, often using wider subcarrier spacing (e.g., 60 kHz) to shorten symbol times and slot durations.
• Massive Machine Type Communications (mMTC): Characterized by many devices with sporadic traffic, sometimes adopting numerologies optimized for energy efficiency and robustness.

To accommodate these simultaneously, 5G NR employs multiple numerologies on the same carrier or frequency range, necessitating effective multiplexing strategies.

Frequency Division Multiplexing (FDM) of Numerologies

The coexistence of different numerologies on the same spectrum is commonly achieved by assigning separate frequency bands or subbands to each numerology. This is referred to as Frequency Division Multiplexing (FDM) of numerologies.

In FDM, each numerology occupies its own frequency portion so that the different numerologies operate side by side in the frequency domain. This approach allows the base station (gNB) to flexibly allocate resources according to the service needs without time-sharing or additional complex scheduling constraints.

Basic Concept

Imagine a frequency bandwidth of 100 MHz. A portion of it (e.g., 60 MHz) can be assigned to a numerology with 15 kHz SCS (for eMBB), while the remaining 40 MHz is allocated to a numerology with 60 kHz SCS (for URLLC). Each segment is then independently managed with the appropriate waveform parameters. This separation in the frequency domain avoids overlap in time and frequency of signals with different numerologies.

Advantages of Frequency Division Multiplexing for Numerologies

• Isolation: By separating numerologies in frequency, interference between them is reduced compared to other multiplexing methods.
• Scheduler Flexibility: Enables independent scheduling of different numerologies, simplifying resource allocation.
• Network Efficiency: Supports diverse service types without compromising specific QoS requirements.
• Simplified Implementation: Receiving devices can demodulate based on their assigned numerology frequency segment without complex synchronization to others.

Challenges and Considerations in FDM of Numerologies

Although FDM provides a practical way to multiplex multiple numerologies, it introduces several technical challenges. The primary concern is inter-numerology interference (INI) occurring at the boundaries of frequency segments.

Inter-Numerology Interference (INI)

INI arises because the different numerologies have different subcarrier spacings and symbol timings. This leads to imperfect orthogonality between subcarriers of different numerologies especially near the frequency edges of the assigned bands. The OFDM waveforms with different parameters do not align perfectly, resulting in spectral leakage and interference.

INI can degrade the performance of the users, particularly those located at the edge of frequency partitions, and makes coexistence less efficient if not managed properly.

Guard Bands

To mitigate INI, 3GPP recommends introducing guard bands small unused frequency gaps between the numerology allocations. The width of the guard band depends on:

• Numerology spacing difference
• Transmit filter characteristics
• Required interference levels
• Overall spectral efficiency constraints

While guard bands reduce interference, they also reduce usable bandwidth, which lowers spectral efficiency. Thus, network designers must balance interference management and capacity.

Filtering and Windowing Techniques

Another approach to reduce INI is the use of advanced filtering or windowing methods at the transmitter and/or receiver. Examples include filtered-OFDM (f-OFDM), universal filtered multi-carrier (UFMC), and windowed-OFDM:

• Filtering: Applies subband-level filters to shape the spectrum and reduce out-of-band emissions.
• Windowing: Smooths transitions between OFDM symbols to control spectral leakage.

These techniques increase complexity and processing latency but are considered potential enhancements for reducing INI in FDM numerology multiplexing.

Synchronization Issues

In multi-numerology FDM, synchronization among numerologies must be carefully managed, especially when:

• Numerologies have different symbol lengths and timing offsets
• User Equipment (UE) is capable of supporting multiple numerologies simultaneously

Precise timing and frequency alignment minimize cross-numerology interferences and improve system robustness.

Example of Numerology Multiplexing in 5G NR Spectrum

Consider a 5G NR deployment in the sub-6 GHz band with a 100 MHz channel bandwidth. The network operator might assign:

• Carrier bandwidth from 0 to 60 MHz: 15 kHz numerology for coverage-oriented eMBB services.
• Carrier bandwidth from 60 MHz to 90 MHz: 30/60 kHz numerology for URLLC slices requiring low latency.
• Carrier bandwidth from 90 MHz to 100 MHz: Reserved as a guard band to minimize interference.

This kind of frequency partitioning ensures each service type operates optimally with the suitable waveform parameters and minimizes their mutual interference.

Standardization and Implementation in 3GPP

The 3GPP specifications (TS 38.211 and TS 38.213) define the framework for supporting multiple numerologies in 5G NR and discuss multiplexing strategies. The concept of Bandwidth Parts (BWPs) plays a key role in multiplexing numerologies where the carrier bandwidth is split into BWPs, each configured with a specific numerology.

BWPs enable:

• Assignment of different numerologies within the same carrier
• Dynamic switching among numerologies for user equipment
• Efficient use of spectrum by activating only necessary bandwidth

The flexibility embedded in the BWP architecture makes frequency division multiplexing of numerologies a deployable feature that can evolve dynamically with user demands.

Future Trends and Research Directions

As 5G NR further evolves and as we progress towards 6G, multi-numerology multiplexing will likely become more sophisticated. Areas of emerging interest include:

• Adaptive guard band allocation: Dynamically adjusting guard bands based on real-time interference measurements.
• Advanced waveforms: Incorporating new multicarrier schemes with better spectral confinement to reduce INI.
• Machine learning techniques: Utilizing AI to optimize scheduling and multiplexing for multiple numerologies in real-time.
• Cross-numerology interference cancellation: Developing receiver algorithms that can decode and cancel interference from neighboring numerologies.
• Integration with mmWave and beyond: As mmWave bands use very high subcarrier spacing, efficiently combining those with sub-6 GHz numerologies requires innovative FDM strategies.

Conclusion

Frequency Division Multiplexing of numerologies is a fundamental mechanism in enabling 5G NR to support its diverse set of service requirements. By allocating distinct frequency ranges to different subcarrier spacings and symbol durations, 5G NR can optimize performance for eMBB, URLLC, and mMTC simultaneously.

This approach, however, introduces challenges especially related to inter-numerology interference, requiring careful design of guard bands, filtering, and synchronization techniques. Ongoing standardization efforts and research continue to enhance the efficiency and flexibility of multi-numerology FDM, ensuring that 5G networks remain robust and scalable as demands grow.

For network operators, understanding and effectively managing frequency division multiplexing of numerologies is key to unlocking the full potential of 5G NR and future wireless generations.

References and Further Reading

• 3GPP TS 38.211 - NR Physical Channels and Modulation
• 3GPP TS 38.213 - NR Physical Layer Procedures
• Sesia, S., Toufik, I., & Baker, M. (2011). LTE - The UMTS Long Term Evolution: From Theory to Practice. Wiley.
• Dahlman, E., Parkvall, S., & Skld, J. (2020). 5G NR: The Next Generation Wireless Access Technology. Academic Press.
• Ghosh, A., Ratasuk, R., Mondal, B., Mangalvedhe, N., & Thomas, T. (2014). LTE-advanced: next-generation wireless broadband technology. IEEE Wireless Communications, 17(3), 10-22.