Subcarrier spacing is a crucial parameter in both 5G (Fifth Generation) and LTE (Long-Term Evolution) wireless communication systems. It refers to the frequency separation between individual subcarriers within a communication channel. The choice of subcarrier spacing has implications for system performance, spectral efficiency, and compatibility with different deployment scenarios. Let’s explore the subcarrier spacing in detail for both 5G and LTE:
Subcarrier Spacing in LTE:
- LTE OFDM (Orthogonal Frequency Division Multiplexing):
- In LTE, the downlink (DL) and uplink (UL) transmissions use OFDM, which divides the available frequency spectrum into multiple orthogonal subcarriers.
- LTE typically employs two subcarrier spacings: 15 kHz and 7.5 kHz.
- The 15 kHz subcarrier spacing is the standard configuration, providing a balance between spectral efficiency and system complexity.
- The 7.5 kHz subcarrier spacing is used in scenarios that require higher precision, such as positioning services.
- Cyclic Prefix:
- LTE subcarriers are spaced in a way that allows them to be orthogonal to each other, minimizing interference.
- A cyclic prefix is added to each symbol to guard against inter-symbol interference and multipath fading.
- The cyclic prefix duration depends on the chosen subcarrier spacing.
- Downlink Control Channels:
- The downlink control channels in LTE, such as Physical Downlink Control Channel (PDCCH) and Physical Control Format Indicator Channel (PCFICH), are mapped to specific resource blocks based on the subcarrier spacing.
- Uplink Control Channels:
- The subcarrier spacing influences the structure and mapping of uplink control channels, such as Physical Uplink Control Channel (PUCCH).
Subcarrier Spacing in 5G:
- 5G NR (New Radio) OFDM:
- 5G NR also uses OFDM as the foundation for both downlink and uplink transmissions, but it introduces flexibility in choosing subcarrier spacings.
- 5G NR supports multiple subcarrier spacings, offering more adaptability to diverse deployment scenarios.
- Common subcarrier spacings in 5G NR include 15 kHz, 30 kHz, 60 kHz, and 120 kHz.
- Numerology in 5G NR:
- 5G NR introduces the concept of numerology, which defines the subcarrier spacing, slot duration, and other time-frequency parameters.
- Different numerologies support different use cases. For example, a smaller subcarrier spacing may be suitable for wide-area coverage, while a larger spacing may be used for enhanced data rates in specific environments.
- Mini-Slots and Flexibility:
- 5G NR introduces mini-slots, allowing for more flexible scheduling of communication resources.
- The flexibility in subcarrier spacing and numerology supports diverse deployment scenarios, including enhanced mobile broadband (eMBB), massive machine-type communication (mMTC), and ultra-reliable low-latency communication (URLLC).
- Enhancements for URLLC:
- The ability to configure smaller subcarrier spacings, shorter slot durations, and reduced transmission time intervals (TTIs) in 5G NR supports ultra-reliable low-latency communication, addressing the stringent requirements of URLLC use cases.
- Frequency Range Extensions:
- 5G NR allows for frequency range extensions to millimeter-wave bands, where wider subcarrier spacings can be employed to leverage the available spectrum efficiently.
Summary:
- LTE:
- Common subcarrier spacings: 15 kHz (standard) and 7.5 kHz (for higher precision).
- Fixed subcarrier spacings with less flexibility compared to 5G.
- 5G NR:
- Multiple subcarrier spacings: 15 kHz, 30 kHz, 60 kHz, 120 kHz.
- Introduction of numerology for flexibility in time-frequency parameters.
- Supports diverse deployment scenarios through configurable numerologies.
In conclusion, both LTE and 5G leverage OFDM with subcarrier spacing, but 5G NR introduces greater flexibility with multiple spacings and numerology. This flexibility allows 5G to address a wider range of use cases and deployment scenarios, contributing to the adaptability and efficiency of 5G networks.