This eight-part series delves into the world of 5G, with this article being the fourth in the series. The aim of this series is to clarify key terms and technologies surrounding 5G, while also giving an overview of both current and future applications for 5G connectivity. Originally published in an e-magazine, the articles have since been revised and updated by Wevolver for distribution on their platform. The series is made possible by the support of Mouser, an online distributor of electronic components, who are dedicated to fostering innovation and knowledge for a more connected future.
The International Telecommunication Union Radiocommunication Sector (ITU-R) envisioned a unified mobile communication framework called Fifth-Generation (5G) standard in September 2015. This standard supports high data-speed applications, mission-critical communications with ultra-reliable and low-latency requirements, and the connectivity of billions of devices that can communicate with each other autonomously. Since then, standard-setting organizations have been racing to develop the technology components necessary to meet the long-term objectives of 5G while supporting urgent short-term market needs.
In late 2015, the 3rd Generation Partnership Project (3GPP™) began defining a new Radio Access Network standard called 5G New Radio (NR) to meet ITU-R requirements. The standardization process involved channel modeling of spectrum up to 100 GHz, which allows for larger bandwidth allocation, higher throughput, and lower latencies. 3GPP uses a phased approach to standardization, with each phase called a release. The first phase of 5G NR, known as Release 15, supports enhanced mobile broadband and basic ultra-reliable low-latency communications (URLLC) in spectrum up to 52.6 GHz. The first version of the Release 15 specification was available in December 2017.
Just Completed: Release 16
In October 2020, the 3GPP completed the second phase of 5G NR, called Release 16, after working tirelessly to meet its objectives.
Release 16 was completed in two stages: the first stage in late 2019 focused on physical layer aspects, while the second stage in late 2020 dealt with higher layer aspects.
Release 16 introduces new capabilities that cater to vertical markets, improving the operational efficiency of the Radio Access Network and further enhancing capacity and spectrum efficiency. This article provides an overview of the spectra available in 5G NR and the physical layer technologies that make 5G NR possible.
5G NR Spectrum
In response to the increasing demand for mobile data and faster data rates, along with the plentiful spectrum available in the 3 to 100 GHz range, regulators and standard organizations logically decided to explore the utilization of centimeter wave (cmWave) and millimeter wave (mmWave) spectrum for mobile communication. Additionally, they considered a framework for shared spectrum with existing technologies.
In Release 15, 3GPP defined two frequency ranges: Frequency range 1 (FR1) spanning from 450 MHz to 7.125 GHz, and frequency range 2 (FR2) covering 24.25 to 52.6 GHz (as depicted in Table 1).
Ongoing studies are underway to assess the availability and regulatory requirements for spectrum within the 52.6 to 114.25 GHz range, as well as potential use cases and deployment scenarios.
Adopting millimeter waves for communication poses challenges due to the unique characteristics of the radio channel and the higher susceptibility to attenuation caused by atmospheric conditions. However, by leveraging shorter wavelengths and densely-packed, smaller antenna arrays, it becomes feasible to design antenna systems comprising numerous elements that offer substantial antenna gains and focused beams. This strategic approach compensates for the amplified losses associated with millimeter waves and potentially mitigates interference through the implementation of narrow beams.
In order to achieve higher throughput, NR utilizes several techniques, including:
- Higher spectrum utilization: NR can reach up to 98 percent spectrum utilization compared to LTE’s 90 percent.
- Higher post-FFT subcarrier occupancy: NR increases the number of occupied subcarriers by 25 percent, resulting in a 25 percent increase in channel bandwidth.
- Larger FFT size: NR supports a maximum FFT size of 4,096, which is double that of LTE.
- Higher subcarrier spacing: NR allows for a maximum subcarrier spacing of 120kHz, which increases channel bandwidth by a factor of eight without increasing the number of subcarriers. In FR1, subcarrier spacings of 15, 30, and 60kHz are allowed, while in FR2, subcarrier spacings of 60 and 120kHz are allowed.
Table 2 provides examples for FR1 and FR2, illustrating how these techniques increase throughput.
5G NR Physical Layer Aspects
The air interface for 5G NR is based on orthogonal frequency-division multiplexing (OFDM), where each subcarrier in the radio signal is allocated to channels and signals for transmission. NR uses OFDM access as the multiple-access scheme. For both downlink and uplink transmissions, without multiuser multiple input, multiple output (MIMO), different users are assigned to different subcarriers.
The waveform of the transmitted signal in NR uses cyclic prefix (CP)-OFDM for both downlink and uplink transmissions. In the uplink, a discrete Fourier transform-spread OFDM (DFT-S-OFDM) waveform is also used due to its low peak-to-average power ratio (PAPR) for user equipment (UE) in power-limited regions. DFT-S-OFDM is used only for single-layer uplink transmissions. In this case, the signal goes through a DFT precoder before reaching the subcarriers allocated for the transmission. A CP is appended to the OFDM symbol after conversion back to the time domain.
5G NR is a versatile air interface that can operate over a wide range of frequencies, from less than 1GHz to tens of gigahertz.
The bandwidth is adjusted in proportion to the carrier frequency to maintain a reasonable bandwidth-to-carrier frequency ratio. However, using only the number of subcarriers to support different channel bandwidths can result in a lack of multiplexing granularity for small channel bandwidths and increased hardware complexity for large channel bandwidths.
To address this, the subcarrier spacing can be scaled, which allows for a reasonable range of subcarriers to be maintained while still supporting a variety of channel bandwidths.
The subcarrier spacing is designed to scale in powers of two, with supported spacings of 15, 30, 60, 120, or 240kHz (with the 240kHz spacing used only for synchronization signals and the broadcast channel).
5G NR incorporates several technology improvements to facilitate low-latency operations:
- Higher subcarrier spacing allows for shorter symbol and slot durations, resulting in reduced latencies.
- Short physical downlink-shared channel and physical uplink-shared channel transmissions can be as brief as two symbols.
- A flexible scheduling and timing framework, along with support for different UE processing capabilities, enables a 5G NR network to optimize downlink and uplink transmission times based on UE processing capabilities and corresponding traffic latency requirements.
- Preconfigured uplink resources, such as configured grants, allow the UE to transmit uplink data autonomously.
The 5G NR air interface has introduced new channel coding techniques to improve data and control channel reliability:
- For data channels, the 5G NR air interface uses Low-Density Parity-Check (LDPC) codes.
- For control channels with a payload of 12 bits or more, Polar codes are used.
- For control channels with a payload of 3 to 11 bits, Reed-Muller codes are employed.
- For control channels with 1 or 2 bits, Repetition or Simplex codes are respectively used.
Massive MIMO in 5G NR
5G NR utilizes massive MIMO, the latest extension of MIMO technology, to establish a larger set of antenna arrays with a greater number of elements. This enhances the efficiency of the 5G network infrastructure, with the aim of improving wireless capacity through higher order spatial multiplexing, and coverage through beamforming. To achieve this, 5G NR has developed a flexible and scalable framework for massive MIMO, which supports carrier frequencies ranging from less than 1GHz to mmWaves, and different kinds of antenna array architectures (digital, analog, and hybrid), in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) modes of operation.
5G NR employs two mechanisms to enhance spatial multiplexing in its MIMO framework. The first is to support multiple transmission layers through multiple antenna ports for channel transmission. The second is to measure the channel to determine its rank, and then precode information and feed it back to the transmitter, enabling optimal precoding of the transmission layers across the available antenna ports.
In the downlink of 5G NR, the SU-MIMO framework supports eight layers per UE, with up to four layers per code word. Meanwhile, for MU-MIMO, there can be up to 12 orthogonal demodulation reference signal (DM-RS) ports across all UEs.
To determine the channel rank and MIMO precoding coefficients in the downlink, the network can use either the Channel State Information (CSI) framework or the Sounding Reference Signal (SRS) framework. The CSI framework is composed of two parts: CSI acquisition and CSI reporting. CSI acquisition relies on the CSI Reference Signal (CSI-RS) resource setting, which configures the CSI-RS resources. On the other hand, CSI reporting is done through the CSI report setting, which configures the resources on which the CSI reports. Trigger states are used by the network to configure a link between the CSI resource and CSI report settings.
Alternatively, for TDD systems, the network can make use of the reciprocity of the downlink and uplink channels that transmit in the same frequency band, and use the SRS framework. In this framework, the UE transmits SRSs on multiple antenna ports, and the gNB measures the channel quality and rank based on the uplink channel. This information is then used to determine the channel rank and MIMO precoding coefficients in the downlink.