Introduction
NR, short for New Radio, is also referred to as the new air interface. The term "new air interface" contains two parts: "new" and "air interface". This raises two questions: what is the air interface, and what is new about it?
What Is the Air Interface?
A mobile communication system primarily consists of three parts: the terminal, the base station, and the core network, which form two subsystems. The terminal and base station form the radio access subsystem, while the core network comprises many complex network elements forming the network subsystem.
Typically, the terminal is the mobile phone, the base station is the tower that transmits signals, and the core network sits behind them, essentially functioning as a router that links base stations so they can intercommunicate. The interface between the mobile phone and the base station travels through the air as electromagnetic waves, so it is called the air interface. The air interface is specific to the radio access subsystem and does not directly involve the core network.
In the 5G era, the independence between the radio access subsystem and the core network has increased. As a result, 5G is flexible: it can be deployed as standalone or as non-standalone together with 4G. In standalone mode, 5G base stations connect to a 5G core network; in non-standalone mode, 5G base stations and 4G base stations can connect to either the 4G core network or the 5G core network.
5G Network Deployment Options
The 5G radio access subsystem can include both 4G and 5G base stations, and the network subsystem can include both 4G core and 5G core networks. Because 5G base stations can connect to 4G core networks and vice versa, the radio access and core network can evolve independently.
What Is New About the 5G Air Interface?
The diagram below shows the non-standalone deployment options 3 and 7, where both 4G and 5G base stations connect to the same core network, forming a non-standalone 5G architecture.
In this arrangement, regardless of whether the connected core network is the 4G EPC or the 5G 5GC, a single mobile device may have two air-interface connections: the link to the 4G base station is the "old air interface", and the link to the 5G base station is the "new air interface". The 5G air interface is designed based on 4G air interface technology and also uses OFDM modulation. It refines some aspects of the 4G frame structure and adds support for massive connections and low latency, making it more flexible and spectrally efficient.
How Do Operators Deploy 5G NR?
Operators generally follow three deployment approaches:
1. Non-Standalone (NSA): LTE + 5G NR millimeter wave
This approach, used by operators such as Verizon and AT&T, deploys 5G NR millimeter wave on existing LTE networks to supplement hotspot coverage or for fixed wireless access.
2. Non-Standalone (NSA): LTE + sub-6 GHz NR bands
This option can rapidly improve 5G coverage, but introduces complexity in interfaces and carrier aggregation between LTE and 5G NR.
For non-standalone deployments there are two evolution paths:
- Path 1: Option 3 series -> Option 2: Deploy the 5G radio access network first, then deploy the 5G core, and finally migrate the 5G radio access network to the 5G core network.
- Path 2: Option 3 series -> Option 7 series or Option 5: Deploy the 5G radio access network first, then deploy the 5G core, and finally connect both 4G and 5G radio access networks to the 5G core.
3. Standalone deployment
Standalone deployment involves deploying a complete 5G network end to end. This simplifies the migration from NSA to a 5G core, and reduces some complexity, but requires comprehensive 5G coverage and ecosystem support.
How Does 5G NR Define and Allocate Spectrum?
3GPP has specified a list of bands supported by 5G NR. The 5G NR spectrum extends up to 100 GHz and is divided into two frequency ranges:
1. Frequency Range 1 (FR1)
FR1 generally refers to bands below 6 GHz.
- Frequency range: 450 MHz - 6.0 GHz
- Maximum channel bandwidth: 100 MHz
2. Frequency Range 2 (FR2)
FR2 corresponds to millimeter-wave bands.
- Frequency range: 24.25 GHz - 52.6 GHz
- Maximum channel bandwidth: 400 MHz
5G NR supports up to 16 component carrier (CC) aggregation.
5G NR defines flexible subcarrier spacing; different subcarrier spacings apply to different frequency ranges, as illustrated below.
5G NR bands include FDD, TDD, SUL and SDL. SUL and SDL are supplementary bands for uplink and downlink, respectively.
Unlike LTE, 5G NR band numbers are prefixed with "n". For example, LTE Band 20 (B20) is referred to as n20 in 5G NR.
3GPP-specified 5G NR bands include the following ranges:
5G NR includes some LTE bands and also introduces new bands. The bands most likely to be prioritized globally are n77, n78, n79, n257, n258, and n260, corresponding to 3.3–4.2 GHz, 4.4–5.0 GHz, and millimeter-wave bands at 26 GHz, 28 GHz, and 39 GHz.
