Approximately once each decade, wireless communications standards have marched forward, advancing through 2G, 3G, 4G and now moving into 5G. The 5G New Radio (NR) standard creates a whole new era of wireless communications. The promise of everything connected, all the time, with extremely fast download speeds and ultra-low latency will require massive changes across the 5G ecosystem.
The 5G NR standard adds new operating bands with advanced ways to package and transmit signals. mmWave operating bands, wider modulation bandwidths, scalable numerologies and new initial access procedures introduce many changes to understand and implement in new infrastructure and mobile designs. Chipsets and devices will operate at higher frequencies. Devices and base stations will use new technologies to make connections and networks will evolve to handle more data, more users and different levels of service. 4G and 5G NR networks must initially work in harmony to provide seamless service for users. To fully capture 5G opportunities through new use cases and new business models, it is important to understand the 5G NR standard and develop techniques for successful implementation.
WHO DEVELOPED THE 5G NR STANDARDS?
The International Telecommunications Union (ITU) worked with operators, network equipment manufacturers (NEM) and standards organizations to define the International Mobile Telecommunications 2020 (IMT-2020) vision. The 5G NR standard specifies new features that require the development of new technologies to meet the aggressive goals shown in Table 1. The Third-Generation Partnership Project (3GPP) is responsible for the development of the 5G NR access technology specifications to meet the recommendations of IMT-2020.
The ITU and 3GPP are using a phased approach to enable widespread commercialization of 5G NR by 2020. In phase 1, 5G NR Release 15 focuses on setting the foundation for the enhanced mobile broadband (eMBB) and ultra-reliable and low latency communications (URLLC) use cases. Phase 2 will continue the evolution of 5G NR, optimizing new features like unlicensed spectrum access and connected vehicle-to-everything (V2X) communication, beginning in 5G NR Release 16, which is projected to be completed by the end of 2019. 5G NR Release 15 is forward compatible with NR Release 16 but not backward compatible with 4G Release 14. 3GPP will also continue to define enhancements to LTE-Advanced Pro (initially specified in Releases 13 and 14) in Releases 15 and 16 (see Figure 1).
The expectation is that 5G NR will operate alongside 4G LTE and deliver enhanced services. 5G NR Release 15, approved in June 2018, specifies the 5G radio access network (RAN) that will operate with both the 5G NR next-generation NodeB (gNB) and LTE evolved NodeB (eNB) base stations. 5G NR supports both standalone (SA) and non-standalone (NSA) modes of operation in phase 1. In NSA mode, the user equipment (UE) requires a legacy eNB base station with a connection to the evolved packet core (EPC), so the control plane can support 5G NR communication. In SA mode, the 5G network operates independently from the 4G core network (see Figure 2). Seven different connectivity options are defined in the 5G NR specifications, enabling different upgrade paths to the next-generation core network for NEMs.
While 5G NR Release 15 is considered complete, there is a NR late drop freeze planned for December 2018 and an abstract syntax notation (ASN) drop in March 2019 to address NR architecture connectivity options not completed in September 2018. Conformance test definitions, one of the biggest areas still in development, have a target completion date of May 2019. Release 16 will begin identifying new types of services, devices, deployment models and spectrum bands with an emphasis on URLLC enhancements for industrial IoT, utilization of unlicensed bands, V2X, UE positioning and UE power efficiency.
WHAT IS IN THE 5G NR 3GPP SPECIFICATIONS?
RAN working groups define the 5G NR specifications. The workgroup outputs are public: all documents, meeting reports and published specifications are available on the 3GPP website.1 The 5G NR specifications appear in the 38.xxx series documents. The 5G NR RAN study items and specifications define the functions, requirements and interfaces of the networks. RAN study items are followed by work items that are followed by the release of specifications.
The radio interface between the UE and the network consists of layers 1, 2 and 3 of the communications stack, commonly known as the physical layer, the data link layer and the network layer. The physical layer, defined in TS 38.200, represents the interface to the “real world”and includes the hardware and software to control this linkage. The physical layer provides a transport channel and specifies how information is transferred over the radio interface. Layers 2 and 3, defined in the TS 38.300 series (see Figure 3), work in conjunction with the physical layer. The data link layer, also known as the medium access control (MAC) layer, enables data transfer between the different networks. The MAC layer provides different logical channels to the radio link control (RLC) in the network layer. Layer 3, the radio resource control (RRC) layer, connects with the nodes in the network so that the UE can travel seamlessly throughout the network.
The RAN working groups are responsible for developing the 5G NR specifications in certain areas, such as the 5G NR physical layer. TR represents a technical report or study, and TS represents a technical specification. The 5G NR RAN working groups and technical specifications are:
- RAN1 (radio layer 1, TS 38.201–38.215) is responsible for the physical layer (layer 1) of the UE and the data transport to the radio interface protocol architecture (layers 2 and 3). It includes specifications of the physical channel structures, mapping of the transport channels into physical channels, multiplexing, modulation and channel coding, as well as the physical layer procedures, such as cell search, power control and beam management.
- RAN2 (radio layers 2 and 3, TS 38.300–TS 38.331) is responsible for the radio interface architecture and protocols. This includes interfaces between the 5G NR and the 5G core network. It covers the network interfaces, the physical layer and connections to MAC, RLC and the packet data convergence protocol (PDCP). RAN2 is also responsible for the RRC protocol, the strategies of radio resource management (RRM) and the services provided by the physical layer to the upper layers.
- RAN3 (radio network, TS 38.401–38.474) is responsible for the overall architecture and the protocol specifications. TS 38.2xx and TS 38.3xx in RAN1 and RAN2 define the radio interface protocols, and RAN3 defines the next-generation interface protocols.
- RAN4 (radio performance and protocol, TS 38.101–38.307) is responsible for the RF aspects of the communications and the development of the minimum requirements for 5G NR transmission and reception, as well as the parameters for channel demodulation. RAN4 also provides test procedures for base station conformance and specifications for electromagnetic compatibility (EMC), radio link, cell selection/reselection and performance supporting RRM.
- RAN5 (mobile terminal conformance tests, TS 38.508–38.533) is responsible for the specifications of conformance testing at the radio interface for the UE, based on the specification defined in RAN4 for signaling and protocol test cases. RAN5 has the responsibility for RF and signaling subgroups, including RF conformance and inter-radio access technology (RAT) procedures.
The RAN specifications introduce new frequencies and techniques for 5G NR signal creation, transmission and reception:
- Operating bands extend into mmWave: frequency range 1 (FR1) from 450 MHz to 6 GHz and frequency range 2 (FR2) from 24.25 to 52.6 GHz.
- Wider channel bandwidths up to 100 MHz for FR1 and 400 MHz for FR2, which can be aggregated to produce even wider transmission bandwidths.
- Scalable numerology with flexible allocation of resources to support many use cases and services (e.g., subcarrier spacing that scales, enabling variable slot duration for low latency, time-sensitive applications).
- Dynamic time-division duplex (TDD) and bandwidth, which deliver flexibility in resource assignments and better spectrum utilization.
5G NR signals are more complex than 4G signals. A signal’s modulation properties can be evaluated by viewing IQ constellation diagrams and error vector magnitude (EVM), EVM per symbol and EVM by subcarrier. This analysis provides some insight into the signal’s performance. The radio layer protocol tests ensure a device is performing as expected. A network emulator can provide the protocol messages with specific numerologies and frame structures to test the key performance indicators (KPI) and validate the performance of a device prior to commercial deployment.
Equally important, the 5G system architecture must evolve to keep pace with the radio access changes. Key to success are supporting the variety of 5G services, the many different types of devices and the varied traffic loads. Many network operators are moving to software-defined networking (SDN) and network functions virtualization (NFV). Distributed cloud, network slicing and self-optimizing networks (SON) are key enabling technologies. These new technologies help virtualize the network architecture and management plane to create enhanced communication capability. In parallel with 5G NR, the 3GPP system architecture (SA) work identifies the features and functionality needed to deploy a services-based operational network for 5G. These specifications are contained in 3GPP TS 23.xxx documents.
CONFORMANCE, PRE-CONFORMANCE AND DEVICE ACCEPTANCE TESTING
A major milestone in the development of devices and base stations is passing the conformance and compliance tests outlined in the 3GPP RAN4 and RAN5 specifications. All UEs and base stations must pass the required conformance tests before being released to the market. Conformance tests, however, only provide a minimum pass/fail result, offering no indication of how the device will perform when integrated into a wireless communications system. Device and base station manufacturers will test a wider set of parameters using verification and regression testing to ensure quality and sufficient margins. Pre-conformance testing is also done to check the confidence of a “pass” before conformance testing. This reduces the time and expense of rework in case the device fails official conformance tests (see Figure 4).
UE conformance tests involve connecting a device to a wireless test system and performing the required 3GPP tests:
- RF transmission and reception to a minimum level of signal quality.
- Demodulation to determine data throughput performance.
- RRM to assure initial access, handover and mobility.
- Signaling to assess the upper layer signaling procedures.
Even though 5G NR Release 15 is complete, most of the 5G NR conformance tests will not be defined until May 2019. One of the key challenges will be testing the radiated performance of the device antenna. The use of mmWave multi-element antenna arrays integrated into RFIC requires over-the-air (OTA) testing to validate beam steering and performance. It is important to validate designs using OTA test methods approved by 3GPP. To date, 3GPP has approved three RF performance OTA test methods for UE devices (see Table 2). Base station tests are still in development. An OTA test solution typically includes an anechoic chamber, probing and the test equipment to perform a wide range of RF, demodulation and functional performance tests at sub-6 GHz and mmWave frequencies. Third party labs conduct conformance tests to ensure equipment vendors do not influence the results. Test systems used to perform conformance tests must be validated and calibrated to ensure they perform the tests under controlled conditions with known uncertainties.
Once a UE passes conformance testing, the device is validated on a specific network. Device acceptance testing is operator specific and used to evaluate whether the device has sufficient performance to meet the goals set by the operator. For example, some networks make claims such as the “fastest network” or the “most reliable” network. In these cases, the operator acceptance tests include performance and functional tests to ensure the device will deliver the promised service on their network. Many operators expect to have 5G NR acceptance tests available in 2019.
It will be especially important for device and base station manufacturers to test the radiated RF performance of their designs early in the design cycle. Multi-element antennas will need validation of their 3D beam performance and measurements such as EVM and adjacent channel power ratio (ACPR) during movement to ensure performance across the antenna range. Initial access and beam management tasks like handovers and 4G fallback procedures evaluated early in the design cycle will help ensure proper operation across the wireless network.
5G NR promises everything connected, all the time, with extremely fast download speeds. With 5G NR Release 15 approved, developers are working on 5G NR devices and base stations. 5G NR expanding into higher frequencies and using new technologies adds complexity in the way signals are built, transmitted and processed. As the standard continues to evolve, more features are added and conformance and acceptance tests are completed, 5G NR designs need to be flexible and ready to adapt to the higher mmWave frequencies with wider bandwidths, denser waveforms and a growing number of test cases.
- 3GPP Specifications Groups, www.3gpp.org/specifications-groups.