New, unified, scalable, and configurable 5G air interface

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5G is the next innovation frontier of the global mobile communications industry and ICT ecosystem. 5G will greatly expand and enhance mobile Internet application fields and support extensive IoT applications in the future.

From the perspective of technical features, 5G, compared with 3G/4G, has such advantages as higher rates, lower latency, higher reliability, stronger support for big connection, broader optional spectrums, more diversified business models, more types of deployment patterns, and more elastic network architecture.

From the perspective of application scenarios, 3G/4G mainly meets the requirements for voice and data services, and 5G focuses on enhanced mobile broadband, IoT with mass connections, and vertical industry applications. Compared with 3G/4G, 5G can reflect the deep convergence of IT and CT much more and is the key bearer to achieve IoT in the future world.

Advanced 5G applications will dramatically increase system capacity in comparison with 3G/4G. Several typical technical features include an ultra-low latency of 1 ms, ultra-large throughput of 10 Gbps and above, and ultra-large access of a million terminals.

System capacity improvement is always accompanied by challenges. Available 5G low-frequency spectrums are very limited. To lift the restriction, 5G will expand resources in high-frequency spectrums. A 5G system supports more diversified services and more types of system deployment patterns, so the 5G system needs to flexibly support the deployment of low-frequency, high-frequency, and hybrid networks. At the same time, the 5G system also needs to flexibly support ultra-large-capacity, ultra-large-scale, and ultra-low-latency applications.

Therefore, the 5G system needs to have a flexible air interface and network architecture design. After a long period of in-depth study and exploration, ZTE systematically proposed a unified, scalable, and configurable air interface and network architecture. This article discusses air interface technologies.

Key Features of 5G NR
The air interface includes the physical layer, link layer, and network layer. The physical layer is the most important layer. The main difference among 2G, 3G, and 4G lies in the physical layer of the air interface. The physical layer involves a variety of technologies including waveform, modulation, coding, duplex, antenna, and access technologies, and is highly complex to implement. Therefore, the development of physical-layer technologies of the air interface represents the development of mobile communication systems.

Compared with the 3G/4G air interface, the 5G air interface is commonly known as the new 5G air interface, and also 5G New Radio Access Technology (5G NR). 3GPP has already started the 5G NR R&D project, which is aimed at formulating an OFDM-based brand-new 5G radio access standard.

As shown in Figure 1, a good air interface design for 5G consists of three layers: a bottom layer, a middle layer, and a top layer. At the bottom layer, an abstract physical layer using the waveform and frame structure of a unified framework design is added. The bottom layer flexibly adapts itself to the requirements of various services and full-frequency-band deployment and is completely transparent to different services and frequency bands. The middle layer can provide flexible network slicing and elastic resource allocation based on service types. The top layer has the service sensing function and achieves dynamic and intelligent service convergence.

Figure 1 ZTE UAI Architecture

Through the new air interface technologies, 5G will connect everything, from smart mobile phones, cars, or even to the infrastructure of the entire city. Interconnection and interoperability of all these things will be achieved through 5G NR. In addition to serving as a unified framework for connection, 5G NR will also raise network data transmission rates, capacity, latency, reliability, efficiency, and coverage to a new level, and will make full use of every bit of the available spectrums.

According to the 5G vision proposed by the ITU, in the future, 5G networks will provide necessary support for three scenarios: Enhanced Mobile Broadband (eMBB), Massive Machine Type Communication (mMTC), and Ultra-Reliable Low Latency Communications (uRLLC). To entirely overcome the technical challenges from the scenarios, 5G NR uses a series of new technologies and designs, such as the Massive MIMO antenna technology, self-contained frame structure, OFDM-based extensible waveform technology, low latency–based timeslot structure, and advanced coding and modulation scheme.

Compared with Pre5G, the Massive MIMO technology is more flexible and forward compatible, gives consideration to both high and low frequency deployment, and achieves rapid switch between various transmission mechanisms.
The self-contained subframe design contains both uplink and downlink transmission in the same subframe, implementing data transmission and decoding confirmation, and realizing a lower latency. The self-contained subframe design also supports forward compatibility and adaptive UL/DL configuration, and satisfies the requirements of the Massive MIMO technology.
Based on the OFDM waveform technology, in the eMBB and uRLLC scenarios with a frequency band lower than 40 GHz, 5G NR will use the CP-OFDM waveform in the downlink and CP-OFDM (as primary waveform) and DFT-S-OFDM (as supplemental waveform) in the uplink. Research has shown that the advantages of the DFT-S-OFDM and CP-OFDM waveform supported in the uplink are combined, and scenario-based adaptive switching benefits both DFT-S OFDM link budgets and MIMO spatial multiplexing. In addition, the sub-carrier interval of 15–480 KHz is flexible and scalable.
To support highly reliable and low-latency services, 5G NR will adopt the ultra-short frame design in either of the following methods: (1) decrease the number of OFDM symbols in a timeslot and designs new mini-slots, for example, the timeslot with two OFDM symbols; (2) use a larger sub-carrier interval, for example, an interval of 30/60 KHz.
5G NR supports arbitrary code length and code rate, and new 5G LDPC codes matching the limited circulating cache rates.

5G NR Moving Toward Commercial Use
With the solid research strength in the field of 5G, at the MWC 2017 held in Barcelona, ZTE launched its 3.5 GHz and 28 GHz 5G NR AAU products, which are the most compact in the industry. The products support the latest 5G NR technology, and have the capability of software upgrade according to the full R15 of the NR standard. In addition, the combined high/low frequency networking ability will strongly enhance network coverage and bearing of operators.

Figure 2 5G High/Low Frequency Series from ZTE at the MWC2017

Integrated with innovation technologies (such as 5G beam shaping and Massive MIMO) into networks, 5G NR base stations expand network capacity and coverage and dramatically reduce network interference, providing "clean" networks and enhancing user experience.

In February 2017, ZTE, together with China Mobile and a subsidiary of Qualcomm, announced that the companies plan to jointly perform 5G NR–based interoperability tests and OTA field tests.

The tests will be performed based on the 5G guidelines proposed by China Mobile and use ZTE's base station solutions and the terminal prototypes integrated with Qualcomm technologies to simulate real scenarios and support a wide range of 5G NR cases and deployment scenarios. The test will involve multiple advanced 3GPP 5G NR technologies, such as the Massive MIMO antenna technology, adaptive standalone TDD, multiple beam technologies, and OFDM-based waveform. These technologies meet the data connection requirements of the emerging services, such as AR, VR, and cloud computing, and can also be used in some low-latency services, for example, unmanned vehicles and aircraft.

Performed based on the 3.5 GHz frequency band, the tests aim to drive large-scale rapid verification and commercial use of 5G NR in the radio field, making the 3GPP Rel-15–compliant 5G NR infrastructure and terminals available for immediate deployment of commercial networks. The interoperability test will begin in the second half of 2017 in China.

5G NR has gradually realized the transition from theoretical research to commercial use, marking around-the-corner commercial use of 5G. Let us look forward to the new era of Internet of Everything in the future.

New, unified, scalable, and configurable 5G air interface

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