Understanding the first version of 5G standards in one article

Today, 5G is like an abstract painting, and everyone has a different understanding of it. This article hopes to show you the most realistic 5G by briefly describing the first version of the 5G standard R15.

5G defines three major scenarios: enhanced mobile broadband (eMBB), ultra-reliable low-latency communication (URLLC), and massive machine type communication (mMTC).

For these three scenarios, the 3GPP R15 standard completed in June 2018 not only defines 5G NR (New Radio) to meet 5G use cases and requirements, but also defines a new 5G core network (5GC), as well as extends and enhances LTE/LTE-Advanced functions.

One picture to understand the series of 5G R15 standards...

R15 5G NR mainly defines new specifications for two major scenarios: eMBB and URLLC.

For eMBB scenarios, NR mainly defines three categories of technologies: high-frequency/ultra-wideband transmission, Massive MIMO, flexible frame structure and physical channel structure.

High frequency/ultra-wideband transmission

High frequency: NR specifies two major frequency bands, FR1 and FR2, FR1 (450MHz-6GHz), FR2 (24.25GHz-52.6GHz).

Ultra-wideband: Channel/single-carrier bandwidth up to 100MHz for FR1 and up to 400MHz for FR2.

In addition, the physical layer also supports carrier aggregation (CA) and dual connectivity technology, which can aggregate up to 16 carriers to achieve higher-speed transmission.

The LTE frequency band is no higher than 3GHz, and the single-carrier bandwidth is only 20MHz. Therefore, high frequency and ultra-wideband are the main differences between 5G and 4G.

Since NR introduces higher and wider frequency bands, high-frequency signals are more sensitive to multipath fading and phase noise. If the OFDM subcarrier spacing of all frequencies is the same as LTE, it is obviously no longer adaptable. Therefore, NR also supports multiple OFDM subcarrier spacings of 15, 30, 60 and 120kHz for data transmission.

Massive MIMO

The Massive MIMO standardization work defines technologies such as reference signal design and beam management, with the aim of supporting up to 256 antenna elements on the base station and up to 32 antenna elements on the terminal side to achieve massive MIMO transmission in high frequency bands.

In order to achieve high-speed data transmission, the downlink supports up to single-user 8-layer and multi-user 12-layer MIMO transmission, and the uplink supports up to single-user 4-layer MIMO transmission.

For high frequency bands, beamforming is a key technology that can enhance coverage. In the 4G era, due to the low frequency bands used, digital beamforming technology can be used to achieve it. The beamforming is generated in the digital domain, but this method cannot cope with 5G high frequency band Massive MIMO. 5G NR uses a hybrid of digital and analog to achieve beamforming.

Flexible frame structure/physical channel structure

As mentioned earlier, NR supports multiple subcarrier spacings. The subcarrier spacing can be wider in the frequency domain and the OFDM symbol can be shorter in the time domain. For example, the subcarrier spacing of LTE is 15KHz, and now the subcarrier spacing of 5G NR can reach 120KHz. Compared with LTE, the OFDM symbol length is shortened by one eighth, thereby achieving lower latency transmission.

5G NR can also flexibly change the number of OFDM symbols in the allocation unit of the control and data channels, and can flexibly change the uplink and downlink time slot ratio in the frame structure according to the uplink and downlink service ratio.

URLLC is designed to support or assist in completing some mission-critical businesses with near real-time and high reliability requirements, such as autonomous driving, industrial robots, and telemedicine.

As mentioned earlier, lower latency communication can be achieved by using a wider subcarrier spacing and reducing the number of OFDM symbols. On the other hand, in order to achieve high reliability, R15 also defines new CQI (channel quality indicator) and MCS (modulation and coding scheme) for URLLC.

4G LTE/LTE-Advanced has expanded and enhanced its functions for the three major scenarios of eMBB, mMTC and URLLC. Among them, the 5G mMTC scenario is mainly based on the expansion of LTE/LTE-Advanced technology to adapt to large-scale IoT communications.

For eMBB scenarios, LTE/LTE-Advanced feature enhancements mainly include:

1024QAM support

To further increase the peak data rate, R15 defines 1024QAM and reduces the DM-RS (demodulation reference signal) overhead.

Enhanced CoMP (Coordinated Multi-Point Transmission)

Enhanced CoMP supports non-coherent joint transmission, where two base stations can send different data sequences without knowing each other's channel state information (CSI).

8 Antenna Technology

The terminal is equipped with 8 receiving antennas, which can expand the downlink coverage of the cell. At the same time, the downlink rate can be greatly improved with the 8-layer MIMO.

Various interference suppression techniques

R15 also defines a number of enhanced LTE/LTE-Advanced functions to reduce interference between cells. One of the functions is to reduce CRS (cell reference signal) transmission when the cell is under low load to reduce interference and save base station power consumption. In addition, some interference suppression technologies for base stations and terminals are also defined.

Enhanced Carrier Aggregation (CA) functionality

In the early days of carrier aggregation, there was a terminal processing delay due to the need to measure the quality of candidate carriers and start the RF channel. To solve these problems, R15 defines a mechanism to measure the wireless signal quality of candidate carriers in advance when the terminal is in idle state and initialize the RF channel in advance before SCell.

Uplink data compression

In TDD mode, the uplink to downlink ratio usually emphasizes the downlink, so the wireless resources available for uplink transmission are limited. In order to improve the uplink spectrum utilization, R15 defines an uplink data compression mechanism, which mainly compresses the packet headers at the IP layer and above.

Video QoE measurement function/content caching

With the rise of mobile video, improving the video experience (QoE) in mobile communication environments has become a key concern for operators. In order to measure the QoE of real networks, R15 defines a mechanism that can directly collect QoE measurement values ​​from terminals, called Minimized Drive Test (MDT).

At the same time, R15 also studies the mechanism of caching video content to a server close to the base station to reduce the delay when downloading videos. Through this mechanism, the terminal downloads data directly from the base station or a nearby content server without having to go back to the core network, thereby reducing the delay.

For mMTC scenarios, LTE/LTE-Advanced feature enhancements mainly include:

Drone terminal detection/interference suppression

Drones are being widely used in all walks of life, but they have problems such as limited flight range and difficulty in supervision. In the future, networked drones connected to mobile communication networks are an inevitable trend. In order to meet future needs, 3GPP has studied providing wide-area communication support for drone terminals through LTE/LTE-Advanced networks.

Since the UAV will introduce uplink interference from the UAV to the base station after it is connected to the Internet, R15 defines open-loop power control parameters for the UAV terminal. In addition, the use of mobile communication networks to detect whether the UAV has obtained a flight license is also studied.

Enhanced LTE-M and NB-IoT

R15 further enhances the functions of cellular IoT technologies LTE-M and NB-IoT, mainly by adding TDD support and low power consumption capabilities.

(a) Idle mode power saving technology (wake-up signal)

To reduce power consumption in idle mode, R15 defines a new wake-up signal. Usually, IoT terminals in idle mode will periodically decode the downlink control channel to obtain paging information. Since the terminal does not know whether there is a paging message before channel decoding, this process must be performed regularly, which will increase power consumption. Therefore, R15 defines a simple process for detecting wake-up signals, so that IoT terminals can directly determine whether there is a paging message without periodic decoding, thereby further reducing power consumption.

(b) Reducing the latency of small data packets

Many IoT applications transmit extremely small data packets, such as smart meter reading. For these applications, R15 defines the direct addition of small data packet transmission during the random access process to reduce communication delays.

(c) TDD support

In R13 and R14 versions, LTE-M and NB-IoT only support FDD mode, and R15 adds support for TDD mode.

Enhanced V2X (Vehicle to Everything)

R15 expands the V2X communication functions released in R14. In order to improve the data rate and bandwidth of V2X communication, CA is introduced in Mode 4, allowing terminals to autonomously select transmission resources from the resource pool. At the same time, support for 64QAM modulation is added, and new terminal performance specifications are added to meet low latency requirements.

4G networks must also be able to support low-latency services such as VR and autonomous driving. To this end, R15 defines the function of achieving low-latency and high-reliability communications on LTE/LTE-Advanced.

mainly include:

Improve the transmission quality of downlink control channels and uplink and downlink data channels

In the downlink control channel of traditional LTE/LTE-Advanced, PCFICH (Physical Control Format Indicator Channel) is used to indicate the number of symbols occupied by PDCCH in a subframe. The PCFICH channel needs to be detected to identify the number of PDCCH OFDM symbols. However, in this case, the quality of the entire downlink control channel is constrained by PCFICH error detection. For this reason, R15 specifies a way to improve the quality of the downlink control channel, which directly notifies the number of PDCCH OFDM symbols through higher-layer signaling, thereby avoiding the impact of PCFICH detection errors.

Reducing LTE latency

In order to reduce latency, a new short TTI is defined. The 1ms TTI of traditional LTE contains 2 time slots and 14 OFDM symbols. Each time slot consists of 7 OFDM symbols. Based on Short TTI, 2~3 OFDM symbols can be scheduled, thereby reducing the one-way air interface latency from 10ms to 1ms.

At the same time, the processing delay from receiving data to sending HARQ feedback, as well as the processing delay from receiving the downlink control channel to sending uplink data, is reduced from the previous minimum of 4 milliseconds to 3 milliseconds.

As we all know, from the perspective of wireless or terminal side, 5G networking includes independent networking (SA) and non-independent networking (NSA).

Simple explanation:

NSA means that the terminal is connected to the mobile communication network through multiple wireless access technologies (such as LTE and NR). If the terminal is connected to the mobile communication network through LTE and NR, this is called "dual connection"; SA means that the terminal is connected to the mobile communication network through only one wireless access technology.

From the perspective of the core network, 5GC (5G core network) will also provide two solutions for independent networking and non-independent networking: EPC extension solution and 5GC solution.

The EPC extension solution supports EPC dual connectivity. Its main feature is that it reuses the S1 interface between the 4G base station (eNB) and EPC and the non-access layer interface (NAS) between the terminal and EPC to minimize the modification of 4G core network equipment and enable the rapid introduction of 5G NR.

On the one hand, 5G's early voice services and cellular IoT services need to rely on the 4G LTE network that has achieved continuous coverage to carry them; on the other hand, due to the higher frequency band of 5G NR, base stations are limited to local deployment in the early stage, making it difficult to quickly form continuous coverage. Therefore, in order to introduce 5G as soon as possible to increase network capacity, some operators will adopt the EPC dual-connection deployment method, continue to provide 4G data, VoLTE and cellular IoT services through the EPC equipment of the existing 4G core network, and introduce 5G NR to meet the needs of large-traffic services such as high-definition video.

However, now that 5G NR has been introduced, considering future 5G packages, roaming and other issues, 4G EPC certainly cannot remain unchanged. It also needs to introduce some new functions to flexibly provide 5G NR services.

These new functions refer to 5G NR service identification (control) functions, which mainly include:

•5G area notification function

When the terminal connects to the network, such as attach, the core network will confirm whether the user has subscribed to a 5G package. If so, based on the base station cell configuration information, it will determine whether the terminal is in the 5G NR coverage area and whether a 5G NR connection can be provided for the terminal.

•5G NR connection decision function

As mentioned above, if the terminal user has subscribed to a 5G package and the coverage base station also supports 5G NR, the next step is to establish a 5G NR connection for the 5G terminal.

•5G Gateway (GW) selection function

The core network will provide GW equipment optimized for 5G capacity and give priority to connecting 5G NR-enabled terminals to the GW equipment.

•5G data reporting function

In the LTE and NR dual connection mode, the base station will flexibly allocate data traffic to the 4G base station and the 5G base station based on factors such as the wireless environment. It is also necessary to count how much data traffic the 5G base station has transmitted and report it to the core network.

5GC, or 5G core network, will no longer use the 4G core network. It is worth mentioning that the 5G core network has undergone great changes and can be said to be a disruptive design.

The 5G core network is based on a service-oriented and software-based architecture, and through technologies such as network slicing and control/user plane separation, it enables network customization, openness and service-orientedness to serve the Internet of Things and all walks of life.

SBA (Service Based Architecture) is a service-based architecture designed based on the Cloud Native architecture. Cloud Native mainly consists of microservices, DevOps, and agile infrastructure represented by containers, with the goal of achieving elasticity, repeatability, and reliability of delivery.

The 5G core network's service-based interfaces and APIs enable operators to agilely create "network slices" for various industries, transforming the role of operators from "pipelines" to "platform providers" in the future.

5GC not only supports connection to 5G base stations, but also supports connection to 4G base stations. However, the 4G base station connected to 5GC is no longer called eNB, but ng-eNB, which uses the new N2 interface together with the 5G base station.

In addition, voice fallback technology will also be provided, and when the terminal makes a VoLTE voice call, the terminal will connect to the 4G EPC.

The full text is over.

R15 is the first phase standard of 5G. In view of the second phase of 5G, 3GPP started to formulate the R16 standard in October 2018. In terms of NR, R16 will promote the multi-beam/MIMO technology of millimeter wave and expand the application areas of URLLC and IoT. In terms of 5GC, R16 will further study the functional evolution of 5GC to face the future diversified business applications of 5G.

In short, the first version of the 5G standard R15 takes into account both the smooth evolution of 4G and the new future needs of 5G; it not only enhances 4G functions, but also adds 5G capabilities, fully demonstrating a steady and pragmatic pace towards the 5G era.

This may not match the sci-fi version of 5G that some people imagine, but this is the most real and authoritative 5G. There is no need to praise 5G to the sky. As the next generation of mobile communication technology, 5G is the future ICT infrastructure. To change the world, it needs to integrate multiple technologies and cooperate with multiple ecosystems. We should not be pessimistic about 5G. Like any previous G, it will eventually mature and become popular step by step.

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