5G & IoT – A Match Made in Hell or Heaven?
5G is entering uncharted territory. 3rd Generation Partnership Project (3GPP) Release 16 enables entirely new applications, expanding commercial mobile communications from familiar mobile broadband to the vast internet of things (IoT). 5G engineers stand before their greatest challenge yet: achieving the International Mobile Telecommunications 2020 (IMT-2020) vision for 10−5 reliability and 1 ms end-to-end latency. Will it be an engineering marriage made in hell or heaven? Part of the answer, like in marriage, is to find out what you are getting into.
Release 16 is fascinating. It leverages real-time technology to bring new devices and revolutionary services to consumers. The release builds on Release 15 to deliver new IoT use cases to consumers. These use cases form the canvas for connecting billions of “things.” “Things,” however, can refer to a number of devices ranging from a smart watch to an industrial machine or a car. This distinction is important to understand, as it has significant implications for design and test.
Massive Machine-type communications (mMTC) applications refer to the traditional IoT space. Here, focus areas include low rates, low costs, and long battery life. mMTC applications involve sending small bits of data in the network to deliver useful services, such as connecting a refrigerator to the Internet to reorder milk. As a result, such applications do not require 5G. 5G is mostly embracing the approaches used by 4G for this use case.
The heart of the 5G and IoT story lies with ultra-reliable low-latency communications (URLLC). Key applications include the following:
- Augmented reality (AR), which superimposes information on the real world
- Industrial IoT (IIoT), which brings cyber-physical systems and integration between computing, networking, and physical processes
- Autonomous driving with the introduction of self-driving cars
- Tactile Internet that enables real-time human and machine interaction via the Internet
These applications have stringent requirements and most present high risks to those considering offering such services. Launching the network and optimizing it later is not an option.
Figure 1. 5G New Radio use cases.
Release 16 completion is coming up in March 2020. It will introduce significant enhancements for URLLC, but many do not realize that URLLC components are already available in Release 15. Even though Release 15 focuses on mobile broadband, it features important URLLC aspects. These aspects have just not been implemented in commercial networks yet.
Building on Release 15 for Improved Reliability
Achieving ultra-reliability represents a paradigm shift for mobile communications networks because the focus has been on optimizing for efficiency. URLLC applications require sacrificing some efficiency to increase reliability. Most URLLC components in Release 15 do just that.
Blind repetitions, for example, involve sending multiple copies of a given packet rather than relying on the network to request retransmission when a packet is lost or arrives with errors. This differs significantly from the historical process, during which the sender waits for the destination point to check the packets and asks for a retransmission if not correct.
Figure 2. Blind repetition process.
Diversity, another Release 15 feature, allows the same information to be sent at different frequencies or using different antennas - a technique known as spatial diversity. Once again, this feature aims to provide multiple opportunities for the receiver to get the intended information.
Figure 3. Base station sending same information to mobile phone via multiple beams.
Release 15 also includes a channel quality indication (CQI) table optimized to achieve 10−5 reliability for URLLC. Typically, user equipment (UE) sends CQI information to tell the base station about the level of quality of the radio channel. It then advises on the modulation scheme to ensure that the data can be received with few errors. While this process was optimized to a packet error rate (PER) of 10−3 in the past, Release 15 allows configuration to 10−5. The UE therefore becomes more conservative and deliberately chooses an easier modulation scheme to lower the error rate.
In addition, Release 15 provides two ways to duplicate data across multiple carriers – carrier aggregation (CA) and dual connectivity. CA adds carriers at the media access control (MAC) layer. Dual connectivity connects carriers at a higher layer — the packet data convergence protocol (PDCP). CA and dual connectivity duplication use multiple carriers to send the information multiple times.
Figure 4. CA versus dual connectivity duplication.
New Release 16 includes improvements to physical uplink shared channel (PUSCH) repetitions. It also offers compact downlink control information (DCI) for the physical downlink control channel (PDCCH) to increase the reliability of the downlink (DL) control channel.
Building on Release 15 for Lower Latency
Reducing latency involves more complex concepts than achieving higher reliability. Release 15 defines a set of tools that can be collectively used to meet IMT-2020 latency requirements. Different numerologies align with the needs of the application, while bandwidth parts (BWPs) manage the different numerologies in the UE or enable the use of simple lower cost devices. Lower latency applications also require UL grant-free scheduling to enable URLLC services to send data without having to previously request UL resources.
Mixed numerology enables signals to adopt different symbol lengths and frequency separation for their respective subcarriers. This concept is key to address the requirements from different services – mMTC, enhanced mobile broadband (eMBB), and URLLC. Legacy long term evolution (LTE) / narrowband IoT (NB-IoT) technologies use a fixed numerology for subcarrier spacing and symbol duration. Mixed numerology allows for changes to the structure to accommodate different subcarrier spacing and symbol durations in order to fit different types of service.
For example, some IoT use cases do not have a low latency requirement. These IoT services can use longer symbol periods (i.e., smaller subcarrier spacing) and multiple symbols, as low latency is not critical. In contrast, other use cases require very low latency. Shorter symbols (i.e., larger subcarrier spacing) and shorter time assignments are used for these use cases to enable low-latency communications. In addition, the network could allow URLLC services to puncture a “hole” in other lower priority services’ allocations, such as eMBB. This is a significant change compared to previous operation, as it allows high-priority traffic to be delivered and received by the network without interference from other lower priority services, thereby increasing reliability.
Another key element for low latency is grant-free transmissions. Here, devices are pre-assigned a set of resources which they can use for transmission right away when needed, eliminating the need for explicit indication by the network. In the past, any service would have made a request to the network, which would have looked to see if there were spare resources in that allocation before granting access to the requesting service. Grant-free scheduling allows the low-latency service to bypass that process and access the resources it needs much faster, decreasing latency as a result.
Multiple BWPs are also important, as they allow a UE to manage different numerologies to both achieve low latency and support low rate non-critical IoT. In addition, BWP allows the network to support low cost devices, which can only support specific numerologies. For an IoT service, for example, the BWP will tell the UE to only focus on a certain group of subcarriers. This service can therefore have a simpler receiver that does not have to work as quickly or have the capacity to deal with a lot of data or look at broad bandwidth. The UE receiver can also be cheaper and consume less power.
Figure 5. Scheduling choices for LLC.
Figure 6. Multiple BWPs and mixed numerology for different services.
Like the reliability aspect of URLLC, many features for enabling lower latency are already available in Release 15 including the following: new numerology / slot size / mini slots, multiple BWPs and mixed numerology, configured grant, pre-emptive scheduling, and eMBB puncturing. The industry has just not implemented these features yet. Release 16 enhancements for lower latency include:
- UL pre-emption indication, where the gNB can indicate eMBB UEs to stop previously scheduled UL transmission and clear the channel for a higher-priority, low-latency traffic from other UEs.
- Improvements to PUSCH repetitions.
- Improvements for UL grant-free transmission to allow UEs to send data sooner.
- Uplink control information (UCI) changes to introduce the concept of traffic priority.
Let’s Get Married
3GPP has had URLLC in mind from the beginning of the standards development process. Release 16 builds on Release 15 and introduces new concepts to achieve the IMT-2020 requirements. Still, URLLC presents significant technical and business challenges to the 5G mobile ecosystem. Understanding the URLLC components included in Release 15 and the enhancements brought by Release 16 is the first step to preparing for URLLC.
Keysight is a recognized leader in 5G, supporting the development of this technology since 2012, actively participating in standards bodies like 3GPP, the Global Certification Forum (GCF) and PCS Type Certification Review Board (PTCRB). Keysight's collaborations with industry leaders have led to the launch of more than 100 5G devices spanning varied formats: smartphones, robotics, laptops and dongles, as well as applications: fixed wireless access (FWA), mobile connectivity, factories and automotive.