SDRs for Low Latency and Time Sensitive Industrial Internet of Things (IIoT) Applications

May 31, 2022

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Time-sensitive Industrial Internet of Things (IIoT) networks require ultra-low latency links to be able to properly function. One solution to this is the integration of high performance software defined radio (SDR) platforms, which feature field programmable gate arrays (FPGAs) that allow for the implementation of low latency networks. These platforms also offer high interoperability and reconfigurability that can greatly benefit IIoT networks, especially since the technological advancements are rapidly progressing.

SDR platforms with FPGA are known to be deterministic and low latency, but they also featurewide tuning ranges and broad flexibility that assist with interfacing devices witth a range of protocols used in the IIoT ecosystem.

What is IIoT?

IIoT is revolutionizing factories by providing powerful connectivity between various devices. Until recently, wired communication has dominated connectivity in industries. Factories are replacing wired connections with wireless networks because the latter allow higher mobility and quick reconfiguration, require less installation, and lower maintenance costs. Satisfactory performance in industrial environments demands more than just basic installation of 4G and WiFi.

IIoT employs a wide range of network protocols and standards to interconnect various devices in factory environments. Some of the most popular network protocols for IIoT applications include Bluetooth, Zigbee, and LoRaWAN. For instance, a protocol stack  for a connected factory floor worker can have a physical layer (Layer 1), a link layer (Layer 2) and a networking layer (Layer 3). The physical layer can have wireless protocol; the link layer can have 3GPP, 4G/5G, IEEE 802.11 and IEEE 802.15.4; and the network layer can have internet protocol, cloud, and edge services.

Some of the key factors to consider when designing an IIoT network include network architecture, network function layers, communication stack limitations, type of spectrum, coverage, mobility, and technology lifecycle requirements (Figure 1). IIoT applications require a common network architecture to ensure interoperability and allow devices to connect to data centers. They also require a network function that is based on a common layer to ensure forward compatibility and enhance interoperability.

Networks for use in IIoT applications are required to take the limitations of the communication stack used in end devices into account. To ensure reliability, it is critical to consider the trade-offs of using licensed and unlicensed spectrum when implementing an IIoT network. An IIoT network should have a range and reach that is capable of meeting the needs of a factory. In addition, the network should be capable of meeting the mobility needs of a factory environment.

Figure 1: IIoT network design considerations

Why Deterministic Low Latency is Important

Network latency refers to the delay experienced by signals as they propagate through a communication network. In a typical communication system, latency can be viewed as the time required for capturing a packet of data, transmitting it, and processing it through multiple components of a network system until it is received and decoded at its destination.

Conventional wireless network protocols are designed to allow exchange of huge amounts of data that do not have strict time constraints or need for synchrony. Some signals used in industries such as single control commands have strict delay constraints and demand network infrastructure with deterministic latency. The aim of developing deterministic Ethernet and time-sensitive networking is to meet the tight timing requirements of such applications. Figure 2 shows some of the main causes of latency in IIoT networks.

Time Sensitive Networking for IIoT

Time-Sensitive Networking (TSN) refers to a collection of standards aimed at providing precision timing and synchronization. TSN components can be broadly classified into three: time synchronization, traffic rules, and path selection. The time synchronization component requires all the devices involved in real-time communication to have the same understanding of time. The traffic rules component requires all devices involved to stick to the same rules when processing and forwarding data packets. Lastly, TSN requires all devices to stick to the same rules when selecting communication paths and reserving time slots and bandwidth.

TSN offers an array of benefits to time-sensitive applications. It is optimized to minimize latency when transporting time-stamped and latency-sensitive data under various traffic environments. To maximize interoperability, TSN employs standard components that are available in large volumes. This helps to enhance scalability and reduce the overall cost of deploying and maintaining networks.

TSN integrates several mechanisms to ensure deterministic performance across similar settings. Some of these features include improved precision time control, bandwidth reservation, redundant paths for transporting data streams, and integrated quality of service (QoS) features for communications over Ethernet links. These features can help to ensure deterministic latency and tight synchronicity in IIoT applications.

TSN is engineered to provide more bandwidth, making it suitable for industrial applications that require a lot of Ethernet bandwidth such as 3D scanning and machine vision. Its design helps to simplify network infrastructure while its deterministic Ethernet networking approach allows a single Ethernet network to be used for transporting mixed traffic.

Figure 2: Network latency contributions in IIoT

SDRs for IIoT

An SDR system allows various radio signal processing components such as modulators, demodulators, coders, and equalizers to be implemented in software instead of dedicated hardware. A typical SDR features a radio front-end (RFE) and a digital back-end. The RFE performs transmit (Tx) and receive (Rx) functions and is designed to provide a wide tuning range. The highest performance SDR platforms offer multiple independent channels, each with a dedicated analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). Moreover, these platforms are designed to provide a very high instantaneous bandwidth.

Most high performance SDR platforms feature an FPGA with various-board digital signal processing (DSP) capabilities such as modulation, demodulation, upconverting, and data packetization over Ethernet. In addition, SDR platforms are capable of supporting mixed traffic, simplifying network infrastructure, and providing sufficient bandwidth.

The architecture of an SDR platform enables implementation of low latency solutions for time sensitive applications. FPGAs have a highly parallel architecture that enables them to perform processing tasks much faster than host PCs. Embedding application logic on this device helps to improve the overall latency performance of a system. For applications that demand ultra-low latencies, implementation of custom interface protocols utilizing SFP+ connectors can help to further reduce the time delay between the host machine and the SDR platform.

SDRs for TSN

Tests have shown that low end-to-end latency of 3.75 ms can be achieved in SDR-based solutions. This means that SDR-based implementations can be used in IIoT applications that demand low latency and time synchrony, such as Human-Machine-Interaction (HMI), sensor data collection, and automated guided vehicle (AGV) systems.

Combining SDRs with software defined networking (SDN) technology can help to realize complex TSN for use in IIoT applications. This technology provides resource and security orchestration and helps to resolve congestion and other latency related issues. In addition, SDN is capable of using real-time predefined requirements to dynamically reconfigure a network.

Many SDR-based TSN prototype solutions have been developed and tested. Tests with a prototype of an advanced radio receiver system for the IEEE 802.15.4 offset-quadrature-phase-shift keying (OQPSK) physical layer have shown that SDR-based implementations are suitable for low power IIoT applications using protocols such as WirelessHART, ZigBee, and 6LoWPAN.

Tests with a prototype SDR-based next generation network have shown that low latency network solutions can be realized by using an SDR with FPGA. This implementation enabled an SDR to take advantage of various features of the IEEE 802.1 TSN standard including time scheduling and latency-optimized scheduling.