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Synchronizing the 5G Network
The 5G (5th Generation) mobile network connects machines and devices at higher data speeds, and with ultra-low latency when compared with its predecessor, 4G. In order to maintain consistent operation and high reliability, network components must be synchronized. Accurate timing (clocking) plays a crucial role in maintaining synchronization within a mobile network. Inaccuracies in synchronization and timing can lead to interference between nodes in a radio access network (RAN). Error-prone oscillators and clocks can cause time shifts that result in deterioration of performance and reliability. This article explores the importance of stable clocking in network synchronization, especially with arrival of 5G, and techniques that engineers are using to meet the challenges.
What is an Oscillator?
An oscillator is an electronic circuit that uses a crystal to generate a periodic electrical signal with constant frequency, also known as a clock or timing signal. Most digital circuits rely on clock signals in order to synchronize the different components within them. The following section describes how crystal oscillators are used in 5G application devices for network synchronization.
Clocking and Synchronization in 5G Applications
The goal of a radio access networks (RAN) is to optimize service performance and reliability. A RAN is comprised of different commponents that are synchronized, each contributing functionality in order to deliver required services. Timing accuracy and availability, as well as cost, are important considerations for the end product.
Figure 1 shows an open RAN architecture. It consists of a remote radio unit (RRU), fronthaul switch, and a distributed unit (DU). The block diagram shows a crystal oscillator (TCXO and OCXO) generating the clock signal to synchronize these devices. The remote radio unit (RRU) serves as the consumer's access point to the network. The distributed unit (DU) connects the central unit (CU) and the mobile core. The fronthaul switch routes traffic between the RRU and DU. These components need to be precisely synchronized to avoid data packet loss and system interruptions.
Figure 1: Block diagrams of the role of an oscillator in the RRU and DU systems
Image Source: Embedded Computing Design
Many 5G networks rely on Time Division Duplexing (TDD), a method of duplex communications where inbound signals are separated from outbound signals based on time allocation in the same frequency band. This requires all devices to be precisely synchronized. This synchronization is typically based on the IEEE 1588 Precision Timing Protocol (PTP). Synchronous Ethernet (SyncE) is another standard that can be used synchronizing frequency. The network architecture dictates whether PTP, SyncE, or both are used for synchronization.
In PTP, a device called the grandmaster uses a sync source (usually GPS-based) to create several timestamped PTP packets that are delivered to follower clocks at other locations. The packets are used to calculate the time offset between the grandmaster and follower clock, after which a local clock signal can be generated by the follower clock at its own location. PTP is an intelligent system, and is able to adapt to grandmaster loss.
SyncE is an older system in which a high quality clock reference (usually from a GPS or Cesium clock source) is used to time the output from the core of the network. At other locations on the network, clock recovery takes place, analyzing the signal edges of the output signal and using them to derive their own clock signal for the receiving equipment.
The DU must pass a precisely synchronized timing signal to the fronthaul switch and the RRU. Clocking in the DU should be resistant to issues such as heat under a heavy load, or the addition of a fan to the system. The RRU needs to be the most environmentally robust, because they are usually placed outdoors, on rooftops and poles, and near roads and highways. Because precise synchronization needs to be maintained between the DU, fronthaul switch, and RRU, jitter cleaners are used to reduce any jitter that might occur due to environmental factors.
Challenges with Oscillators in 5G Applications
Environmental stress can influence oscillator performance. 5G networks rely on a large density of radios, and as such, radios are often mounted in locations that are subject to vibration from multiple sources, including trucks, trains, cars, wind, and thunder. Devices mounted outdoors are also subjected to large temperature ranges, including extreme heat and cold. Oscillators deployed in these radios must maintain stable performance during environmental stress to prevent dropped links.
Cost and availability are also concerns, as well as size, heat, and power considerations. TCXO and MEMS-based oscillators are replacing other types due to their performance and affordability, however, low-cost MEMS oscillators can introduce additional constraints. They may not react well to physical-layer rearrangement, and typically do not support the necessary bandwidth for PTP G.8275.2, limiting them to the lower-bandwidth used in the G.8275.1 PTP profile.
Types of Clocks (Oscillators)
Temperature-compensated quartz crystal oscillators (TCXOs) are crystal oscillators with a temperature-sensitive reactance circuit in their oscillation loop, which is used to compensate for the frequency-temperature characteristics inherent to the crystal unit. Figure 2 shows a block diagram of a TCXO unit. A Voltage Controlled Crystal Oscillator (VCXO) is an important TCXO component that links to a temperature sensing circuit and applies minute correction voltages to the oscillator. TCXOs provide stabilities of 1 part per million (ppm) to 0.1 ppm. A notable advantage of a TCXO is its relatively high stability while consuming minimum power (several milliwatts). These can thus be ideal for multiple communications and telecom applications, such as point-to-point RF, GNSS/GPS, mobile phones, and other precision RF connectivity systems.
TCXOs are well-suited for multiple communications and telecom applications, such as point-to-point RF, GNSS/GPS, mobile phones, and other precision RF connectivity systems. Wireless infrastructure equipment is becoming physically smaller, enabling it to be installed in locations that are space constrained. Such locations may also be remote and have limited connectivity to power sources, requiring equipment to maintain low power consumption. TCXOs provide the necessary performance, in a small package with low power consumption. Since TCXOs are temperature-compensated, they are able to adapt to and maintain their stability through temperature changes.
Figure 2: A TCXO Block Diagram
Image Source: Electronics Notes
Abracon TCXO Oscillators
The Abracon ASGTX5 TCXO series are factory-programmable oscillators that are jitter and stability optimized. They provide stable, high-frequency clocking at a small footprint of 5.0mm x 3.2mm, necessary characteristics for driving a communications infrastructure. Designed for +/-3ppm over a broad -40 °C to 85 °C temperature range, these devices meet the Stratum 4 clocking requirements used in edge networking equipment. They are also capable of improving phase-locked loop (PLL) lock time and performance in broadcast and professional video applications.
Figure 3: Abracon ASGTX5PAF1-156.2500T2
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Abracon’s TCXO oscillators are factory configurable to output frequencies from 15MHz to 2.1GHz, and support LVPECL, LVDS, HCSL, or CML output logic types with voltage options to 1.8V. Figure 4 shows recommended testing circuits for different output logic types. The enabled output can be configured for any combination of pin 1 or pin 2 and active high or active low functionality. The ASGTX5's design flexibility is important in applications that require backward compatibility with specific pinout or firmware combinations. ASGTX5DAF1-250.0000 , ASGTX5PAF1-156.2500T2 , and ASGTX5DAF1-125.0000 are some examples from this series.
Figure 4: Abracon ASGTX5 TCXO series Oscillator and Recommended Test Circuit for LVPECL, LVDS, HCSL, and CML logic Output
Image Source: Abracon
Abracon has also recently introduced the Ultra-Performance TCXO series (AST3TDA), offering clocks as stable as ±50 ppb at +105ºC, in a 7.0 mm x 5.0 mm x 2.2 mm package.
Summing up: Network Synchronization
Effective network synchronization is vital for stable and reliable network performance. Various compelling use cases for 5G, such as IoT and Industrial Automation, require precise timing, and the need for accurate synchronization will continue to grow in the near future. The ASGTX5 TCXO and Ultra-Performance TCXO series oscillators from Abracon provide stable clocks at high frequencies in a small footprint, making them well-suited candidates to handle clocking duties for 5G applications.