May 2, 2017 —
Urban small cells deployed at street level are the next logical step to meet growing data traffic demand in city centers. Practical solutions need to be quick and easy to install, adapt seamlessly with tactical evolution and be resilient during outages. SoftBank installed CCS Metnet self-organizing microwave backhaul in the challenging metropolis of Tokyo, delivering valuable insight into the behavior of 5G microwave.
Planning the deployment of urban small cells involves trade-offs between three key capabilities: backhaul, power and site availability.
Seven nodes were installed on existing telegraph poles, with two directly connected by gigabit fiber backhaul. The simplest links were direct line-of-sight along the street, which coped with reflections from buildings on both sides. The most remote node had three wireless links (hops) to reach fiber backhaul. The topology of the deployment shown in Photo 1 is a live screenshot from the management system. Photo 2 shows Metnet nodes installed on the streets of Tokyo.
The site preferred by RF planners may not have electrical power; fiber backhaul may be unavailable or have long lead times to install; some sites may be much more costly or inaccessible than others. Flexibility is critical for success.
In the SoftBank implementation, CCS Metnet simplified the planner’s dilemma by providing high-capacity, highly resilient wireless backhaul that could be quickly deployed in the rapidly changing environment of Tokyo.
Operating in the 26GHz-band where radio-wave propagation normally is constrained to a line of sight, live trials in downtown Tokyo have proven NLoS (non-line-of-light) propagation to work as part of the mix.
No skilled tuning or calibration was required for these links. The self-organizing microwave backhaul automatically senses, filters and adapts to optimize performance and minimize interference. This is a continuous on-going process, adapting to environmental changes such as traffic movement, weather and foliage growth.
All nodes use identical hardware with a single radio per location. Single-RF-channel operation does not require any RF planning, making installation faster and simpler.
The node shown in Photo 3 is a typical street alley deployment with its neighboring node farther along the canyon.
More unusual were NLoS links, such as one shown in Photo 2, where the self-organizing microwave backhaul resolved and adapted to multipath reflections from nearby buildings. The 270-degree, wide-angle, 16-antenna array built into each backhaul node captures a wide range of signals for extensive processing and filtering. This is much easier and more flexible than point-to-point radio links that require alignment, where instead Metnet nodes with their 270-degree antenna array automatically detect all possible LoS and NLoS links and select them accordingly.
Dense environments such as this are likely to have small cells spaced closer than 50 meters apart. One link was 32 meters measured as a direct path with NLoS reflected signals travelling 49 meters. The adjacent nodes had 37 different possible RF links with varying attenuation levels for which the best options are automatically calculated and continuously reviewed. This demonstrated the high degree of multipath reflection that occurs in a typical street-level urban canyon. All potential self-organizing microwave backhaul links are shown in Figure 1.
This link achieved an average signal-to-noise ratio/modulation of 12 dB, supporting backhaul data throughput of 135 Mbps. The link achieved 100 percent availability throughout the trial period. Round-trip latency ranged from 0.5 milliseconds (1 hop) to 0.88 milliseconds (3 hops). The latest dual-channel Metnet radios are capable of delivering 1.2 Gbps using 256 QAM in a typical 112-MHz-wide single-channel pair.
Operating in a single channel pair without the need to radio plan, the self-organizing network algorithm selects the best possible links and topology and runs a dynamic spatial time-division, multiple-access schedule that dictates exactly which node and antenna pair can transmit and receive so as not to cause interference in the cluster. This is made possible by the thousands of measurements that are taken every second to determine the real-time state of the mesh. This cognitive approach to microwave networking provides a robust and flexible platform in the most challenging or urban environments.
Metnet addresses many of the issues of urban small cell deployment through simplified adaptation while ensuring resilient high-capacity backhaul can become available to all.
“A live deployment in downtown Tokyo is the ultimate challenge for a small cell backhaul system,” said Tomohiko Furutani, an engineer in Softbank’s strategic technology office. “Our engineering team was impressed with Metnet’s unique multipoint-to-multipoint mesh architecture and approach, and we wanted to test how this self-organizing, self-optimizing and self-healing microwave solution performed in a dense urban area.
“We designed the trial principally to assess the self-organizing microwave backhaul’s automatic support for NLOS, and its ability to cope with multiple RF paths in a multipath propagation environment — all while delivering optimal performance and quality of service,” Furutani said. “Metnet’s capabilities fulfilled our expectations. The trial has provided a valuable insight into the behavior of microwave and millimeter-wave systems in future 5G bands, and the results of our trial demonstrate that self-organizing microwave backhaul can be well positioned to support these fifth-generation networks.”
David Turner is head of technical sales at CCS in Cambridge, United Kingdom. CCS manufactures the Metnet self-organizing microwave backhaul. Visit www.ccsl.com