Move to centralized RAN faster with optical fronthaul

May 7, 2016
The Nokia Mobile Fronthaul solution helps operators move to centralized RAN and meet the strict new transport requirements it imposes.

Introduction

Mobile broadband usage is projected to double annually and grow to 8x current rates by 2018. To address this growth, operators are deploying more cells to augment mobile network capacity and moving to new centralized radio access network (RAN) architectures. These architectures improve RAN performance by enabling better cell coordination and more efficient use of precious spectrum resources.

According to a recent Bell Labs business model, operators can reduce total cost of ownership (TCO) by 23 percent by moving away from traditional, distributed architectures and centralizing baseband processing. They can increase these savings to 28 percent by moving to virtualized RAN resources that make use of general processing platforms in a cloud-like environment. In both cases, operators generate savings by reducing the amount of equipment – and, therefore, the amount of space and power – they use at cell sites. In addition, operators can avoid having to over-provision the network by taking advantage of pooled resources that better balance the traffic load across the network.

Transport network challenges

The move to a fully centralized RAN brings new challenges to the transport network. The link between the remote radio heads (RRHs) and the baseband unit (BBU) carries raw, uncompressed antenna samples using a Common Protocol Radio Interface (CPRI) or an Open Base Station Architecture Initiative (OBSAI) interface. This approach is inherently inefficient. It requires high bandwidth – 10 Gbps –regardless of whether user traffic is being transmitted. It also requires operators to uphold strict latency and jitter requirements to prevent radio performance degradation.

Early fronthaul deployments have tended to use point-to-point fiber to connect cell sites to the central hubs that house the baseband pool. While relatively simple, this approach is costly and fiber intensive. It requires operators to use dark fiber to connect each radio uniquely to the centralized BBU.

Because operators have limited fiber resources, they have also turned to passive wavelength-division multiplexing (WDM) solutions. These solutions allow operators to increase utilization of fiber by supporting multiple channels on a single fiber, thereby removing the one-to-one relationship between RRHs and fiber.

To support passive WDM solutions, operators must update the RAN infrastructure at both the cell site and central hub. This means deploying WDM-compatible colorized optics in both the radio and BBU and using close coordination to ensure the wavelength frequencies across the radios do not interfere with one another. Operators that deploy passive WDM solutions face the risk of increased downtime. These solutions typically lack operations, administration and maintenance (OAM) capabilities, so operators have no way to proactively monitor and manage the network.

Accelerating deployments through optimized fronthaul

The Nokia Mobile Fronthaul solution addresses these challenges by supporting passive, semi-passive, and active transparent configurations. The active transparent configuration (Figure 1) enables operators to take advantage of the high degree of reliability and transparency that passive WDM provides. It also allows operators to add unique capabilities that make fronthaul economically and operationally viable in a wide range of deployments. These capabilities include wavelength translation, robust OAM and fiber optimization, centralized monitoring and management, and cell site infrastructure monitoring.

Figure 1: Active transparent WDM configuration

Wavelength translation

The Nokia 1830 Versatile WDM Module (VWM) Translation Line Unit (TLU) supports a wavelength translation capability. This capability eliminates the need for operators to change the optics within the RAN at both the cell site and central hub. With wavelength translation, operators can speed and simplify deployments and keep using their existing installation procedures or processes.

The 1830 VWM TLU executes the translation from “gray” optics to “colorized” or WDM-compatible optics with virtually no latency (a few nanoseconds). This fast translation keeps the link span lengths from being compromised. It enables operators to maximize the distance between the radio and central BBU pool, which can be upwards of 20 km away, while maintaining the strict latency and jitter requirements of the CPRI/OBSAI protocol.

Wavelength translation does not alter the underlying CPRI/OBSAI data in any way. For example, it does not perform any electrical framing, which tends to be proprietary and can add incremental latency. The solution is transparent and can work with RAN equipment from any vendor.

The solution makes colorized optics much more accessible and less prone to failure by locating the small form-factor pluggable (SFP) transceivers within the transport equipment (TLU) instead of at the RRHs on the top of the tower. Operators need specialized personnel and equipment to replace the SFPs at the top of a tower. Replacement costs for these hard-to-reach radios can reach several thousand dollars. As shown in Figure 2, this is about 8x higher than the cost of replacing SFPs on the fronthaul equipment at the base of the cell site.

Figure 2: Impact of wavelength translation on replacement costs for SFPs, FIT rate, and power use

The failure in time (FIT) rate of coarse wavelength-division multiplexing (CWDM) optics is about 2.4x higher than gray optics. With DWDM optics, the FIT rate is roughly 5x higher than gray optics (Figure 2). Given the higher FIT rates for the WDM optics, it is advantageous for operators to keep the RAN gear untouched. They can do so by leveraging existing gray optics and executing translation within the fronthaul equipment, as supported by the Nokia Mobile Fronthaul solution. In addition, there are cases – particularly with small cells – where a higher-powered WDM-compatible SFP cannot be used due to power constraints. The solution’s innovative wavelength translation capability enables operators to avoid these constraints by keeping higher-power, WDM-compatible SFPs within the transport equipment.

Robust OAM and fiber optimization

The Nokia 1830 VWM Photonic Managed Unit (PMU) adds robust OAM capabilities to passive WDM with management features such as link monitoring, remote power level measurement, inventory management, and software downloads. The PMU serves as a local management hub for the other fronthaul equipment at cell sites, including TLUs and site monitoring modules (SMMs). It enables the optical supervisory unit (OSU) at the central hub site to have complete visibility of all the fronthaul components.

The PMU also leverages WDM to aggregate multiple channels or wavelengths. It optimizes fiber resources while maintaining the strict latency, jitter, asymmetry, and synchronization requirements of the underlying CPRI/OBSAI data. The PMU supports up to 18 channels (CWDM). Future releases will include DWDM to support higher channel counts.

Centralized monitoring and management

The Nokia 1830 VWM Optical Supervisory Unit works with the PMU to add robust OAM capabilities to passive WDM. Together, these technologies support end-to-end fronthaul network monitoring and management. Through an out-of-band optical supervisory channel, the OSU can use fiber link monitoring and per-channel power monitoring to detect the network’s health. This enables operators to proactively monitor and maintain the entire network from a centralized location.

The solution adds reliability by ensuring that power failures do not impact the underlying CPRI/OBSAI traffic. Only the management link is lost in a power-failure scenario. The solution also permits network demarcation between the mobile and transport networks. This allows operators to maintain SLAs that support fronthaul-related wavelength services. The OSU interworks with the Nokia network management system to provide close coordination across the broader transport network (including backhaul) and the RAN.

Cell site infrastructure monitoring

Traditionally, the BBU that performs digital processing is situated at the cell site. It is responsible for monitoring housekeeping alarms relating to local devices or equipment at the site. In a centralized RAN architecture, the BBU moves to a central hub site. This move creates the need for technologies that can monitor local alarms, such as those relating to doors, power, temperature, or humidity.

New modules are available to take up this local monitoring role. With the Nokia Site Monitoring Module, operators can monitor and manage alarms at remote site locations from a central location. The SMM can also interface with other equipment using telemetry information. It can turn devices such as power generators, air conditioning systems, and pumps on and off to improve network up time.

A complete fronthaul services portfolio

Nokia backs the Mobile Fronthaul solution with extensive experience and a comprehensive set of services. These services enable operators to enhance network performance by quickly and cost effectively implementing fronthaul solutions. The fronthaul services offer includes:

  • Deployment services: Engineering, installation, and commissioning services that enable seamless network rollouts
  • Professional services: High and low-level design, network element integration, on-site engineering, installation and commissioning, and element management system/network management system (EMS/NMS) software upgrades
  • Maintenance services: Technical support, repair and exchange, field maintenance, and proactive services that optimize network performance and ensure always-on networks
  • Managed network services: Network operations services, operations assistance services, field operations, multivendor maintenance services, and network security services

Collectively, these services help facilitate the move to new centralized RAN architectures and ensure that the fronthaul network is designed and optimized for peak performance.

Summary

The move to centralized RAN architectures enables network operators to leverage better coordination among cell sites to increase scalability, lower costs, and improve performance. However, these architectures impose strict transport requirements, including high bandwidth, low latency, and the need to minimize jitter to maintain clock reference accuracy and synchronization.

The Nokia Mobile Fronthaul solution is based on the 1830 VWM portfolio. It delivers the industry’s most comprehensive mobile fronthaul solution, offering support for passive, semi-passive, and active transparent transport options.

With Nokia Mobile Fronthaul, operators benefit from a solution that:

  • Meets the strictest latency and jitter requirements and scales to support all CPRI/OBSAI rates
  • Accelerates and simplifies deployments with vendor-agnostic wavelength translation capabilities
  • Cost-effectively adds OAM capabilities to passive WDM to maximize channel performance and support proactive maintenance and monitoring capabilities
  • Simplifies operations by supporting common network and services management across the transport portfolio (backhaul and fronthaul)
  • Is backed by a comprehensive set of fronthaul services, including deployment, maintenance, and professional and managed services

To learn more about the Nokia Mobile Fronthaul solution, please visit: http://networks.nokia.com/portfolio/solutions/fronthaul

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