Fronthaul: Small Cells’ New Best Friend (pt 2)

Cellular Trends Impact Network Architectures (Part 2 of a 2-Part Series)

Unfortunately, in reality, fiber is not nearly as ubiquitous as would be required for a full fronthaul network. Even in the best of cases, fiber may be present to the curb, but may not be economically available to the rooftop, pole, or towertop where the RRH/RRU needs to be deployed. In other cases, installing fiber on an existing tower may prove to be a challenge. So, even in areas where fiber may be present in The Last Mile, it is the last 1,000 or 100 feet that represent the most challenging problem. Keep in mind that the majority of cell sites are deployed today in suburban and road areas (where more capacity is needed to meet fast growing traffic demand) where fiber is far from being ubiquitous. Therefore, the significance of the "fiber gap" is obvious.

Carriers are finding it impossible to deploy traditional macro cell sites in these areas. Neither traditional small cells nor outdoor DAS solutions are realistic solutions for these areas that form a high percentage of an operator’s footprint. Ideally, a new way of expanding coverage from the existing macro infrastructure would be welcome news for the carriers.

Wireless fronthaul is a tool in the carriers’ toolbox to complement outdoor DAS, fiber and traditional small cells. Wireless fronthaul can extend the reach of fiber and enable new site solutions with quick deployment and much lower total cost of ownership.

To begin, we need to understand the elements of a system that would benefit providers most. The ideal fronthaul solution provides a highly efficient wireless solution to connect baseband units and remote radio heads. By reducing the amount of spectrum needed, the best solution could interconnect up to 3 LTE or UMTS RRH to a BBU using a single wireless link. (See Figure 1.)

Figure 1. Fronthaul and wireless fronthaul.

The ideal system would be compatible with either unlicensed or licensed bands to implement a wireless fronthaul link. For example, 5.8 GHz becomes a practical transmission medium thanks to its favorable transmission characteristics and availability of a wide frequency band. Therefore the ideal system would be able to use the 5.8 GHz band and other bands as they become available.

It would also have the capability to be deployed using existing base stations (UMTS or LTE) and RRH/RRUs. In addition, it would be compatible with emerging and future smaller and lower power RRH/RRUs as well as integrated antennas. (See Figure 2.)

In the ideal solution, carriers would be able to use smaller antenna access points and place them close to subscribers. These smaller antennae would replace the large and higher cost amplifiers needed to drive a big cell tower. With no need to build a big tower, the operator could then avoid — or at least defer — the costs and zoning headaches associated with towers, as well as the expense of equipment rooms. Smaller RRH/RRUs are an attractive and efficient alternative to all integrated small cells. (See Figure 3.)

To explain further, the lower power RRH or integrated antennas would be deployed as coordinated small cell site solutions at a variety of sites (light poles, billboards, wallmounts, etc.). (See Figure 4.) By quickly deploying these smaller antennas close to areas where RF coverage is needed, wireless fronthaul supplements site acquisition and fill-in strategies.

Figure 4. Wireless Fronthaul can be deployed in a variety of situations.

Why It Works

Because the baseband unit remains in control of the remote sectors in a fronthauled system, performance can be optimized because the centralized baseband unit continues to manage handovers between the sectors quickly and efficiently. As the density of cells and sectors increase due to increased need for capacity, the frequency of handovers grows rapidly.

In a backhauled scenario, each handover takes more time, resulting in the potential loss of user data and an increase in overhead resources. In a high-density network of small cells, performance and capacity could potentially be impacted given that increased time for handoffs. Conversely, a fronthaul system delivers smoother and more efficient handovers, even in a dense network of small cells and sectors.

Enhanced interference cancellation and mitigation techniques are additional challenges providers face today in their wireless networks. Thankfully, new techniques such as Enhanced Inter Cell Interference Cancellation (eICIC) are being developed for LTE and LTE-Advanced systems. Newly developed cooperation techniques such as CoMP (Coordinated Multipoint Transmission) are also able to take advantage of neighbor cells or sectors signals to enhance performance of a mobile device at the cell edge (where signal quality is generally the worst).

The higher latencies in a backhaul scenario make such methods unpractical for cells or sectors not sharing the same baseband unit. And since backhaul scenario, sectors are interconnected after the baseband stage, they cannot share real-time RF data. This means that cell edge or sector edge interference is bound to be higher in a network of backhauled cells and sectors. In high-density cell networks, lack of efficient interference cancellation or mitigation techniques make a backhauled solution impractical. (See Figure 5.)

Figure 5. Coordinated Multipoint Transmission (CoMP) and wireless fronthaul.

Another benefit of centralized baseband and fronthaul is that existing network management systems and operational processes can remain unchanged. Indeed, the same base station management systems continue to be the basis for all configuration, alarms and performance reporting. By contrast, a heterogeneous network of macro and small cells with distributed baseband and control requires an overhaul of the network management system and operational processes within the carrier’s organization.

The Future for Fronthaul

As the need for capacity increases, larger and larger fronthaul networks can be deployed off of the same centralized baseband unit. This leads to high capacity pooled baseband servers or base station hotels that can serve hundreds of sectors. A hybrid network of fiber and wireless fronthaul can also be used to interconnect with a dense network of distributed RRHs in the field.

Pooling all the baseband resources in a central location not only enhances performance and lowers deployment costs, but also reduces electric power consumption and maintenance cost. Since only radios are deployed in the field, maintenance crews can be reduced and intervention processes can be simplified. Even better, technology upgrades can be done "in the cloud" as opposed to the field at every cell site. To a large degree, the operational expenses of the cloud RAN become integrated with the cost of operating the core network.

Wireless fronthaul indeed provides a cost-effective and flexible solution for macro and small cell sites. One fronthaul solution, EBlink’s FrontLink 58 system, meets the criteria described in this article. Providers looking to evolve their wireless networks quickly and efficiently should incorporate systems such as this into their network evolution roadmap.

Frédéric Leroudier is General Manager, North America, EBlink Inc. He has more than 20 years of experience in the wireless industry, and has led network design, technology selection and business development for a variety of international carriers and system integrators. Eblink, founded in 2005, and based near Paris, France, is a pioneer and provider of wireless fronthaul technology. For more information, visit www.e-blink.com.