Delivering on the 4G user experience promise with micro-cellular networks

Global mobile data growth is driving innovative solutions from service providers that include everything from billing schemes to technology solutions.  This accelerated growth of mobile data traffic is being fuelled by the continued advancement of smartphone and “superphone” technology and the advanced applications they enable. Additionally, picture and video intensive networking, including social networking, have further compounded capacity demands on mobile networks.

From a technological standpoint, mobile equipment and operator communities have responded with “3G” and “4G” solutions capable of delivering increased capacities. A key element to meet the capacity demand is ensuring there is readily available Radio Access Network (RAN) spectrum. Globally, regulatory bodies are working to free up or re-purpose spectrum for mobile broadband networks.

Due to the propagation characteristics of “4G” RAN technologies, it is unlikely that traditional “large cell” networking solutions will yield the required coverage or deliver the sufficient in-building penetration needed. What’s become evident is that in order to best meet the coverage demands at the street level, the industry is looking towards implementing small-cell (or micro-cellular) networking structures that overcome propagation challenges in high-access, crowded, urban environments.

Interest in Micro-cellular Networking is Escalating

Micro-cellular mobile network architectures have evolved primarily to address the shortfall in available RAN spectrum resources relative to end user capacity requirements. 

When considering RAN spectrum, there are various cases with resulting considerations related to micro-cell networking.

Case 1: Move to higher RAN frequencies for 4G

  • Examples:  2.5 GHz in the US; 2600 MHz Europe
  • Lower cell coverage performance (less diffraction, higher absorptions) which reduces cell size
  • Improved inter-cell isolation allows higher frequency reuse
  • Smaller cells & some reuse improvements mean large per-square capacity delivery into the service area

Case 2:  Use of lower RAN frequencies

  • 700 MHz AWS spectrum; or Re-purposed 2G spectrum (i.e., 850 or 900 MHz.)
  • Lower frequencies allow for better penetration of buildings, foliage and other obstructions, but also cause low frequency reuse (due to reduced inter-cell isolation performance
  • These spectrum segments are generally not considered “broadband”, making them a better solution for rural applications than high capacity micro-cellular networks

Mostly, the need for high capacities within dense urban areas is the dominant motivator for micro-cell deployments (Case 1 above). In this case, cellular infrastructure is deployed in complex, street-level environments where backhaul is a key design feature in building this type of network.

Enabling Micro-cellular Backhaul Solutions

Attempting to leverage the growing capabilities of new RAN technologies, many operators are now experiencing significant bottlenecks in their backhaul network. Modern RAN equipment is primarily backhauled across packet transport (Gigabit Ethernet).  There are essentially two options to backhauling: Fiber and High-Capacity Wireless.

While fiber offers the best capacity, it also has its disadvantages, such as having a high cost of installation and operation (leased), being slow to install, and it faces a number of roadblocks to deployment whether it is hung, trenched or installed in conduit. What’s more, leased fiber is often sourced from a cell operator’s competition.

Another option is wireless backhaul, which, over the past five years, has seen significant advancements in capacity and spectral efficiency. Using high-capacity Point-to-Point (PtP) implementations, current systems are capable of delivering multi-gigabit capacity, making them amenable to virtually all backhaul topologies in a network, including resilient rings.

Wireless PtP links require exclusive spectrum allocation in order to deliver highly available, interference-free operation, which depends upon the use of licensed radio spectrum In the case of licensed wireless backhaul deployment, the links are first analysed (or “co-ordinated”) to ensure there is no interference when the N+1 PtP radio link is installed amidst existing links.

Many mobile backhaul networks rely on leased fiber installations or existing microwave infrastructure. Microwave is becoming increasingly popular for backhaul networking due to its cost (see Figure 1) and time-to-deploy benefits.

Figure 1 – Cost comparison analysis between popular micro-cell backhaul networking options.[1]

Because wireless backhaul faces technical challenges due to a complex “street-level” propagation environment, the use of high frequency line-of-sight engineering is combined with advanced multipath suppression technology to address these challenges. While line-of-site engineering sounds complex, it is simplified by using visual verification tools, such as Google Earth StreetView. Defined backhaul paths in a

downtown/dense-urban scenario can then be verified for line-of-site attributes using conventional visual aids.

Typical Micro-cellular Network Implementation

In many scenarios, the operator looks to micro-cellular networks as a means to provide:

  • High capacity in high-user density areas (usually downtown urban areas)
  • High degree of building penetration at or near the street level (particularly when high frequency RAN operation is considered)

Cell spacing can range from an operating radius of a few hundred meters up to approximately 1 km. At 1 km spacing, cell radiuses are approximately 500m.  In a typical downtown urban area, his infrastructure can be achieved by deploying micro-cell nodes every few blocks.

In a micro-cellular backhaul subnet, there is essentially a ring combined with several spurs used to connect outlying base stations.  Inside the ring is an aggregation link used to connect the micro-cellular subnet layer with a rooftop macro-cellular site.  This subnet topology might be repeated many times to achieve coverage of a given service area.

The use of the ring and spurline topology optimizes network availability, resiliency and self-healing attributes and minimizes equipment costs. When considering the propagation obstacles inherent in a urban operating environment, these networking attributes become extremely important.  

Ring topologies aggregate large amounts of traffic from the various base station sites that are located as nodes on the ring. This drives a requirement for very high capacity microwave ring technology. Further optimization to meet capacity demands is achieved through such features as bulk data compression, statistical multiplexing and ring load-sharing – all of which help future-proof the network.

Outdoor Micro-cell Base Station Node

Outdoor micro-cell nodes are essentially small-scale versions of a macro-cellular node, but with some primary differences, including:

  • Single sector RAN designs (typically “omni”), with reduced transmit power capability
  • All outdoor electronics
  • Traffic Light Pole (TLP), Street Light Pole (SLP) or “store front” mounted near street-level
  • Limited or no battery backup

Deploying micro-cellular infrastructure involves using a high number of nodes to obtain complete coverage in a given service area.  In contrast to macro-cellular base station nodes, micro-cellular nodes require simple, “one box” nodes that are easily and quickly installed with minimal on-site configuration.  It is also essential that microcells meet city’s zoning codes by being fully integrated systems contained in aesthetic, lightweight packages able to withstand the outdoors. 

Given the unique characteristics of microcell underlay networks, operators will need to adopt flexible backhaul strategies to minimize capital and operational costs, as well as deployment timelines.  Network planning for microcellular solutions requires that new tools and techniques are employed to ensure optimal site selection and compliance with city zoning requirements.

While there is clearly no one singular solution to deploying micro-cellular networks, what’s clear is that fully integrated, all-outdoor microsites can deliver both access and backhaul within a single unit, offering operators cost and operational advantages that will lead to this nascent architecture becoming a reality.

[1] Comparison between new fiber install (leased) and capitalized microwave backhaul.  “low” and “high” indicate approximate cost range.  Fiber costs include installation, NID, leasing/foot. Microwave costs include equipment, radio license (FCC), co-ordination fee, installation, real estate lease (TLP or SLP mounted) .