Small Cells, Big Returns

Sometimes small things can get big results. As wireless carriers seek cost-efficient ways to boost both coverage and revenue, many are adopting an innovative small-cell approach that can deliver big returns on network investment.

Hierarchical cell structures consist of one or more small, low-powered cell sites underlying higher-powered macrocells. These flexible and highly affordable smaller cells are ideal for spot coverage and providing capacity relief on congested sectors of a growing wireless network.

The market success of the IS-136 TDMA and other digital standards has fueled a rapid growth in small-cell network configurations. This innovative approach has been applied successfully in many of the world's most densely crowded wireless markets, including Latin America, Israel and major U.S. metropolitan areas. As capacity pressures grow in crowded wireless networks, small-cell underlays will become even more popular.

Existing AMPS and IS-54 Revision B (IS-54B) technologies offer limited hierarchical cell support, but those standards require that the underlay provide the best and strongest channel to effectively steer a phone to that cell. With the current IS-136 standard, setting up low-power underlay cells is no longer based on a best server or strongest signal design. In fact, the IS-136 standard embeds support for hierarchical cells. These embedded IS-136 cell reselection parameters gracefully steer mobiles based on factors such as mobility, RF link quality, preferred or non-preferred cell type, and several operator-selected public, private and residential identifiers.

Small hierarchical cells can be deployed in an outdoor or indoor configuration and serve a community of interest such as an office building, a pedestrian mall or a residential neighborhood. They improve capacity, service offerings and customer satisfaction.

By understanding the benefits and deployment challenges of the small-cell underlay, you can better position your networks to meet future capacity and quality demands.

BENEFITS OF HIERARCHICAL CELLS Perhaps the most powerful benefit of hierarchical cells is that they deliver network capacity when and where you need it. These relatively low-cost cells are flexible and easy to install, and you can redeploy them to meet evolving network requirements. As networks add users, features and sophisticated new billing structures, underlays are a viable and cost-efficient alternative to cell splitting. Hierarchical cells pull traffic from the overlay, freeing capacity on the macrocell while supporting cost-effective growth on the entire network.

You can use hierarchical cells to create a highly customized mix of private, semi-private and residential sub-networks. For example, you might use an underlay to create a private dedicated service area -- offering a reduced per-minute or a flat monthly rate -- for a corporate location, theme park or university. A semi-private service area, featuring a mix of public and private users, can be created for applications such as shopping malls and hotels. Residential cells, apartment complexes, homeowner associations or country clubs might offer customized features at a monthly flat rate. By custom-tailoring attractive pricing and applications for these tiered service cells, you can increase customer retention and offer exclusive services within the underlay.

Small hierarchical cells provide a flexible and cost-efficient deployment option. Because of their compact design and varied mounting options, you can deploy small cells strategically in applications where installing, splitting or adjusting a macrocell is impractical.

Small, self-contained cell-site enclosures can be deployed quickly and at a fraction of the cost of a macro-site installation. Conventional or distributed antenna systems often can be deployed on existing structures, rather than new wireless towers, providing highly effective RF coverage at a competitive cost. When compared to a macrocell application, small hierarchical cells typically provide dramatically reduced real estate costs, fewer zoning requirements and lower installed site costs.

In some applications, low-power in-building underlays can be exempt from regulatory filing requirements, which further reduces the time and cost of deployment. Hierarchical cell sites also can be redeployed to support a rapid and affordable response to changes in network requirements.

Hierarchical cells can improve coverage in cold spots -- or geographical areas such as building interiors, underground parking areas or tunnels -- where RF coverage from the macrocell is limited or blocked entirely. By using an underlay to fill in cold spots, you can boost coverage and improve voice quality.

Small low-power cells also can be used to off-load wireless traffic in hot spots -- peak-use areas such as busy intersections, city centers and pedestrian malls -- which can overburden the macrocell and lead to serious network blocking difficulties. With strategically placed hierarchical cells, you can handle more calls while significantly improving end-user service.

Wireless handsets operate at lower power levels in these small cells, thus extending battery life in many applications. And because small cells are isolated and transmit at low-power levels, you can reuse channels with fewer interference problems.

DEPLOYING HIERARCHICAL CELLS To fully appreciate the opportunities of the small-cell approach, you also should understand how best to deploy small cell sites. Before deploying a hierarchical cell, make sure that the macro network has been optimized for the target location. If a network is experiencing bad originations, dropped calls or poor handoffs at the macrocell level -- and these difficulties can be traced to an antenna, azimuth, downtilt, effective radiated power (ERP) or other coverage problems -- those situations should be corrected before deploying a smaller cell. Putting smaller cells on a poorly performing network only will make those problems worse.

You should start by analyzing potential network triggers, which indicate a given geographical area is a good candidate for a small-cell underlay. Network activity logs, drive tests or other measurement tools can take a broad overview of a network and identify problem areas with high rates of blocked or dropped calls, frequent origination or termination failures, high bit-error rates and areas with high interference.

Customers are perhaps the best quality triggers of all. You can identify trouble spots and potential hierarchical cell locations simply by listening to feedback from wireless users.

Once you have identified a problem area, more precise equipment and methods can refine and evaluate potential small-cell locations. Tools such as test receivers and drive test equipment can monitor the received signal strength of originations, path loss and interference.

You also should consider traffic mobility. Due to the high hand-off rate requirements of fast-moving users, small-cell applications can be customized to address the requirements of high- and low-mobility traffic. Offices, shopping centers, pedestrian malls, neighborhoods, campuses, downtown corridors, throughways and other similar locations are prime locations for small cell sites.

During the early planning stages of a hierarchical cell underlay structure, you should formulate a strategic approach to network frequency usage. Frequency planning is a key variable affecting the growth of a wireless network. A poor frequency plan inevitably leads to capacity bottlenecks, quality breakdowns and angry customers. A well-conceived frequency strategy, on the other hand, creates a smooth and efficient pathway to capacity growth, profitable new services and satisfied customers.

There are three main frequency strategies associated with hierarchical cell structures: non-segregated, segregated and adaptive. Each offers a unique set of demands and opportunities. If your hierarchical cell opportunities are isolated, you may want to use a non-segregated, or fixed, frequency plan to maintain capacity on the microcell network.

In a segregated frequency scheme, you create a contiguous web of small hierarchical cells, which together act as the primary carrier for the low-mobility traffic. In this model, the macrocell network ensures the smooth flow and handoff of high-mobility traffic. Segregated frequency strategies most often are used in areas with numerous in-building networks, high-traffic zones and multiple unique applications.

An adaptive frequency-planning scheme relies on the small cell's ability to scan the surrounding environment and determine the best channels for use. This self-engineering method can take advantage of the changing traffic patterns in a network and provide capacity improvements for peak traffic hours. This method is best suited to situations where segregation has too great an impact on macrocell capacity, and fixed reuse is not feasible.

When selecting a frequency strategy, examine existing usage patterns, capacity measurements, growth forecasts and their unique network-management philosophy.

Because the algorithms used in traditional prediction tools are calibrated to evaluate macro-level issues, these prediction tools yield limited results at the hierarchical cell level. For this reason, you should employ more accurate drive- and walk-testing methods to examine the coverage and interference ramifications of a proposed small-cell underlay. Test personnel key up channels on surrounding cells and sectors to analyze the effect of a hierarchical cell on the surrounding network. You also can use test transmitters to evaluate the ERP at the proposed underlay cell site.

The digital control channel (DCCH) of the IS-136 standard contains built-in traffic acquisition algorithms that support the creation of hierarchical cell networks. The IS-136 DCCH supports wireless traffic management based on mobility, cell preference, time filtering and RF link quality -- giving carriers powerful and flexible control over these custom-tailored cell underlays.

Once a hierarchical cell is installed, you can use a variety of technologies and methods to verify and fine tune the application. Network managers revisit operational activity logs to confirm that the underlay cell is meeting traffic-acquisition and capacity requirements, and that it has not created new interference problems. Network drive and walk tests also can be used to optimize antenna placement ERP and to ensure smooth handoffs between the new cell, the macrocell and other adjacent hierarchical cells. Virtually all aspects of a small underlay cell can be adjusted at this verification stage to ensure maximum efficiency.

Concurrent with the deployment of hierarchical cells in existing networks, you will be able to capitalize on advancements in system features that allow for automated optimization and self-engineering. Leading-edge vendors will offer self-engineering capabilities for hierarchical cell deployments.

Small hierarchical cells allow carriers to maximize network coverage, stimulate higher usage and generate revenues from profitable new cell-specific services. Subscribers benefit from improved network performance, more effective portable mobility and an array of personalized wireless services.

This innovative new network strategy offers a clear and compelling alternative to the high cost and difficulty of macro-level implementation. You can reap big returns through the use of small cells.

Hierarchical cell underlays also are closely associated with GSM and CDMA. GSM provides highly intelligent algorithms, which manage the smooth flow of traffic between small-cell underlays and the larger umbrella cell. Cell underlays can be used to quickly boost capacity in small areas of a GSM network, and these microcells, minicells and picocells are ideal for adding coverage in high-traffic hot spots, in-building applications and other key locations. Compact, plug-and-play base-station units make adding small cells to a GSM network an affordable alternative.

CDMA technology offers a highly scaleable network architecture that also makes use of hierarchical cell structures. Because CDMA is implemented with broadband RF techniques, you can add network capacity and hot-spot relief by adding frequency overlays or underlays. CDMA is a high-capacity technology, so hot spots do not surface as quickly. When hot spots do occur, CDMA can provide relief through sectorization or through adding an additional carrier to the hot-spot location. With the introduction of EVRC, CDMA capacity increases even more by using 8kb/s voice coding while maintaining 13kb/s voice quality.

Offering customized location-based services requires more than just network efficiency. In order to meet the full array of needs, you will need to consider the requirements of location-based billing as well.

Generally, today's wireless billing plans are based on a 2-dimensional model where type of usage and time of day determine the per-minute rate. State-of-the-art wireless carriers often bill based on the cell sites and zones. In a next-generation environment (based on Phase II of FCC Rule 94-102), the geographic information will be more specific and will be tied to real world parameters, enabling carriers to associate call-event records to specific geographic locations.

Next-generation, location-based billing may provide greater momentum for wireless carriers to provide local loop services. With location-enabled rating, a carrier could offer a residential customer a set of individualized rates in a customized package -- one rate for home, one for the office and another for a frequently visited location such as a vacation house or the mall.

Beyond the residential market, location-based billing will provide a greater push for wireless providers to enter the commercial PBX market. Carriers would be able to offer PBX service that includes individualized rates within the user's home office, at a related branch office or from a client location. The use of a wireless phone both within and outside the office would reduce maintenance costs of both wireline and wireless phones and offer greater communication efficiency through a single point of contact.

But there are still many unanswered questions about location-based services. For example, what will it take for billing systems to support location-based services? Answering this question is just as important as settling on a location-determining technology.

To prepare for next generation location-based billing capabilities, you need to answer this question: Does your current or prospective billingsystem rate calls based on network cell sites or zones? Answering this initial q uestion also will offer some insight into the fundamental geographic rating capability of the system. For example, if cell site or zone is one of a fixed number of predefined rating keys (and no others can handle geographic information), then you probably will face customization costs. If the system features open rating keys that can be configured to accept user-defined geographic information from a call-detail record, you will be able to carry out next-generation location-based billing more easily.

You must be able to create customer-specific rate plans for location-based billing. You should be certain that these plans can be created for individual accounts. CSRs will have to configure multiple rates for customers at the time of activation. You must be sure that these rates can be created quickly and easily during the customer-acquisition process.

Finally, you must be confident that your billing and customer-care system can support the provisioning and usage-based rating of next-generation location-based enhanced services. Although these services might be offered as monthly recurring charges, carriers could maximize these services by offering customers a choice of monthly recurring charges, pay as you go or a combination of both. By providing more service options, carriers will have a better chance of increasing sign-up rates for services that would further differentiate them from wireline providers.

The continued growth of wireless services and their ultimate ability to challenge wireline services will depend on a number of factors -- price, service quality, coverage, product and service mix. Wireless carriers can seize location-based billing as an opportunity to provide competitive pricing and differentiated services to the end user.

Small hierarchical cell underlays are designed to deliver maximum deployment and service flexibility. Typically powered by commercial ac power, a hierarchical cell base-station unit consists of radios, multicoupling and power supply. These compact cell stations are configured for indoor or outdoor placement and can be mounted on the floor, wall or rack.

In-building systems can use omni, directional or leaky coax antenna to promote consistent RF illumination throughout a structure.

For high-volume hierarchical cell placements, more than one T1 may be used. Multiple radios supported by the multiplexer can be configured as a single underlay or segregated into a number of separate partitions.

Depending on your equipment and functionality requirements, you will need various switch software to manage and operate the remote cell underlay site.