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The Demands on In-Building Solutions
The Demands on In-Building Solutions

Most buildings have blank spots where your phone won't work. Here's a look at how to increase mobile phone coverage in buildings, without annoying the network provider or the authorities.

While cellphones are becoming ubiquitous, and advanced new wireless services are increasingly popular, access to the wireless network is frequently blocked inside buildings. This prevents phone calls from being completed and lowers the throughput of wireless data services.

For very large public spaces like airports and casinos, wireless service providers can justify the significant investment required to deploy new cell sites and distributed antenna systems. But for the more than 4.2 million buildings in North America with less than 100,000 square feet, there have been no widely accepted solutions, preventing the use of phones in many work environments. In order to address the coverage problems being experienced by mobile users in these smaller locations, in-building solutions must satisfy three key criteria:
1.   They must always protect the spectrum from interference.
2.   They must be cost-effective and easy to deploy.
3.   They must be able to react to a changing environment without any manual intervention.

Protect the Spectrum from Interference
Wireless spectrum used for public voice and data services is licensed by the wireless service provider and, under FCC regulations, the service provider must authorize all equipment that uses the licensed spectrum. These regulations reduce the risk of interference affecting other users throughout the network.

Today, systems using traditional repeaters and bidirectional amplifiers (BDAs) are sometimes deployed for in-building coverage. Unfortunately, if not properly configured, or if the environment changes, they may oscillate, causing harmful interference. Depending upon the severity of the interference, a traditional solution can knock out an entire base station sector affecting a large number of users. This is certain to upset service providers.

While some systems can align themselves, others need to be fitted with care to ensure such interference is avoided. Influences such as building materials, local geography, and nearby mast placement may have to be taken into account.

Cost Effective and Easy to Deploy
Service providers must justify each solution deployment based on its projected return on investment. Since the revenue potential relating to small locations is generally small, the initial investment must also be small. Solution costs include the equipment and cabling, as well as the labor costs for site surveys, planning and engineering, deployment, commissioning, and testing.

Devices requiring manual alignment will need skilled radio technicians to fit and configure, which will often cost more than the hardware itself. Any costing for such a system must include an investigation of how much this fitting is going to cost. If a system offering automated alignment is considered, the cost saving on fitting must be weighed against the cost of the hardware.

React to a Changing Environment Without Manual Intervention
Wireless networks are dynamic given that signal levels continuously change:

  • In the macro network, signals change due to growth of the network (e.g., base stations are added or removed), ongoing network optimization to decrease interference or improve coverage, and new services such as GPRS and 1xRTT.
  • Inside a building, signals can change due to external factors, like construction of a neighboring building that blocks reception.
  • Inside the building signals also change more frequently, when windows and doors are opened, and as building interiors are modified.

    This requires that static solutions based on traditional repeaters and BDAs be continually monitored and manually reconfigured as the network changes. Again, this running cost can push such a solution beyond being economically viable. While lost calls can be expensive to your business, maintaining a staff of trained radio technicians might cost even more! Some modern systems can automatically adjust themselves to the changing conditions, which should eliminate running costs even if they are expensive in the short term.

    Creating a radio repeater that can modify itself to suit the conditions should enable anyone to fit it with minimal running costs, and provide a solution for smaller scale customers. This is what Spotwave has done with what we term an "adaptive repeater."

    In order to better understand the role of adaptive algorithms and the capabilities of adaptive repeaters, we first need to understand the basic operation of the traditional BDA.

    Traditional BDA Operation
    Simplistically, traditional BDA-based systems work by gathering signals from outside the building and amplifying them inside; similarly, signals from inside the building are amplified outside the building. These solutions must be designed, engineered, and monitored to ensure they have no impact on the macro network. Their installation requires specialized tools, like spectrum analyzers, and a priori knowledge of the network. Unfortunately, BDA solutions are static solutions in a dynamic environment.

    Adaptive Repeater Operation
    In contrast, adaptive repeaters operate by continuously scanning and monitoring the RF signaling environment and dynamically changing their power and gain. Included in the software is a knowledge-based algorithm that automatically assesses its environment, learns about changes, and adapts itself in real-time to the changes.

    Adaptive repeaters introduce enormous benefits to repeaters, but unfettered, could cause concerns. To meet the requirements, any kind of adaptive repeater will need to adhere to the following rules:
    1.   Maintain the integrity of the spectrum: They are guaranteed to protect the integrity of the spectrum - they never oscillate - because they automatically respond to all changes in the environment.
    2.   Accommodate changes in the network and the local environment: Should the network change, adaptive repeaters can rapidly rescan the spectrum and again lock in on the best available signals.
    3.   Provide a consistent coverage area: Changes are under the control of a new, adaptive algorithm, which allows the coverage area to remain constant, while avoiding any potential for oscillation (i.e., subject to Rules 1 and 2).

    Providing a consistent coverage area depends on a number of factors with the received signal strength from the base station and the level of isolation being the most important, as they tend to be the most dynamic. In response to changes in the received signal strength, an adaptive repeater will maintain a consistent coverage area by increasing the gain to compensate for a lower signal level or decreasing the gain if the received signal increases. However, changes in isolation will affect the size of the coverage area if the installation is isolation limited.

    For example, if the level of isolation changes such that integrity of the spectrum is in jeopardy, the system gain will be reduced accordingly to reestablish the appropriate stability margin. This will reduce the coverage area, but will only be noticeable to users if the available coverage area equals the required coverage area. That is, in many locations the available coverage area will exceed the required coverage area so the change does not impact users. Once the level of isolation is restored, the system increases the gain to achieve the maximum coverage area.

    This combination of adaptive techniques and smart antennas also simplifies the installation and maintenance of in-building solutions. Adaptive repeaters can scan the wireless frequencies across the spectrum, looking at both wanted signals and potential interference from unwanted signals. Using smart antenna techniques, the optimal location can be automatically determined and indicated to the installer, eliminating the need for special tools and a priori network knowledge.

    The Hardware
    An adaptive repeater consists of two main units, the Donor Unit and the Coverage Unit, connected by a flexible, coaxial cable. The Donor Unit communicates with the macro network and the Coverage Unit communicates with the wireless device.

    The Donor Unit contains an embedded high-gain antenna capable of seeking out the weakest of cellular signals, with sufficient gain stages to communicate to and from the Coverage Unit. Overall system gain is distributed between the units to allow inexpensive small diameter cables to be used for cable runs of up to 100 meters. The Donor Unit is so sensitive that it can work with received signal levels as low as -105dBm, allowing deployments completely inside buildings without needing roof access. A small indicator on the Donor Unit turns red, amber, or green to indicate its optimum alignment.

    Most processing is contained in the Coverage Unit (see Figure 1). The adaptive algorithm continually monitors external and internal operating conditions and adjusts the operating parameters to provide a consistent coverage area while guaranteeing spectrum integrity. The device continually monitors the signal received from the base station and level of isolation between the Donor and Coverage Units.

    Whenever there is a change in the received signal strength, the algorithm adjusts the system gain to ensure a stable coverage area and manage noise levels. It should be possible to put several adaptive repeaters within the same base station range without any interference, as each one will adjust its power to avoid just that.

    If the signal from the base station was to disappear (during maintenance, for example) the Donor Unit would automatically shift to the best available signal, ensuring continued service inside the building. The Donor Unit will also automatically return to the initial signal when it is restored.

    The device determines the system isolation between the Donor and Coverage Units and establishes the maximum system gain by detecting its own feedback signal. Because it continuously monitors the amount of system isolation, it can react immediately to any reduction in isolation by reducing gain. If the isolation level ever becomes very low, the entire system will shut down until the isolation is restored, thereby preventing network noise.

    Innovative Solution for Small Spaces
    Solving in-building coverage problems in millions of small locations requires solutions that protect the spectrum, eliminate deployment costs, and react automatically to the changing network environment. By making use of an adaptive repeater it is possible to spread wireless access into buildings and other low signal areas and ensure contiguous connectivity.

    About Steve Adams
    Steve Adams is the vice president of marketing at MyFax. He has extensive experience in creating and marketing new products and expanding high-tech organizations. He has over 15 years of experience in the high-tech and software industry, most recently with Ottawa-based Spotwave Wireless, a provider of in-building wireless coverage solutions. Prior to Spotwave, Steve was with CrossKeys Systems Corporation, a global telecom software solutions provider. Steve is a graduate of the University of Waterloo and the Ivey School of Business.

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