As you continue to fine-tune network build-out and expand coverage areas, you might be contemplating in-building coverage as well. Providing coverage to campus and office environments is key to reaching the enterprise market. The overall goal is to offer feature-rich wireless office services (WOS) that allow workers to use wireless phones instead of their wired desk sets. The critical issue is: How can you implement complete in-building and office coverage cost-effectively?

One of the critical concerns of a cost-effective WOS system is the method used to distribute RF signals throughout the facility. There are two radio architecture approaches: a distributed radio approach and a distributed antenna system (DAS) interfaced to a centralized picocell or mini base station (MBS). In the distributed radio approach, MBS units are distributed throughout a building, interconnected through standard in-building data networking.

In the centralized architecture, the radio capacity is centrally located, and a DAS provides RF coverage. Each of these approaches has benefits depending on the building size you want to cover and the traffic that you need to support. DAS used to be unattractive because it required bulky coaxial cable, and each system required a custom design. But new technology has eliminated this limitation.

DISTRIBUTED RADIO ARCHITECTUREFor a distributed radio architecture, you provide RF coverage by deploying base-station cells throughout a building. The cell is a group of one or more base stations that have the same footprint and for which a single digital control channel is broadcast. Antennas are integrated directly into the base-station enclosure to create a compact unit. Base stations are located throughout a building to provide sufficient coverage.

You can add additional capacity to the established cells as needed. For a single cell, only a single control channel will be active regardless of the number of base stations in that cell. You can increase capacity by adding traffic-only frequencies as necessary either by expanding an existing base station or through adding traffic-only base stations in existing cells, or both. There is a variety of possible cell configurations for the distributed radio architecture.

The base-station line interface connects the base station to the base-station controller. Messaging associated with call processing and control, operation, administration, maintenance, provisioning and self-engineering functions flow over this interface cable. Dc power is supplied through the signaling cable, except in installations where long cable runs preclude power feeding. In the distributed radio architecture, distributed picocells or MBSs provide coverage and capacity.

The spectrum efficiency of a distributed-base-station system is based on the number of base-station transceivers, where there are multiple transceivers per base station, and the entire system's frequency reuse factor. The required number of base-station transceivers depends on the system's coverage and capacity requirements. The total capacity of a distributed-base-station system equals the average capacity per transmitting location for all transmit locations. Typically, when the subscriber density per transmit location is high, and/or the frequency reuse factor is high, a distributed-base-station architecture is more spectrally efficient than a distributed-antenna architecture.

DISTRIBUTED-ANTENNA ARCHITECTUREIn the distributed-antenna architecture, picocells or MBSs are centralized, and a DAS provides coverage. The spectrum efficiency of the distributed-antenna architecture is based on the number of frequencies required to achieve the capacity requirements divided by any frequency reuse factor. The total capacity for a DAS is equal to the sum of the capacity of all pooled radios in each cell. Typically, when you have a low-to-medium subscriber density per transmit location, a distributed-antenna architecture is more efficient than a distributed-base-station architecture. When the density of subscribers per transmit location is high, DAS may or may not be more efficient than the distributed-base-station architecture, depending on the reuse factor for the distributed-antenna architecture.

DASs used to be implemented with leaky coaxial cable, single-mode fiber or other types of specialty coaxial cables as the RF interconnecting infrastructure. These cable installations, which are non-standard for premise wiring, proved too expensive and difficult to install. The need to provide an RF interconnecting infrastructure (which is foreign to most premises) vs. data interconnecting infrastructure (which is common to almost all premises) was considered the Achilles heel of the distributed-antenna approach.

But new DASs use standard Category 5 Ethernet cables as interconnecting infrastructure, or RF-over-twisted-pair technology. With this technology, installing a DAS is virtually identical to the distributed-radio approach. The technology allows you to reap the benefits of a centralized base-station approach while maintaining standard in-building cabling.

The primary benefit of the centralized-base-station approach is a gain in radio trunking efficiency. This reduces the cost and reduces the number of radio channels that you need to allocate to the building for a given number of users. You can use the standard trunking theory based on the Erlang traffic model to quantify the cost advantage of the distributed-antenna/centralized-radio approach.

Using RF-over-twisted-pair technology allows you to take advantage of a pre-installed LAN to deploy in-building service. With this new approach:

* There is no bulky coax, which is difficult to install and requires custom RF design.

* Cabling and system installation costs are 50% lower than other cable schemes.

* Deployment takes less than one day.

* You do not need a cable-installation crew, so you won't disrupt daily office activities.

* System adjustments, such as relocating antennas, are simple because the system is plug-and-play.

* Because product hardware is modeled after LAN equipment, hardware costs are lower.

* The system is format independent and is compatible with all major air interfaces.

* The system is ideal for campus environments where you need to connect multiple buildings to the same WOS controller.

DISTRIBUTED VS. CENTRALIZEDThe distributed radio architecture has complete radio coverage, but does not have the same total traffic-handling capacity of a system that centralizes all radio channels. Because traffic is statistical, it is inevitable that certain radios will experience congestion and call-blockage while others remain unused. To guarantee quality of service, you have to assign additional channels to each radio to handle traffic spikes as they occur.

On the other hand, a distributed-antenna/centralized-base-station approach can reduce blocked calls. Furthermore, with the centralized base station (N) you will need fewer radio channels for installation and will save on hardware costs.

You can assume that the radio coverage for an antenna located on a MBS is identical to an antenna in a DAS. The need for adequate radio coverage requires subdividing the building into multiple coverage areas (M). In the distributed-radio approach, each of these radio coverage areas constitutes a cell, where it is served by a single remote antenna in a distributed-antenna/centralized-base-station approach. The number of such coverage areas depends on the size of the premise. The traffic load in each coverage area, on the other hand, depends on the number of users in the building and the average traffic load each user offers.

The total traffic load is A Erlangs for each of the coverage areas. The number of channels N(A) required to handle this assumed homogenous traffic with a 1% grade of service is given by the Erlang B distribution. This is the number of channels each radio requires in the distributed-radio approach. If a DAS antenna, which routes the entire floor's traffic to a central base station, services each coverage area, the traffic load will be M*A. You can obtain the number of channels required N (M*A) from the Erlang B distribution for the same 1% grade of service.

Assume that each pico base station contains five voice channels (two radio channels, three TDMA voice channels in each radio channel, one channel used for traffic control within the cell). The amortized cost of each picocell is Cp = $3,000. In the distributed antenna architecture, many radios are co-located to form the centralized base station, while each remote antenna costs $2,000 amortized over the cost of hubs that make up the complete distributed-antenna system, with a minimum system cost of $6,000. For a system with 200 users and for building sizes ranging from 25,000 to 1 million square feet, the distributed-antenna/centralized-picocell approach can provide a cost advantage.

The DAS can reduce implementation costs, including hardware, installation and maintenance.

The design requires minimal RF engineering and planning, because each remote antenna unit (RAU) outputs the same amount of power, making the distribution design much easier than classic coaxial distribution systems. You do not have to compensate for cable loss and amplifiers in the design or account for them in the system design. In addition, you don't have to rely on specialized coaxial contractors. Instead, you or the customer can install the equipment.

PERSPECTIVESDASs using RF-over-twisted pair technology are operating across various industries including electronics manufacturing, newspaper, healthcare, insurance and hospitality. An installation at Cambridge Hospital in Boston employs a DAS to provide cellular coverage with six antennas in two buildings.

"The WOS was a very easy system to install," said Michael Murray, Cambridge Hospital director of telecommunications. "With the help of the project manager, we installed all of the Category 5 cables ourselves and used existing multimode fiber to connect the second building."

Cellular One is offering distributed-antenna-based WOS to San Francisco's Tech Museum. Providing in-building coverage was particularly difficult because the museum is located below ground. But with a DAS, the museum is able to provide hands-on demonstrations in its cellular exhibit. The installation, which covered more than 132,000 square feet, used one main hub, an expansion hub and four RAUs.

"(The service) is allowing us to provide engaging and informative demonstrations in the Communications Gallery," said Greg Brown, Tech Museum spokesman.

With advanced WOS systems now available, carriers and end users are realizing there are many solutions available to meet any in-building coverage need cost-effectively.

Average traffic per user -- 0.05 Erlangs

Total number of in-building users -- 200

Picocell/remote-antenna radio coverage area -- 25,000 feet

Building size -- 6 cases, 25, 50, 100, 250, 500, 1,000 feet

MBS cost (14 voice channels) -- $3,000 (including amortized cost of controller)

Remote antenna cost -- $2,000 ($6,000 minimum) (amortized over DAS system including all installation)

Blockage probability -- 1%

More carriers are recognizing the true benefits of offering wireless office services (WOS), evidenced through several announcements over the past few months. Cellular One of San Francisco joined the bandwagon earlier this year when it signed a joint marketing agreement with AG Communication Systems. The agreement allows Cellular One to offer mobile solutions in the office environment and gain a competitive edge over local carriers not yet offering WOS.

Cellular One customers will have standard TDMA wireless phones, which act as cordless extensions of the their desk phones. The system allows users to make and receive calls and use all features of the office phone. Calls within building range incur no per-minute airtime charges. Subscribers also can use the same phone in the external wireless environment.

Rogers Cantel also has signed a joint marketing agreement with AG Communication. Rogers Cantel now offers a solution that supports up to 2,000 registered users and up to 300 digital traffic channels.

Rogers Cantel also signed an agreement with Hughes Network Systems for its AIReach office system, which provides coverage through a network of low-power picocells.

AG Communication has signed a joint marketing agreement with U.S. Cellular, which will allow the carrier to offer the Roameo solution in 145 markets.

"Beyond convenience, Roameo dramatically increases efficiency because it allows you to become untethered from your traditional work space," said Larry Spovic, U.S. Cellular product manager.