Wireless Propagator: In-Building Wireless

Attach the word "wireless" to any topic and you're likely to pique the interestof your colleagues, technical or otherwise. But mention "in-building wireless,"and you'll just as quickly lose their attention as your own mind drifts toimages of hot and itchy insulation-lined ceilings and narrow elevator shafts.

In light of the growing enterprise wireless infrastructure market, pervasivelydeployed Wi-Fi networks remain relatively rare. The cost/coverage issue inlarge open areas such as warehouses and stadiums is just one reason. In acompletely wired office with a small number of mobile workers, it can bedifficult to justify building out beyond the foyer, a few conference rooms andexecutive offices.

The other wireless, cellular, faces similar challenges. Both business andpersonal mobile phones are in use by most employees, but buildingconstruction--whether modern with cinder blocks, steel trusses andsheetrock-lined metal studs or historical with foot-thick concrete walls andfloors--often prevent cellular signals from penetrating beyond more than one ortwo walls. This limits the use of cellular for both voice and 2.5/3Gapplications.

My previous columns have discussed the concept of dual-mode handsets thatuse the cellular network when outside and the Wi-Fi network on the inside. Thatworks fine when the enterprise can justify such an infrastructure, but theimmaturity of the technology and the reality that it ignores existing GSM- orCDMA-based devices provides reasonable consideration for pause.

In-building wireless systems can address the challenge of pervasive andconsistent Wi-Fi and cellular coverage by distributing the wireless signal toremote locations from a central site. For example, rather than have accesspoints mounted in ceiling tiles, they can be secured in that floor's wiringcloset and the signal transported over the in-building wireless system. In acellular example, a larger base station can be used to provide service to anentire airport over many long and separate strands of cable rather than usingseveral pico cells, which need both power and space and are used suboptimally.In-building wireless systems are most commonly found in public areas such asairports, subway stations and sports venues and in institutions such ashospitals and universities.

Two main categories make up in-building wireless: passive systems and activesystems.

Passive systems have been around for the longest time and are most closelyassociated with the phrase "leaky coax." It's a system that includes the coaxcables, splitters, combiners and taps. As the category describes, limited or noactive technology is used to connect the radio to the antenna. Coax has theadvantage of being a broadband medium that simultaneously carries a broadrange of spectrum. Vendors offer a mix of different coax cable optimized forsome of the following: low coupling loss, low attenuation, high frequencies, smallbending radii and retarding fire. Some of the cons of passive systems includethe complexity of calculating and predicting RF loss over the wires, amplifiersand connectors, limited expandability and installation difficulty. According toLGC Wireless, labor generally makes up about 60 percent of the cost, while theequipment accounts for the remainder. This means that in countries wherelabor rates are low, passive systems may be more attractive than activesystems.

Popular vendors of passive systems include Andrew Corporation, RFS (RadioFrequency Systems) and InnerWireless.

Active system indoor wireless solutions include fiber-fed repeaters, pico cellsand DAS (distributed antenna system). Fiber-fed repeaters merely replicate RFsignals over a fiber-optic link. Pico cells are small versions of regularcellular base stations and require the cooperation of the cellular carrier forinstallation and maintenance. DAS uses a mixture of multi- or single-mode fiberand CAT5/6 cable to transmit a narrow band or range of spectrum. Single-mode fiber can extend the RF signals over tens of thousands of feet, andCAT5/6 copper cabling capitalizes on existing cable plant and installationexperience. The DAS architecture consists of a core unit that modulates theradio signal onto fiber or coax and then is remodulated into RF at the far endusing a remote access unit. Some sites already have spare fiber or coppercabling, and rather then pull a separate coaxial wiring infrastructure, theexisting plant can be utilized. This results in a significant cost savings. DASsystems are by nature narrow band. For example, serving 802.11a, which usesspectrum in the 5.3 to 5.8 GHz range, generally requires a separate overlaysystem. On the other hand, DAS allows for zero-loss-of-signal transport, whicheliminates the need for calculating RF loss, allows flexibility in reconfiguringsystems and permits the concentration of equipment. The use of activecomponents carries additional cost, but it does allow for system monitoring,something that coax usually does not offer.

Popular vendors for active systems include Andrew Corporation, LGC Wireless,Powerwave and MobileAccess.

Along with the labor cost and the fact that it may not support requiredfrequencies, another criticism aimed at in-building wireless systems is thecapacity issue for 802.11-based wireless. If the wireless network is designedwith microcells using low-powered access points every 2,500 square feet, itdoesn't make sense to home run either an active or a passive system back toa central location--labor and equipment costs will far exceed the cost ofdeploying multiple access points. On the other hand, users who need access totheir GSM- or CDMA-based BlackBerrys or 3G applications while within the innerrecesses of the building will lack coverage.

Many organizations expect wireless carriers to step up to the plate andprovide better in-building coverage at no cost. Although some companies havesuccessfully negotiated an agreement, it usually requires signing afixed-length contract for a certain number of handsets. Depending on whetherit's a pico cell, amplifier, repeater, directional antenna or in-buildingwireless system, if employees carry a personal cellular device from a competingcarrier, it may not work, thereby marginalizing the value. In the case of mostairports, convention centers, sports venues and shopping malls, the buildingowner may develop a neutral-host system and negotiate leases with interestedcarriers. The carriers benefit from better coverage, capacity and increasedsubscriber satisfaction, while the host generates a recurring revenue stream(perhaps in addition to what has already been negotiated with the carriers), abetter system for public safety and the safety of their own personnel, andincreased customer satisfaction.

Admittedly, in-door wireless doesn't conjure images of sexy dual-modehandsets, but it can provide good pervasive wireless access where otherwise itwould be neither affordable nor possible.

Frank Bulk is a contributing editor to Network Computing Magazinecovering wireless and mobile technologies and works for atelecommunications company based in the Midwest.0