Getting In

As cells become smaller and indoor wireless use increases, RF penetration into buildings from outside sources has become one of today's hottest issues. Will a nearby cell provide reliable service throughout the entire office, or will the conference room be a "dead zone?" On the other hand, will your indoor wireless PBX be blown away by the co-channel user in the building next door? The cost of in-building and microcell devices is decreasing, and the workload for RF engineers and technicians to install these on-premises systems soon will skyrocket. Rapid engineering design and deployment will be vital for cost-efficient build-out.

Thanks to new wireless engineering software, service providers can model co-channel interference's effects on indoor systems and find dead zones within the building that the outdoor macrocellular system serves. Even before setting foot into the building, you can assess the RF environment in a systematic, organized fashion by plotting either signal strength or interference levels.

New programs use 3-D computer-aided design (CAD) renditions of buildings or collections of buildings. These renditions show partitions and are assigned appropriate RF attenuation values. For example, outside walls may be assigned a 10dB loss. Signals passing through thinner and more RF-friendly interior walls may drop by 3dB, and windows may show a 2dB RF penetration loss.

You can extract estimated partition loss values from extensive in-building measurements. You can deduce these measurements from field experience or you can measure the partition losses in the actual building directly and optimize them with a software program. Once the appropriate partition values are specified, you can place any RF sources, either within or outside the building, in a 3-D building database, and plot received-signal-strength identification (RSSI) or C/I ratios directly onto the CAD drawing. With the right software, you also can carry out traffic-capacity analysis, frequency planning and co-channel-interference analysis simultaneously within the propagation model.

SIGNAL STRENGTH Will the macrocell located several miles north of the office building provide sufficient signal strength for indoor use in your customer's sixth-floor office? If you scale the CAD drawing to include the macrocell at its actual distance, the building itself will appear too small in the drawing to be useful. Although site-planning programs offer a scaleable zoom-in capability where actual macrocell maps may be represented in the 3-D world, it is often more convenient to model the external macrocell in a virtual location closer to the building. To do this, you should use a standard path-loss propagation formula to calculate that cell's lower transmitted power as if it were located just outside the building's wall.

WHAT ABOUT INTERFERENCE? With wireless systems springing up almost everywhere, interference, instead of simple signal strength, is the dominant performance-limiting factor in many locations. Modeling macrocell interference to an established or contemplated indoor wireless system also is relatively straightforward.

Suppose, for example, that an indoor base station is assigned a frequency set identical to an outdoor macrocell. Although the indoor node may provide sufficient RSSI throughout its coverage area, interference from the outside cell may render the indoor wireless system unavailable in certain parts of the building.

Be careful, however, when modeling and analyzing interference, because its detrimental effect also may depend on signal processing at the receiver, not just raw signal levels. For example, a geographic area could have similar AMPS and CDMA signal-strength levels in the 800MHz band, but phones using either method may co-exist because their respective demodulation processes reject interference that the undesired cell provides. A proper C/I model in this situation examines AMPS interference to the CDMA network by reducing the effective narrowband signal strength by a factor related to the CDMA processing gain.

ANALYZING IN-BUILDING SIGNALS Analyzing in-building signal penetration is critical. Will the existing outdoor macrocell adequately supplement its coverage throughout the building? If not, will other existing macrocells adequately supplement its coverage, or must you add indoor picocells? Where should you place these cells? If you are planning an indoor wireless system that competes with signals from an outside macrocell, how much interference can you expect, and where will it occur within the building? As in-building wireless systems proliferate, you must answer questions quickly and inexpensively in a systematic and repeatable manner. You can address these issues by modeling the situation with today's latest engineering programs and examining the resulting RSSI or C/I contours.