Wireless Search Area Design
Site acquisition projects are created by first designing a search area, often referred to as the search ring. Search areas are designed by RF engineers. The most pertinent RF design concepts in relation to site acquisition include three geographic location perspectives: existing structures, air navigation, and the potential for radio frequency interference. RF engineers originate search area assignments representing the client’s RF engineering criteria for site acquisition agents. Search areas might be rings, squares, rectangles, ovals, odd shapes, or even two or more distinct areas of geography.
Search areas include geography from where the RF engineer determines a macrocell site or multiple microcell sites that will provide the best coverage. Locating specific geography is the primary objective of the search area project. We’ve all heard about the relevance of “location, location, location” in real estate. The same concept applies in relation to finding a site for each new wireless facility. The three references to a location that form the context for each search area designed by RF engineering are:
(1) the global location defined in the US by the United States Geological Survey (USGS) coordinate system
(2) the wireless operator’s existing grid or system of sites
(3) the search area purpose or objective
Search Area Design based on Global Location Criteria
The most common method used to express the location of existing, potential, and target cell sites in the United States is the USGS coordinate system. While the search area boundary is identified on a local map, specific site locations are identified using three measurements in relation to the surface of the earth. The first measurement is expressed in the number of degrees, minutes, and seconds (DMS), or degrees plus the decimal fraction of a degree (DD) for the location north of the equator (latitude).1 The second measurement is expressed in the number of DMS or DD west of the prime meridian (longitude). The prime meridian is an imaginary line between the North and South Poles passing through Greenwich, England. It also extends around the globe, where it is called the International Date Line in the Pacific Ocean. The third measurement is expressed in the number of feet or meters that the earth is raised above mean sea level (AMSL) at the precise location of any given latitude and longitude. This is called ground elevation (GE).
Search Area Design based on System Location Criteria
As discussed in Module 4 Wireless System Design new search areas are assigned to establish either a coverage site or a capacity site. In either case, each search area is designed for a new macrocell site or multiple microcell sites to optimally blend into the existing system grid of wireless sites and to serve the desired coverage area. Imagine that a new search area is assigned for one or more sites between three or four existing sites, one in each cardinal direction. Whether this will be a capacity site or a coverage site may depend on the distance of the new search to the existing sites. If each of the nearest surrounding sites is ten miles away, the new search area is likely intended to be a coverage site since the geography within five miles of the new search area probably doesn’t have adequate coverage from the existing sites at a distance. If each of the nearest surrounding sites is only one mile away, the search area is probably intended to be a capacity site to add to the existing capacity of the nearby sites.
Search Area Design based upon Coverage Location Criteria
RF engineers use marketing and network intelligence data regarding traffic demand and network performance to judge where new search areas need to be located. Search areas designed for microcell sites are intended to cover urban activity centers of high customer demand, such as a neighborhood, a convention center, a sports complex, a shopping mall, or a large office building. While macro cell sites may exist nearby, the ability to continue developing macro cell sites in the locale may be limited due to zoning or space restrictions. Since microcell sites are an extension of an existing network, they offer greater opportunity for strategic placement with respect to the desired coverage area. Therefore, whether a macro cell or a microcell, the third location requirement for RF engineering is the new site’s ability to serve a specific geographic coverage area.
If feasible, it is preferable to locate new cell sites on existing structures. This saves both time and the expense of obtaining local and federal permits. Therefore, the first due diligence question to ask is whether there are existing structures for collocation in or near a search area that might prove viable.
Air Traffic Navigation
Antenna structures represent obstacles and potential hazards for air traffic. Flight approaches for takeoffs and landings may extend for miles from runways. This limits the height of structures that may be built near the approach paths. Proposed antenna structures require a determination of no hazard to air navigation (a favorable determination) from the Federal Aviation Administration (FAA) that no protection threshold concerning obstructions to air navigation will be exceeded by the proposed construction.1
For collocations on existing towers where other wireless antennas are already present, an evaluation of existing frequencies in use determines the impact existing antennas will have on the proposed antennas and vice versa. To accomplish this, RF engineers perform intermodulation (intermod or IM) studies that evaluate the potential for signal interference between antennas. Most well-managed multiple-user antenna sites have technical standards that specify the newest operator on the site as responsible for avoiding interference with existing site users and correcting any reported interference to other radio systems. This is the general rule for new users on multi-user sites. Clearing proposed new frequency uses with nearby existing users prior to system activation is referred to as radiofrequency coordination or prior coordination. Prior coordination is a normal operating procedure in wireless environments.
It’s common for AM radio stations to use a whole tower as a transmitting antenna that propagates signals over the air. These are called hot towers, and the quality of the signals they propagate can be impacted by the installation of nearby steel structures such as wireless towers.1 Developers of new towers near hot AM towers may be required to make structural modifications to mitigate the impact of the new tower structure on the propagation pattern of the preexisting AM station.