The case for testing electromagnetic radiation at mobile radio sites The measured field strength around base stations is a potential silent liability that can be addressed with simple hand- held test equipment.
In a well-designed mobile radio communications system, electromagnetic energy is directed toward its intended receivers, rather than toward people operating or maintaining the transmitter site or living nearby. Despite an otherwise good design, when the system is not functioning properly, or when it is inadequately shielded, it will radiate energy in unwanted areas. This leakage can reduce system performance, cause surrounding equipment to malfunction and shorten equipment life. In severe cases, it can pose a health hazard. It can raise questions about employer liability and about safety for employees working nearby. Fortunately, it is simple to monitor and track radiation levels around the transmitting system. (See “Sniffing It Out” below.) Being diligent in periodically examining electromagnetic levels near the base station demonstrates a service provider’s care for employees and the environment in which its transmitters are operating. Does this sound altruistic? Well, the cold facts stand for themselves: Electromagnetic-field-related litigation is on the increase, and more employees and citizens file lawsuits against communications system operators every year. Inattention to the electromagnetic environment created by communications systems can cost far more than taking the simple steps required to keep it in check. This trend toward litigation and the benefit of monitoring and controlling electromagnetic fields will become more apparent in coming years as base stations for various types of personal communications services (PCS) proliferate. Service providers already are scurrying to erect PCS infrastructures (albeit at lower transmitter power levels) to speed their systems to market. Mobile communications systems based on low-earth-orbit (LEO) satellites will add to the electromagnetic content in the airwaves, most significantly by creating the need for additional base stations and communications links.
Reviewing the scenario All electronic equipment generates an electromagnetic field whenever current flows through a conductor. In a mobile radio transceiver or cellular telephone, the most visible conductor (and source of electromagnetic field) is the antenna, although even in low-power, programmable equipment, the microprocessor can generate electromagnetic fields of moderate intensity. An electromagnetic field includes electric and magnetic components (See Figure 1 above.) An electromagnetic field actually consists of electric and magnetic field vectors, with specific magnitudes and directions at every point in space. Each part of an electromagnetic field has X, Y and Z vectorial components at a given point in space that can be measured with the proper field probe or antenna. An electromagnetic emitter in free space radiates energy in a three-dimensional, spherical pattern. The power per unit area decreases as the distance from the radiating source increases. The reduction is proportional to the square of the distance from the electromagnetic source. Of course, if the electromagnetic source is directional, such as a parabolic antenna, the power density level will depend on the bearing from the electromagnetic source, too. Electromagnetic fields generally are measured as field strength readings at specific distances from an emitter. The resulting measurements are given in volts per meter (V/m) for electric fields and amperes per meter (A/m), or tesla (T), for magnetic fields. Electromagnetic field levels also are often expressed in terms of power densities as power levels per unit area, such as mW/cm2 or W/m2. High voltage with relatively low current will generate an electromagnetic field with a strong electric field component and a weak magnetic field component. Conversely, a high current flow at relatively low voltage will yield a strong magnetic field and weak electric field. It is well known and documented that high-level electric fields at some frequencies cause pronounced heating effects in organic tissue through the excitation of water molecules. The microwave oven, with an operating frequency of 2,450MHz, is based on this heating principle. The microwave oven, industrial heating, food processing, product packaging, and medical treatment and diagnostic systems all use high-frequency electromagnetic fields for beneficial purposes. These same high-frequency electromagnetic fields can damage the human body. It is this potential hazard that diligent monitoring of systems using RF energy attempts to reduce or eliminate. Although high energy levels unquestionably cause harm, many epidemiological studies have been conducted to determine at what level damage begins to occur and what other, more insidious, effects this radiation can have. There is no consensus on what the threshold level may be, or whether other maladies have their root in exposure to low- level, high-frequency radiation. At very low frequencies, the situation is even murkier, because solid evidence has not been unearthed to link radiation at 60Hz with an increased incidence of cancer and other diseases. The levels of both types of fields generally are tested in accordance with standards established by various national and international health and safety organizations, including the International Radiation Protection Association (IRPA), the World Health Organization (WHO), the Institute of Electrical and Electronic Engineers (IEEE) and the American National Standards Institute (ANSI). To the service provider that has been operating communications systems since “wireless” meant shortwave, this consciousness-raising discussion may seem like much ado about nothing. Although future generations of scientists may disprove most possible health risks, concern is growing that electromagnetic radiation produced by communications antennas is at least partly responsible for the increased incidence of some diseases. This perception makes it that much more important for service providers to pay attention to electromagnetic radiation emitted from their base stations. The alternative is vulnerability to accusations of laxity concerning the protection of employees and citizens.
Surprising emitters Even low-power receiver circuitry can develop surrounding fields of moderate strength. Modern mobile transceivers include an increasing amount of computing power in the form of microprocessors and digital signal processing (DSP) components that generate high levels of noise. Although the direct levels of such fields are relatively low, the field strength can be boosted by coupling through transmitter amplifiers. The frequency of the digital noise depends upon the clock speeds of the microprocessors, the speeds of sample clocks for analog-to-digital converters (ADCs) and digital-to-analog converters (DACs), and the switching speeds of supporting circuitry.
Finding the sources Analysis of electromagnetic fields, whether as interference for a mobile communications system or as a potential health hazard, starts with the identification of electromagnetic radiation sources. Modern measurement equipment is available in the form of hand-held instruments capable of accurately measuring magnetic and electric field strength levels, both manually and automatically under remote control. (See “Taking Electromagnetic Testing to the Field” on page 16.) These instruments can be used to monitor field levels continuously at cell sites or base stations to provide a warning to service technicians when field levels rise above recommended safety limits. The same instruments can be used to detect radiation leaks in portable equipment and for isolating breaks in insulators or leaky connectors within mobile handsets.
Summary As the world heads toward an age of global communications, sources of electromagnetic radiation will continue to increase, creating an environment saturated with emitters at different frequencies and field strength levels. As more is understood about the biological effects of both electric and magnetic fields, the need to accurately characterize such fields and upgrade current safety limits will grow as well.
Cellular communications system operators want to reduce user churn–the act of customers switching back and forth between competing carriers–to save the expense of deleting and adding the same subscribers and to increase revenue growth by retaining new subscribers. Cellular systems include networks operated by the familiar cellular telephone companies, as well as enhanced specialized mobile radio (ESMR) systems and personal communications systems (PCS). Customers may switch carriers for a lower rate, to obtain a new “free” phone every so often or because of dissatisfaction with how well their phones seem to work. Dissatisfaction with cellular communications service may be caused by hearing noise on the line (weak signal), hearing voices from other conversations (interference) and having calls interrupted (dropped calls caused either by a weak signal or interference). Strengthening the wireless connection between cell sites and cellular phones helps to overcome these problems. In the past, system operators have installed more cell sites and repeaters to strengthen the connection, but this equipment is expensive. An alternative is to use an intelligent antenna system (IAS) at the cell site. An IAS can greatly improve existing system performance at a cell site for a fraction of the cost of adding cell sites.
Proven technology Although some people working in the commercial sector think of intelligent antennas as an “emerging technology,” they are, in fact, being produced today for use in cellular networks. Moreover, these antennas have been used by the military for more than three decades. Techniques formerly used by the military, such as phased array antennas, adaptive interference rejection, switched beam steering and adaptive beam steering, now are being used in commercial applications. For example, Nextel Communications, Rutherford, NJ, accepted our first 10 production systems for use in its ESMR network. Improvements in range and coverage can be achieved with intelligent multibeam antenna systems that provide greater antenna gain and discrimination against interference. A cell site’s range is extended because the IAS improves the available signal-to-noise ratio. Tests on cellular systems using advanced mobile phone system (AMPS) and ESMR technology substantiate these results, as do data taken from network operations. The signal-to-noise ratio improvement is further evidenced by a computer simulation that uses terrain-based propagation models. In the simulation, a real cellular system lay-down and the results of interference-reduction measurement predict a signal-to-interference ratio improvement greater than 8dB to 10dB with multibeam antennas compared to sector antennas. In most cases, this deployment will nearly double the system’s traffic capacity or coverage while either maintaining the same quality of service or improving it. For example, it will substantially improve the quality of service deep inside buildings, and it will substantially improve the quality of service with half-watt handsets. Intelligent antenna systems can be implemented only to receive at the cell site, or to receive and transmit. Receive-only models: extend base station range as much as 40%. extend the service area for portables in a system designed for mobiles. increase coverage in shadowed areas or within buildings. decrease co-channel interference at the base station. improve overlapping coverage between cells or allow the use of fewer base stations or both. Where the use of fewer base stations is preferred, the cost of infrastructure can be decreased as much as 25%. decrease battery drain on portables. When the transmit mode is implemented, receive-transmit models: decrease co-channel interference at the “mobile” (the hand-held, transportable or installed mobile unit). increase system channel capacity and trunking efficiency. The combination of features used at a given site is a function of cell-site objectives and circumstances. In the increasingly competitive wireless market, carriers delivering service with higher signal-to-noise ratios in fringe areas and other troublesome areas will have an advantage. Their users will have better wireless connections and will experience less frustration.
Why IAS works so well Conventional three-sector antennas provide equal gain for the signal from the desired mobile as well as for the interferer. The intelligent antenna, though, has a narrower radiation beam-pattern that provides increased gain and that can be pointed in the direction of the mobile. Simultaneously, it presents less gain to the interfering signal. As a result, the desired signal is aligned near the center of the main beam, and interferers are more likely to be aligned where the selected beam has less gain. An IAS’s higher gain on the uplink (receive) will improve coverage for building penetration, especially for low-power portables, and will extend the base station’s range. It overcomes the greater propagation loss at the higher frequencies used for PCS telephony. When operating on the uplink and the downlink (receive and transmit), IAS achieves the same coverage and range improvements as with the uplink (receive-only) system, plus the narrower transmit beams reduce interference at both the base station and the “mobile.” Using uplink and downlink IAS at co-channel sites (two or more sites with the same channels) increases the system’s signal-to-interference ratio and offers the opportunity for increasing the system capacity with tighter frequency reuse. Theoretically, the overall traffic capacity increase can approach 100%. In a multibeam-omnidirectional configuration, IAS will increase trunking efficiency by increasing the number of radio channels available in any direction from the base station. Interference reduction can be enhanced further by an auxiliary subsystem that steers a null in the antenna pattern in the direction of the interferer. This technique can reduce the interference by as much as 40dB.
Implementation How an IAS is implemented depends on what benefits are desired, what problems are being addressed and what equipment is required at a particular site. For example, the carrier must decide what RF channels and transmitter power output to use, what type of antenna diversity to use, whether to use receive-only or receive-transmit IAS and, in the case of receive-transmit IAS, whether to use duplexed antennas or separate receive and transmit antennas. Based upon these choices, site engineering can proceed for the multibeam antennas, cabling, lightning protection, equipment room space, power and, if necessary, towertop pre-amplifiers. Experience indicates that IAS installation considerations resemble those for base stations and sector antennas. Typically, a well-planned installation takes less than a week for the antenna, towertop pre-amplifiers, cabling, equipment and check-out. Depending on certain antenna systems options and the particular manufacturer’s architecture, some base station intersystem cabling may be required in addition to the cabling for the receivers and transmitters. System maintenance and operation requires minimal effort with the IAS’s built-in test capability, built-in redundancy and remote monitoring and control.
Summary Today, intelligent antenna systems are real, and they have real economic benefits for commercial systems. Infrastructure costs are reduced by coverage fill-in and range extension, and by capacity increase. Customer satisfaction is improved by reduced interference and better quality of service. With better-performing cell sites, mobiles and portables function better. User satisfaction is increased and churn rates are decreased. In addition, the mobile and hand- held subscriber will be less critical of wireless technology. Improved profitability is a natural consequence when operators choose to install IAS. Lower infrastructure costs, better quality of service and a growth in the subscriber base means more on the bottom line.