How Clear Are Your Channels? Careful planning can mitigate the numerous sources and types of interference.

Transcription

1 How Clear Are Your Channels? Careful planning can mitigate the numerous sources and types of By Dr. Perry L. Schwartz Achief of police couldn t communicate with his local officers because of interference on his channels. After troubleshooting with help from his radio engineer, the chief discovered the source of interference: a 300-foot tower, 100 miles away in a different state sitting on a 1,300-foot mountaintop. The tower atop the mountain was transmitting at the same frequency and in direct line of sight with the chief s local tower. The chief s only solution was to change frequency, because whenever state police in the neighboring state used their radios, the chief couldn t communicate with his officers. In another city, police officers were becoming frustrated. With portables in hand, they could see but not hear each other due to adjacent-channel Public safety professionals often use analog radios that can be heard easily by others but fail to work properly or at all on their own networks. Again, the best solution was to carve out a new band of frequencies; the officers moved from VHF to UHF. Recently, it seems any discussion on interference has focused on 800 issues, but what about other sources of interference? A multitude of environmental and man-made conditions, such as lightning, sun spots, weather, terrain, urban landscapes, and even shrubs and trees can cause Broadband and Mesh Networking At a recent public safety regional conference, one-third of the presentations focused on broadband wireless and mesh networking. Everyone seems to be moving toward Wi-Fi networks, but as network engineers know, Wi-Fi still follows the rules of physics. Any agency considering the use of unlicensed spectrum of 2.4 GHz, 5.2 GHz, and 5.8 GHz without the benefit of regulated power requirements is playing Russian roulette with An example, of course, is the interference issue that has afflicted so many agencies as a result of the Sprint Nextel development of legally licensed systems in the 800 band. The emerging use of other bands, especially unlicensed ones, does not preclude similar problems. Based on past experience, Wi-Fi and WiMAX may end up introducing interference potential from a whole new set of sources. Public safety organizations are rightfully leery of technologies that operate in unlicensed spectrum. The main way to protect yourself, as in any wireless implementation, is with a properly designed network. By measuring in-band interference, you can design around potential sources. If you encounter interference in your broadband mesh networks, changing to an alternate channel may be a viable solution. Installing more access points minimizes the risk of inband

2 Interference Figure 1: Co-Channel, Adjacent Interference, and Thermal Noise (all signals are referenced to zero-power level) Power Thermal noise Figure 2: Cases of Potential Base Station Interference Sources F H G Desired signal Out-of-channel interferer Fixed antenna site C E Receiver filter characteristic Out-of-channel interferer D B A Frequency This figure shows the three typical types of interference scenarios: co-channel, adjacent channel, and thermal noise. Target unit Target unit This figure summarizes interference sources to a base station. The interference victim is the antenna site shown as the black triangle on the left, with its radiation pattern represented as ellipses, both primary (in front) and sidelobe and backlobe as shown. The letters represent potential cases of Textbook Definition Radio waves propagate in an unbounded medium. Interference occurs when undesired signals affect reception of a desired signal. Interference among wireless systems operating in the same frequency band is inevitable. The three typical types of interference scenarios are co-channel, adjacent channel, and thermal noise. Prevalent in any system, these produce a background condition that must be overcome. In the co-channel (in-band) scenario, two operators are either in adjacent territories or territories within radio line of sight and have the same spectrum allocation. The co-channel interference bandwidth may be wider or narrower than the desired signal. With a wider cochannel interferer, only a portion of its power will fall within the receive filter bandwidth. The rest is The most problematic co-channel interference occurs between base stations over the same frequencies. Careful deployment reduces interference between licensed systems within 37 miles of each other. Obviously, increasing the distance between transmitter and receiver decreases You can accomplish the same effect by lowering transmitter power. An alternative method is to retain and use spare frequencies if you detect Licensed territories may overlap and be assigned adjacent frequency bands. In this case, interference comes from a channel near your own. The interferer s sidelobes or transmitter output noise floor may fall next to your desired signal in the receiver filter s passband. You need to conduct simulations to identify the proper guard frequency to prevent this from occurring. Intermodulation is another common type of Detecting intermodulation can be challenging. Your own transmitter or receiver may be part of the problem, combining with an unknown signal. Other radiated intermodulation sources may include rusty bolts on a tower or other dissimilar metallic junctions. Finally, harmonic interference occurs due to unplanned sums and differences of the receive frequency. Harmonics are always fundamental frequency multiples. Sometimes when you locate and filter the offending source, you may only be moving it to another harmonic. Only after you have passed the fifth harmonic does the level of impact become minimal. Software programs are available to help you calculate interference through the many combinations of

3 harmonics. You will need a complete listing of all transmit and receive frequencies and their users within a reasonable distance of a site (1.24 miles). Unfortunately, most computations and filtering systems address up to only the third harmonic. Interference Sources Interference typically results in a loss of signal. Although signal loss is often straightforward, sometimes it can be random or sporadic, especially in trunked operations. Interference symptoms commonly include hissing sounds, garbled messages (unintelligible, but you know someone is saying something), and nothing (you don t even know you have an incoming message). Many factors, such as separation between antennas, extent of data traffic, power levels, and modulation types, can affect the degree of interference between two wireless systems. Figure 2 summarizes interference sources. With base-station-to-basestation interference, the best solution is to prevent transmitting on frequencies used for reception at a nearby base station. Case A: Base station antenna is in the main beam of one or more base stations. This can be significant, because base station antennas tend to be elevated with a high probability of line of sight like a mountaintop. Case H: Interference is from another base station s sidelobe or backlobe. Case F: Two base stations are interfering backlobe to backlobe or sidelobe to sidelobe. This typically occurs when systems are deployed on the same rooftop, and you can eliminate it with a coordinated frequency plan. Several examples represent basestation-to-target-unit Case B: The base station s highpower antenna is in the main beam of the target unit s low-power antenna. If the two signals are on frequency, interference occurs. Case C: Interference is due to Snapshot Survey Snapshot Survey: Interference This issue we asked our readers about interference and whether they think it s a serious problem. Seventy percent said interference is as much or more of a problem than it was a year ago, with less than 20 percent of respondents saying it is not a problem. Fifty percent favor regulatory fixes for interference, while 30 percent say users should tackle the problem themselves (Page 42). Most users said they still operate in the VHF and UHF bands. rain or other weather conditions (sleet, fog, etc.). Case D: Interference is from a repeater s sidelobe. Because it sees rain in its main path, it does not turn down its power. Case G: Interference comes from a satellite system. Case E: Interference is due to foliage. Field-intensity coverage maps are useful for determining the strength of an interfering signal from a given transmitter compared with the desired signal from another transmitter. Desensitized antennas. One form of interference occurs when the output antenna of the transmitter and the receive antenna on a different frequency are too close to each other. The transmit power will desensitize the receive antenna, even though they are not at the same frequency, and prevent reception. Equipment-related spurious emissions. Interference from spurious emissions can be attributed to many factors in a transmitter. Equipment with short circuits or improperly Is interference a problem for your communications systems? I don t have a problem with interference 19.2% It s improved from a year ago 5.8% It s a bigger problem than it was a year ago 30.8% It s about the same as it was a year ago 44.2% tuned systems can generate spurious emissions. Other sources include power lines and fluorescent lighting. Spurious frequencies can be due to nonlinear characteristics in a transmitter or physical placement of components and unwanted coupling. Mechanical parameters of antennas also affect interference: wind and ice loading, water, temperature and humidity, and vibration. Antenna parameters can also be mitigation techniques: antenna-to-antenna isolation, orientation, tilting, directivity, and polarization. Weather-related emissions. Atmospheric noise is the main cause of radio interference on low frequencies. Lightning discharges in thunderstorms cause atmospheric noise, which has interference potential dependent on frequency, time of day, weather, season, and geographical location. At frequencies above 30 megacycles, the noise falls to levels generally lower than receiver noise. Rain can cause fading in radio communications. Parameters such as frequency of operation, desired link

4 Interference Snapshot Survey What is the best solution for interference? Other 19.1% User resolution change frequencies or bands of operation 31.9% availability, and bit error rates (BERs) dictate the maximum radius of operation. The extent of rain attenuation, which can impact system performance, is different for the desired signal and the interfering signal. Other natural causes. Spurious signals can also be produced by sun spots, trees, and terrain. Pine needles can cause one-quarter wavelength of interference at certain frequencies, absorbing radio transmission. Cyclical sun spots cause significant interference every seven years by generating spurious emissions through a magnetic field. Digital transmission may be less impacted than analog systems. Digital radios propagate a more robust signal and maintain higher quality transmission than analog radios under adverse conditions such as heavy rainfall because they use smaller antennas and automated power control. If an interfering signal blocks a digital signal, the radio s message will still be affected, however. Multipath reflections. Buildings, especially ones coated in metal, produce Regulatory mandates like the 800 reconfiguration 14.9% Improved FCC rules for new/modified transmitters to prevent interference 34% significant reflection and multipath In this case, two or more signals of the same origin arrive at the receive antenna delayed in time because they traveled different path lengths or reflections and scattered in the environment. The signal may be severely degraded, and a strong reflection may cancel the transmitted signal. Increasing power is not an option because both the direct and reflected interfering signal will increase; a redirection of the antenna may solve the problem. Bidirectional amplifiers (BDAs). BDAs are designed to extend radio coverage into enclosed spaces experiencing coverage deficiencies, such as buildings, tunnels, garages, and other underground facilities. BDAs amplify in both directions; for example, two antennas, one outside a building and one inside, transmit a signal into the building, where it is amplified with a BDA and reradiated inside the structure. This process can produce interference because the BDA is a broadband device, which provides power gain across a wide range of frequencies. Unfortunately, if an interfering frequency is in the band, it will be amplified the same as all others and may become even more magnified. To minimize this effect, filtering technologies narrow, sharp filters can block or attenuate the interfering frequency so it cannot mix with the desired signal frequency. Calculating A number of techniques are available for calculating interference: worst-case analysis, Monte Carlo simulations, or interference area. The best method depends on the interference source. Worst-case analysis is appropriate when you experience one dominant source. Many times, worst-case analysis is of limited value, because if there could be several potential sources of interference, the worst case may be severe but unlikely. Monte Carlo simulations provide a means of assessing the probability of a range of interference levels to determine the most likely cause. Interference Protection What can a frustrated, overwhelmed engineer do to mitigate potential causes of interference? The response takes us back to physics. Carefully plan in advance: collaborate in good faith with other known operators before initial deployment and before every system modification; conduct analyses to evaluate potential interference sources and identify mitigation options; and develop a formal channel plan. Carefully design new systems or changes/upgrades to existing systems: Properly position antennas so they are not too close. Ensure sufficient horizontal displacement between antennas (not too close to prevent desensitization). Follow FCC rules regarding channel spacing and distance before reusing a frequency. Ensure receivers have high intermodulation specifications (sharp filter skirts). Increase signal strength above local noise levels (carefully calculate them).

5 Analyze antenna radiation patterns relative to footprint and use two polarization orientations, horizontal and vertical, to mitigate interference between adjacent systems. With existing systems you need to identify potential system modifications: retune channels, power levels/ antenna height, and antenna characteristics. Ensure that base station equipment operates to improve signal strength. Incorporate filters into the equipment. You must also segregate public safety and other spectrum assignments. Interference has many sources. Although careful system design is critical, you must also thoroughly know and understand your local environment. Choose bands suited to your terrain, and check frequencies being used in your area. Refer to the universal licensing site (ULS) on the Web site of the FCC: Dr. Perry L. Schwartz, P.E., is a Freehold, N.J.-based public safety consulting engineer and chief technology officer of Intertech Associates. Contact him at ps@intertechassociates.com. Percentage of users in frequency band In which frequency band(s) does your system operate? 69.2% 63.5% 0% VHF UHF % 800 Frequency band 13.5% % 7.7% 4.9 GHz Other RadioResource MissionCritical Communications delivers wireless voice and data solutions for mobile and remote mission-critical operations. The magazine covers business, public safety, and regulatory news; case studies; in-depth features; innovative applications; product information and comparisons; emerging technologies; industry reports and trends; and technical tips. In addition, each issue contains Public Safety Report, a special section devoted solely to the needs of the public safety community. Editorial content targets organizations in the United States and Canada with mobile and remote communications needs, including public safety, government, transportation, manufacturing, utility/energy, business, and industrial entities. To request a FREE subscription or get more information, go to RadioResource MissionCritical Communications is published by the RadioResource Media Group. Pandata Corp., 7108 S. Alton Way, Building H, Centennial, CO 80112, Tel: , Fax: , Copyright 2005 Pandata Corp. All rights reserved. Reprinted from the July 2006 issue of RadioResource MissionCritical Communications. For more information about MissionCritical Communications and the RadioResource Media Group please call or visit