1、 TSB-176-2009 APPROVED: APRIL 1, 2009 REAFFIRMED: FEBRUARY 4, 2014 TSB-176 April 2009Radiowave Propagation- Path Loss- Measurement and Validation NOTICE TIA Engineering Standards and Publications are designed to serve the public interest through eliminating misunderstandings between manufacturers an
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20、i INTRODUCTION . iv 1.0 SCOPE 1 2.0 REFERENCES . 1 3.0 REFERENCES DEFINITIONS however, this will partially be mitigated by the presence of multipath transmission. 1TSB-171 provides a detailed background on mobile antennas with respect to the size of an vehicles roof dimensions and the effect of heig
21、ht of the vehicles roof AGL. See also TIA 329.2-C for the standard on vehicle antennas. 5 TIA TSB-176 At the base site, the optimum antenna arrangement is an omni-directional radiator located atop a mounting structure2. However, side mounted antennas can be accommodated if the antennas position with
22、 respect to the tower and the towers geometry are well defined. The following antenna arrangements cannot be adequately characterized. Therefore, they ought not be used for research purposes: Variable down-tilt antennas Antennas mounted such that a portion extends above the tower and a portion is pa
23、rallel to the antenna structure. Antennas within the near field3of another antenna Because of the near-impossibility of determining the source transmitter, simulcast systems ought not be used as signal sources for measurements used for research purposes. 4.1.2 Receiver Considerations 4.1.2.1 General
24、 Communications receivers frequently lack dynamic range at either the low end or the high end, or at both. To correct for the low-end problem, a pre-amplifier can be employed. To correct for the high-end problem, a programmable attenuator can be employed. Measuring receivers always put out signal st
25、rength in digital form. Communications receivers, on the other hand, output signal strength in either digital form as Received Signal Strength Indication (RSSI), or in analog form as limiter voltage. In the latter case, employ a Digital Voltmeter (DVM) or an A/D converter to interface between the re
26、ceiver output and the computer. See Figure 1. The best tool for making a signal measurement is a receiver designed specifically for that purpose. This type of receiver has numerous advantages and two disadvantages when compared to a communications receiver: A specialized measurement receiver is expe
27、nsive. The measurement bandwidth is somewhat inflexible. This can be serious, since the measurement bandwidth is required to 2See TIA 329.1-D for Base Antenna standards. See also TIA 845-A-1 for a method of describing base antenna mounting configurations. 3*The near-field far-field boundary is given
28、 by 22d, where d is the effective length of the antenna and is the wavelength. Antennas that are closely-spaced, yet outside the near field of any other antenna are in accord with this Bulletin, but are not recommended for research purposes. 6 TIA TSB-176 accommodate the modulation from the transmit
29、ter. However, if an unmodulated carrier is used, measurement bandwidth is unimportant. A communications receiver can also be used for making signal measurements. Although they do not have the many features provided by a measuring receiver, they are adequate for the job when properly applied and do h
30、ave a small number of advantages over measuring receivers, including low cost and having the exact bandwidth that is needed for the given application. Before using a communications receiver for this purpose, the user ought to ensure that the receivers output characteristic (either RSSI or limiter vo
31、ltage) is monotonic. 4.1.2.2 Extending dynamic range If a communications receiver is to be used, it is recommended that consideration be given to adding a low noise preamplifier to increase the measurable range at the low end. In implementing a preamplifier, exercise care such that significant inter
32、modulation products are not produced, distorting the measurements. To address the opposite issue, saturation problems experienced at very strong signal levels, a programmable attenuator can be employed. When the computer sees that the signal strength is approaching saturation, it can switch a PIN di
33、ode coaxial switch that controls the attenuator. Capturing the activiation of the attenuator allows the receivers calibration to be extended for processing. 4.1.2.3 Calibration issues Calibrate the communications receiver to its antenna input port using a signal source whose absolute level accuracy
34、is specified as within + 0.5 dB. Specify and calibrate out coaxial cable losses. Ensure that the calibration signal source has been calibrated within the time interval recommended by its manufacturer, but in no event more than one year prior to calibrating the test receiver. Prior to calibrating the
35、 receiver, warm up the calibration signal source according to its manufacturers recommendation for guaranteed amplitude accuracy, but in no event for less than 30 minutes. Since communications receivers can have rather non-linear calibration curves, it is desired that calibration points be taken at
36、1 dB intervals. Additionally, each point in the calibration table ought to be the mean of at least 30 individual measurements. This latter recommendation is necessary because, at some points on the calibration curve, the signal level is close enough to the thermal noise level which can cause instabi
37、lity in the reading. Most measurement receivers are self-calibrating. Follow the manufacturers recommendations in performing the calibration. 7 TIA TSB-176 Be sure the receiver and base transmitter are properly netted to eliminate this potential error. It should be noted that temperature and voltage
38、 variations can affect receiver calibration. It is recommended that the receiver be tested for sensitivity to these variables. A regulated power supply may be necessary if the sensitivity to supply voltage variations is excessive. There is no practical means of addressing excessive sensitivity to te
39、mperature variation. 4.1.3 Position Information Considerations A source of position information is essential to a successful test. Position information is commonly derived from a Global Positioning System (GPS) receiver. The level of accuracy delivered by this system (22.5 meters at the 95thpercenti
40、le) may be adequate in some cases. However, it is strongly recommended that some form of DGPS (differential GPS) be employed when making measurements for research purposes. Some such systems give results as good as 1 meter. One notable DGPS system is the Wide Area Augmentation System (WAAS), supplie
41、d by the U.S. Federal Aviation Administration. It has 1 meter accuracy; however, because the difference signals are transmitted from geo-synchronous satellites some ground-based receivers may have difficulty in receiving the differential signal. The U.S. Coast Guard Navigation Center provides somewh
42、at less accurate DGPS information, on the order of 10 meters rms. Differential corrections can be processed either in real time or post-processed. Given that the speed calculations made by GPS receivers include a certain amount of hysteresis, a speed transducer, such as a Hall-effect device inserted
43、 in the speedometer cable or a “fifth wheel”, can be useful to provide more accurate speed information. 4.2 Data Gathering It is only practical that the signal be transmitted from the base site and measured at a mobile receiver, rather than the opposite. This stems from two considerations: First, re
44、ceivers located at base sites are typically more subject to signals that can produce desensitization and intermodulation interference than are those in mobile locations. Desensitization reduces the overall dynamic range available to be measured. Intermodulation interference can do so, as well. Inter
45、modulation is also more apt to be intermittent, causing the receiver to become intermittently uncalibrated. 8 TIA TSB-176 A second reason for the preference for the base-to-mobile direction is that this mode of operation makes it practical to make multiple simultaneous geographically separated measu
46、rements by the use of mobile receivers mounted in multiple vehicles. It is understood that there may exist a need for documenting data gathering methods in the mobile-to-base direction. Because of the multiplicity of issues involved and the uncertainty of the magnitude of the need, mobile-to-base me
47、asurements will be left for a future issue of this bulletin. 4.3 Data Reduction The output of the receiver used in making the measurement may take any of the following three forms: a signal strength value (from a measuring receiver) a limiter voltage a received signal strength indication (RSSI) data
48、 stream (from a communications receiver) and the status of an attenuator if utilized. Convert the outputs from the communications receiver into a signal strength value by making reference to a calibration curve. At this point, incorporate all antenna system gains and losses to create an output value
49、 that represents the value that would appear at the output terminals of a half wave dipole antenna. In accordance with user requirements, this can be converted to signal strength in dBi, field strength, power density, or (after taking into account transmitter ERP) into path loss relative to dipoles or isotropic radiators. However, it is recommended that the value shown in the output file be expressed in dBm. Having diligently removed all systematic sources of error from the measurement system, the remaining measurement errors can be assumed to be uncorrel
