Continuous-Wave Propagation Channel-Sounding Measurement System - Testing, Verification, and Measurements
Summary
This report describes the setup, validation and verification, and results from propagation measurements using a continuous-wave, radio frequency channel-sounding measurement system.
Abstract
Channel sounders are used to measure channel characteristics for radio systems. There are several types of channel sounders used today: continuous-wave (CW), direct pulse, frequency domain using a vector network analyzer (VNA), correlation-based, and swept-time delay cross-correlator. Each of these has unique advantages and disadvantages. CW systems have a larger dynamic range than other systems with a signal that can propagate further into the environment. As the audio sampling rates allow smaller file sizes than other systems, data collection can be continuous and last for several hours. This article discusses a CW-channel sounder system, which has been used to make numerous propagation loss measurements in various cities in the United States of America. Such propagation measurements should be accurate, reproducible, and free of artifacts or biases. This article shows how to set up the measurement, how to validate and verify that the system is making reliable measurements, and finally, it shows results from some of the measurement campaigns such as repeatability measurements, clutter loss measurements (where clutter loss is defined as the excess loss from free-space transmission loss), and reciprocity measurements.
Introduction
The Institute for Telecommunication Sciences (ITS) is the research laboratory of the National Telecommunications and Information Administration (NTIA), an agency of the U.S. Department of Commerce. ITS has a long history of conducting accurate, well-regarded radio frequency (RF) propagation measurements. The increase in spectrum-sharing has been accompanied by the need for accurate, reproducible measurements that provide a better understanding of the radio environment that multiple services will have to share. For the past few years, the military services have been developing spectrum-sharing arrangements with commercial wireless carriers in the Advanced Wireless Services (AWS)-3 band (1755-1780 MHz)1. This will allow commercial wireless carriers to use the AWS-3 band prior to phasing military services out of the band. The use of the band will be coordinated by both isolating systems geographically and by modeling frequency interference scenarios. To share this band of spectrum, propagation measurements are necessary to develop and improve propagation models for the evaluation of RF interference between the military and commercial wireless systems within the band.
The Defense Spectrum Organization (DSO) is responsible for the management of the AWS-3 transition and has tasked ITS and others with performing a series of channel-sounding measurements. These measurements will be used to build new models for the calculation of the impact of foliage and man-made structures in the environment (collectively known as clutter). Improved propagation modeling that accounts for clutter could lead to fewer restrictions on commercial transmitters in the vicinity of military systems. The CW-channel-sounder system discussed in this article has been used for the past five years to collect radio propagation measurement data and calculate the clutter attenuation. This measurement system produces accurate, repeatable, and unbiased results, and DSO encouraged ITS to share its institutional knowledge-including best measurement practices for the measurement and processing of RF propagation data-with the wider technical community.
Best measurement practices require understanding a system from the component level to the assembled-system level. These best measurement practices have been documented in the recently published NTIA Technical Memorandum TM-19-5352 that describes a set of best practices for the preparation and verification of radio propagation measurement systems. ITS recently completed a JoVE article on calibrating a VNA used to measure component losses and identify bad components for this measurement system3. This article is a continuation in documenting these best measurement practices for the wider community. Although best practices are discussed in this article for a CW-channel sounder, these same techniques can be used to verify other channel sounder systems: VNA systems; CW systems; full-bandwidth, correlation-based systems; direct pulse systems; and sliding correlator-based systems4,5,6.
This article describes in detail how to setup a CW-channel sounder measurement system using a vector signal analyzer (VSA), a spectrum analyzer (SA), two rubidium oscillators, a power meter, a vector signal generator (VSG), and various filters and power dividers for measurements in an outdoor measurement environment7,8. The transmitting side of the system consists of the VSG, which generates a CW signal that is boosted by a power amplifier. This is then split by a directional couple to divert some of the signal to the power meter, which allows the user to monitor the system output. The rest of the signal is sent to the receiving side of the system via the propagation channel. The receiving side consists of a low-pass filter to reduce interference and harmonics produced by the power amplifier. The filtered signal is split in a power divider and fed into the SA for monitoring during the measurement along with a time stamp and Global Positioning System (GPS) location. The other half of the signal is sent to the VSA to be downconverted into in-phase quadrature (I-Q) data in the range of 1-5 kHz. The sampling rate is determined by the instrument span9 and is guided by the expected Doppler spectrum shifts, which are a function of the speed of the vehicle. The resulting time series is then transferred to a computer for postprocessing and data analysis.
Rubidium clocks are used at both the transmitter and receiver to provide highly accurate measurements and highly stable frequencies. The rubidium clock at the receiving end has a fine frequency adjustment for the precise alignment of the transmitting and receiving frequencies. Typically, the frequencies are adjusted to be within 0.1 Hz of each other for testing. Rubidium clocks are essential for high-accuracy CW propagation measurements. They ensure precise time base accuracy over the course of the measurements and prevent frequency drift of the transmitter and receiver. This article also details how to validate and verify that a system is making accurate measurements in a laboratory setting, both with and without an antenna, prior to making measurements in an outdoor environment. The system has been used for an extensive series of outdoor and indoor tests at frequencies ranging from 430 MHz to 5.5 GHz and for many different transmitting powers7,8,10.
Protocol
Representative Results
Discussion
It is very important to test a system as described in this protocol before attempting to make measurements in an outdoor environment. In this way, any bad components or instabilities can be traced and identified in the measurement system and can be resolved. The critical steps in this protocol are to 1) test the individual components first, and verify that they are operating within their specification, 2) assemble transmitting and receiving sides separately and test the chain of components, 3) assemble the transmitting and receiving side by inserting a stepped attenuator and measuring the signal levels as the attenuation is changed to make sure the received signal levels in the VSA and SA are as calculated. Further troubleshooting can be performed by using a VSG, such as the one shown in the Table of Materials, that has an option to generate fading simulations, which can be used to test the system using simulated waveforms in various fading environments encountered in real-world propagation environments. Once the measurement system is operating correctly, measurements can be made in an outdoor environment with the confidence that measurements will be accurate.
Another important step is to monitor the transmitting power throughout the measurement to verify that the system is operating correctly. The power amplifier is characterized and tested separately to understand its linearity and out-of-band emissions spectra. The power amplifier may be validated on the benchtop with the rest of the setup, but care must be taken to reduce the signal power below the maximum rated power input to the VSA using appropriately rated attenuators. Neither the GPS antenna nor its settings should be used for lab verification and validation. As the VSA's screen is not capable of providing real-time monitoring of the environment, the addition of an SA as a real-time monitor helps determine the current state of the system. There are several types of channel-sounding measurement systems to capture channel characteristics for radio systems: CW, direct pulse, frequency domain using a VNA, correlation-based, swept-time delay cross-correlator.
One limitation of this system is that a CW signal probing the local environment does not contain time-domain information such as time-delay profiles. A time-delay profile gives information about the timing of source reflections of the signal in the local environment. However, an advantage of using a CW signal is that it is easier to obtain permission to transmit on one frequency in various bands using the narrowband CW signal rather than trying to transmit a wide-band signal. CW systems can have a larger dynamic range than other systems, and the signal can usually propagate further in the environment. A CW signal also has audio sampling rates that result in smaller file sizes than other types of channel-sounding systems. With this system, data collections are continuous and can last for several hours. The CW-channel sounder measurement system discussed in this article can be used at different frequencies, depending on the range of the various assembled components. The system can be used in an outdoor propagation environment or an indoor propagation environment15.
Acknowledgements
Thanks to the Defense Spectrum Office (DSO) for funding the work presented in this article.
Materials
| Cabling | Micro-Coax | Various lengths | |
| Directional Coupler | Anatech Electronics, Inc. | AM1650DC833 | |
| Filter 1 | K&L Microwave, Inc. | 8FV50-1802-T95-O/O | |
| GPS Antenna | Trimble | SMA connection to SA | |
| Instrument Control & Processing Software | MATLAB | Used to store and process measurement data | |
| Power Amplifier | Ophir RF | 5263-003 | |
| Power Divider | Mini-Circuits | ZAPD-20+ | |
| Power Meter and Power Sensor | Keysight | E4417A/E4412A | |
| Receiving Antenna | Cobham | OA2-0.3-10.0V/1505 | |
| Rubidium Frequency Standard | Stanford Research Systems | FS725 | |
| SA | Agilent | N9344C | |
| Transmitting Antenna | COMTELCO | BS1710XL6 | |
| Vector Signal Generator | Rohde & Schwarz | SMIQ | |
| VSA | Keysight Technologies | N9030A |
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