Originally published in Federal Lab Test & Measurement Tech Briefs supplement to NASA Tech Briefs, May 1995. Copyright 1995 Associated Business Publications. All rights reserved. NASA Tech Briefs.

Digital Sampling Channel Probe

May 1995

Jeff Wepman

An innovative probe provides impulse response characterization of radio propagation channels and enables performance prediction of radio communication systems.

ITS, in a joint effort with Telesis Technologies Laboratory, Inc., has developed a digital sampling channel probe (DSCP). The probe is ideal for making outdoor impulse response measurements to characterize wideband propagation in the radio channel. If the noise and interfering environment is known, impulse response measurements can be used to predict radio communication system performance either through computation of various parameters from the measured data (such as RMS delay spread) or through wireless link simulation. Therefore, these measurements aid in the design, development, and planning of radio systems, including new technologies such as personal communications services (PCS) and digital cellular systems. Because of this, the DSCP has been used extensively for propagation measurement studies in many locations for various commercial and governmental organizations.

The basic configuration of the DSCP (see figure) uses a maximal-length pseudorandom-noise (PN) code to modulate an RF carrier and produce a binary phase-shift-keyed (BPSK) signal. This signal is then filtered, amplified, and transmitted through an antenna. The transmitted signal, modified by the propagation channel, is then received, downconverted to an intermediate frequency (IF), and digitized.

The in-phase and quadrature-phase baseband components of the received signal are determined via software. The complex impulse response is generated by cross-correlating, in software, a simulated copy of the transmitted PN code with the in-phase and quadrature-phase components of the received signal. An interval of discrimination (ratio of the power in the correlation peak to the peak noise power in the complex impulse response) within 2 or 3 db of the maximum theoretical value (54 dB for a 511-bit PN code) can be achieved. The DSCP can also measure an impulse response much faster than the traditional analog sliding correlator probe. This allows better characterization of rapidly changing propagation channels.

Many different system configurations are available with the DSCP. Current configurations use commercially available equipment such as RF-signal generators, spectrum analysers, digital oscilloscopes, and personal computers to achieve a high degree of flexibility and to minimize system setup time for specific field studies. Both the transmitter and receiver have a dual-channel capability allowing for transmission and reception with various combinations of two different PN codes, carrier frequencies, antenna polarizations, and antenna spacing.

In the typical configuration, the null-to-null bandwidth of the probe is 20 MHz, providing a delay resolution of 100 ns and a maximum measurable delay of 51 microseconds. A processing gain of 27 dB is achieved with a receiver noise figure of approximately 8 dB. The probe can be easily configured for wider bandwidths (finer time resolution) and different maximum delays. Recent improvements to the probe include the ability to measure absolute time and Doppler spread. Future plans include expanding the probe to multiple channels to help analyze the potential benefits of advanced antenna systems and antenna signal processing.

This instrument was developed by Kenneth C. Allen, Jeffery A. Wepman, J. Randy Hoffman, Lynette H. Loew, Peter B. Papazian, and Yeh Lo of the Institute for Telecommunication Sciences (ITS) and Andrew-Lindsay Stewart of Telesis Technologies Laboratory, Inc.. A patent has been issued.

• J. A. Wepman, J. R. Hoffman, and L. H. Loew, Analysis of Impulse Response Measurements for PCS Channel Modelling Applications, IEEE Transactions on Vehicular Technology, VOL. 44, NO. 3, August 1995.
• J. A. Wepman, J. R. Hoffman, and L. H. Loew, "Characterization of Macrocellular PCS Propagation Channels in the 1850-1990 MHz Bands", Proceedings of the 3rd International Conference on Universal Personal Communications, September 1994.
• J. A. Wepman, J. R. Hoffman, and L. H. Loew, "Impulse Response Measurements in the 1850-1990 MHz Band in Large Outdoor Cells", NTIA Report 94-309, June 1994, (NTIS Order No. PB 94204906).
  
  • Abstract: Mobile impulse response measurements were taken in the 1850-1990 MHz band in three different macrocellular (cell radii of 5 km) environments: flat rural, hilly rural, and urban high-rise. Spatial diversity with a 15-wavelength separation was employed by using a dual-channel receiver. All antennas were omnidirectional and vertically polarized. The data were analyzed to provide delay statistics; spacial diversity statistics; multipath power statistics; number of paths, path arrival time, and path power statistics; and correlation bandwidth statistics. The urban high-rise cell showed more multipath components (out to 4 or 5 microseconds in delay) than the rural cells. Very long delays (greater than 10 microseconds), while not seen often, were seen more frequently in the rural cells than in the urban high-rise cell. Parameters to help design a tapped delay model of the radio channel in the different environments are given.
• J. A. Wepman, J. R. Hoffman, L. H. Loew, W. J. Tanis, M. Hughes, "Impulse Response Measurements in the 902-928 and 1850-1990 MHz Bands in Macrocellular Environments", Proceedings of the 2nd International Conference on Universal Personal Communications, October 1993.
• J. A. Wepman, J. R. Hoffman, L. H. Loew, V. S. Lawrence, "Comparison of Wideband Propagation in the 902-928 and 1850-1990 MHz Bands in Various Macrocellular Environments", NTIA Report 93-299, Sept. 1993.
  
  • Abstract: Impulse response measurements were taken simultaneously in both the 902-928 and 1850-1990 MHz bands using a wideband measurement system consisting of a fixed transmitter and a mobile receiver. Four different macrocells representing typical semi-rural, suburban, urban, and urban high-rise environments were used for the measurements. Vertically polarized transmit and receive antennas were used for all cells; circularly polarized transmit antennas were also used in the suburban and urban high-rise cells. RMS delay spread, correlation bandwidth, and various other multipath power statistics were used to characterize the wideband propagation and to provide a comparison between the two frequency bands, the two transmit antenna polarizations, and the different cell environments. Major differences were not seen in the propagation behavior between the two frequency bands. The urban high-rise cell exhibited the most multipath, showing more delayed signals having higher power and longer delays than in the other cells. An improvement in propagation (less multipath) was seen when using the circularly polarized transmit antennas instead of the vertically polarized ones for 1920 MHz.

Disclaimer: Certain commercial equipment, components, and software are identified in this report to specify adequately the technical aspects of the reported results. In no case does such identification imply recommendation or endorsement by the National Telecommunications and Information Administration, nor does it imply that the equipment or software identified is necessarily the best available for the particular application or uses.

For technical information concerning this report, contact:
Jeffery A. Wepman, Electronics Engineer
Institute for Telecommunication Sciences
Voice: (303) 497-3165
jwepman@its.bldrdoc.gov