Testing Land Mobile Radios for Public Safety Use with NI LabVIEW

Author(s):
Eric Nelson - Institute for Telecommunication Sciences (ITS)
Julie Kub - Institute for Telecommunication Sciences (ITS)

Industry:
Telecommunications

Products:
LabVIEW,

The Challenge:
Automating labor-intensive and inaccurate manual testing of land mobile radios.

The Solution:
Using LabVIEW automation with databases for accurate, cost-effective, and well documented results.

The Emergence of Project 25 Land Mobile Radios
Whether responding to cataclysmic events such as natural disasters and terrorist attacks, or performing day-to-day operations it is critical that public safety personnel can communicate. Cellular and land-line communication systems, though convenient, are often overloaded and unusable when they are needed most. Dedicated, reliable land mobile radios (LMR) systems are necessary for public safety. For many years analog FM modulation techniques predominated in public safety communications. More recently, though, spectrum congestion has driven a move to more spectrally efficient digital modulation. To address the problem of interoperability and make better use of scarce radio frequencies, in 1989 the Association of Public Safety Communications Officials International (APCO) established Project 25 (P25).

As the public safety community gradually transitions from analog to digital P25, a practical coexistence of digital P25 and analog radio systems must be ensured. While interference rejection from like modulation sources is well known, the interactions between the legacy analog systems and P25 are not.


Identifying LMR Interference Measurements
The Institute for Telecommunication Sciences (ITS) was contracted to determine the interference rejection performance of a sample population of eight modern P25 radios in the presence of interference of different modulation types. The four most common modulation schemes employed in LMR (analog FM with 2.5, 4 and 5 kHz peak deviation, and digital Project 25) yield 16, two-emitter combinations. For each combination, rejection performance measurements were performed at 13 different interferer frequency offsets (including the co-channel case which has zero offset). Originally these measurements were performed manually, requiring 15 minutes per measurement.

To ensure accurate and repeatable results, these measurements must be performed in a laboratory environment in accordance with the Telecommunications Industry Association (TIA) standards TIA-603 and TIA-102. These standards call for the use of signal generators to create both desired and interferer signals, which are then fed into an LMR antenna terminal. Interference rejection is determined by adjusting these power levels until an LMR operating 3 dB above its receive sensitivity performs as though it were operating at its receive sensitivity level. The receive sensitivity is defined as either 12 dB SINAD (the ratio of signal-plus-noise-plus-distortion to noise-plus-distortion) for analog or five percent bit error rate (BER) for digital) .

To manually perform all 1664 measurements required two and half person-months. Repeating the process multiple times for statistical assurance was out of the question. Also, the process was prone to human error, because the users had to visually average readings. In contrast, the automated solution averaged 150 values for each of the 1664 measurements. More than 150 values were required because of high data variance (e.g. 2 dB for analog) and non-Guassian data statistics from noise, interferer signal, and desired signal components.

Automating the LMR Measurement Process
After performing the measurements manually, the need for automation was clear. First, the instruments were reset and cleared. Then the desired signal source was configured to deliver the appropriate modulation at a moderate signal level. For analog modulation, the user adjusted the audio level to the radio manufacturer’s specification, typically the loudest volume delivered at the minimum distortion. Next, the desired signal was decreased to either 12 dB SINAD or five percent BER. Then the desired signal was increased by 3 dB, which increased SINAD or decreased BER.

Finally, the power level of the interference source was adjusted until a 12 dB SINAD or five percent BER was reached. This measurement was repeated for 13 interferer frequency offsets, 16 two-emitter modulation pairs, and eight LMR’s.

SINAD was measured using a GPIB controllable modulation analyzer whereas BER data were extracted directly from the LMR via its RS-232 interface. The desired and interferer signal sources were controlled using GPIB. The computer and GPIB controlled equipment was connected through a GPIB-ENET/100 GPIB to Ethernet converter box.

The selected equipment did not have IVI (Interchangeable Virtual Instruments) drivers. Therefore, a database solution was implemented to easily manage and control all GPIB/RS232 information. We used LabVIEW VIs to select multiple database commands and send VISA commands to an instrument. Support for new instruments only required a new “class” VI to identify command sequences for the instrument. During development, the existing interferer signal generator was replaced with a different model. Incorporating the new instrument only took two days – one to identify the GPIB commands and one to create and test a new “class” VI.

A second database stored measurement results. The user selected the LMR type, brand, and firmware version from the database. LabVIEW automatically linked the desired and interferer measurement data to the selected LMR.

The instrument and measurement databases can reside on either a local or a remote server; the LabVIEW software is the same in either case. Only the ODBC connections need to be changed.

Using NI LabVIEW to automate this process, only five minutes per measurement was required and the software could run for 12 hours rather than eight hours because human interaction was only required every four hours. This resulted in a measurement time of less than three weeks with only 30 minutes per day of human interaction.

Improving LMRs with LabVIEW
The advent of digital Project 25 (P25) LMRs has forced system designers to exhaustively characterize the interactions between P25 and legacy analog systems. These interference rejection tests, performed manually, were time-consuming and are prone to human error. The use of LabVIEW automation ensured accurate, cost-effective, and repeatable LMR measurements through the use of a database library of selectable GPIB/RS232 instrument commands and database measurement reporting. The only section of the software that required human interaction was adjusting audio levels and radio modulation settings. Engineers reduced their measurement workload from 2.5 months of eight-hour a day shifts to less than three weeks of 30 minute a day shifts. Programmatically averaging 150 rapidly appearing values was extremely accurate. Finally, the entire suite of measurements now can be repeated multiple times for statistical assurance.

For more information, contact:
Julie Kub
Institute for Telecommunication Sciences
325 Broadway
Boulder, CO 80303
Tel: (303) 497-4607
E-mail: jkub@its.bldrdoc.gov

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