Spectrum Engineering Reports

We describe the design, implementation, and analysis of a speech intelligibility test. The test included five codec modes, four frame-erasure rates, and two background noise environments, for a total of 40 conditions. The test protocol required twenty listeners to repeat all words that they heard in short messages with median length of seven words. Each condition was tested using approximately 1100 words total. Listeners’ responses were scored against the original message transcripts to produce a count of words correctly repeated and thus a measure of speech intelligibility. We present results that show exactly how this measure of speech intelligibility drops as frame-erasure rate increases for three of the five codec modes. The remaining two codec modes did not produce valid results due to defects in the reference software provided to us.

Keywords: background noise; speech coding; packet loss; speech intelligibility; audio coding; frame erasures; acoustic noise

• TR-17-522

Version three of Advanced Wireless Services (AWS 3) radio systems will soon use spectrum that is adjacent to bands currently used by airborne telemetry links at U.S. government test and training ranges (TTRs). Spectrum sharing analyses need to be performed to determined how much off-tuning (number of megahertz) and distance separation (number of kilometers) are needed between AWS 3 transmitters and telemetry receiver stations to avoid harmful interference to those receivers. To complete these studies, detailed wide dynamic range emission spectrum measurements of representative models of the soon-to-be-deployed AWS 3 transmitters have been performed. This report describes those measurements, and the method used to obtain them. These measurements have been performed with over 100 decibels (dB) of dynamic range. The measurements have been collected in a variety of resolution bandwidths, transmitter modulations, and types of transmitter loading (i.e., number of resource blocks used) for two measurement detector modes. In general, AWS 3 eNB and UE transmitted emission spectra are found to be insensitive to variations in transmitter configurations. The measured power spectra of the eNBs and UEs vary in direct proportion to measurement (or receiver) bandwidth (i.e., as 10 log measurement bandwidth) with an approximate offset of about 10 dB between peak and average levels. The measurement results indicate that AWS 3 eNB and UE power spectra are suppressed by at least 100 dB in the adjacent telemetry bands for the devices tested. These results can now be factored into EMC analyses for AWS 3 transmitters operating in proximity to telemetry receivers.

The work described in this report was performed by the National Advanced Spectrum and Communications Test Network (NASCTN). Results were published simultaneously as NASCTN Report 4, NTIA Technical Report TR-18-528, and NIST Technical Note TN 1980.

Keywords: band sharing; emission spectrum; spectrum sharing; spectrum measurements; interference analysis; out-of-band (OOB) emissions; 1755-1780 MHz; 2155-2180 MHz; AWS-3; aeronautical mobile telemetry (AMT); Band 66; band sharing analysis; eNodeB (eNB); telemetry links; user equipment (UE)

• TR-18-528

Frame erasures and background noise are two factors that can interact with speech coding to reduce speech intelligibility and thus impair public safety mission-critical voice communications. We conducted two tests of intelligibility in the face of these factors. The tests covered five adaptive multi-rate (AMR) and enhanced voice services (EVS) speech coding modes, each using a bit rate near 13 kb/s. Two EVS Channel Aware (CA) modes were included. Both tests use the Modified Rhyme Test (MRT) protocol and together they comprise over 150,000 trials. The first test used frame erasures targeted at critical consonants for maximum sensitivity and the second used frame erasures generated at random by a two-state Gauss-Markov model. By using these large numbers of MRT trials we found that the CA codec modes offer small but statistically significant speech intelligibility improvements in numerous frame-erasure environments.

Keywords: noise; speech coding; speech quality; modified rhyme test (MRT); packet loss; speech intelligibility; frame erasures; AMR; EVS; channel aware; frame loss

• TR-18-529 

Spectrum is a limited resource upon which the world makes continually increasing demands. It is therefore natural and compelling to study the efficiency with which radio systems use spectrum. Spectrum efficiency studies reveal how future systems can make better use of radio spectrum, and allow spectrum engineers and managers to make better allocation and sharing decisions. This report provides a 53-year historical review of previous domestic and international spectrum efficiency studies. Based on this review, we recommend possible future spectrum efficiency work to extend the state of knowledge in this area.

Keywords: spectrum efficiency; band sharing; spectrum utilization; spectrum sharing; out-of-band (OOB) emissions; spurious emissions; frequency bandwidth; spectrum efficiency metrics

• TR-18-530 

This report describes emission spectrum and time domain measurements of a contraband wireless device micro-jammer that was operated temporarily in four Commercial Mobile Radio Service (CMRS) bands at a Federal Correctional Institution (FCI) at Cumberland, Maryland. The four jammed CMRS bands were between 730 MHz and 2.155 GHz. The micro-jammer targeted CMRS service indoors, in a single medium-security prison cell. Spectrum measurements of the jammer emissions were performed at two places inside the targeted prison cell and at two non-targeted nearby locations outdoors. Jammer emission measurements were performed at each location with multiple measurement bandwidths and detectors across a frequency range of 300 MHz to 4.34 GHz. Measurements at each location were performed twice, with the jammer device on versus off, so as to show the relative power levels of the jamming signal versus the ambient CMRS signals at each location. Aggregate emissions from multiple micro-jammer devices such as would be required to cover an entire prison facility were not measured. Jammer emissions are presented in units of power per unit bandwidth in measurement system circuitry; a table for conversion of those data to units of incident field strength in space is provided.

Keywords: electromagnetic compatibility (EMC); harmful interference; aggregate emissions; cellular communications jamming; denial-of-service jamming; emission bandwidth; radio jamming; in-band emissions; commercial mobile radio service (CMRS) jamming; communications jamming; micro-jammer; radiation hazard (RADHAZ); wireless device jamming

• TR-18-533

Interference protection criteria (IPC) determine the interfering signal power a system can tolerate when sharing spectrum with other services. IPC are typically determined by measurements, but good measurements are often hindered by restrictions on equipment availability and inaccessible intermediate signals, performance metrics, and operational parameters. The purpose of this research is to determine if radio system software simulation can accurately emulate these measurements and alleviate their hindrances. Our approach is to use commercial off-the-shelf (COTS) radio system simulator software to model previous IPC measurement test fixtures and compare simulated to measured results. Measurements of mutual interference between SPN-43C radar and LTE systems are compared. The comparison revealed that 1) when the SPN-43C pulse repetition interval was the same as the LTE subframe period SPN-43C interference in the LTE UE was highly dependent on which OFDM word within the LTE subframe the SPN-43C pulse was repeatedly placed on and 2) simulation is more accurate than measurement for IPC tests with fixed threshold radars such as SPN-43C. These revelations show that simulation is a useful addition and potentially viable alternative to IPC measurement.

Keywords: spectrum engineering; spectrum sharing; electromagnetic compatibility (EMC) analysis; Long Term Evolution (LTE); interference protection criteria (IPC); Citizens Broadband Radio Service (CBRS); radio system software simulation; surveillance radar

• TR-19-540

Received signal power measurements were performed on the Common Air Route Surveillance Radar (CARSR) operating in the 1300–1370 MHz band in Parker, Colorado and on the Airport Surveillance Radar (ASR 9) operating in the 2700 2900 MHz band in Platteville, Colorado. The measurements were taken along five radials extending from each radar transmitter. Four or five fixed locations were chosen along each radial where predicted received signal power varied from relatively strong to weak levels. Multiple peak received power measurements were made at each location to provide statistically significant results. In another effort, these measurements will be used to validate spectrum usage contours and the methodology used to generate them as developed by the Office of Spectrum Management (OSM) of the National Telecommunications and Information Administration (NTIA).

Keywords: radar measurements; airport surveillance radar (ASR); received signal power measurements; peak received power; Common Air Route Surveillance Radar (CARSR) ; spectrum usage contours

• TR-19-542 

This report describes emission spectrum measurements of a wireless jammer device operated temporarily inside a South Carolina state prison maximum security housing block. The measurements were intended to demonstrate the operation of the jammer in four commercial mobile radio service (CMRS) bands between 730 MHz to 2.155 GHz. Spectrum measurements of the jammer emissions were performed indoors and outdoors with two measurement bandwidths. Measurements at each location were performed with the jammer on versus off, so as to show the relative power levels of the jamming and ambient CMRS signals at each location. This report’s data can be applied in future electromagnetic compatibility (EMC) analyses. However, the data provide no information as to whether a CMRS wireless device can or cannot perform its intended communications function in the presence of a competing signal of specified strength. Only thorough theoretical analysis, well-engineered simulation and modeling, plus selected measurements in controlled (laboratory) environments can objectively quantify the impact of interfering transmissions on CMRS wireless devices.

Keywords: electromagnetic compatibility (EMC); harmful interference; denial-of-service jamming; radio jamming; commercial mobile radio service (CMRS) jamming; communications jamming; micro-jammer; wireless device jamming

• TR-19-541 

Received signal power measurements were performed on the Common Air Route Surveillance Radar (CARSR) operating in the 1300 1370 MHz band in Cedar City, Utah, and on the Airport Surveillance Radar (ASR 9) operating in the 2700 2900 MHz band in Trout Creek, Utah. The measurements were taken at sites relatively far from each radar transmitter. The measurement locations represent different predicted propagation modes (such as line-of-sight, diffracted, tropospheric scatter, etc.) and varying predicted received signal powers (from strong to weak). Distances from the transmitter to the measurement locations varied from roughly 56 km to 141 km for the ASR-9 and 17 km to 194 km for the CARSR. Multiple peak received signal power measurements were made at each location to provide statistically significant results. In another effort, these measurements will be used to validate spectrum usage contours and the methodology used to generate them as developed by the Office of Spectrum Management (OSM) of the National Telecommunications and Information Administration (NTIA).

Keywords: radar measurements; airport surveillance radar (ASR); received signal power measurements; peak received power; Common Air Route Surveillance Radar (CARSR) ; spectrum usage contours

• TR-20-543 

This report is a case-history of the development, deployment, and operational experiences associated with 5 GHz unlicensed national information infrastructure (U NII) devices that incorporate a detect-and-avoid approach to spectrum sharing. Such dynamic frequency selection (DFS) technology was authorized by the Federal Communications Commission (FCC) to accommodate co-band operation of U NII transmitters among other incumbent radio systems, specifically radars. DFS-equipped U NII systems are designed to detect frequencies occupied by radar transmissions and then command their own transmitters to avoid operation on those occupied frequencies. Examining the historical and technical aspects of the development and deployment of 5 GHz DFS-equipped U NIIs, this report focuses on issues encountered with the deployment of this nascent DFS technology, particularly with respect to two government radar systems that have experienced harmful interference: Terminal Doppler Weather Radars (TDWRs) and Range Instrumentation Radars (RIRs). These interference interactions and the likely underlying causes are described, along with steps that have already been taken in an effort to mitigate existing and potential future interference interactions. This report’s narrative summarizes the DFS experience and shares the Lessons Learned from these experiences that may be applied to future similar spectrum-sharing approaches.

Keywords: radar; electromagnetic compatibility (EMC); band sharing; spectrum sharing; radio interference; out-of-band (OOB) emissions; spectrum measurement; unlicensed national information infrastructure (U-NII); terminal Doppler weather radar (TDWR); dynamic frequency selection (DFS); emission limits; spurious emissions; 5 GHz band; access point (AP); detect and avoid; range instrumentation radar (RIR)

• TR-20-544