Publications

This report details a method that was developed to identify all potential forms of interference that could occur with a proposed collocation of three Federal systems in the 1675–1695 MHz frequency band. The incumbents are the National Oceanographic and Atmospheric Administration’s (NOAA) Geostationary Operational Environmental Satellites (GOES) and receivers and radiosonde systems. The entrant is the Department of Homeland Security’s (DHS) Video Surveillance System (VSS). The primary objective is that the quality of the mission-critical communications for each service is maintained.

A detailed electromagnetic compatibility (EMC) analysis is used to identify both the highest potential interference scenarios and those scenarios that have little to no effect. Two primary interference mitigation techniques can be implemented to achieve electromagnetic compatibility: frequency offset (Δf) and separation distance. Based on the frequency dependent rejection (FDR) between the interference source and the victim receiver, the Δf and separation distance necessary for a desired level of interference rejection can be calculated. For all potential interference interactions, the Δf and the separation distance can be adjusted to arrive at a solution for operation on a non-interference basis. It is not the intent of this report to make pronouncements on how to achieve coexistence within a shared band. The intent is to examine and illuminate the engineering questions that need to be answered so that those who are responsible for Federal services in a band may negotiate and cooperate with their colleagues who are responsible for other Federal services in the same band.

Keywords: electromagnetic compatibility (EMC); spectrum sharing; interference mitigation; frequency dependent rejection; frequency offset; separation distance

• TR-15-516 

This report describes a comprehensive series of tests that were conducted by engineers and researchers from the U.S. Department of Commerce’s Public Safety Communications Research (PSCR) program and the University of Colorado during the period of July 2013–May 2014. The report presents results obtained at two buildings located on the campus of the University of Colorado at Boulder. Indoor coverage was measured using the PSCR Band 14 LTE outdoor macro network. We also explored methods for improving in-building coverage using a cell on wheels and small cell feeding either discrete antennas or a distributed antenna system. The results indicate that the PSCR macro network by itself does not provide complete coverage inside these buildings and that coverage needs to be supplemented with combinations of a small cell deployed indoors and a cell on wheels (COW). The results indicate that significant system in-building performance improvements can be realized using small cells and a COW.

Keywords: modem; antenna; building attenuation; indoor propagation; signal strength; spectrum analyzer; Long Term Evolution (LTE); small cells; test methodology; backpack measurement system; macro network; Band 14; cell on wheels; channel analyzer; in-building

• TR-15-518

Great emphasis is seen on the networking and data management aspects of spectrum monitoring, but far less attention is given to the radio frequency (RF) sensor systems used to collect the spectrum data. This report focuses on these sensor systems and, in particular, the commercial-off-the-shelf (COTS) RF sensors used in the sensor systems. A test plan for evaluating the RF performance of COTS sensors is outlined. Evaluation of COTS sensors is an ongoing task of the Center for Advanced Communications (CAC) Spectrum Monitoring Program. The intent is to build a comprehensive cost/capability/performance matrix to help guide the selection of the appropriate COTS sensor for a given monitoring scenario. The test plan strives to standardize the tests and metrics, so that results can be compared from sensor to sensor.

Keywords: software defined radio; spectrum monitoring; RF sensor

• TR-15-519 

We describe a major effort to quantify the speech intelligibility associated with a range of narrowband, wideband, and fullband digital audio coding algorithms in various acoustic noise environments. The work emphasizes the relationship between these intelligibility results and analogous ones for an analog FM land-mobile radio reference. The initial phase of this project includes 54 noise environments and 83 audio codec modes. We use an objective intelligibility estimator to narrow the scope and then design a practically sized modified rhyme test (MRT) covering 6 challenging yet relevant noise environments and 28 codec modes for a total of 168 conditions. The MRT used 36 subjects to produce 432 trials for each condition. Results show that intelligibility depends strongly on noise environment, data rate, and audio bandwidth. For each noise environment we identify codec modes that produce MRT intelligibility values that meet or exceed those of analog FM. We expect that these results can inform some of the design and provisioning decisions required in the development of mission-critical voice applications for LTE.

Keywords: background noise; speech coding; modified rhyme test (MRT); speech intelligibility; audio coding; acoustic noise; ABC-MRT

• TR-15-520

A spectrum sharing scheme is considered in which ship-based radar stations are operating in the same spectrum band as on-shore communication transmitters, and in which the communication transmitters will cause interference to the radar receivers when interference, I, to noise, N, ratios in the radar receivers exceed a given level (e.g., I/N >= -6 dB). The problem is that on-shore environmental sensing capability (ESC) monitors need to determine whether interference is occurring at off-shore radar receivers based only on information from the radars’ transmitters, with no information available from the victim radar receivers themselves. We describe an on-shore monitoring approach in which the principle of reciprocal propagation between the directions of radar-to-ESC and ESC-to-radar provides a simple go/no-go (single-bit) output from the ESCs to an associated Spectrum Access System (SAS) controlling the communication network, to perform on-shore channel changes for protection of the off-shore radar receivers. The ESC station outputs are based on a power-detection threshold of radar signals at the ESCs (e.g., -64 dBm peak-detected power in 1 MHz bandwidth). Examples are provided in which ship-based radar receivers are protected by a simple algorithm applied to a group of on-shore ESCs and a SAS controller for the terrestrial communication network channel frequencies.

Keywords: radar; radio propagation; antenna gain; spectrum sharing; spectrum access system (SAS); Citizens Broadband Radio Service Devices (CBSD); environmental sensing capability (ESC); interference monitoring

• TR-16-521

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

Future spectrum sharing between high-power radars and Citizens Broadband Radio Service Device CBSD in the 3550–3650 MHz (3.5 GHz) band could expose radio frequency (RF) receiver front-end low noise amplifiers (LNAs) to high peak power radar pulse signals in the band under certain situations. In this band, radar effective isotropic radiated power (EIRP) peak levels can exceed 1 gigawatt. Previous experience with LNAs exposed to high-power radar pulses in spectrum near 3.7 GHz has shown that non-linear effects can be induced in the LNAs, leading to service interruptions. To assess the level of risk for similar LNA overload at 3.5 GHz, NTIA performed gain overload (e.g., compression) tests on two representative 3.5 GHz LNAs and a small-cell base station receiver. The tests determined the pulsed radar signal power levels that caused overload (1 dB gain compression) for these devices. Approximate distance separations that would be necessary to preclude potential overload interference effects are presented, based on the measurement results and propagation modeling.

Keywords: radar; spectrum sharing; low noise amplifier (LNA); effective isotropic radiated power (EIRP); Long Term Evolution (LTE); 3.5 GHz band; Citizens Broadband Radio Service Devices (CBSD); non-linear effects; LNA overload; 47 C.F.R. Part 96; General Authorized Access (GAA); Priority Access Licensed (PAL)

• TR-17-525 

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