Spectrum Engineering Reports

In response to proposals to introduce new Long Term Evolution (LTE) radio systems into the 3550–3650 MHz (called 3.5 GHz) portion of radio spectrum in the United States, a joint team of National Telecommunications and Information Administration (NTIA) and U.S. Navy electronics engineers performed emission spectrum measurements on a 3.5 GHz (LTE Band 42) wireless access point (WAP), or hotspot. The hotspot was packaged for indoor use but similar systems could be deployed outdoors. The authors measured the hotspot emission spectrum with 110 dB of dynamic range across 1.5 GHz of spectrum (from 2.7 to 4.2 GHz). Other data outputs include: spectra measured with the device tuned to its lowest, highest, and middle available operational frequencies; comparative peak-to-average spectra; and spectra measured when the hotspot was operated with 10, 15, and 50 resource blocks. The emission spectrum is plotted against proposed in band, out-of-band (OOB) and spurious emission limits; the spectrum meets those limits by at least 10 dB at all points. The results presented here may be used in electromagnetic compatibility analyses for future 3.5 GHz spectrum sharing between LTE-based transmitters and incumbent systems such as radar receivers.

Keywords: radar; electromagnetic compatibility (EMC); band sharing; spectrum sharing; out-of-band (OOB) emissions; spectrum measurement; Long Term Evolution (LTE); 3.5 GHz band; LTE band 42; emission limits; resource blocks; spurious emissions; wireless access point (WAP); wireless local area network (WLAN)

• TR-15-512

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