Publications

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

Spectrum reallocations may place broadband radio services (BRS) near spectrum used by 2900–3100 MHz band marine radars. Signals from the BRS base stations can potentially cause the radar front-end to overload and cause interference. This report provides a method that can be used to estimate front-end filter attenuation required at various radar to base station separation distances. The attenuation is the difference between the interfering power at the radar low-noise front-end (LNFE) and the allowable interference power. The BRS signal was emulated with 10 MHz bandwidth Gaussian noise. The allowable interference power IPC is determined from probability of detection measurements with a custom test fixture. Two front-ends were tested. One was an off-the shelf magnetron radar front-end assembly consisting of a circulator, limiter, and low-noise front-end. The other, referred to as the reference front-end, was constructed of discrete components. The reference front-end was tested without the frequency selectivity of the circulator and limiter. Results showed that the allowable interference power is -11.5 and -9 dBm for the reference front-end and magnetron front-end assembly, respectively. Additional front-end filtering is not required for either front-end at distances as close as 400 meters. Distances less than 400 meters were not analyzed due to near-field effects. Gain compression and noise enhancement metrics, which are simpler to measure than performance degradation, were also evaluated to determine if they could reliably predict allowable interference power. Only the noise enhancement metric could reliably predict the performance degradation. This result is important since many front-end overload studies are based on the gain compression point metrics.

Keywords: radar; interference; radio wave propagation; front-end overload; interference protection criteria (IPC); broadband radio service; marine radar; radio spectrum engineering; front-end filter; gain compression; noise enhancement

• TR-15-515

Spectrum reallocations may place broadband radio services (BRS) near spectrum used by 2900–3100 MHz band marine radars. Signals from the BRS base stations can potentially introduce unwanted emissions in the radar detection bandwidth and cause interference. Interference protection criteria (IPC) are needed to mitigate this effect. The primary IPC of concern are the interference to noise power ratio (INR) and the reliability of the radar link at a specified radar signal to noise power ratio (SNR). Reliability is determined by radio wave propagation path loss variability which increases with distance. Distance separation between base stations and radar for various spurious attenuations were calculated using reliability expressions for short radar to target ranges with constant SNR and for longer radar to target ranges with variable SNR. For a magnetron radar under clutter free conditions, 90 dB of spurious attenuation is needed to obtain 90% radar operational reliability at1.2 km when the INR is -6 dB and the SNR is constant. Reducing the INR to -9 and -12 dB increased the separation distance to 1.7 and 2.7 km, respectively. At longer base station to radar separation distances, variable SNR required less spurious attenuation than constant SNR. Consequently, constant SNR analysis can be considered worst case. Distance and frequency separation were calculated using frequency dependent rejection (FDR). These calculations showed that for constant SNR 92.8 MHz of frequency separation is required to meet the 90% reliability IPC at 1.2 km separation distance when the INR is -6 dB. Only 54.6 MHz frequency separation is needed when the separation distance is increased to 32.8 km.

Keywords: radar; interference; radio wave propagation; frequency dependent rejection; interference protection criteria (IPC); spurious emissions; broadband radio service; marine radar; radio spectrum engineering

• TR-15-514

Spectrum reallocations may place broadband radio services (BRS) near spectrum used by 2900–3100 MHz band marine radars. Interference effects from these reallocations include unwanted emissions in the radar detection bandwidth and front-end overload. This report provides background information for subsequent reports that analyze these effects. Interference protection criteria (IPC) are identified, an interference scenario is described, and models for the radar system, BRS system, radar target, and radio wave propagation are presented. The BRS signal is shown to be reasonably emulated by Gaussian noise. A method for determining the aggregate power distribution using a realistic propagation model and Monte Carlo analysis is described. The aggregate power from the base stations was found to be as much as 6 dB more than power from a single base station. Finally a method for incorporating a variable SNR, caused by variable radar to target path loss, into interference analysis is shown.

Keywords: radar; interference; radio wave propagation; front-end overload; unwanted emissions; interference protection criteria (IPC); aggregate power; broadband radio service; marine radar; radio spectrum engineering; signal characterization

• TR-15-513 

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