According to the DOJ, the National Telecommunications and Information Administration (NTIA) has posted a report detailing its findings from the Jan. 17, 2018 test of micro-jamming technology conducted at the Federal Correctional Institution at Cumberland, Maryland.
Data from the test show that the micro-jammer’s signal disrupted commercial wireless signals inside the prison cell, which meant that if cellphones were operating inside the cell, they would have been rendered inoperable. At 20 ft. and 100 ft. outside the cell, however, the micro-jammer signals did not disrupt the commercial wireless signals.
Under the Communications Act of 1934, it’s still illegal for states to jam public airwaves — only the federal government can authorize federal agencies to do it — but micro-jamming technology is starting to break down arguments against amending the law.
Telecommunication companies have long resisted any change in the law, saying jamming technology could block authorized calls outside the proscribed area.
Department officials present during the January 17, 2018, test reported that while their cellphone signals were blocked inside the cell, their cellphones were operable when standing several feet from the cell’s window. The data in the report will be used by BOP and the Department to understand the efficacy of micro-jamming, conduct further evaluation of jamming technology, and develop recommendations for strategic planning.
Here are the key findings from that report. We have highlighted some the key issues and concerns yet to be addressed as the technology is further evaluated.
Of note is finding Number 10 – With 50 to 100 micro-jamming units required per housing unit, electrical power requirements, capital costs and detailed propagation modeling prior to installation may be significant issues in any real world deployment:
1) A contraband wireless device micro-jammer that was operated in four CMRS bands was temporarily installed and operated under an STA for a single day inside a Cumberland, Maryland FCI medium security housing unit. This micro-jammer, installed inside a utility closet, radiated its signal through a wall (concrete and steel, unspecified thickness) to a targeted prison cell on the other side. The cell was located on the ground floor. It had a single barred window to the outside. A commercially available light-reflective coating with unspecified electrical characteristics was temporarily installed on the outside of the window for the purpose of reducing the amount of jamming power that radiated outdoors.
2) The jammer consisted of a single unit containing four transmitters and associated antennas. The transmitters simultaneously produced sawtoothed FM (chirped) carrier waves that covered the CMRS bands 729–757 MHz, 869–894 MHz, 1930–1990 MHz, and 2110–2155 MHz. The jammer transmitters each produced 0.9 W per band, delivered into respective radiating antennas (combination of vertically and horizontally polarized) of +3 dBi gain. The unknown electrical characteristics of the wall through which the jammer radiated makes the unit’s ERP unknown, other than that it was less than the sum of the transmitter power and the antenna gain, that is, it was less than +32.5 dBm.
3) The unit was operated in on versus off states while emission measurements proceeded at four locations: two places inside the targeted cell and two places outdoors, adjacent to the targeted cell. The outdoor locations were 6.1 m (20 ft) and 30.5 m (100 ft) outside the building, with clear LOS to the window of the targeted prison cell.
4) NTIA performed in-band (CMRS band) and adjacent-band, OoB, spurious, and harmonic band measurements of the jammer emissions relative to the ambient CMRS signal levels at the four specified measurement locations. The results of those measurements are provided in this report.
5) Our data show that inside the targeted jamming zone (the prison cell interior), the jammer signal power levels substantially exceeded those of the CMRS signals, as summarized in Table 8 and Table 9 of this report. At the outdoor measurement locations, the jammer signals were substantially lower than indoors and the ambient CMRS signals were substantially higher in power than indoors. The range of signal power levels observed for the jammer versus the ambient CMRS signals at these locations are summarized in Table 8 and Table 9.
6) Lack of accepted quantitative engineering criteria for jammer effectiveness within a targeted jamming zone and for harmful interference outside a targeted jamming zone make it impossible for us to state, based solely on the measured jamming signal power levels and ambient CMRS signal levels, the effectiveness of the micro-jammer or its potential for causing harmful interference. The data in this report could be used for such analysis if (or when) such criteria are ever developed.
7) Noting this gap in knowledge, we recommend that quantitative engineering criteria for jammer effectiveness against contraband wireless devices (e.g., S/J thresholds) and for harmful interference to non-targeted CMRS receivers (e.g., S/I thresholds) should be environments. Theoretical analytical and numerical models should be used in conjunction with selected laboratory measurements to determine these criteria.
8) Outside the targeted CMRS bands, the only spurious signals observed from the jammer were second harmonics of the 729–757 MHz band and the 2110–2155 MHz band. These harmonics, falling in aeronautical mobile telemetry and airborne radio altimeter bands, respectively, were about 24 dB lower than the intentional emission level for the associated 750 MHz band and 7 to 15 dB lower than the intentional emission for the 2.1 GHz band.
9) The results presented in this report are idiosyncratic to the technical particulars of this jammer transmitter and the housing unit building in which its signal was radiated. Different results would be expected for any given jammer installation at any given location.
10) Aggregate emissions from the numbers of micro-jammers that would be required to provide denial-of-service coverage to entire facilities such as Cumberland FCI will represent the sum total of emission power for those groups of jammers in such facilities. Seen at a distance from the facility, the total, aggregate power for such assemblages will tend to increase in direct proportion to the number of such transmitters deployed in each facility. It has been estimated that 50 to 100 jammer transmitters like the one observed in this report might be required to cover a single, large-size housing unit in a prison facility. There are eight such medium security housing units at Cumberland FCI, for example. Assessment of the aggregation effects of multiple jammer transmitters is beyond the scope of this report. Such assessment would require either a full micro-jammer deployment at a prison facility, or else a detailed theoretical-analytical study for a given facility, including detailed propagation modeling of all of the facility’s walls, ceilings, and floors.
11) Measured characteristics in the time domain are consistent with the chirped modulation of the micro-jammer transmitter.
12) Measured emission spectra have a lobed structure that is consistent with the chirped modulation of the micro-jammer transmitter.
13) Jammer transmitter detected peak power goes as 16.7?log(receiver IF bandwidth) for any given receiver. Detected average power will go as 10?log(receiver IF bandwidth) for any receiver. See Appendix B for experimental confirmation and further discussion of this topic.
14) The total-power emission bandwidth for the jammer transmitter goes, band-by-band, according to the values of Table 2. For peak detection the power-limiting bandwidths run between 1.4 and 2.5 MHz; for average detection the power limiting bandwidths are slightly wider than the chirp bandwidth, Bc. See Appendix B for further discussion.
15) The data in this report can be used with the unit conversion information provided in Section 5.3 and the supplemental material of  to compare with radhaz recommendations and limits.