Monday, September 17, 2007

NATO Issues Warning on BPL and the HF Radio Spectrum

NATO's (North Atlantic Treaty Organization) research and technology arm recently issued a report warning of the potential impact that can be expected by the widespread deployment of BPL (Broadband over Power Lines - also referred to as PLC or PLT) in the HF portions of the RF spectrum.

What's important to understand here (and what most people don't realize) is that BPL technology, in case you were unaware, has never-the-less been approved for deployment in the U.S. by the Federal Communications Commission and the NTIA, to the dismay of many shortwave radio listeners, amateur radio operators, public safety communication system managers, high seas marine radio users, and military LMR and airborne radio system operators. (Take some time to review these comments from the FCC proceedings on this matter for further background and insight)

The entire report (9MB) titled "HF Interference, Procedures and Tools" includes this Executive Summary:

This Report presents the results of the work carried out by IST-050/RTG-022, the Research Task Group (RTG) on “HF Interference, Procedures and Tools”, to address the concerns raised by the potential for unintentional radio interference to be caused by the widespread operation of broadband wire-line telecommunications systems.

PowerLine TeleCommunications (PLT, PLC) and various forms of Digital Subscriber Line (xDSL) transmissions use the existing mains electricity or telephone wiring including in-premises cables for telecommunications with data rates higher than 1 MBit/s. As these lines were not designed for such broadband transmissions, they will cause unintentional RF emissions which may adversely affect the established radio noise floor directly, or by cumulative propagation from many such sources. The existing HF background noise possibly may be increased via ground wave and/or sky wave propagation.

Increase of the existing HF noise floor by widespread use of PLT and/or xDSL will bring up problems for Military Radio Users as well as for HF Communication Intelligence (COMINT) in all NATO countries. The signal-to-noise ratio thus may be reduced for tactical and strategic HF radio as well as for fixed sensitive COMINT sites.

Exact calculations of HF radio noise emissions from the new broadband wire-line telecommunications networks were impossible because of missing models for these transmission systems. Therefore methods have been investigated to find procedures, models and tools applicable for being able to determine the influence of PLT and xDSL on reception of HF radio signals. These are described in this report.

The RTG addressed itself to the HF radio emission effects of the new broadband cable transmissions. It investigated and found means that allow calculation of cumulative field strengths of HF noise radiated by PLT or xDSL. This will enable NATO and its nations to determine the threat to military HF radio communications and COMINT systems from PLT and xDSL and to take the appropriate steps. It should be noted here that the determination of the nature and the severity of any possible detrimental effect upon the military systems was outside the RTG’s expertise and ToR.

The RTG chose to concentrate its work on the PLT issue rather than xDSL because PLT will produce the most problems regarding HF interference (power lines have less symmetry and will have impedance discontinuities), they will be deployed in large numbers, and finally the current versions of xDSL have no documented HF interference-causing problems, while the VDSL variants covering the entire HF range are still in the definition phase.

In the course of the studies, the RTG determined that ITU-R P.372-8 noise curves (based on measurements carried out in the 1970s) are still valid in Europe. Recent measurements carried out in Germany and Great Britain indicated that there is no remarkable difference between these measurements, specifically no increase of the ambient noise in quiet rural zones within the last 30 years.

Based on these measurement results, the cumulative interference field strengths far away from telecommunication networks should not be higher than 15 dBìV/m (9 kHz bandwidth) across the entire HF range, if no measurable increase in minimum noise levels are to be tolerated. The RTG refers to this criterion as the Absolute Protection Requirement. It should be noted that this value is in the range of 10 to 1 dB below the ITU-R P.372-8 Quiet Rural noise curve, which are median values, across the HF band.

A couple of important tasks in the RTG’s work, namely, the appropriate measurement techniques and the most suitable propagation path loss models for interference studies, were addressed and completed.

The quantity of interest when considering cumulative effects in the far-field is the EIRP (equivalent (or effective) isotropic radiated power) per unit bandwidth caused by each signal source, in units of dBm/Hz, at different frequencies. The radiation pattern might also be of interest in some cases, but when summing up many different sources with different wiring geometries over a wide area, it is reasonable to approximate the average radiation pattern as isotropic (in elevation as well as in azimuth).

In modeling the emissions from an overhead Access PLT line, the PLT wires can be modeled as a successive set of dipoles, assuming that the standing waves present are the dominant emission source. Given the PLT geometry, the cylindrical coordinate system is more practical rather than the spherical coordinate system generally used in electromagnetics. In the vicinity of a PLT, up to 200 metres, the use of the expression for the exact solution of a dipole is recommended, which is valid at any distance in both near-field and far-field.

The RTG has developed a “Cumulative PLT Tool”, which was used to perform cumulative PLT noise calculations at several hypothetical sensitive receiver locations. For each receiver location and frequency, the percentage of parameter combinations was computed where the estimated cumulative PLT noise level is above the quiet rural level, above quiet rural +6 dB, and above the rural noise level. The results indicated the following:

a) High probability that PLT would cause increased noise levels at sensitive receiver sites given the projected market penetration; and

b) The percentages are highly influenced by assumptions on transmitter EIRP, PLT market penetration, and duty cycle.

The percentage of parameter combinations was also computed where the estimated PLT noise level is above the Absolute Protection Requirement. Again, the probability of the cumulative effect of PLT exceeding the Absolute Protection Requirement is predicted to be relatively large for all frequencies and receiver locations investigated.

Currently, there are no commonly accepted regulatory emission limits for PLT. While it is highly desirable that the regulatory limits on PLT emissions be harmonized throughout the NATO countries, the RTG recognizes that NATO, by itself, has no regulatory authority over the emission limits. Therefore, it is recommended that NATO seek the implementation of this goal by working together with the national and international regulatory authorities."

The complete NATO report sheds additional detailed information on this matter and should be of interest to those concerned with "spectrum pollution" matters.

Please pass along the link to this report to others who may not be aware of the potential disruptive and destructive impact of BPL technology on the RF spectrum.


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