Industry Canada's BPL Consultation Paper on BPL Communication Systems
Section 6 of the Consultation Paper, Discussion and Proposals is really the heart of the document. In it IC is asking for feedback on several aspects of BPL systems. For each section of the IC document, the following presents an analysis and some recommendations.
6.1 Equipment Standard and Approval Process
Today's urban environment is home to many interference-causing devices and systems. Potential devices range from automotive ignition systems to noisy electric motors, and electronic devices such as computers or television sets. Many of these sources can be found in rural areas although their density will be much lower than in an urban setting. As varied as these possible interference sources are, they have common features; each noise source will affect only a limited area and they are not continuous producers of interference. The situation is quite different in the case of BPL. BPL signals are continuous and the interference that is produced will also be continuous.
Radiated BPL noise can be produced at any point in the system, starting at the BPL signal injector, and following along the cable. Due to the extremely varied environments in which BPL systems will be installed, it will be difficult to predict radiated field patterns accurately unless detailed modeling, based on detailed geographic models, is performed. This is hardly practical for something as extensive as the proposed BPL network.
- Since the cabling is usually the major source of the radiated field, all signal-carrying cables must be considered part of the BPL system.
- Each BPL system should be required to undertake field strength measurements along the full length of any signal-carrying cables and obtain certification of compliance with emission limits before operation is commenced. Experience with systems in the U.S. has shown that any testing must be comprehensive and thorough if it is to fully document the electromagnetic field in the region of a BPL system.
- Concerning equipment, BPL signal injectors, repeaters and any other signaling equipment should be subject to type approval testing to verify that they will operate, in all reasonably expected operating conditions, in a way that does not lead to an unexpected rise in the level of the radiated BPL signal. Particularly important would be the verification of proper operation of power control and frequency notching/filtering mechanisms.
6.2 Prospective Technical Requirements
Experience in the U.S. has shown that BPL systems operating in the 1.705 to 80MHz range are strong sources of interference. The similarity between the U.S. and Canadian power distribution systems indicates that, without question, H.F.-based BPL systems that comply with U.S. regulations will, if installed in Canada, produce unacceptable levels of interference.
What can we do if Canadian BPL systems are to avoid the problems that have occurred in the U.S.?
- Canadian BPL systems must either avoid the 1.705 to 30MHz portion of the 1.705 to 80Mhz frequency range or be designed and operated so that they will meet emission limits that are greatly lower than the U.S. limits which are the same as those contained in the Consultation Paper.
a) Emission limits
Although BPL systems radiate a signal, they would be able to co-exist with HF radio communication and receiving systems as long as the emission limits are set appropriately. The best guide for this is to use the existing ambient radio noise levels and ensure that BPL signals do not raise that level.
The emission limit question is really three questions. First, should BPL systems be allowed to raise the ambient noise level? Secondly, what should the field strength limit be? And last, at what distance from a BPL line should the limit be measured?
The noise level question:
There would seem to be no good reason to allow BPL providers to raise the noise level over large areas of the country by many tens of dB. Other network technologies are able to operate without the collateral damage that would be caused by BPL networks. The correct operation of communication systems and future radio-based devices depends on a noise-free environment. BPL's emissions can only compromise the integrity of all communications in the 1.705 to 80MHz range and the future of radio-based devices that operate within this frequency range.
- The field strength question:
An acceptable field strength depends to some extent upon your location. Referring to the BPL Signal and Ambient Noise graphs in the Discussion Points section, emissions produced by a BPL line at a distance of 30m would have to be reduced by 50dB to approximately 0.1uV/m in order bring the received BPL signal down to the level of the ambient noise in a quiet rural location. For rural and residential locations the emission limit contained in the Consultation Paper needs to be reduced by about 35 to 40dB to between 0.5 and 0.3uV/m. However, it must be realized that a 0.5 to 0.3uV/m field will produce a signal that is about 10dB above the receiver's sensitivity, and quite detectable under quiet band conditions. So, pick your own emission limit field strength - 0.1uV/m if you want to have a single number that will ensure that BPL does not significantly raise the noise level in any location, or 0.5 to 0.3uV/m for a compromise that will work in most locations.
- The distance question:
The 30m distance specified in the Consultation Paper is of little use when most antennas will be much closer to a BPL line. If the purpose of the emission limits are to limit interference to radio receivers, then the above field strength must be specified at the location of the receiving antenna. In most cases this will be much less than 10m, and more typical antenna-to-BPL-line distances are in the range of 3 to 1m. So, pick your distance - 10m is too large, but a 1 or 3m distance would seem to be in the right range. It should be noted that other investigators also consider 1 and 3m distances to be appropriate. As it is possible for BPL field strengths to rise under some circumstances as you move away from a BPL line, the field strength value should apply at the reference distance and outwards.
Putting it all together:
A conservative figure, interference very unlikely: 0.1uV/m at 1m or 3m from a BPL line.
A city-dwellers number: 0.3 to 0.5uV/m at 1m or 3m.
If you live near a factory: 1.5uV/m at 10m.
If you are prepared to live with the nominal S-9 noise levels allowed by Industry Canada's Consultation Paper: 30uV/m at 30m
Please note that while the above has been developed for the HF region of the BPL band, similar figure apply in the 30 to 80MHz range. Check back later for any new values for the 30 to 80MHz range.
b) Interference Mitigation Techniques for Access BPL Systems
Interference to radiocommunication and other radio users would be most easily prevented by establishing emission limits that will not result in any significant increase in ambient noise levels. Filtering, notching and frequency avoidance parameters would have to be established during system certification and startup. It is unrealistic to visualize a system in which power levels and/or frequencies-in-use will be dynamically varied during operation to suit complaint levels or time-of-day noise intensity. It would be best to use remote power and frequency control to set system operating parameters, on a line-by-line basis, in a given BPL system.
However, is it realistic to expect that BPL segments will actually be shut down when conditions so warrant? Is this the "service" that BPL operators are offering? Would anyone tolerate a phone or cable TV system that goes on and off during the day and perhaps stays off for extended periods? Compared to DSL/ADSL and cable modems, BPL seems to offer a service of very questionable reliability.
- Remote shut-down will be essential to cover situations in which remote power or frequency controls have failed or are not providing the required reduction in the level of an interfering BPL signal. Also, BPL's emissions might compromise the reliability of emergency HF communications during a disaster. Shut-down capability would remove the BPL-generated interference.
6.3 Operational Requirements
a) Prohibited Frequency Bands.
Prohibiting operation would be more effective at ensuring BPL-interference free communications than emission limits. Characterizing the fields radiated by a BPL line is a complex procedure and it would be easier and far more effective to prohibit operation in the bands mentioned below.
- Operation in all amateur radio and short-wave broadcast bands, and any bands or frequencies used for safety-of-life communications within the proposed BPL frequency range should be prohibited.
b) Geographical Frequency Restrictions and Coordination Requirements
The resolution of interference problems is an art and a science. The process can only be aided by having easy access to information on potential local sources of interference.
- BPL operators should make the details of their operations available to radio users. The best way to do this would be to make the information freely available on the Web.
c) Interference Resolution.
- Having a point of contact for the resolution of problems within the BPL providers' local organization would, in all likelihood, facilitate the solution of interference problems and hopefully lead to speedy solutions.
- A mechanism should be in place to track all complaints and compile statistics on interference issues. Tracking the number and type of complaints, and the time required to resolve them, will allow one to judge the effectiveness of remediation efforts.
- Industry Canada would have to set time frames for responding to and resolving complaints, and criteria for establishing when a problem has been resolved.
- Any database of complaints should be maintained by Industry Canada, and be publically available.