From Telecom Introduction Broadband over Power Lines (BPL) represents a substantial business opportunity for power providers and the service personnel that work with them. BPL deployment has already begun and each system requires extensive testing before and after it is placed into service. This article is based on prior work performed by the author for the Electric Power Research Institute (EPRI) and gives the reader an overview of the requirements for testing BPL systems.
What is BPL? Access Broadband over Power Lines (Access BPL) is a new type of carrier current technology that provides high-speed broadband access using electric utility companies’ power lines. Power lines reach virtually every residence and business in the US and Access BPL service could be made available in most suburban areas.
Access BPL deployment will also enable a variety of more sophisticated power distribution applications, including automated outage detection, restoration confirmation, remote monitoring and operation of switches and transformers, more efficient demand-side management programs, and power quality monitoring to detect faulty components. New BPL systems being built in Manassas, VA, Cincinnati, OH and Honolulu, HI include advanced utility functions to improve their business case for deployment. [1]
Background Carrier current systems are not in themselves new. Pre-BPL carrier current systems transmit radio frequency energy by conduction over an electric power line to a receiver on the same power line. See 47 C.F.R. § 15.3(f). They use AC power lines to carry communications by coupling low power RF signals onto the wiring and operate unlicensed under Part 15 of the Commission’s rules. As unlicensed devices operating under this Part, these devices operate “at sufferance.” That is, they may not cause harmful interference to authorized radio services and must accept any interference that they receive. Traditional power line carrier systems for utility communications were narrowband and operated below 500 kHz.
The development of newer, faster digital processors and sophisticated modulation schemes has allowed the creation of carrier current devices that overcome the earlier obstacles caused by the noise and impedance mismatch of power lines. BPL systems have been developed that use spread spectrum or multiple carrier techniques with adaptive algorithms to counter noise present on the lines. These new unlicensed BPL systems provide high speed communications by coupling RF energy onto either the power lines inside a building (“In-House BPL”) or onto the medium voltage (MV) power distribution lines (“Access BPL”). Electric utility companies can also use Access BPL systems to monitor and manage their power distribution operations. Since Internet service can be made available in conjunction with the delivery of electric power, BPL may provide an effective means for “last-mile” delivery of broadband services and may offer a competitive alternative to digital subscriber line (DSL), cable modem services or other high-speed technologies.
How can we avoid interference from a system that will traverse so many locations? The “spread spectrum” and “multiple carrier” techniques used in proposed BPL systems employ modulation that spectrally resembles noise and can employ system architectures capable of minimizing radiated interference. The multiple carriers can be controlled in amplitude, turned on and off, or “notched” to remove energy radiating at specific frequencies. Typical Access BPL frequencies start above AM radio (1.7 MHz) and end just below FM radio (up to 80 MHz). Usually the entire available range is not used—the upper frequency is typically less than 50 MHz. There are many licensed, incumbent services that must be protected in this frequency range, including:
Amateur Radio – 1.8 – 54 MHz Short-wave radio – 5.9 – 26.1 MHz CB Radio – 26.96- 27.41 MHz TV Channels 2-6 Public Safety bands Federal Government bands Aeronautical Stations Coast Guard Stations Radio Astronomy Stations
Network Architectures Access BPL systems typically use three basic types of equipment: (1) injectors, (2) repeaters, and (3) extractors. Injectors have bidirectional connections between the Internet backhaul point and the MV lines feeding a neighborhood via overhead or underground power lines. A typical BPL signal only travels for 1,000 to 3,000 feet down line, so a repeater may be employed. Some systems don’t use repeaters because they decrease system performance. This distance shortcoming limits BPL deployments to suburban settings rather than rural long-haul applications. An extractor pulls the broadband signal from the MV lines and places it onto LV (low voltage) “in-house” systems or onto a wireless WiFi link into the customer’s premises.[2]
One system uses Orthogonal Frequency Division Multiplexing (OFDM) to modulate the broadband signal onto many narrow-band sub-carriers which are injected onto a single MV wire. End-users employ standard “In-house” (Home Plug) modems to provide an Ethernet connection. Another OFDM implementation uses separate frequencies for upstream and downstream traffic with a wireless WiFi connection into the customer’s building. A third type of architecture uses Direct Sequence Spread Spectrum (DSSS) techniques to transfer data from a number of repeaters (extractors) to a concentrator for interface to the high-speed Internet backbone. Injectors on this third type use two couplers, one on Phase and the other on Neutral. An experimental microwave surface wave system has demonstrated 200Mbps transmission speeds using multiple WiFi chipsets over MV power lines. [3] A simplified matrix of these BPL system architectures is given in Table 1. [4]
Table 1: BPL System Architectures
Effective 10/14/04, the FCC Report & Order recognized the concerns of authorized radio users that radio frequency (RF) energy escaping from BPL signals on power lines doesn’t cause harmful interference to licensed radio services. The historical record and FCC investigations indicate that BPL network systems can generally be configured and managed to minimize and/or eliminate this harmful interference potential. The Commission’s goals in developing the rules for BPL deployment were to facilitate the rapid introduction and development of BPL systems and to protect licensed radio services from harmful interference. The Commission adopted:
1. New operational requirements for Access BPL to promote avoidance and resolution of harmful interference; 2. New administrative requirements to aid in identifying Access BPL installations; and 3. Measurement guidelines and certification requirements to ensure accurate and repeatable emissions measurements of Access BPL and all other carrier current systems.
In the Commission’s words: “We believe these actions will promote the development of BPL systems by removing regulatory uncertainties for BPL operators and equipment manufacturers while ensuring that licensed radio services are protected from harmful interference.” [1]
A database of BPL systems deployed or pending in the US is maintained by the United Power Line Council and mapped in Figure 1. [6]
Figure 1: BPL System Deployments in the US
Types of BPL Access BPL-Overhead: Access BPL systems carry high-speed data signals to neighborhoods from the “backhaul point” connection to a telecommunications network. The point of network connection may be at a power substation or at an intermediate point between a substation and users. Some systems complete the connection between the medium voltage lines and their subscribers by using WiFi wireless links, while other implementations employ an extractor at the distribution transformer to transfer the Access BPL signals across them (In-House BPL).
Access BPL extractor installed on MV power line.
In the illustration in Figure 2, the broadband signal is brought to the utility substation (or further downstream) to a “backhaul point” and BPL injector by fiber optic or wireless links. Access BPL – Overhead lines are shown between the poles. Delivery of the broadband signal to the end-user’s home or business over the “last mile” can be either by
1. Wireless Link: A WiFi (IEEE 802.11) link is represented by the yellow lightning bolt. This method is useful if there are several users clustered near the transformer, such as in a typical residential backbone setting.
2. In-House BPL: the signal is coupled from the medium-voltage lines and “bridged” around the transformer by the extractor and placed onto the low-voltage feeders to the building(s). These broadband signals on the home or business low-voltage wiring are decoded by power-line modems into the traditional “Ethernet” (RJ-45) connection. Use of the existing wiring facilitates the implementation of computer networks.
Figure 2: BPL System Layout
Access BPL-Underground is the same as Access BPL run on overhead lines, except that the lines are run underground between transformer vaults, thus limiting access to the lines and shielding some of the interference caused by signals radiating from them. As such, Access BPL equipment may operate at higher injected currents in this situation while still maintaining its conformity to the radiated emissions limits imposed under the Report and Order.
Emissions Limits Power lines are neither shielded nor well-balanced, so it is likely some of the RF energy they carry will be radiated. This RF “leakage” can become harmful interference if not carefully managed. Licensed users on the same frequency bands as local Access BPL signals could possibly receive harmful interference from this signal leakage if adequate precautions aren’t taken, and BPL operators are required to investigate and mitigate any instances of harmful interference.
The radiated emissions limits are separated into two frequency ranges. Below 30 MHz, the intentional radiator limits are given in 47 CFR 15.209. From 1.705 to 30 MHz, the limit is 30 uV/m (29.5 dBuV/m) at a measurement distance of 30 meters. The FCC allows extrapolation of values measured at different distances to the 30 meter regulatory specification using a correction factor of 40 dB/decade (inverse square scaling) below 30 MHz. The actual measurement distance from the pole-line or transformer is the slant distance from the wires to the measuring antenna.
Carrier current devices operating above 30 MHz must meet the radiated emission limits of Section 15.109(a), (b) or (g) for digital devices. Access BPL systems that operate on medium voltage lines external to residential environments are considered Class A devices, per FCC § 15.109(b). The “Unintentional Radiator” Class A limits above 30 MHz in 47CFR § 15.109 (b) are graphed in Figure 3.
Figure 3: FCC Radiated Emissions Limits For CLASS A Equipment For measurements above 30 MHz, inverse distance scaling (20 dB/decade) is used for comparison to the limits. Quasi-Peak detection is used up to 1,000MHz if the data rate is greater than 20 Hz. (Peak detection is used otherwise). Average detection is used above 1,000MHz. 47CFR § 15.33 gives guidance on the upper frequency range of measurement as shown in Table 2.
Table 2: Unintentional Radiator Upper Frequency Range of Measurement
There are no Conducted Emissions limits. Access BPL operates above the AM band and therefore is not subject to the conducted emissions limits of Section 15.107. Besides, it would be difficult and potentially dangerous to make low-level line conducted measurements on the MV lines, which carry kilovolt voltages.
Measurement Guidelines The FCC Report & Order requires that Access BPL systems, including all component devices (couplers, injectors, extractors, repeaters, boosters, concentrators) installed on utility overhead or underground medium voltage lines be measured “in situ” to demonstrate compliance under the Part 15 limits. Measurements are made at three overhead and three underground representative locations using the measurement guidelines in Appendix C of the R&O. Access BPL measurements are made with the system set for maximum power and maximum burst rate. Notations are made in the Test Report of the reduction to lower power and data rates needed to meet the Part 15 emission limits.
The NTIA Phase 1 Study 04-413 suggested an un-workable requirement that all measurements above 30MHz be made while searching for the peak emissions during a vertical scan of the receiving antenna from 1 to 4 meters. This would be impractical even using a mast-equipped utility EMI van, since the roof-mounted mast could not be lowered to 1 meter. In crowded urban settings the lateral measurement distance may be reduced to 3 meters, thereby placing the antenna too close to the wires for safety when raised to 4 meters height. The FCC recognized these problems but leaves this “NTIA method” in place as an alternate. The preferred FCC method of measurement allows a 1m high tripod to be used at 10 meters lateral distance from the pole line (or transformer case). The lateral measurement distance can be decreased to 3 meters if the received BPL signals are less than 6dB above the ambient radio signals at that location. In either case a 5dB correction is added to the received field strength, which is slant-range distance corrected using the appropriate scaling factor to compare to the limits for the frequency being measured (i.e., 40 dB/decade below 30 MHz, and 20 dB/decade above).
Access BPL Overhead lines require measurements at fixed horizontal distances from the power line where the Access BPL signal injection source is installed. The receiving antenna is moved down line, parallel to the power line, starting from the Access BPL signal injection equipment location, to find the maximum emissions at each frequency within the frequency range of the Access BPL device. The down-line measurement distances are specified in terms of the wavelength of the Access BPL mid-band frequency.
Here is a practical example: If the device injects frequencies from 3 to 27 MHz, the wavelength of the mid-band frequency of 15 MHz is 20 meters, and the wavelength of the lowest injected frequency is 100 meters. Measurements would be performed at 0, 5, 10, 15, and 20 meters down line, from zero to one wavelength of the mid-band frequency. Because the mid-band frequency exceeds the minimum frequency by more than a factor of two, additional measurements are required at 10-meter (which is one-half of the mid-band wavelength) intervals until the distance down-line from the injection point equals or exceeds 1/2 of 100 meters. Additional measurements are therefore required at 30, 40, and 50 meters down line from the injection point. Testing is repeated for each Access BPL component (injector, extractor, repeater, booster, concentrator, etc.), as these will typically not be co-located. See Figure 4.
Figure 4: Example of Access BPL Overhead Measurements
Antennas are those normally employed in FCC compliance measurements, and are summarized in Table 3.
Table 3: Typical antennas for BPL Certification testing
Measurements are performed at a horizontal separation distance of 10 meters from the pole-line centerline. If necessary, due to ambient emissions, measurements may be performed a horizontal distance of 3 meters. Distance corrections are to be made in accordance with Section 15.31(f) of the Rules, based on the slant range distance, which is the line-of-sight distance from the measurement antenna tripod to the overhead line. (This can be measured with an optical rangefinder). In the example of Figure 5 the measurement is made at a horizontal distance of 10 meters with an antenna height of 1 meter and the height of the BPL-driven power line is 11 meters. The slant range distance is 14.1 meters [10 meters vertical distance and 10 meters horizontal distance]. At frequencies below 30 MHz, the measurements are extrapolated to the required 30-meter reference distance by subtracting 40 log10(30/14.1), or 13.1 dB from the measured values. For frequencies above 30 MHz, the correction uses an inverse scaling factor (here, 20*log10(10/14.1) = -2.9 dB) and the 10-meter reference distance specified in section 15.109 of the rules.
Figure 5: Example of Slant Range Distance
BPL system settings determine the detector to be used in the EMI Receiver. Data rates above 20 bursts per second require the use of Quasi-Peak detection, and lower rates require Peak detection. Certification tests are run with the BPL system set for maximum output power and maximum duty cycle and then re-run with the system set for Part 15 compliant output power (or less), also with maximum duty cycle. [7] Frequency “notching” is used to prevent interference. For example, if the system has 5MHz of maximum modulation bandwidth and can be tuned from 5-30 MHz, it is set to sequentially produce carriers at 5, 10, 15, 20, 25 and 30 MHz, each with full data-rate modulation. This meets the requirement that the Certification test be run “sequentially” with the BPL devices “operating at all frequencies at which they are capable.”
Underground line installations are those in which the BPL device is mounted on a pad-mounted transformer housing or junction box and couples only onto underground cables. Access BPL Underground installations are tested by measuring radiated fields along 16 radials at 10 meters distance from the edge of the above ground transformer where the Access BPL equipment is located. If necessary, due to ambient emissions, measurements can be performed at a distance of 3 meters. Distance corrections are made in accordance with Section 15.31(f) of the Rules as discussed above. If directional radiation patterns are detected, additional azimuth angles should be investigated. See Figure 6.
Figure 6: Example of Access BPL Underground Measurements
In-House BPL equipment includes modems used to transmit BPL signals on low-voltage lines, associated computer interface devices, building wiring, and overhead or underground feeder lines connected to the electric utility. “In situ” testing is performed with the EUT installed in a building on an outside wall on the ground floor or first floor. Testing is run on three typical installations, including a combination of buildings with overhead-line(s) and underground line(s). The buildings cannot have aluminum or other metal siding or shielded wiring in conduit or BX cable.
Measurements are run at positions around the building perimeter where the maximum emissions occur. If directional radiation patterns are detected, additional azimuth angles should be investigated. Measurements are performed at a horizontal distance of 10 meters from the building perimeter. Again, if necessary due to ambient levels, measurements may be performed at a distance of 3 meters. Distance corrections are made in accordance with Section 15.31(f) of the Rules.
In addition to testing radials around the building, testing is also performed at three positions along the overhead feeder line connected to the building (the service wire). These measurements are performed starting at a horizontal distance 10 meters up the line from the connection point on the building. The conditions of each measurement (offset from the line, slant distance, and correction factor) are the same as for overhead Access BPL, but only three measurements are taken along the line.
Interference Mitigation – Other Conditions of BPL Operation Access BPL systems incorporate adaptive interference mitigation techniques to remotely reduce power and adjust operating frequencies, in order to avoid site specific, local use of the same spectrum by licensed services. These techniques may include adaptive or “notch” filtering, or complete avoidance of frequencies, or bands of frequencies, locally used by licensed radio operations. Access BPL systems employ a remote-controllable shut down feature to deactivate, from a central location, any unit found to cause harmful interference, if other interference mitigation techniques do not resolve the interference problem. The FCC’s Report and Order places the following conditions on BPL systems:
- Consultation with government users: Access BPL system operators must notify and consult with public safety users in the BPL service area at least 30 days prior to starting operations.
- Excluded Bands: Access BPL operations on overhead medium voltage power lines are excluded in the frequency bands listed in Table 4 to protect Aeronautical (land) stations and aircraft receivers.
Table 4: Excluded Frequency Bands
- Exclusion zones: These regulate the operation of any Access BPL system within 1km of the boundary of coast station facilities or within 29 km of the coordinates for the ten Very Long Baseline Array facilities or within 11 km of the coordinates for the ten Very Long Baseline Array facilities listed in Allocation US311. Within the exclusion zones for coast stations, Access BPL systems cannot use carrier frequencies within the band of 2173.5-2190.5 kHz. Within the exclusion zone for Very Long Baseline Array radio astronomy observatories, Access BPL systems cannot use carrier frequencies within the 73.0-74.6 MHz band.
- Existing coast station facilities: Access BPL systems cannot operate in the frequency band 2,173.5 – 2,190.5 kHz, within 1 kilometer (km) of the boundary of coast stations at the coordinates listed in the Report & Order.
- Access BPL Database: Access BPL operators must supply an industry recognized entity information on all their existing and proposed Access BPL systems for inclusion into a publicly available data base (Web site) within 30 days prior to beginning service. The United Power Line Council maintains this site at http://www.uplc.org. The information must include:
(1) The name of the Access BPL operator
(2) The frequencies used by the Access BPL operation
(3) The postal zip codes served by the Access BPL operation
(4) The manufacturer and type of Access BPL equipment and its FCC ID number, or with Access BPL equipment that was subject to verification, the Trade Name and Model Number as shown on the equipment label.
(5) Telephone number and email address of the person at the BPL operator’s company who facilitates the resolution of any harmful interference complaint.
(6) The proposed or actual date of Access BPL operation.
Here’s what the FCC says about the duties of BPL operators to maintain a database and handle interference complaints:
Equipment Authorization of Access BPL equipment: Certification testing is specified in Section 15.101 of Title 47. The certification procedure is carried out by the equipment manufacturer. (Not the Utility). Telecommunications Certification Bodies (TCBs) are not yet authorized to approve BPL systems – certification by the FCC is the only path to authorization. As the FCC puts it:
Maintaining Compliance Initial Installation (installer control settings): While initial Certification testing of a BPL system is the responsibility of the manufacturer of the equipment, the final legal responsibility for the compliance of the finished installation rests with the power provider. This is the reason behind the FCC’s recommendation that operators perform (or technically witness) the initial installation Certification testing, as well as maintain compliance with periodic testing.
The FCC hasn’t indicated how often ongoing testing should be performed, but it seems reasonable to assume that every few years would be sufficient for such maintenance. Since interference with Government users requires a response (or shutdown) within 24 hours and interference with other licensed services require mitigation “within a reasonable time,” some capability for the investigation of interference may need to be retained. Service organizations are available to assist power providers in this area.
Figure 7: Example of In-House BPL Measurements
Conclusions BPL is being widely deployed and is probably here to stay in some form. Broadband over Power Lines is a new technology that will help the United States accelerate broadband penetration. Currently, the US is rated #11 in the World in broadband deployment. BPL should improve this situation.
Because of widespread demand for Entertainment, VoIP telephony and Video-On-Demand products, the growth potential is good for power providers who widen their offerings to include BPL. The benefits of advanced utility applications are also driving recent deployments. On the downside is the need to prevent widespread interference. This balancing of issues lead to the FCC’s careful deliberations, and its generation of detailed and thorough BPL system qualification and operating requirements. g
References 1. Broadband over Power Line 2004: Technology and Prospects, Electric Power Research Institute, An EPRI White Paper, http://www.epriweb.com/public/000000000001011264.pdf
2. The National Association of Regulatory Utility Commissioners (NARUC) report from The Broadband over Power Lines (BPL) Task Force, February, 2005, http://www.naruc.org/associations/1773/files/bplreport_0205.pdf
3. NTIA Report 04-413, Potential Interference from Broadband Over Power Lines (BPL) Systems to Federal Government Radiocommunications at 1.7 – 80 MHz, Phase 1 Study; National Telecommunications and Information Administration, U.S. Department of Commerce, April 2004, http://www.ntia.doc.gov/ntiahome/fccfilings/2004/bpl/
4. American Radio Relay League, Ed Hare, in private communications with the author, April, 2005, w1rfi@arrl.org
5. Federal Communications Commission FCC 04-245 Report and Order, In the Matter of Amendment of Part 15 regarding new requirements and measurement guidelines for Access Broadband over Power Line Systems, Adopted October 14, 2004, http://hraunfoss.fcc.gov/edocs_public/attachmatch/FCC-04-245A1.pdf
6. United Power Line Council, updated map of BPL deployment and trials in the US, April, 2005 http://www.uplc.org
7. FCC R&O, ibid., paragraph 113.
About the Author Jerry Ramie is the President of ARC Technical Resources, Inc. which provides training, equipment, systems and services for EMC. Jerry is a Member of the IEEE-EMC Society, the dB Society and is a NARTE-certified EMC technician. He can be reached at http://www.arctechnical.com or at (408) 263-6486. © Copyright 2007 Conformity |
