BPL interference evaluation tool usage notes

This is the documentation to accompany the BPL interference evaluation tool.

An overview of BPL is presented to assist in application of the BPL interference evaluation tool to a specific situation.

The model variables and modelling technique is explained, assumptions detailed and errors analysed.

BPL overview

Broadband over Power Lines is the latest incarnation of attempts to utilise power transmission and distribution infrastructure for electronic communications. Earlier efforts included PLC which was principally used for supervision, telemetry and control, and PLT which included extension to more generalised telecommunications (eg voice channels).

Although BPL offers the opportunity for power industry telemetry and control (eg remote meter reading, dynamic load management etc), it depends on delivering commercial Internet access to subscribers for its economics.

The Good

Broadband over Power Lines (BPL) is a telecommunications technology for transmission (by conduction) of high speed data over existing power lines. The technology could be applied to the "transmission" segment of the power network for long distances, but it is more likely to find deployment in the "distribution" segment which distributes power at the street level, and "in-premises" for delivery right to the end-user's workstation.

The greatest attraction for BPL is to compete with traditional telecommunications technologies for the Customer Access Network (CAN), the so-called "last mile" which accounts for a sizeable portion of the cost of telecommunications services, and is often non-competitive in that a single dominant carrier owns the in-street cable infrastructure (pit and pipe, copper and fibre cables). The promise of BPL to be deliver not only the "last mile" to the customer's premises, but to deliver right to each of the customers workstations is seen as attractive in reducing the cost of cabling within the customer's premises which is an expensive capital cost that is a barrier to take-up and that usually has to be substantially subsidised by the would-be carrier.

The Bad

Power utilities have also enjoyed domination of the market, and non-competitive ownership of in-street infrastructure.

It is no wonder that, at a time when governments focus on the performance on these non-competitive utilities, and when governments try to encourage cost effective and efficient power for industry, commerce and residential users (eg by opening up distribution infratructure to power resellers), that the power utilities would see an attraction in diversification into telecommunications, especially where they can leverage their locked-up infrastructure that is not available to competitors and to bundle Internet access with power to muddy the waters.

The power utilities may be easily seduced by hardware vendors looking desperately for a market for telecommunications equipment when the traditional telcos are travelling slowly and more cautiously after the dramatic downturn of three to five years ago.

The Ugly

Back to the statement "Broadband over Power Lines (BPL) is a telecommunications technology for transmission (by conduction) of high speed data over existing power lines". There are three key items that bear further consideration:

High speed data

Broadband means high speed data. The current market is for data speeds from 256Kb/s to in excess of 10Mb/s, and the "need for speed" grows rapidly. Asymmetric services (where the upstream speed is lower than downstream) are commonly deployed and well suited to residential users browsing the Internet, but are less suited to business, and are likely to be supplemented to reasonably priced symmetric services.

The power lines are a bus architecture, meaning that every subscriber on the bus shares the bus bandwidth. Another example of a bus architecture is the pay TV cables deployed in Sydney and Melbourne.

To be able to deliver megabit rate services to multiple subscribers without contention requires very high speed data on the power line "bus". The power lines are not suitable for raw carriage of very high speed data, the data stream needs to be encoded and modulated on carriers for transmission on the power lines. The proposed modulation schemes utilise radio frequencies up to and including the low VHF frequencies, typically from near to DC up to 80MHz or more.

Transmission by conduction

BPL technologies do not depend on radiation of radio frequency energy for their operation. The designed mode of operation is that of conduction, albeit over conductors that are far from ideal radio frequency transmission lines.

Existing power lines

Minimisation of radiation and immunity to external interference can be achieved by using shielded cables, or unshielded balanced conductors. In the case of balanced conductors, the performance depends critically on how perfectly the balance is achieved and preserved. For example, part of the specification of Cat 5 twisted pair cable for data stipulates that the pairs are twisted, that the twist rate is high enough to give good balance in the proximity of other conductors, and that adjacent pairs are twisted a different rates to further reduce cross talk between pairs. Power lines are far from ideal as balanced lines for radio frequency transmission. They were designed to work adequately at 50Hz to 60Hz, and do not perform well at a million times that frequency. Although Aerial Bundled Conductors (ABC) may perform better at RF than flat open wire configurations, the practice of tapping the distribution lines with single phase, two phase and three phase taps, and the variable terminations of the tee offs, the earthing of the neutral conductor at every consumer's main switchboard, all prevent achievement of any kind of balance.

Figure 1 shows a typical LV distribution pole. Note the lack of symmetry, mix of line characteristic impedance, tees of one or more conductors of the main distribution line. Some of the branches (those connecting to a main switchboard or to a metal street light pole) have the neutral conductor grounded at the far end, unbalancing the cable. By comparison, the multi-pair Cat 5 cable used for a 50Mb/s VDSL has very low radiation due to the careful attention to design of Cat 5 cables as balanced transmission lines.

Figure 1: A typical power pole in a Canberra back yard.
The top tier is typical of LV distribution in Australia, it is a four wire three phase system at 400/230V 50Hz. The neutral wire is typically bonded to the premises earth system at each premises main switchboard in an MEN configuration. There is also a switched active for street lighting. Newer installations tend to use Aerial Bundled Cable (ABC) rather than open wires. Depicted to the right is a single phase tee from the neutral and switched active (1 x open wire) and three three phase tees (3 x ABC) off the open wire distribution. Newer installations would tend to tee only one or two phases to individual premises. The second tier is the Transact advanced digital network which is a Fibre To The Curb (FTTC) architecture. It connects each subscriber premises to a "Hub" site using a dedicated VDSL 50Mb/s ATM stream carried over single Cat 5 copper pair up to 300m to a "Node" and then multiplexed on fibre upstream. The drop cable is 4pr Cat 5 copper. The lowest tier is Telstra's Plain Old Telephone Service (POTS) which is currently used to deliver ADSL services at up to 1.5Mb/s (though ADSL technology is capable of 6 to 8Mb/s). The cable is 25pr and 50pr Cat 3 distribution and single pair drops to the customer premises (though one of the customer drops here is a 10pr).

The poor performance of the power lines as conductors of wide band radio frequency signals and the high inherent noise of the power lines can be offset somewhat by pumping more signal power into the transmitters to provide sufficient signal to offset noise at the distance receiver.

Radio frequency energy is radiated by BPL systems not as a functional requirement, but as a result of cavalier conducted transmission on unsuitable media that just pollutes the radio spectrum resource used by others (including competitors).

The role of standards

At this point in time in Australia, licenced radio communications services are protected from interference by the Radiocommunications Act 1992 (Cth), including s192, s193, s194, s197.

Since BPL technologies do not depend on radiation of radio frequency energy for their operation, it is doubtful if any regulatory change is need to enable implementation... except that it is unlikely that current BPL technology could be deployed without setting aside the interference provisions of the Radiocommunications Act 1992 (Cth).

The introduction of a standard such as the draft CENELEC standard "Electromagnetic emissions from access powerline communications networks" and its enabling legislation / regulation might well override the existing protection enjoyed by licensed radio communications services from interference where the source of the interference complies with that standard.

History indicates that we might reasonably expect pressure on the regulators by industry (equipment vendors and power utilities) to make the regulatory changes necessary to give them shelter from the existing interference provisions in the law.

This calculator allows modelling of scenarios of the maximum permitted field strength permitted under the draft CENELEC standard "Electromagnetic emissions from access powerline communications networks" for the purpose of obtaining an understanding of the likely impact of introduction of such a regime.

If a standard of this nature was introduced, it may subtly, but effectively nullify existing legislated anti-interference provisions that protect all uses of radio communications.

The BPL interference evaluation tool

This calculator is designed to model the impact of permitted radio interference resulting from a BPL systems that complies with the April 2004 voting draft of proposed European standard CLC/prTS 50437 entitled "Electromagnetic emissions from access powerline communications networks".

The proposed standard sets the maximum permissible field strength, generally at 3m from the network cable. The following tables show the maximum permitted radiated fields under the draft standard.

Table 1 – Limits of radiated disturbances from networks below 30 MHz
Frequency range MHz	Field strength limits dB (µA/m)		Reference measurement distance m	Measurement bandwidth kHz
Frequency range MHz	Quasi-peak	Average	Reference measurement distance m	Measurement bandwidth kHz
0,15 to 0,5	14 to 4	4 to -6	3	9
0,5 to 30	4	-6	3	9
NOTE In the frequency range 0,15 MHz to 0,5 MHz, the limit decreases linearly with the logarithm of frequency.

This calculator extrapolates the maximum permitted field strength and calculates the likely strength of the interference signal at a user's radio receiver system.

Input values / formats

Limits on range of valid data are shown adjacent to the input field label, ** means a large number.

The input fields may support flexible input format. In general, the formats supported include traditional floating point number (50.00), scientific notation (5.05E1), in combination with qualifiers decibels (-3dB), percentage (50%), per unit (0.5pu).

Model Variables

Title

Title for documentation purposes only, appears as a sub heading on the results form.

Frequency

The frequency in MHz. The model implements the reference technique for frequencies from 0.15MHz to 1GHz.

Rx Antenna Gain

Antenna gain is the gain of the antenna in the direction of interest (usually the main lobe of the antenna pattern) above an isotropic radiator. Antennea gain is often expressed with reference to a dipole (dBd), in which case 2.14dB needs to be added to obtain the isotropic gain (dBi).

Rx Bandwidth

Rx bandwidth is used for calculating receiver noise floor, and received signal strength in the "spread" scenario.

Rx Noise Figure

If you do not know the Noise Figure for your receiver system, you should be able to find your receiver sensitivity and bandwidth in the specifications section of your manual. Figure 2 plots Noise Figure against Sensitivity for a range of receiver bandwidths for a linear receiver.

In the absence of any information, an SSB/CW HF receiver will probably have a Noise Figure around 7dB, and an outstanding VHF receiver may be as low as 1dB.

Rx IMD

Rx Inter Modulation Distortion (IMD) is used to calculate the strength of "in-band" interference products that result from "out-of-band" interference signals as a result of imperfections in practical receiver linearity. Most (but not all) amateur transceivers achive IMD of better than -60dB.

Antenna distance from source

The antenna distance is is specified in metres (1 metre = 3.28 feet). It is used to extrapolate the power density at the antenna location from the measurement point specified in the standard. The model (in keeping with the standard) assumes a point source radiator and inverse square law, but this may underestimate the power density (and consequently, the received signal strength) where the radiator is a line source (such as relatively long power lines) rather than a point source.

Calculated results

Scenarios

The impact of intermodulation products is much lower than the direct products, but may still be troublesome and if so, indicates that proposed techniques such as notching (where priority frequency bands were notched out of the spectrum used by the BPL system) would not as effective as purported by the proponents of BPL.

Spectral distribution of BPL interference

The proposed standards sets the maximum permitted field strength measured in a designated bandwidth at a frequency. The measurement bandwidth is 9kHz below 30MHz, and 120kHz above 30MHz. The BPL interference within the measurement bandwidth could range from a single discrete relatively narrow band signal to a continuous "white noise" distribution. The result table columns "Discrete" and "Continuous" show calculated values for those two cases as broad limits for the likely impact of real modulated signals. There is no guarantee that a BPL system would operate at either end of this continuum, and may in fact operate over the range depending on load.

Rx interference - Watts

This is the maximum BPL interference power at the receiver terminals expressed in Watts for the entire scenario.

Rx interference - dBW

This is the maximum BPL interference power at the receiver terminals expressed in dBW (or dB relative to 1 Watt) for the entire scenario.

Rx interference - µV in 50 ohms

This is the maximum BPL interference voltage at the receiver terminals expressed in µV for the entire scenario.

Rx interference - S meter

This is the maximum BPL interference power at the receiver terminals expressed in S Meter units (50µV in 50 ohms is S9, 6dB per S point) for the entire scenario.

Rx interference - dB above Rx noise

This is the maximum BPL interference power at the receiver terminals expressed in dB above Rx noise (or minimum discernable signal) for the entire scenario.

Assumptions

The BPL interference power at the receiver terminals is calculated in line with Friis' formula for path loss, where the received power is calculated as the power density at receiver antenna times the capture area of the receive antenna, and that the ratio of power density at the receive antenna wrt the permitted power density at the reference measurement distance is inversely proportional to the square of the ratio of the distances.

Errors

This model assumes a point source radiator, and plane wave conditions. This implies that the power density at any point can be calculated from either the electric field strength or the magnetic field strength using the characteristic impedance of free space, and that the ratio of power density at two distances is inversely proportional to the square of the ratio of the distances. The reality is that power lines will be relatively line source radiators in most situations, and the assumption of plane wave conditions introduces some error in the near field zone (less than about a quarter wave from the radiator).

The model does not specifically consider multiple paths (eg reflections from ground) which may result in an underestimate of the strength of the interference, depending on the extent to which such multipathing contributes to measurement of the BPL system during compliance checking.

The model does estimate the impact of internal receiver intermodulation on BPL interference, but does not consider external intermodulation sources, and in so doing underestimates the impact of intermodulation products in a general sense.

The assumption of free space losses (ie no atmospheric losses) and lossless receive antenna and feedline would result in an overestimate of the strength of the BPL interference, but the magnitude in a good implementation would be less than 1dB.

The model is more likely to underestimate the strength of interference than to overestimate, and the extent of the total error is likely to be less than 10dB at distances of less than 10m, and become less with increasing distance.

The uncertainty due to estimated errors will in most cases not significantly affect the outcome.

Implementation

For the technically minded, the calculator is written in a combination of HTML, PHP, and Javascript, using Microsoft Frontpage as the development tool.

PHP is a server side scripting tool (or Dynamic HTML). Most of the smarts are in PHP, and you won't see them directly.

Javascript is used to a very limited extent because of the lack of support for recent Javascript features in IE, especially dynamic objects and string objects. It is mainly used for initialisation of the form and linking transactions in a session-less pseudo-conversational style.

Glossary

Term	Meaning
BPL	Broadband over Power Lines, a technique for high speed data transmission over power infrastructure
Interference	From the Radiocommunications Act 1992 (Cth), interference means: (a) in relation to radiocommunications - interference to, or with, radiocommunications that is attributable, whether wholly or partly and whether directly or indirectly, to an emission of electromagnetic energy by a device; or (b) in relation to the uses or functions of devices-interference to, or with, those uses or functions that is attributable, whether wholly or partly and whether directly or indirectly, to an emission of electromagnetic energy by a device.
PLC	Power Line Communications
PLT	Power Line Telecommunications

Disclaimer

Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.