Sunday, April 25, 2010

broadband over power lines

Abstract:
This paper describes the current broadband over power-line or BPL system. For many years now researchers and electric companies have tried to find a reliable method for passing data over the power lines. The one key advantage of doing this is that power lines are running to nearly every home. This would enable people in rural areas to obtain broadband access, which they previously couldn’t from services such as cable modems and DSL lines.
Broadband over the power lines sounds promising but there is a significant problem of interference with this technology. BPL emits radiation from the power lines at an unlicensed frequency of 4MHz to 30MHz and this is the same frequency in which amateur radio is broadcast at, creating interference. This issue will be addressed along with the advantages and disadvantages of the technology.



Introduction:
Today's information age is driven by the need for data about productivity, inventory, materials, and performance. Fact-based decision-making reduces uncertainty, provides insights into complex processes and investment opportunities, and adds structure to our knowledge base. The more intelligence we can glean from processes and events help us to manage our careers, business and daily lives.
The need for information extends from boardrooms and seats of government to production floor supervisors that run factories, generate energy, transport resources, and support our daily lives. Indeed, the broad spectrum of everyday devices and machines has the potential to yield a wealth of knowledge that can be utilized if it can be accessed. The most common way of getting information to users via the Internet is for large ISPs to lease fiber-optic lines from the phone company to carry data around the Internet and eventually to another medium (phone, DSL or cable line) and into homes. Trillions of bytes of data a day are transferred on fiber-optic lines because they are a stable way to transmit data without interfering with other types of transmissions.
The idea of using AC (alternating current) power to transfer data is not new. By bundling radio-frequency (RF) energy on the same line with an electric current, data can be transmitted without the need for a separate data line. Because the electric current and RF vibrate at different frequencies, there is no interfering with each other. Electric companies have used this technology for years to monitor the performance of power grids. There are even networking solutions available today that transfer data using the electrical wiring in a home or business. But this data is fairly simple and the transmission speed is relatively slow.

ABOUT DIGITAL COMMUNICATION

System Model


Figure 1 shows a simplified model of a digital communication system. The objective of the communication system is to communicate digital information (a sequence of binary information digits) over a noisy channel at as high bit rates as possible. The data to be transmitted could origin from any source of information. In case the information is an
analog signal (speech) then an A/D converter must precede the transmitter.

FIGURE 1: A model of a digital communication system.


The source encoder outputs data that are to be transmitted over the channel at a certain
Information bit rate, Rb. Bit error probability, Pb, is a measure of performance and defined as the probability that a bit is incorrectly received at the destination.

Source Coding

Most data contains redundancy, which makes it possible to compress the data. This is
done by the source encoder and minimizes the amount of bits transmitted over the
channel. At the receiver the source decoder unpacks the data to either an exact replica of
the source (loss less data compression) or a distorted version (loss data compression). If
the received sequence does not have to be an exact copy of the transmitted stream then the degree of compression can be increased
.
Channel Coding

In order to reduce the bit error probability the channel encoder adds redundancy (extra
control bits) to the bit sequence in a controlled way. When an error appears in the bit
stream the extra information may be used by the channel decoder, to detect, and possibly
correct, the error. The redundancy added is depending on the amount of correction
needed but is also tuned to the characteristics of the channel
Two coding techniques often used are block codes and convolution codes
Modulator

The modulator produces an information-carrying signal, propagating over the channel. At this stage the data is converted from a stream of bits into an analog signal that the channel can handle. The modulator has a set of analog waveforms at its disposal and maps a certain waveform to a binary digit or a sequence of digits. At the receiver, the demodulator tries to detect which waveform was transmitted, and convert the analog
information back to a sequence of bits.
Several modulation techniques exists,
e.g., spread-spectrum, OFDM (Orthogonal Frequency Division Multiplex),
GMSK (Gaussian Minimum Shift Keying),
FSK (Frequency Shift Keying), PSK (Phase Shift Keying) .

Channel

The channel might be any physical medium, such as coaxial cable, air, water or telephone wires. It is important to know the characteristics of the channel, such as the attenuation and the noise level, because these parameters directly affect the performance of the communication system

Bandwidth

The frequency content of the information-carrying signal is of great importance. The frequency interval used by the communication system is called bandwidth. For a specific communication method, the bandwidth needed is proportional to the bit rate.
Thus a higher bit rate needs a larger bandwidth for a fixed method. If the bandwidth is doubled then the bit rate is also doubled. In today’s environment bandwidth is a limited and precious resource and the bandwidth is often constrained to a certain small interval. This puts a restriction on the communication system to communicate within the assigned bandwidth. An advanced telephone modem can achieve a bit rate of 56.6 kb/s using a bandwidth of 4 kHz and the bandwidth efficiency is 14.15 b/s/Hz. A meter reading system for the power-line channel that has a bit rate of 10 kb/s and communicates within the CENELEC A band has a bandwidth efficiency of 0.11 b/s/Hz, thus the performance of the telephone modem is much higher.

Diversity

To reduce the error probability of harsh channels, diversity techniques may be used.
Examples are time diversity and frequency diversity
In time diversity the same information is transmitted more than once at several different time instants. If the channel is bad at some time instant the information might pass through at some other time when the channel is good (or better). This is especially useful on time-varying channels.
Frequency diversity transmits the same information at different locations in the frequency domain. It can be compared to having two antennas transmitting at different frequencies, if one of them fails the other might work


What is BPL?
Broadband over Power Lines (BPL), also known as Power Line Communications (PLC) is a quickly evolving market that utilizes electricity power lines for the high-speed transmission of data services. The new low-power, unlicensed BPL systems couple radio frequency energy onto the existing electric power lines to provide high-speed communications capabilities (FCC, 2004).
Broadband over Power Lines transmit high frequency data signals through the same power cable network used for carrying electricity power to household users. In other words, power lines are just one component of electric companies' power grids.
Furthermore, power grids use generators, substations, transformers and other distributors that carry electricity from the power plant all the way to a plug in the wall. When power leaves the power plant, it hits a conduction substation and is then distributed to high-voltage transmission lines.

Why BPL?
By providing high-speed data transmission between all of the electrical plugs in a house, there is the potential to network all kinds of common appliances in a household. The capacities to improve, decrease of costs are crucial advantages relate to broadband power lines.
Broadband power lines allow simple phase of deployment in comparison to other broadband technology; cost-efficient and low risk are other vital attributes of broadband power lines. In relation to scalability and flexibility, broadband power lines provide both due to avoiding stranded investment and relatively stable platform respectively.
BPL is a broadband delivery service that could be accessible to everyone with constant electricity supply. Currently residents in rural communities are not able to obtain broadband services, because they are too far to be reached by cable or DSL access.
However, since power lines are in nearly every place in the nation, it would be easy to reach these rural communities through a medium that was already installed such as the power lines. BPL opens an entirely new market of customers to broadband access and capabilities. This is one of the driving forces between the BPL industry and voltage transmission lines.


How does it work?
Broadband over Power Lines (BPL), also known as Power Line Communications, is a rapidly evolving market that provides high speed broadband data communications at low cost. BPL systems couple radio frequency (RF) data signals onto the existing electric power lines to provide high speed data communication. The high frequency (1MHz – 30 MHz) data signals are transmitted through the same power lines that carry low frequency electricity (50 or 60 Hz) to household or business. This enables both the signals to coexist on the same wire.
BPL provides effective data communication through the combination of the electric network within the home or office, the power distribution grid, and the backbone network which transfers the data signal from the Internet Service Provider (ISP) onto the power lines. One of the primary advantages of BPL is that most of the last mile infrastructure is in place. The last mile refers to the final segment of delivering communications connectivity to the home or office. BPL system takes advantage of one of the largest and the most pervasive networks on the earth, the power grid. Power lines pass within 100 meters of almost every home or building in the United States. BPL offers to the residential and the business customers not only voice, video, and data services but also other incentives such as mapping and home management abilities that work more reliably and faster than in the past.

Components of BPL
The broadband signals have to follow the same path as that of the power lines: the transmitter or the generator, the transmission medium i.e. the power lines, the transformers, the substations and then into household. Three different ways to feed broadband to consumers using BPL. With the coupler, data can move easily from the 7,200-volt line to the 240-volt line and into the house without any degradation. At the transformer (which is nearest to the consumer) one of three things happens
The first option is to “blast-through” the transformer. In this case, there is no additional electronics required to connect consumer homes to the broadband service. The end user merely needs some type of device capable of interacting with the broadband being supplied through the electricity lines. The drawback of using this approach is that existing transformers limit the usable bandwidth delivered to the home.
The second option is called “bypass” which involves a type of bridging device that is co-located near the transformer. This bypass can re-route broadband signals around the transformer and deliver increased bandwidth to each endpoint. Connectivity between the bypass and the end user could be coax, fiber, twisted pair, or even the power lines

The third option is to use something called a “wireless step off”. This option allows the utility to create a wireless hotspot near each transformer. Depending on the strength of the wireless transmitter within the step off, this method can effectively create a mesh wireless network the power lines
Hundreds of thousands of volts of electricity don't vibrate at a consistent frequency. That amount of power jumps all over the spectrum. As it spikes and hums along, it creates all kinds of interference. If it spikes at a frequency that is the same as the RF used to transmit data, then it will cancel out that signal and the data transmission will be dropped or damaged en route.
BPL bypasses this problem by avoiding high-voltage power lines all together. The actual delivery of BPL starts by interconnecting all the substations (many of which already have some type of fiber interconnecting them). At the substation, broadband is injected or coupled onto the circuits feeding the into the much more manageable 7,200 volts of medium-voltage power lines. Once dropped on the medium-voltage lines, the data can only travel so far before it degrades
Repeaters take in the data and repeat it in a new transmission, amplifying it for the next leg of the journey.
Transformers
The transformer's job is to reduce the 7,200 volts down to the 240-volt standard that makes up normal household electrical service. There is no way for low-power data signals to pass through a transformer, so you need a coupler to provide a data path around the transformer. capable of interconnecting end users with multiple overlapping wireless service areas.
Types of BPL Systems
Access BPL
Access BPL systems utilize the power distribution network, owned, operated and controlled by an electricity service provider, as the means of broadband delivery to and from premises such as the home or office. Access BPL systems use injectors, repeaters, and extractors to deliver high-speed broadband services to the end-user.
Injectors provide the interface between the Internet backbone and the MV power lines. Once the signal has been injected onto the MV power line, it is extracted to deliver the information to the end-user. Extractors provide the interface between the MV power lines which carry the signals to the customers in the service area. Extractors are generally installed at LV distribution transformers that service groups of homes. Since the BPL signal loses strength as it passes through the LV transformer, extractors are required to retransmit the signal. In other cases, couplers on the MV and LV lines are used to bypass the LV transformers and relay the signal to the end-user. At least one company has designed a third type of extractor, which transmits a wireless signal directly from the MV power line to end-users.
To transmit signals over long distances, repeaters are employed to overcome losses resulting from physical characteristics of the power line.
Access Broadband over Power Line (Access BPL):
A carrier current system installed and operated on an electric utility service as an unintentional radiator that sends radio frequency energy on frequencies between 1.705 MHz and 80 MHz over medium-voltage lines or over low-voltage lines to provide broadband communications and is located on the supply side of the utility service’s points of interconnection with customer premises.
Multiple Formats of Access BPL
The Department is aware of various implementation/deployment architectures of Access BPL systems. However, the Department believes that Access BPL systems can be generally classified as either: (1) an end-to-end system, or (2) a hybrid system.


End-to-End Access BPL:

End-to-end Access BPL systems use either a combination of MV and LV power lines or LV power lines only. These systems represent the classical architectures for Access BPL. In this case the BPL signal is injected onto and carried by the MV power line. The BPL signal is then transferred to the LV power line via couplers or through the LV transformer and delivered directly to the end user.
In the case of LV only BPL systems, the BPL signal is injected onto the LV power line at the transformer or the utility meter.
Hybrid Access BPL
Hybrid systems use a combination of power lines and wireless transmission. For example, a hybrid system may inject a BPL signal onto an MV power line and use a special extractor to translate the signal into a wireless channel, which is delivered, to the end-user.
More recently, a second hybrid system has been developed. These systems capture wireless signals and inject them directly onto the LV power line. The signal is distributed using the LV power line and in house wiring to the end-user.
the hybrid Access BPL system uses repeaters and extractors which are capable of transmitting and receiving wireless signals to and from end-users.
In-house BPL
In-house BPL systems utilize electric power lines not owned, operated or controlled by an electricity service provider, such as the electric wiring in a privately owned building. Broadband devices are connected to the in-building wiring and use electrical sockets as access points (see Figure 1).
In-house BPL technologies are largely designed to provide short-distance communication solutions, which compete with other in-home interconnection technologies. Product applications include networking and sharing common resources such as printers.

Advantages of BPL:
 The biggest advantage of BPL is that the infrastructure is already in place and no change in business or household wiring is necessary to implement the BPL system. As a result, the BPL services can be rendered at a faster rate and the amounts of capital expenditures required are reduced.
 BPL service is a cost-effective way to reach rural customers that do not have access to cable or DSL. BPL services are offered to the customers at lower prices than DSL or cable.
 BPL enables networking machines within a building.
 It provides broadband connection in every socket in every room making it possible to network all kinds of common appliances in a household. .
 BPL offers to the residential and the business customers not only voice, video, and data services but also others such as mapping and home management abilities.
 From the government’s point of view, BPL increases national security. Wide scale BPL would provide another layer of redundancy for communications systems and allow more careful monitoring of the power grid.

Interference: The biggest potential hurdle for BPL appears to be interference. Ham radio operators have claimed that BPL makes it difficult to impossible to operate their devices. Recently, however, this issue has subsided somewhat, as the FCC (Federal Communications Commission in U.S) has endorsed BPL and is (establishing testing requirements for the equipment .The problem with interference is multi-faceted and to some extent difficult to fully comprehend. A very low powered signal can propagate hundreds or thousands of miles. The power lines are not designed for data transmission at high frequency, thus they are not shielded. When data is transmitted at high frequencies there is a loss of signal or leakage of signal through the wires because they are not shielded. This again results in signal loss.
According to radio amateurs and some broadcasters, BPL is said to be a polluter of the radio spectrum, causing a large rise in the `noise floor' in urban areas akin to "radio smog". BPL providers are basically hedging their bets that they can coexist in HF spectrum by tweaking their systems to not use frequencies that aren't in use locally. The assumption is that they won't interfere locally with anyone, but due to the propagation characteristics of HF, it could cause likely long distance interference or an increased noise floor across the HF bands.
Sharing bandwidth might be a problem because the available bandwidth over the wires is fixed, thus as many users connect to the Internet, the bandwidth will be split among all of them. This indicates that the more number of users, the less bandwidth will be made available for each of the users.

BANDWIDTH:

As bandwidth is proportional to the bit rate, thus a large bandwidth is needed in order to communicate with high bit rates. The bandwidth standards only allow frequencies between 3 kHz and 148.5 kHz. This puts a hard restriction on power-line communications and might not be enough to support high bit rate applications, such as real-time video, depending on the performance needed.
Range is subdivided into five sub-bands. The first two bands (3-9 and 9-95 kHz) are limited to energy providers and the other three are limited to the customers of the energy providers. In addition to specifying the allowed bandwidth the standard also limits the power output at the transmitter. In order to increase the bit rate, larger bandwidth may be needed. Recent research has suggested the use of frequencies in the interval between 1 and 20 MHz [4], [5], [20]. If this range could be used it would make an enormous increase in bandwidth and would perhaps allow high bit rate applications on the power-line. An important problem is that parts of this frequency band is assigned to other communication system and must not be disturbed. Other communication systems using these frequencies might also disturb the communication on the power-line. Examples of communication systems in this interval are broadcast radio, amateur radio and airplane navigating.

Radiation of the Transmitted Signal

When transmitting a signal on the power-line the signal is radiated in the air. One can think of the power-line as a huge antenna, receiving signals and transmitting signals. It is important that the signal radiated from the power-line does not interfere with other communication systems. When using the frequency interval 1-20 MHz for communication the radiation is extremely important because many other radio applications are assigned in this frequency interval. It is not appropriate for a system to interfere with, e.g., airplane navigation or broadcast systems. It has been observed that this problem and tries to set up a maximum power level of transmission. It’s the radiation from the households that makes the major contribution. Wires inside households are not shielded and thus radiate heavily. A solution might be to use filters to block the communication signal from entering the household


Normally, at conventional communication, impedance matching is attempted, such as the use of 50-ohm cables and 50-ohm transceivers. The power-line network is not matched.
The input (and output) impedance varies in time, with different loads and location. It can be as low as milliohms and as high as several thousands of Ohms and is especially low at the sub station. Except the access impedance several other impedance mismatches might occur in the power-line channel. E.g., cable-boxes do not match the cables and hence the signal gets attenuated.
By the use of filters stabilizing the network. The cost of these filters might be high and they must be installed in every household and perhaps also in every cable-box.

The Time-variant Behavior of the Grid

The noise level and the attenuation depend partly on the set of connected loads, which varies in time. A channel, which is time-variant, complicates the design of a communication system. At some time instants the communication might work well but at other times a strong noise source could be inherent on the channel, thus blocking the communication. At any time the characteristics of the channel are estimated, e.g., through measurements, and the effect is evaluated to make a better decision. The cost of this is higher complexity.

conclusion
In spite of the proliferation of broadband technology in the last few years, there are still huge parts of the world that don't have access to high-speed Internet. When weighed against the relatively small number of customers Internet providers would gain with the cost of laying cable and building the necessary infrastructure to provide DSL or cable in rural areas too great, BPL comes as a ready incentive since they could be served through power lines. The existence of BPL would be cost effective with no need to build new infrastructures. Anywhere there is electricity there could be broadband. It would also be interesting to have a ‘smart house’ with all the appliances functioning automatically

Bibliography:

 M. Arzberger, K. Dostert, T. Waldeck, M. Zimmermann, "Fundamental Properties of the Low Voltage Power Distribution Grid", Proc. 1997 International Symposium on Power-line Communications and its Applications", Essen, Germany, 1997.

 A.G. Burr, D.M.W. Reed, P.A. Brown, "HF Broadcast Interference on LV Mains Distribution Networks", Proc. 1998 International Symposium on “Power-line Communications and its Applications", Tokyo, Japan, 1998.

 CENELEC, ", Signaling on low-voltage electrical installations in the frequency range 3 kHz to 148.5 kHz".

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