# SECTION 2 TECHNICAL DESCRIPTION OF BPL SYSTEMS

## INTRODUCTION

Access BPL equipment consists of injectors (also known as
concentrators), repeaters, and extractors. BPL injectors are tied to the
Internet backbone via fiber or T1 lines and interface to the Medium
Voltage (MV) power lines feeding the BPL service area.[^1] MV power
lines may be overhead on utility poles or underground in buried conduit.
Overhead wiring is attached to utility poles that are typically 10
meters above the ground. Three phase wiring generally comprises an MV
distribution circuit running from a substation, and these wires may be
physically oriented on the utility pole in a number of configurations
(*e.g.*, horizontal, vertical, or triangular). This physical orientation
may change from one pole to the next. One or more phase lines may branch
out from the three phase lines to serve a number of customers. A
grounded neutral conductor is generally located below the phase
conductors and runs between distribution transformers that provide Low
Voltage (LV) electric power for customer use. In theory, BPL signals may
be injected onto MV power lines between two phase conductors, between a
phase conductor and the neutral conductor, or onto a single phase or
neutral conductor.

Extractors provide the interface between the MV power lines carrying BPL
signals and the households within the service area. BPL extractors are
usually located at each LV distribution transformer feeding a group of
homes. Some extractors boost BPL signal strength sufficiently to allow
transmission through LV transformers and others relay the BPL signal
around the transformers via couplers on the proximate MV and LV power
lines. Other kinds of extractors interface with non-BPL devices (*e.g.*,
WiFi™) that extend the BPL network to the customers' premises.

For long runs of MV power lines, signal attenuation or distortion
through the power line may lead BPL service providers to employ
repeaters to maintain the required BPL signal strength and fidelity.
Figure 2-1 illustrates the basic BPL system, which can be deployed in
cell-like fashion over a large area served by existing MV power lines.

![](media/image1.wmf){width="5.0in" height="3.2979166666666666in"}Figure
2-1: Basic BPL System

## BPL System Architectures

NTIA identified three different network architectures used by BPL
equipment vendors. These architectures are described below.

### System #1

System #1 (see Figure 2-2) employs Orthogonal Frequency Division
Multiplexing (OFDM) to distribute the BPL signal over a wide bandwidth
using many narrow-band sub-carriers. At the BPL injector, data from the
Internet backbone is converted into the OFDM signal format and is then
coupled onto one phase of the MV power line. An injector also converts
BPL signals on the MV power lines to the format used at the Internet
backbone connection. The two-way data are transferred to and from the LV
lines, each feeding a cluster of homes, using BPL extractors to bypass
the LV distribution transformers. The extractor routes data and converts
between access and in-house BPL signal formats. The subscribers access
this BPL signal using in-house BPL devices. To span large distances
between a BPL injector and the extractors it serves, repeaters may be
employed.

![](media/image2.wmf){width="5.0in" height="3.297222222222222in"}Figure
2-2: BPL System #1

The System #1 injector and extractors share a common frequency band (F1)
on the MV power lines, different than the frequency band (F2) used on
the LV lines by the subscriber's in-house BPL devices. In order to
minimize contention for the channel, Carrier Sense Multiple Access
(CSMA) is used with Collision Avoidance (CA) extensions. This type of
system is designed to accept some amount of co-channel interference
between quasi-independent BPL cells without the use of isolation filters
on the power lines, as all devices on the MV lines operate over the same
frequency band. The BPL signal may be sufficiently tolerant of
co-channel BPL interference to enable implementation of two or three of
these systems independently on adjacent MV power lines.[^2] System #1
couples BPL signals into one phase line.

### System #2

System #2 also uses OFDM as its modulation scheme, but differs from
System #1 in the way it delivers the BPL signal to the subscribers'
homes. Instead of using a device that uses LV power lines, System #2
extracts the BPL signal from the MV power line and converts it into an
IEEE 802.11b WiFi™ signal for a wireless interface to subscribers' home
computers as well as local portable computers (see Figure 2-3).
Technologies other than WiFi™ might also be used to interface to
subscribers' devices with the BPL network, the important point being
that BPL is not used on LV power lines in System #2.

![](media/image3.wmf){width="5.009722222222222in"
height="3.3222222222222224in"}Figure 2-3: BPL System #2

This system uses different radio frequency bands to separate upstream
(from the user) and downstream (to the user) BPL signals, and to
minimize co-channel interference with other nearby access BPL devices.
To span large distances between a BPL injector and the extractors it
serves, repeaters may be employed. Like the injectors, BPL repeaters
transmit and receive on different frequencies, and they use different
frequencies from those used by the injector and other nearby repeaters.
System #2 repeaters may also provide the capabilities of an extractor
when outfitted with a WiFi™ transceiver. System #2 couples BPL signals
onto one phase of the MV power line.

### System #3

System #3 uses Direct Sequence Spread Spectrum (DSSS) to transmit the
BPL data over the MV power lines. All users within a BPL cell share a
common frequency band. In order to minimize contention for the channel,
Carrier Sense Multiple Access (CSMA) is used. Like System #1, this type
of system is designed to accept some amount of co-channel interference
between cells, as all devices operate over the same frequency band. At
one trial deployment of the System #3 architecture, the BPL service
provider independently implements two phases of the same run of three
phase power lines.

Each cell in System #3 (see Figure 2-4) is comprised of a concentrator
(injector) that provides an interface to a T1 or fiber link to the
Internet backbone, a number of repeaters (extractors) to make up for
signal losses in the electric power line and through the distribution
transformers feeding clusters of dwellings, and customer premises BPL
equipment, used to bridge between the user's computer and the electrical
wiring carrying the BPL signal. Adjacent cells typically overlap and the
customers' BPL terminals and repeaters are able to communicate with the
concentrator that affords the best communication path at any time.

![](media/image4.wmf){width="5.0in" height="3.297222222222222in"}Figure
2-4: BPL System #3

System #3 couples the BPL signal onto the power line using a pair of
couplers on a phase and neutral line.

## POTENTIAL FUTURE SYSTEMS

BPL manufacturers and service providers anticipate a wide range of
applications that may be offered to their subscribers. High quality,
multi-channel video, audio, voice over Internet Protocol (VoIP), and
on-line gaming applications are expected to rapidly increase the demand
for additional bandwidth.[^3] To support the typical subscriber at 1
Mbps, BPL systems are expected to operate at speeds of 100 Mbps or more
on the MV power lines in the near future. A number of comments filed in
response to the NOI indicate that the BPL industry is already preparing
for this growth.[^4]

A number of BPL vendors have suggested use of frequencies up to 50
MHz.[^5] At least one vendor is considering use of 4 MHz to 130 MHz,
while excluding frequencies that are actively in use by certain licensed
services.[^6] One solution put forward in an attempt to mitigate
interference with licensed services is to attenuate or "notch" BPL
signals in frequency bands where licensed services are in nearby
use.[^7] Future BPL systems may be able to accomplish this automatically
without system operator intervention. To implement this solution while
simultaneously maximizing the useable bandwidth, BPL systems are
expected to use new modulations that can support more sub-carriers that
are more finely spaced.[^8]

As data rates and bandwidth requirements grow, the BPL systems may
require operation at greater transmitted power levels but not
necessarily with higher power density than is used today. BPL vendors
may employ techniques to dynamically adjust the power level to maintain
a minimum signal-to-noise ratio (SNR) over the entire BPL spectrum,
while limiting emissions to levels compliant with Part 15. One vendor
has proposed such a solution for adjusting transmitted power to maintain
a constant SNR across the BPL spectrum, with a hard limit based on Part
15 rules.[^9] The challenge will be to develop the control mechanism
that can maximize transmitted power while simultaneously limiting the
radiated emissions, perhaps in conjunction with frequency agility.

Nortel has developed and patented a filter that blocks BPL signals while
concurrently passing medium-voltage AC power. The judicious use of such
blocking filters will enable optimal segmentation of BPL networks into
cells of various sizes having low conducted co-channel interference from
neighboring cells. This will enable a greater level of frequency reuse
than what is currently available.[^10]

Another BPL technology utilizes the 2.4 GHz and 5.8 GHz unlicensed
bands.[^11] An implementation using multiple IEEE 802.11b/g WiFi™ chips
sets has been used to demonstrate the concept of carrying data over
medium-voltage power lines at rates exceeding 200 Mbps. However, no
party filed comments contending that this technology and these
frequencies should be considered in the BPL proceedings.

## SUMMARY

Three architectures for access BPL networks were identified: (1) BPL
systems using different frequencies on medium- and low-voltage power
lines for networking within a neighborhood and extensions to users'
premises, respectively; (2) BPL use of only medium voltage lines for
networking within a neighborhood, with other technologies being used for
network extensions to users' premises; and (3) BPL use of the same
frequencies on medium- and low-voltage power lines for networking in a
neighborhood and extensions to users' premises. BPL systems currently
provide data rates ranging from 1 Mbps to 10 Mbps, and operate at
frequencies between 2 MHz and 50 MHz. In the future, BPL systems will
operate at data rates exceeding 100 Mbps, utilizing greater bandwidth
and/or advanced modulation schemes. BPL equipment vendors may employ
additional signal processing techniques to optimize performance while
simultaneously limiting emissions to Part 15 levels.

[^1]: MV lines, typically carrying 1,000 to 40,000 volts, bring power
    from an electrical substation to a residential neighborhood. Low
    Voltage distribution transformers step down the line voltage to
    220/110 volts for residential use. *See* BPL Inquiry at ¶ 13.

[^2]: A degree of coupling occurs between BPL signals on adjacent
    phases.

[^3]: Comments of HomePlug Powerline Alliance, BPL Inquiry, July 7, 2003
    ("HPA Comments") at 3-5.

[^4]: Comments of PowerWAN, Inc., BPL Inquiry, July 3, 2003, ("PowerWAN
    Comments") at 2; Comments of Main.net Communications, Ltd., BPL
    Inquiry, July 7, 2003 ("Main.net Comments") at 3.

[^5]: PowerWAN Comments at 2; Comments of Amperion, Inc., BPL Inquiry,
    July 7, 2003, ("Amperion Comments") at 4.

[^6]: Reply Comments of PowerComm Systems, Inc., BPL Inquiry, August 20,
    2003, ("PowerComm Reply Comments") at 14.

[^7]: Main.Net Comments at 7; Comments of Ambient Corporation, BPL
    Inquiry, July 7, 2003, ("Ambient Comments") at 8; PowerWAN Comments
    at 3.

[^8]: PowerComm Reply Comments at 17.

[^9]: Ambient Comments at 6-7.

[^10]: *System, device, and method for isolating signaling environments
    in a power line communication system*, United States Patent No.
    6,590,493, Rasimas, et al., July 8, 2003.

[^11]: *Corridor Systems Announces Breakthrough Technology For Broadband
    Over Powerlines (BPL), Demonstrates 216Mbps over PG&E's
    Medium-Voltage Grid*, Santa Rosa, CA, September 22, 2003
    <http://www.corridor.biz/0309-corridor-pr.pdf>.