Design of an Analog Electronic Interface for a Power
Line Based Telephony System
Alejandro Barreras Gutierrez
Audley Darmand
University of Technology, School of Engineering
Department of Electrical Engineering
Hope Road, Kingston 6, Jamaica
agutierrez@utech.edu.jm
University of Technology, School of Engineering
Department of Electrical Engineering
Hope Road, Kingston 6, Jamaica
adarmand@utech.edu.jm
Victor Watt
Lucien Ngalamou
University of Technology, School of Engineering
Department of Electrical Engineering
Hope Road, Kingston 6, Jamaica
University of the West Indies, Department of Physics
Electronics and Computer Systems Group
Mona Campus, Kingston 7, Jamaica
lngalamou@eng.uwi.tt
vwatt@utech.edu.jm
on hold among others [13]; The other element is a
multifunction power line-to-phone set transceiver which, by
design, will allow the standard digital phone sets available in
the market to interface with the Private Automatic Branch
Exchange (PABX) board. The PABX board communicates
with each user so each user must have a user phone set
transceiver board. Figure 1 shows the system configuration.
Abstract— This paper considers the use of in-building electrical
wiring in as the communication channel for the deployment of
telephony system for homes and small businesses in Jamaica. The
paper presents the design considerations and implementation of
the analog electronic interface to the power line infrastructure
and presents results from actual powerline channel
measurements in a laboratory setting as well as in an existing inbuilding electrical wiring application.. The paper also presents
some considerations for the integration of the powerline
communication system with existing telephony standards,
ranging for standard analog phone sets to the emerging Voice
Over IP (VOIP) standards such as the Session Initiation Protocol
(SIP).
Keywords: Power Line Communications, Multi-Carrier Code
Division Multiple Access, Private Automatic Branch Exchange.
I.
INTRODUCTION
Communication by telephone for many small companies is
a medium by which they interact with their customers and staff.
These companies use small telephony systems for their daily
activity, but most of the systems that are available are designed
to accommodate large numbers of users and they are extremely
expensive. This system presents a solution to those who are
developing small companies and businesses around the world
and are looking for a low cost and reliable way to
communicate.
Figure 1. System Configuration
The design of the entire system is conceived as a
combination of three different sub-systems, due to the
complexity and the extension of the system design. The block
diagram in Figure 2 shows the system design approach divided
into the three different sub-systems.
The system can be divided in two major elements: the first
element is a Peripheral Component Interconnected (PCIPABX) module, designed for installation in a PCI slot of a PC.
It includes an analog unit to interface with the Public Switched
Telephony Network (PSTN) and a Signal Processing Module,
which will perform the features of a standard switchboard, plus
other specific functions such as caller identification, call
waiting, call transfer, automatic interactive voice interface,
multiple-user account setting, automatic user billing and music
c
1-4244-0113-5/06/$20.002006
IEEE.
Figure 2. The Communication System Design
232
This paper presents a design solution for the analog
electronic interface of the telephony system.
II.
STUDY AND MEASUREMENT OF THE POWER LINE
Attenuation (dB )
CHANNEL
Power Lines are not designed to transmit data at high
frequencies. High frequency transmission by PLC is noisy.
Usually signals above a few KHz experience severe
unbalanced attenuation and frequency fading exists when
transmitting across power lines [1], [5], [9], [10], [23]. The
attenuation increases dramatically with the transmission
distance and network structure. This includes the distribution of
the connected loads in the network, such as the appliances
plugged into the low power grid or sub-circuits in the high
power topology as well as the time of the day and the day on
which the system is being analyzed [11].
0,0
-0,5
-1,0
-1,5
-2,0
-2,5
-3,0
-3,5
-4,0
-4,5
-5,0
-5,5
-6,0
0
5
10
15
20
25
30
35
Frequency (MHz)
Figure 4. Attenuation characteristic of the Isolated Cable
In order to analyze the attenuation characteristics of a given
power line, a general measurement system was implemented. A
diagram of the entire system is shown in Figure 3.
It was observed that even for the highest frequency values
where the attenuation is higher; a communication system
implementation is still possible. This test does not accurately
represent a real power line communication system.
The second test was run using the real physical power line
network present in the lab represented in Figure 5 bellow which
simulates a typical household with appliances connected.
Figure 3. The Power Line Measurement System
The system is composed of a receiver and a transmitter
coupling properly connected to the communication channel.
The function generator sweeps a signal through the coupling
network from 150 KHz to 30 MHz when measuring the
attenuation parameters.
The experimental test carried out to obtain the attenuation
characteristic of the power line was run twice, first over a set of
wires with absolutely no loads connected, transmitting no
power and similar characteristics to the one used for the
electrical installation and the second over the real physical
network existing in the lab. The signal at the input of the power
line before it was injected into the coupling circuit was set to 4
Volts RMS, recorded, and maintained throughout the test. A
variable gain buffer amplifier is placed at the output of the
function generator to prevent impedance mismatching and the
voltage drop of the generator output signal. The voltage at the
output of the receiver coupling circuit changed while the
frequency was swept from the minimum of 250 KHz to the
maximum value of 30 MHz. This voltage was recorded during
the test as well as the frequency values. The attenuation
characteristic graph for the first case, where no 50 Hz main
power wave was present and no loads where connected to the
network, is shown in Figure 4.
Figure 5. Laboratory Power Line Network Configuration
The diagram above shows the distance between each outlet
and the service panel. The total distance between the
transmitter and the receiver is 29 feet. The obtained attenuation
characteristic is shown on Figure 6. It shows that there are
some significant attenuation notches at certain frequency
values. The location and intensity of these signal fading points
will change depending on where appliances are plugged into
the network and on the topology of the network. This behavior
may be expected for all power line networks around the world.
Recent publications from the developed world reveal similar
attenuation characteristics with just a few differences such as
the overall attenuation level and the positions of the notches
[5], [7],[8],[12].
233
The use of MC-CDMA over OFDM specifically for this
system is justified due to the fact that OFDM applies coding to
avoid an excessive BER on sub-carriers that are in a deep
frequency fading. Thus the number of needed sub-carriers is
larger than the number of bits or symbols transmitted
simultaneously. MC-CDMA replaces this encoder by an N x N
matrix operation. In other words, MC-CDMA offers better
frequency diversity to combat frequency-selective fading, one
of the main issues seen in power lines at high frequencies [18].
In MC-CDMA, the number of sub-carriers is increased,
creating a denser signal transmission. MC-CDMA then offers
grater bandwidth utilization for power lines than OFDM.
Considering the fact of bandwidth limitations evidently present
in power lines for high speed data transmission, it can be
concluded that MC-CDMA is the ideal solution, so far, to
transmit over power lines. The power line channel frequency
response is perfectly acceptable up to 30 MHz, even though
selective fading may be present. A comparison of the overall
performance of MC-CDMA and the OFDM systems with the
same features, shows that MC-CDMA performs better than
OFDM in terms of BER for this frequency range
[14],[15],[16],[17]. Figure 7 shows the scheme structure for the
transmitter and the receiver of MC-CDMA scheme [4],[6].
0,0
Attenuation (dB)
-2,0
-4,0
-6,0
-8,0
-10,0
-12,0
-14,0
0
5
10
15
20
25
30
35
Frequency (MHz)
Figure 6. Attenuation Characteristic of the Real Physical Power Network in
Lab, Showing Notches
The critical points of attenuations and the impedance value
of a power line to be used as high frequency data transmission
channel cannot be predicted because the configuration and the
characteristic of each particular network, is network dependent
and time dependent. The characteristic of the test bed showed a
transmission frequency of 25 MHz is best selected as it is in the
region of least attenuation and is a more stable bandwidth.
Depending on the location where the system is tested and the
connected loads these parameters may change. One way to get
around this complex channel behavior and its effects is through
the proper design of band pass filters and coupling circuits. The
implementation of suitable transmission schemes and good
error correction methods will vastly improve the performance
of the PLC system.
III.
TRANSMISSION SCHEME SELECTION
Both, Orthogonal Frequency Division Multiplexing
(OFDM) and Multi-Carrier Code Division Multiple Access
(MC-CDMA) schemes promise to be suitable to transmit high
data rates in frequency-selective fading channels with multipath effects such as the power line channels. The fact that
OFDM converts the data stream to several parallel streams,
increasing the symbols duration, makes the system resistant to
frequency-selective fading and minimizes the complexity of the
architecture. The use of orthogonal codes and frequencies
decreases to a minimum the Inter-Symbol Interference (ISI)
and thus the Bit Error Rate (BER) at the receiver level [20].
Jamming and interference to the transmitted signal are avoided
by the spreading of the spectrum making the transmitted signal
appearing as background noise to the system. The use MCCDMA increases the efficiency of the bandwidth usage and
also decreases the effects that may cause frequency-selective
fading channels over the transmitted signals when compared
with the use of OFDM [3],[18],[19]. The advantages of
frequency diversity and the capability to easily recover lost
information occurring during the transmission make MCCDMA a very good option for high speed data transmission
over power lines. But this requires a balance among the
channel characteristics, the transmission scheme and the
application of the system being designed.
Figure 7. The MC-CDMA Receiver & Transmitter Architecture
In figure 7, the system converts the original data stream
with a symbol rate R s into P parallel sequences with a new
symbol rate R p equal to the reciprocal of the original symbol
rate
1
, similar to what happens in OFDM, and spreads each
P
obtained sequence over a number, N, of orthogonal sub-carriers
234
using a given spreading code with length N, thus each chip of
the Pseudo-Noise (PN) code modulates one of N orthogonal
sub-carriers. This is done by applying the Inverse Fast Fourier
Transform (IFFT).
 N + 1  G PMC  100 + 1  10
 ×
∆W =  c
=
= 10.1MHz
×
 100  1µS
 N c  Ts
Approximately 10.1 MHz is needed to transmit the
information over the power line channel using MC-CDMA
with a band guard at a rate of 1Mbsp. Therefore, the central
frequency for the transmission scheme is 25 MHz and the
necessary bandwidth goes from 19.95 to 30.05 MHz.
The first stage of the design allows only voice signals to be
transmitted over the power line channel. Voice signals do not
demand very high speeds for transmission. The human voice
spectrum goes from about 300 Hz up to 3.5 KHz. The
necessary sampling rate, following the Nyquist theorem for the
approximately 4 KHz bandwidth of the human voice range, this
becomes 8 Kbps. After the digitalization and the logarithmic
mu-Law compression process, the transmitted signal requires a
bandwidth of 64 Kbps to transmit and output in real time.
IV. ANALOG INTERFACE CIRCUIT DESIGN
The analog interface circuit design is divided into two main
blocks, the design of the user transceiver board and the design
of the PCI-based PABX board. These blocks are shown in the
block diagrams in Figure 8 and Figure 9 below which gives a
better idea of the complete design.
However, higher data speed range is selected to prepare the
basis for data transmission [22]. The system is intended to
transmit data at 1 Mbps in the form of 10 bits symbols. In this
case:
Rs = 1Mbps
(1)
Ts = 1µS
(2)
Then:
The serial to parallel converter is then set to a 10 bits
conversion. The new symbol rate per parallel sequence
becomes [18]:
Rp =
1
1
R s = 1Mbps = 100Kbps
10
P
(3)
And in terms of bit duration, from Equation 4:
Figure 8.
Tp =
N c Ts
= Ts × P = 1µS × 10 = 10 µS
G PMC
PCI – PABX Board Diagram
(4)
G
The processing gain PMC is selected in such a way that
the chip frequency is at least 10 times higher than the symbol
rate of the information intended for transmission. Thus,
applying the spreading to each parallel sequence with a PN
code of length 10, the processing gain will be:
GPMC = 10
(5)
With this scheme the number of sub-carriers resulting from
the N x N matrix structure will be:
N c = PG PMC = 10 × 10 = 100
(7)
(6)
The overall necessary bandwidth to transmit 1Mbps can be
calculated as follow:
Figure 9. User Transceiver Board Block Diagram
235
to this board. The block diagram in Figure 11 shows a more
detailed structure of the analog interface for the user
transceiver board.
The power line coupling module is common to both boards
and blocks frequencies lower than 100 KHz. It is designed with
average impedance of output and input of approximately 50
Ohms.
The passive band pass filter is designed with phase
correction. It is placed in the receiver section of the board to
filter signal frequencies which are outside of the transmission
bandwidth such as noise and electrical interference caused by
different operating devices that are usually connected to the
power line network like induction motors, switching power
supplies, dimmers and other noisy appliances. The phase
correction segment of the filter corrects distortion due to nonlinear phase response when filtering the signal of interest which
may increase the bit error rate of the received signal.
The signal power stage module is an adjustable gain linear
wideband signal amplifier which acts as a buffer and amplifies
the signal before injecting it into the channel in the transmitter
and after the signal is filtered in the receiver. A block diagram
is shown in Figure 10 for a better understanding of these
modules.
Figure 11. The User Transceiver Interface Module
The on/off hook signal detector and phone voltage regulator
modules simulate the standard PSTN conditions to create
compatibility between the user transceiver and the regular
phone sets available in the market that are to be used in the
system. When the phone is on hook the voltage across the tip
and the ring terminals is approximately 48 volts. If a call is
detected by the board, a ring signal is generated and inserted on
to the phone line. This ring signal waveform has a very low
frequency of approximately 30 Hz, and a peak to peak value of
about 90 Volts. This is excessively high and cannot be
transmitted through the power line channel. The ring signal in
this case is generated on board and once the call has been
detected it is inserted onto the phone line through a low pass
filter. When the user picks up the phone the on/off hook signal
detector senses a current change in the line and instantly 12
volts is introduced on the phone line. The use of high and low
pass filters prevent any harm to the audio and digital processing
electronic circuits which can be produced by the insertion of
the power ring signal.
Figure 10. The Power Line Coupling Module
The receiver filters and amplifies the signal before it is
received by the Digital Signal Processing / Field Programmable
Gate Array (DSP/FPGA) module. At the transmitter, once the
signal from the DSP/FPGA module is amplified, it is then
inserted into the transmission channel through the power line
coupling.
For the PABX integrated board different modules are
required. A PSTN incoming line ring detector and an on/off
hook status simulator is vital for the PABX operation as shown
in the block diagram in Figure 12.
Another common element to both boards is the hybrid
circuit. Each hybrid circuit gives a bi-directional path to the
audio signal to be transmitted to and/or received by the PSTN
line for the PABX board. In case of the user transceiver board,
it links with the user phone set instead of the PSTN. It creates a
difference of phase between signals, one received by the user
and one sent by the user, thus allowing the bi-directional path
of the signal.
The supply voltage of the PCI-based PABX board is taken
from the PC power supply since the board is already placed in a
PCI slot. The power supply of the user transceiver board is a
design of a 48-volt unregulated DC source which supplies 12
V, -12 V and 5 volts to be regulated. Most circuits on the board
are supplied with 12 and 5 volts. The 48 volts DC present in the
user transceiver board will be only used to supply the user
phone set when it is in the on-hook state [2].
Interfacing whit the standard phone sets with which the
system is intended to work requires some modules to be added
Figure 12. PABX Interface Module to PSTN
236
SX micro-controller from the Scenix family of configurable
communications controllers. The control software is
implemented, improved by stages and tested using the
evaluation board provided with the micro-controller SX28AC
shown in Figure 14.
The PSTN incoming line interface is very similar to the
user phone set transceiver board discussed before but with
some slight differences. In this research, the PCI-based PABX
board interfaces with the incoming line from the PSTN, the
system operator and also with the users within the system. In
the first case, it detects an incoming call and simulate the on/off
hook status in order to identify the system with the public
network and make it compatible with the standard operation.
When an incoming ring signal is identified the off hook
state is simulated and the communication starts. Initially, the
call is attended by the operator and then transferred to the
intended user.
An Interactive Voice Response System (IVRS) must be
implemented as part of the design of the Graphic User Interface
(GUI) and the system drivers [21]. Its function is to initially
attend and process any call from the public network and also to
control the voice mail options for the system users.
The operator communicates directly with the system
through an audio interface placed on the board. Therefore the
signal source varies depending on what links with the PSTN
line at a given time, the operator or a user inside the system.
Switching between the multiple sources is carried out by
using analog multiplexers and de-multiplexers which are
controlled by the DSP/FPGA software shown in Figure 13.
Their function is the multiplexing of the signals coming from
the audio interface and the hybrid circuit from the PSTN.
Figure 14. SX28AC Training Kit
V. MICROCONTROLLER SOFTWARE DESIGN
As shown in Figures 8 and 9, the Scenix SX28AC
microcontroller and the virtual peripheral software control the
entire analog electronic interface for both the user transceiver
board and the PCI-based. The design of the control software is
also an important result of this design. The software carries out
the following tasks [2],[13]:
•
Serial communication through the UART protocol:
serves as communication link between the analog
electronic modules and the DSP/FPGA block.
•
Caller ID receiver: detects the caller ID information
signal and transmits it to the DSP/FPGA Module
through UART communication protocol.
•
Controls read and write operation for the MT8880
DTMF transceiver IC: the IC control is carried out
through 3 bits and the data exchange involves a total of
4 bits. The IC has an analog output terminal which
delivers the intended DTMF tone and an analog input
for reading.
•
Ring detection and on/off hook status simulator: a total
of 2 bits are used to interface with the PSTN incoming
line.
Figure 13. Operator/System Audio Signal Processing Module
The electronic interface uses the 74HC4052 which is a
high-speed dual 4-channel analog multiplexer and demultiplexer CMOS device with common select logic. Each
multiplexer has four independent inputs/outputs, controlled by
three bits for channel select logics, including two digital select
inputs (pins S0 and S1) and an active low enable input (pin E).
Its supply voltage ranges between 2 to 10 V. The analog
inputs/outputs do not exceed 10 V.
The program uses the Virtual Peripheral concept to increase
the performance of the micro-controller making possible the
execution of a higher number of tasks utilizing the same
resources. In this Virtual Peripheral, a multi-threading concept
to execute the Universal Asynchronous Receiver Transmitter
(UART) and all necessary modules is applied. Whenever an
interrupt occurs, the program jumps into the interruption
service routine that contains the interruption multi-tasker. At
every occurrence of an interruption, the interruption multi-
The system uses the MAX 507 digital to analog converter
and the MAX 185 analog to digital converter with 12-bit
resolution to standardize digital and analog signals.
Excepting the last circuit on Figure 13, all the electronic
sub-circuits and peripherals are linked and commanded by the
237
tasker module switches to a different thread following a table
order defined in the program memory. Each Virtual Peripheral
module is located into one of the available threads.
[11]
VI. CONCLUSIONS:
A general PLC measurement system was proposed in order
to analyze the channel characteristics using a prototype of the
coupling network. The paper presents measurements of how
the attenuation behaves, according to the frequency of the
transmitted signal for an ideal power line with no loads and a
real in a power line cable connected to the power grid in
Jamaica. Similar behaviors of the frequency vs. attenuation
curve were found if compared with [5],[7],[8],[12]. The
communication channel has proven to be a multi-path,
impedance varying and noisy transmission channel, thus
modern, powerful and resistant communication schemes are
needed. The necessary analog electronic modules that interface
an analog telephone system with power line communication
technology were discussed. While the paper proposes MCCDMA as the ideal transmission scheme to use in power line
communication, it is acknowledged that most high speed PLC
chips now being marketed [24],[25] uses an OFDM
infrastructure. Work is ongoing to compare in greater detail the
MC-CDMA vs. OFDM performance for practical PLC
channels. There is also ongoing work to use the results of this
paper to design and develop an integrated PLC VOIP phone
using existing high speed PLC chips and VOIP software and
hardware.
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
REFERENCES
[1]
Klaus Dostert, Powerline Communications. Upper Saddle River, NJ,
Prentice Hall PTR, 2001.
[2] Departmental Report, Computerized Local PABX, Department of
Electrical Engineering, University of Queensland, Australia
[3] Vjay K. Garg, Smolik K, Wilkes J, Applications of CDMA in
Wireless/Personal Communications, Prentice Hall
[4] B. Sklar. Digital Communications: Fundamentals and Applications.
Upper Saddle River, N.J., Prentice Hall P T R, 2001.
[5] Departmental Report, Power Line Carrier Communications, Department
of Electrical Engineering, University of Newcastle, Australia.
[6] A. Bateman, Digital Communications: Design For The Real World.
Harlow, England; Reading, Mass., Addison-Wesley, 1999.
[7] C. Papaleonidopoulos, C. G. Karagiannopoulos, D. P. Agoris, P. D.
Bourkas, N. J. Theodorou. HF Signal Transmission Over Power Lines
and Transfer Function Measurement. Proceedings of the Sixth IASTED
International Conference. July 3-6, 2001, Rhodes, Greece. (502-505)
[8] M, Zimmermann, K. Dostert. A Multi-Path Signal Propagation Model
for the Power Line Channel in the High Frequency Range. Departmental
Report from the Institute of Industrial Information Systems. University
of Karlsruhe, Germany.
[9] M. Zimmermann, K. Dostert. An Analysis of the Broadband Noise
Scenario in Power Line Networks. Departmental Report from the
Institute of Industrial Information Systems from the University of
Karlsruhe.
[10] G. Maniatis, N. Pogas, P. Foundas. K. Efstathiou, G. Papadopoulos.
Implementation of Load Management System for houses based on
[22]
[23]
[24]
[25]
238
Power Line Communication. Proceedings of the Sixth IASTED
International Conference. July 3-6, 2001, Rhodes, Greece. (497-501)
P. Sutterlin, W. Downey. A Power Line Communication Tutorial.
Challenges and technologies. Departmental report from Echelon
Corporation.
C. Banwell, S. Galli. A New Approach to the Modeling of the transfer
Function of the Power Line Channel. Document report.
A. Sulkin, PBX Systems for IP Telephony, McGraw-Hill Professional; 1st
Edition, 2002.
Kimura, R. Adachi, E. Comparison of OFDM and multicode MCCDMA in frequency selective fading channel. Electronics Letters
Volume 39, issue 3, 6 Feb. 2003 Page(s):317 – 318
Katsis, P.L. Papadopoulos, G.D. PavJidou, F.-N. Coded MC-CDMA
systems for power line communications. Telecommunications in Modern
Satellite, Cable and Broadcasting Service, 2003. TELSIKS 2003. 6th
International Conference on Volume 1, 1-3 Oct. 2003 Page(s):153 - 156
vol.1.
Wetzker, G. Dukek, M. Ernst, H. Jondral, F. Multi-carrier modulation
schemes for frequency-selective fading channels. Universal Personal
Communications, 1998. ICUPC '98. IEEE 1998 International
Conference on Volume 2, 5-9 Oct. 1998 Page(s):939 - 943 vol.2
Akhtman, J. Hanzo, L. Generic Reduced-Complexity MMSE Channel
Estimation for OFDM and MC-CDMA Vehicular Technology
Conference, 2005. VTC 2005-Spring. 2005 IEEE 61st Volume 1, 30
May-1 June 2005 Page(s):528 – 532.
S. Hara, R. Prasad, Overview of Multi-Carrier CDMA, IEEE
Communication Magazine, December 1997, (126-133).
Kamil SH. Zigangirov. Theory of Code Division Multiple Access
Communication. John Wiley & Sons, Inc, ISBN 0-471-45712-4. 2004.
Juha Heiskala, John Terry, OFDM Wireless LANs: A Theoretical and
Practical Guide, Sams, 1st edition, ISBN: 0672321572, December
2001.
Audley B. Darmand, Gary L. Lebby. Automatic Power System Planning
Using Cooperative Intelligent Adaptive System. IEEE, SoutheastCon
2003, April 4 to 6, 2003
Haniph A. Latchman, Laurence. W. Yonge. Power Line Local Area
Networking, IEEE Communication Magazine, April 2003.
Yu-Ju Lin, Haniph A. Latchman, Jonathan C.L. Liu, Richard Newman.
Periodic Contention-Free Multiple Access For Broadband Multimedia
Powerline Communication Networks
Intellon, The Worl Leader In Power Line Communications.
http://www.intellon.com.
Panasonic. Ideas for live. http://www.panasonic.com.