SFSK Power Line Modem Communications for Automated Metering systems

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S-FSK Power Line Modem Communicationsfor
Automated Metering systems
8 March 2009
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How does it started?

• provide flexibility software driven PLM to suit
  local network needs
• cost effective solution ( 5 )
• open source with possibility to modifiable by
  customer
• CENELEC A band (9kHz 95kHz)
• transformer based Analog Front end solution
• software Physical/Media Access layer (PHYMAC)

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S-FSK Power Line Modem RD based on Anquilla DSC

• Aim
• Fully-functional open-source software based S-FSK
  Power Line Modem (PLM) using the MC56F8025
  controller.

Independent testing completed on the
module Currently being finalised for use in
Power Line system in Europe
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PLM Block Diagram
Powerline Interface Board
Controller Board
Power in
Voltage Regulator
AC Main
From meter
Tx-en
RS232
DSP56F8025
L N
Tx-in
Transformer
Filter and amplifier
Rx-out
ZC
AC Power Connector
Isolation
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Communications requirements

• CENELEC A Band
• Programmable frequency and bit rate
• Spread FSK, better signal demodulated depending
  on signal/noise ratio
• Manchester encoded for improved decoding
  robustness
• Frames can be relayed to achieve longer distance
• 2400bps data-rate
• Zero-crossing synchronizedcommunication

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PLM PHY Layer Software Block Diagram
Zero-cross detector
ZC synchronization
zero-crossing
FSK transmitter
PWM signals
Receiver
FFT Preamble Header detector decoder
Forward Error Checking module
DTFTF0 and F1demodulation
RX signal
ADC conversion_at_ 1.2MS/sec
data
double Manchester decoder
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Bit encoding
f0 57.6kHz f1 76.8kHz
f0
416ms i.e. 1 bit _at_ 2400Bd

• Each bit is Manchester encoded i.e.
• theres always toggle between the states right in
  the middle of the bit
• log. 1 f0 -gt f1
• log. 0 f1 -gt f0

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FSK transmitter
IPBus x 3 (88.4736MHz)

• two FSK signals generated (57600Hz
  76800Hz)or optionally (38400 Hz 57600Hz)
• every 8th PWM reload, interrupt processed
• every 3rd or 4th reload, new PWM valuesloaded
  for next TX bit

PWM channel 0
PWM module
PWM channel 1
PWM channel 2
PWM channel 3
four independent PWM center-aligned signals
generated, summed together via golden-ratio
resistor networkto highly improve harmonic
content of the transmitted signal, with low-cost
simple filters
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Transmit waveforms generation details
PWM0 PWM1 PWM2 PWM3

• outer resistors (PWM0 and PWM3) versus inner
  resistors (PWM1 and PWM2) in a Golden-Cut
  ratio
• such topology gives no 3rd and 5th harmonics
  content together with higher odd harmonics still
  exceptionally loweven harmonics are eliminated
  by symmetry
• highly important to keep filter requirements
  simplestill satisfying CENELEC requirement

http//en.wikipedia.org/wiki/Golden_cut
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FSK/ASK receiver and demodulator, DFFT
DTFT _at_ 76.8kHz
f0
signal energy levels at the discrete frequencies
ADC sample buffer (512 words)
f1
DTFT _at_ 57.6kHz
Real and Imaginary coefficients are windowed to
improve frequency response and also signal/noise
sensitivity Hamming window used
doslc END_MAC mac x0,y0,a x(r3),x0 // a -
real part mac y0,x0,b x(r0),y0 x(r3),x0 //
b - imag part END_MAC
energy sqrt(Re2 Im2) move.w a,y1 mpy
y1,y1,a // a - (real part of
F0)2 move.w b,y1 mac y1,y1,a //
a - (real part imag part of F0)2 square root
(32bit, result 16bit) MCLIB_Sqrt(), 102 clock
cycles
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0xAAAA preamble recognition, bit boundary
synchronization
preamble sync pattern 10101010 10101010 10101010
0xAAAA Preamble is used for bit boundary
synchronization During some tests, there were
zero-cross position fluctuations found causing
reception difficult. Reworked bit boundary
software to accommodate such power-line
fluctuations (measured 200us!). Now FFT over
FFT method used each bit is divided into four
sub-bits energy in both channels stored and
this sample buffer is observed for best
periodical content. After this is found,
another FFT is used to determine the phase of
incoming bit-boundary. Required time shift
calculated and all sub-sequent sampling is done
on a exact bit boundaries.
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0x54C7 header recognition, byte boundary
synchronization
header sync pattern 01010100 11000111
0x54C7 is used for recognizing byte boundary
after the correct bit boundary is found (through
0xAAAA preamble) Since then, a reception of
data block proceeds. Once the block of data is
finished (on the two frequencies independently),
both blocks are being processed by Forward Error
Correction software module
0xAAAA preamble
0x54C7 header
PHY data
Forward Error Checking code
CRC
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Forward Error Correction methods

• two (compile time) alternatives implemented in
  the software
• (24,12,8) Golay code
• each 12 data bits encoded into 24 bits
• with possibility to correct 3 to 4 bits
• variable data lengths possible
• B) Reed-Solomon (63,32) correction code with
  erasures capabilities
• possibly as many as double errors can
  be corrected then
• erasure is known error location, e.g. detected
  from side-information during advanced
  demodulation process
• in total
• 2400Bd Manchester encoding ( header preamble
  error correction) 200ms frame time

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Analogue front end for PLM
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S-FSK Power Line Modem Reference Design based on
56F8025
Results Load noise roughly 380m of cable, TV,
fans, heaters, lamps, bulbs Receiver gain 47
BER 100, PER 100 FEC there were almost zero
errors on the line. Performed extensive tests on
five different sockets with different levels
of attenuation and different kinds of noise on
38.4 kHz 57.6 kHz at a baud rate of 2400bps,
Manchester encoded S-FSK (which can also be
detected as two independent ASK
streams/channels). Transmitting injected signal
which was right at the top of CENELEC limits
(38.4 kHz 123dBµV, 57.6 122dBµV). Receiving
evaluated how attenuation disturbances for the
received signal.
This PLM modem uses the frequency band 'A' of the
CENELEC directive (frequency band 3-95kHz)
reserved for energy providers (not consumer home
networks). The PLM application is based on the
Freescale DSP56F8025/23 (Hawk V2) family. S-FSK
(Spread FSK) modulation is used for
communication, and both S-FSK modulation/demodulat
ion routines are fully handled by the DSC
software. Power Line Modem baud rate is 2400bps
and data consistency is secured with FEC (Forward
Error Correction Reed-Solomon codes with erasure)
methods. Modems use the 'repeating' technique to
reach larger distances in harsh environment
(targeting typically 500-1000m from the central
node).
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S-FSK Power Line Modem Evaluation kit and demo

• This demo serves as PLM evaluation kit one
  client (master) and server (slave) on the other
  side. MODEM TEST SUITE is used to evaluate modems
  behavior. See USER MANUAL with demo usage
  description.
• http//roznov.ea.freescale.net/booking/index.asp?a
  ctionShowDemoSupIDS69

• The Spread FSK (S-FSK) Power Line Modem Reference
  Design provides a complete solution for the
  communication over the power lines. A
  software-based solution is running on a
  Freescales cost-effective 16-bit Digital Signal
  Controller DSC56F8023 which is tied to a simple
  analog front end interface.
• http//roznov.ea.freescale.net/booking/index.asp?a
  ctionShowDemoSupIDS68

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Summary

• Completely software driven Power-line mode (band
  A - for automated metering applications)
  available
• using DSC56F8023 Anquilla low-cost 16-bit Digital
  Signal Controller
• total BOM lt5 incl. DSC, with more than 30
  processing power available for customer
  applications (higher layers of protocols, etc.)
• Software will be completely open-source
• Hardware design will be available as a reference
  design document

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Appendix
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Appendix Why and where weve deviated from
original PLAN IEC61334-5-1 specs?

• FEC (Forward Error Correction)
• original PLAN specs does not use any kind of FEC
  codes. Theres just one single MAC CRC checksum,
  so if a single bit (out of multiple) frames is
  flipped, the whole transaction is discarded.
• This effect is worse for long frames which may be
  repeated up to 7 times Ie. one transaction could
  be up to 7 PHY subframes repeated up to 7 times!
• The probability of successful frame transfer
  sharply falls to zero.
• Strong FEC is always needed on such noisy
  environment as power-line.
• Multiple PHY frames are used to transfer single
  MAC frame (in PLAN)
• removed this feature, thus weve limited the size
  of MAC frame, now one MAC frame is always sent
  using one PHY frame.
• The data overhead was simply too big (MAC
  addressing for each PHY subframe etc.) on such
  slow and unreliable links.
• Each PHY subframe carries the same information
  with no additional value.

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Appendix contd Why and where weve deviated
from original PLAN IEC61334-5-1 specs?

• PLAN depends on so-called Spread-FSK where log.
  1 and log. 0 are sent on the two frequencies
  that are far away (typ. delta gt10kHz) Weve used
  57.6kHz and 76.8kHz.
• It is expected that such signal can be detected
  as FSK signal (simply by comparing energies on
  both frequencies).
• It is also assumed that if one of the frequencies
  is interfered or deleted, the other one is better
  and could be demodulated as ASK signal.
• Both assumptions are incorrect!
• Power-line signals heavily change their amplitude
  over time and over frequencies. Also the transfer
  on both frequencies is typically very different
  and standard FSK demodulation is not possible.
• Stepping down to ASK demodulation is of no added
  value, since it is not possible to find the
  correct threshold to compare the energy in the
  channel.
• Thus weve Manchester encoded the bit stream and
  were just demodulating the signal as the two
  independent Manchester-encoded ASK signals.
• Manchester encoding removes the most of the
  dependency on the amplitude, we just compare the
  energy in the first half of the bit versus the
  energy of the second half of the bit the modem
  is in fact no-longer S-FSK but rather a dual ASK
  modem

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Frequency bands