Speeding Up the Smart Grid:

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Nov 21, 2013 (3 years and 9 months ago)

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© Semiconductor Components Industries, LLC, 2011
November, 2011 − Rev. 0
1 Publication Order Number:
TND6018/D
TND6018/D
Rev. 0, NOV − 2011
Speeding Up the Smart Grid:
Technique for Delivering More Robust,
Higher Data Rate Communications
for Automatic Meter Reading

 

Speeding up the Smart Grid: Technique for Delivering More Robust,
Higher Data Rate Communications for Automatic Meter Reading


Abstract

As demands to improve smart grid performance put the traditional PLC (Power Line
Communication) modulation techniques used for automatic meter reading (AMR) under
pressure, utilities are seeking ways to achieve higher data rates and more robust
communications that meet future requirements while maintaining compatibility with existing
installations. This paper looks at a technique that allows smart grid architects to improve the
speed and robust characteristics of AMR communications while protecting investment in
existing infrastructure.

Introduction

AMR systems have become increasingly commonplace as utilities roll out smart metering to
homes and businesses around the world. The majority of implementations use Power Line
Communication (PLC) technology, not least because it offers low cost, ease of deployment,
and scalability - the same network used for energy distribution provides the basis for smart
metering communication, eliminating the need for additional antennas or communications
infrastructure and allowing the communication network to grow in line with the distribution
network. Utilities looking to deliver the high-speed robust communications that the smart
grid now demands want to evolve their existing IEC61334-5-1 meter infrastructure (widely
adopted and deployed AMR standard also called PLAN+), preferably to a multicarrier
technology that makes more efficient use of the spectral capacity.

Evolution of Power Line Communications Technologies

It is almost twenty years since CENELEC (the European Committee for Electrotechnical
Standardization) defined dedicated PLC frequency bands, including the A-band for energy
distribution and metering. This narrow band PLC channel presents some challenges –

 
transmitted frequencies can be highly attenuated depending on network topology and line and
load characteristics, while wide dynamic range for both transmission and reception means
that PLC modem designers must use complex techniques such as Automatic Gain Control
(AGC) to ensure reliable communications. In addition PLC systems must be robust enough to
withstand periodic, mains-related interference sources.

Spread Frequency Shift Keying (S-FSK) modulation has been widely adopted for PLAN-
based AMR as it offers improved performance to traditional narrowband (FSK) systems (the
gain can be from 5 dB to 15 dB dependant on noise characteristics). Now, demand for higher
data rates is driving modulation techniques that use a number of carriers in parallel. The
result is more complex signals with high peak-to-average power ratios (PAPRs) that pose
new challenges in terms of linear and power-efficient circuitry. Solutions based on
Orthogonal Frequency Division Multiplexing (OFDM) have been proposed, but such
schemes would not be compatible with existing systems. The option of a hybrid scheme
based on a multi-channel FSK approach that can meet requirements for higher data rates and
low power consumption while delivering requisite backward compatibility is, therefore,
highly attractive.



Figure 1: Supported Operating Mode of proposed PLAN++ Technology

Hybrid FSK Scheme

Channel characteristics are defined by factors ranging from time and frequency to network
equipment and location. In Europe, where AMR frequencies are typically between 9 kHz and
95 kHz, PLC is particularly susceptible to interference – both from background and impulsive

 
noise and from narrowband interferers. The most effective improvements in communication
quality can be achieved by using carrier frequencies with the minimum attenuation and
interference – either by permanent transmission over a number of frequencies or through
dynamic selection of the optimum frequencies.

Multicarrier technology (also called PLAN++), based on S-FSK modulation, uses a technique
similar to existing PLC designs but improves the capability to transmit data by using up to
four carrier pairs operating at up to 4800 Baud per pair compared to the one carrier pair
operating at 2400 Baud, which is the basis of many existing PLAN+ designs. At the MAC
level designers can use multicarrier technology to choose to upgrade data throughput by
sending different MAC frames over the multiple carriers or to improve robustness of
communication by sending the same MAC frame a number of times.

In order to ensure backward compatibility with existing PLAN+ deployment it is important
that the new technique operates in accordance with the proven IEC61334-5-1 standard
relating to spread spectrum modulation and, therefore, that only minor changes are made at
the MAC layer level. Furthermore, it is proposed to make the choice between increased
robustness and higher throughput dynamic, allowing management layers to adapt the data
path when line conditions change without complete modem reconfiguration.

Operation in accordance with IEC61334-5-1

The IEC61334-5-1 standard specifies a frame indicator of 0x0000 for a long MAC frame,
while reserving indicators 0x00FF, 0xFF00 and 0xFFFF. The frame indicator 0x0FF0 can be
used for a robust mode frame and a high speed frame can be represented by 0xF00F. The link
layer can correct errors when receiving two frames in redundant/robust mode, while the
payload of two frames received with an increased throughput indicator can be combined into
a single link layer data frame. Bit error rate for both robust and high-speed MAC operation is
further reduced by checking/correcting padding bytes and by using repeated frames to correct
frames with bit errors. Forcing burst errors to represent themselves as random errors, and
techniques for interleaving MAC frames also increase robustness. This combination of
techniques deployed in a multi-carrier modem design reduces computational overheads and
has the added advantage of supporting low-power operation.


 
Developments and Projected Results

Semiconductor companies such as ON Semiconductor have previously developed single-chip
IEC61334-5-1 compliant PLC modems that use S-FSK modulation and that provide a fully
integrated solution for complete handling of the physical and MAC protocol layers. Providing
multicarrier versions of these devices is the next logical phase in the evolution of these
technologies, and based on prototype evaluations it is possible to project some key results.

Figure 2: Standard PLAN+ Frame Spectrum


Figure 3: Projected Spectrum of Proposed PLAN++ Multicarrier Frame

For example, Figure 2 shows the spectrum of a standard, single-channel PLAN+ frame, while
Figure 3 shows the proposed spectrum of a multicarrier, dual-channel frame. As the diagrams
illustrate, it is predicted that the signal is decreased by around 68 dB, corresponding to a

 
channel attenuation of 60 dB and 8 dB that can be attributed to a line impedance stabilization
network or LISN. Performance evaluation is based on computation of Frame Error Rate, with
a frame considered incorrect when the CRC-contained Frame Check Sequence is valid.


Figure 4: Frame Error Rate vs. Signal Attenuation for Multicarrier

Figure 4 represents frame error rate versus signal attenuation for both communication modes.
In the absence of noise in the communication band it is projected that reception will be
ensured for signals above -60 dBV
rms
, with error rate increasing between this level and -90
dBV
rms
. No frame will be received below this level.

In the real world AMR communications have to contend with a number of external factors
that contribute to signal attenuation, not least interference from ‘narrowband’ sources such as
military communications and national radio stations. Such noise can reduce the benefit of a
wide signal dynamic, disturb AGC functionality and reduce potential transmission distances.
An evaluation of the new multicarrier communication technique based around insertion of a
‘noise generator’ source indicates significant resilience to the effects of such narrowband
interference.

Upgrading PLAN+ Infrastructure to Multicarrier PLAN++

The planned availability of next-generation PLC modem ICs that provide support for
multicarrier communication while providing backward compatibility with widely-deployed
IEC-61334-5-1 systems will ensure a simplified, low-cost route to evolving AMR systems.


 
Take, for example, the typical PLAN+ deployment shown in Figure 5. The utility could
replace some of the existing smart meters at the residential or commercial premises with new,
multicarrier meters without replacing the data concentrator. In this scenario the new meters
would automatically configure themselves to operate in PLAN+ mode by using just two
carriers for data communication. At a later stage the data concentrator could be replaced with
a multicarrier alternative, allowing the new meters to operate in ultra robust mode while
existing meters continue to operate normally. Finally, when all meters are upgraded then the
infrastructure will be able to operate in ultra robust or high-speed modes with the decision for
optimum performance lying with the data concentrator based on real-time conditions.

Figure 5: Typical AMR Infrastructure

Summary

Deployment of S-FSK, PLC multicarrier technology in ICs that can manage all physical and
MAC layer processing has the potential to allow improved AMR data transfer in terms of
both speed and robustness of communication. Furthermore, because this technique, unlike
alternatives based around OFDM, is backwards-compatible with existing IEC-61334-5-1
systems, utilities and equipment manufacturers can protect their existing smart grid
investments and choose the time and the speed at which they want to evolve from PLAN+ to
PLAN++.

 
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