Broadband Access Networks

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Broadband Access Networks
EE4541.759 Spring 2006

• Chapter 5
• Power Line based Access Networks
• Byeong Gi Lee
• Seoul National University

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5. Power Line-based Access Network

• Characteristics of Power Line Network
• PLC Network Configurations
• Medium Voltage PLC
• Low Voltage PLC
• Friendly User based PLC
• PLC Network Operations
• Modulation Schemes
• Medium Access Control Schemes
• Power Line based Broadband Access
• Broadband by Modulation Technology Evolution
• Broadband Access Rural Area

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Power Line Network (1)

• Electric Power System
• Power Station generates power at tens of kV
  (e.g., 20kV), step up to hundreds of kV (e.g.,
  154kV or 345kV)
• High Voltage (HV) Region power transmission at
  hundreds of kV
• Load-side Substation steps down to tens of kV
  (e.g., 22.9kV)
• Medium Voltage (MV) Region power distribution at
  tens of kV
• Transformer Substation steps down to hundreds of
  V (e.g., 380V three-phase or 220V single-phase)
• Low Voltage (LV) Region power distribution at
  hundreds of V

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Power Network Configuration
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Power Line Network (2)

• Power Line Communication (PLC)
• Refers to the transmission of communication data
  along an electrical utility network (or power
  distribution grid).
• Power distribution grid is omnipresent at the
  habitation areas (rural, residential and business
  sectors)
• Intends to use power line as a medium to connect
  computer, network, or other telecommunications
  devices to standard AC outlets within residences
  or businesses.
• Modulates the data packets (FSK, DS/SS, OFDM) and
  then injects them onto the power line network
  grid.

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Power Line Network (3)

• Characteristics of Power Line
• Power line is connected to various different
  appliances and equipment
• Power line noise varies in frequency and time,
  and depending on the load condition.
• Four main types of noises in power line
• Colored background noise caused by low-power
  noise accumulation,
• Narrowband noise caused by radio and other
  electromagnetic interferences,
• Asynchronous impulsive noise due to the on-off
  switch noise,
• Periodic impulse generated by appliances

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Power Line Network (4)

• Characteristics of Power Line (contd)
• Power line channel can be characterized as a
  star-shaped bus.
• Branching of power lines causes complex echo
  scenarios, and thus yields notches in the
  frequency response, leading to frequency-selective
  fading.
• Depending on the network structure,
  frequency-dependent attenuation can exist, which
  gets superposed on the frequency selective fading
  
• Attenuation increases with the degree of
  branching, as the service point at each customer
  premises absorbs the transmitted power

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Power Line Network (5)

• Characteristics of Power Line (contd)
• Each power line link takes a low-pass
  characteristic
• PLC channel is a random time-varying channel
  having a frequency-dependent SNR over the
  communication bandwidth.
• Due to the random channel behavior of power line,
  there is a severe attenuation in the PLC system.
• Measured attenuation goes as high as 15dB/km

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Power Line Network (6)

• Advantages of PLC Communications
• Much of the infrastructure, or the power line
  grid, is already in place,
• So the required investment is comparatively very
  low.
• The power line grid is omnipresent
• So PLC services are available to the customers at
  every residence or office.
• The equipment using existing outlets makes
  communication setup easier and less expensive
• The in-premise equipment can be made less
  expensive and easier to install than that of
  other broadband solutions such as cable or DSL
• End users can get always-on connection from
  power socket

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PLC Network Configurations (1)

• Overall PLC Network Configurations
• MV Region PLC Network
• Communication injection point at the utility
  substation
• locates PLC transmitter at the substation
• LV Region PLC Network
• Injection point at the low-voltage transformer
• Multiple homes at the same LV distribution grid
  share the PLC
• Customer Premises PLC Network
• Injection point at the entrance of the customer
  premises.
• End users plug a PLC modem into the power outlets
  
• Can be shared by near-by customers (Friendly User
  PLC)

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PLC Network Configurations (2)
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MV PLC Network Configurations (1)

• Broadband signal injected at the load-side
  substation towards the MV distribution network.
• Travels long distances, so may need to install
  intermediate repeaters along the grid to ensure
  the signal strength
• Need to bridge the PLC signal to the LV side of
  the transformer, as a PLC broadband signal cannot
  pass through a transformer.
• Adequate for the rural areas with sparse
  population density of 100500 customers.
• Useful as the backbone network in the
  metropolitan areas too.
• Place a (capacitive) coupling device at the
  substation and a PLC master modem near by the
  customer premises
• PLC master modem injects and receives PLC
  signals.
• digital section includes the processors for the
  modem function
• analogue section includes a coupling device and
  transmitter/receiver unit

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MV PLC Network Configuration (2)
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LV PLC Network Configuration (1)

• Communication signal injected at the step-down
  transformer towards multiple customer premises.
• Adequate to highly populated areas
• Interconnection with the backbone at the LV
  transformer.
• Usually a shelter is required near by the
  transformer to protect the electronic equipment
• Network architecture differs depending on the
  existence of gateway
• A gateway is a device placed at the entry point
  of the customers premise that acts as an
  interface between the PLC master unit located
  somewhere upstream and the customers indoor PLC
  network.
• The PLC master unit resides close to or at the LV
  transformer and the data signal is injected on
  all three phases. The configuration at the
  customers premises is the same as for the MV PLC
  case

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LV PLC Network Configuration (2)
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Friendly-User Network Configuration (1)

• A commonly used concept in Europe
• Selects one customer premise for housing the PLC
  master unit and brings in the backbone connection
  into this premise.
• Advantageous as no extra effort is needed to
  shelter the equipment
• Any qualified and trained electrician can install
  it without the involvement of electricity utility
  company staff.
• Data signal injected at the in-premise PLC master
  unit propagates reverse over the LV grid to all
  other customers on that line.
• Other customers in the group can access the data
  signal in the same way as for the LV case.
• Since a PLC signal cannot pass through a
  transformer, the communication is confined within
  the group of custormers.

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Friendly-User Network Configuration (2)
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In-Premise Network Configuration
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PLC Network Operations (1)

• PLC can provide 27 Mbps DS and 18 Mbps US (shared
  among the multiple users in the same substation
  area)
• Inside the customer premises, PLC can operate
  using the existing electrical wirings. Also
  possible to use twisted-pair or wireless based
  home networking
• PLC reaches the distance of 399m without
  repeaters,
• Much longer than that for Fast Ethernet (100m) or
  for 802.11b wireless LAN(50m)
• Possible to develop an interface between the
  network and transport layers to give freedom of
  selecting specific protocol to users, enabling
  the connections between a power line LAN,
  Internet and other Ethernet-based systems.

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PLC Network Operations (2)

• Possible to develop a network management system
  such that a PLC modem can act as a traditional
  modem for network connectivity and also as a
  repeater in wide area networks for broadening the
  reach of the networks.
• Frequency range of interest to PLC is two fold --
  the low frequency range (e.g., 50Hz250 Hz) and
  the higher frequency range (e.g., 70kHz35MHz).
• Standards necessary to avoid the conflicts
  between home and utility systems that share the
  same power line (e.g., CENELEC and ETSI)
• Issues to resolve in regulating standards in
  relation to PLC.
• PLC radiation emissions could impact on existing
  communications in the high frequency range.
• Interference with household appliances and other
  communications such as wireless LANs that may
  operate in the same frequency range.

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PLC Modulation Schemes (1)

• Special care is needed in selecting modulation
  techniques for PLC, as the channel
  characteristics of power line differ considerably
  from other communication media
• An automatic adaptation feature is required for
  PLC systems, either in transmitter or in receiver
  or both, as the PLC channel properties vary
  depending in frequency and time and on load
  condition.
• Data rate enhancement possible
• not by extending the bandwidth or by assigning
  new frequency ranges
• but by adopting advanced channel adaptation
  strategies
• and by employing sophisticated modulation
  techniques with improved spectral efficiency.

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PLC Modulation Schemes (2)

• Modulation is imperative for PLC in two different
  aspects.
• Possible to significantly reduce the transmission
  attenuation of power line by modulating a
  baseband signal onto a higher frequency carrier.
• Possible to effectively utilize the frequency
  spectrum of power line by adopting an adequate
  multiplexing technique.
• The bi-directional point-to-multipoint data
  communication in the power distribution network
  can be best achieved by adopting a MAC scheme
  that best matches with the selected modulation
  technique
• Evolution of Modem Techniques
• FSK It modulates multiple signals with
  frequency-separated carriers. Simple but
  inherently limited to use in PLC beyond 1 Mbps
  rates
• DS/SS The modulated signal is virtually safe
  from any interference and allows transmissions
  beyond the 1 Mbps rate over the power line.
• OFDM Effectively overcomes various power line
  problems such as frequency selective fading and
  impulse noises, so most promising

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PLC MAC Schemes (1)

• MAC scheme needed to handle the
  point-to-multipoint aspect of PLC distribution.
• A most appropriate protocol for PLC is CSMA.
  Another is Aloha or slotted Aloha. Variations
  with better throughput are CSMA/CA and CSMA/CRCD
• CSMA is easy to implement, technically mature,
  and relatively inexpensive.
• CSMA based Ethernet is widely accepted as the
  standard system
• PLC is too unreliable to implement CSMA based
  Ethernet, so strengthen it by incorporating some
  techniques like ARQ.
• A synchronized random access MAC mechanism
  developed to make PLC networks more robust and
  efficient, by overcoming the limitations of CSMA
  over PLC.

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PLC MAC Schemes (2)

• Security is an important issue in the MAC layer
  due to P2MP configuration.
• can incorporate some measures against tapping or
  install traffic encryption and message integrity
  verification functions inside the PLC modems.
• also possible to implement a built-in identity
  authentication function in the PLC modems to
  authenticate the users before granting any
  network accesses.

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PLC based Broadband Access (1)

• Evolution to broadband possible by improving the
  MAC schemes and employing advanced modulation
  techniques.
• First evolved to CSMA scheme with collision
  avoidance or collision detection capability
• CSMA/CRCD scheme employed a spread spectrum
  technology in conjunction with a frequency
  chirping function to synchronize the transmitter
  and receiver
• Different types of narrowband spread spectrum
  CSMA systems followed that employed multi-bit
  correlators
• Spread-spectrum systems have limitations in
  providing higher data rate services with broader
  bandwidths.
• Inefficient in transmitting high data rate
  signals in noisy channel environment
• Tends to cause averaging effect error when
  transmitting high rate data
• Not effective in overcoming ISI caused by
  multi-path delay spread

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PLC based Broadband Access (2)

• OFDM technology was considered as the enabler of
  broader bandwidth PLC
• It divides the total bandwidth into N parallel
  sub-channels, and allocate different number of
  bits to each sub-channel in proportion to its SNR
  performance. So it can maintain the quality of
  transmitted data.
• It helps to overcome interferences from other
  channels such as amateur radio transmissions,
  which otherwise could jam the PLC signal.
• Allow for high data rates even in frequency
  selective channels, such as the power
  distribution network.
• Can help to overcome the multi-path interference
  problem, as it takes longer symbols to
  demodulate, longer than the delayed paths,
  thereby allowing for correct decoding and
  corrections.
• As a consequence, data rate can increase to
  45Mbps on the medium voltage power grid and up to
  14Mbps in the customer premises.

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PLC based Broadband Access (3)

• Data rate can be further enhanced by adopting
  multiple carrier code-division multiple access
  (MC-CDMA)
• It is an advanced and mixed version of the CDMA
  and OFDM technologies
• It is an OFDM modulation with CDMA overlay,
  spreading in frequency domain
• Results in a denser signal transmission, yielding
  a higher bandwidth utilization for PLC network.
• Data rate can increase to the 100Mbps range or
  higher

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Broadband Access to Rural Areas

• In rural area, deploying broadband communication
  infrastructure is not economically justified but
  electricity infrastructure is readily existing.
• PLC can be designed to effectively cover the
  rural areas, yielding an economical solution to
  providing broadband access to rural areas
• MV power distribution network is especially
  efficient for rural area broadband, with data
  traffic carried over the backbone network
  injected at the substation towards the MV power
  distribution network
• Possible to install a wireless transmitter at the
  step-down transformer, providing broadband
  services to rural users wirelessly beyond that
  point.
• The modem equipment at the customer premises
  receives the service and distributes to the
  in-premise networking equipment.
• 45100Mbps data rate can be achieved over the MV
  distribution network and an aggregate data rate
  of 18Mbps inside the customer premises.

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Comparison of PLC with other solutions