The document discusses green wireless communication using relays. It explains that traditional wireless networks focus on bandwidth efficiency over power efficiency, but high power usage has ecological and economic drawbacks. Relays can help make wireless networks more energy efficient by reducing transmission distances between nodes. The document outlines the basics of relaying and provides a case study on how relays can improve energy efficiency in cellular networks.

1. Green Wireless Communication
with Relays
Aniruddha Chandra
Electronics & Communication Engineering Department,
National Institute of Technology, Durgapur.
aniruddha.chandra@ieee.org
Bangalore, India 01 August, 2012

2. Green Communication
=
Energy Efficient Communication
A new timely idea
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3. Outline
Energy Efficiency – Why?
Energy Efficiency – How?
Basics of Relaying
Case Study
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4. Outline
Energy Efficiency – Why?
Energy Efficiency – How?
Basics of Relaying
Case Study
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5. Energy Efficiency
Why?
 Traditional perspective:
- Reduced Tx power → reduced reliability.
- To maintain QoS, Tx rate should be reduced.
 Ecological perspective:
- Reduce greenhouse gas emission.
 Economical perspective:
- Reduce OPEX cost.
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6. Energy Efficiency
Traditional Perspective
 Value BW most:
- Ever increasing subscriber base.
- Strict spectrum regulations.
- R&D focus on BW efficient radio access techniques.
- These complex techniques demand more processing power.
A typical MIMO-OFDM Tx Rx ckt
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7. Energy Efficiency
Traditional Perspective
 What about energy?
- Battery powered mobile terminals → limited energy.
- Limited energy → limited reliable data rate.
BER vs. SNR curves (M = 16) BW efficiency vs. power efficiency
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8. Energy Efficiency
Ecological Perspective
 2007 statistics on environmental impact:
- A cellular network ~ Energy for 1,70,000 homes.
- 3% of the energy consumption.
- 2% of CO2 emission.
- The figures are going to triple by 2020.
Objects in
Mirror are
Close than
they Appear
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9. Energy Efficiency
Sources of Greenhouse Gas Emission
 Operation of radio access network:
- RF transmission.
- Fossil fuel powered BS.
- Charging of devices.
 Device/ equipment
production.
 Backbone network
operation.
A. Fehske et al. “The global footprint of mobile communications: the ecological and
economic perspective,” IEEE Commun. Magz., 49 (8), 55-62, 2011.
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10. Energy Efficiency
Economical Perspective
 Decreasing revenue:
- Vodafone annual ARPU decreased from € 30 (2000) to € 16 (2009).
 Increasing fuel cost:
- Diesel cost has doubled since 2008.
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11. Energy Efficiency
Energy Consumption
 Cost components:
 Energy components:
- Feeder network.
- RF conversion.
- Climate control (e.g., air conditioning). Energy components for BS
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12. Energy Efficiency
Energy Cost Calculation
 Revenue generated: Cell site
- No. of subscribers per cell site ~ 800.
BS
- ARPU ~ 3$ / month. Subscriber
Subscriber
- Monthly revenue ~ 800 x 3$ = 2400 $.
 Cost for energy:
- Energy cost ~ 0.20 $/ kWh.
Power plant
- Power requirement per BS ~ 1.7 kW.
- Cost per month ~ 30 (days) x 24 (hours) x 1.7 x 0.20 $ = 244 $.
 10% of total revenue (even before tax, interest, depreciation)!
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13. Outline
Energy Efficiency – Why?
Energy Efficiency – How?
Basics of Relaying
Case Study
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14. Energy Efficiency
Energy Savings in Base Stations
 Improvements in PA:
- Linear PAs → 90% wastage.
- DPD, Doherty, GaN based PA.
 Power saving mode:
- Sleep mode, discontinuous Tx/ Rx.
 Optimization:
- BS placement, cell size. Z. Hasan et al. “Green cellular networks: a survey, some research issues and challenges,”
IEEE Commun. Surveys Tuts., 13 (4), 524-40, 2011.
V. Mancuso et al. “Reducing costs and pollution in cellular networks,” IEEE Commun.
Magz., 49 (8), 55-62, 2011.
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15. Energy Efficiency
Energy Savings in Base Stations
 Renewable energy:
- Sustainable bio-fuel.
- Solar energy.
- Wind energy.
 New BS architecture:
- Short, low power RF cable between Amp. & Ant.
- Feeder less site.
Solar powered BS (Italy)
 Reduce no. of BS?
C. Lubritto et al. “Energy and environmental aspects of mobile communication systems,”
Energy, 36 (2), 1109-14, 2011.
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16. Energy Efficiency
New Communication Strategies
 MIMO / beamforming:
- Diversity.
- More sectors per cell site.
 Cognitive radio:
- Find unused spectrum, BW traded off for power.
 Use a third node:
- Reduce effective transmission distance.
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17. Outline
Energy Efficiency – Why?
Energy Efficiency – How?
Basics of Relaying
Case Study
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18. Basics of Relaying
What is a Relay?
 A simple repeater: Receive, boost, and re-send a signal.
 Cellular network: Different node, carrier owned infrastructure, tree topology.
IEEE 802.16j (mobile multihop relay).
Sensor network: Identical node, subscriber equipment, mesh topology.
IEEE 802.15.5 (WPAN mesh)/ 802.11s (WLAN mesh).
Relay #1
Relay Station
(RS) Source Relay #2 Destination
Base Station Mobile Terminal
(BS) (MT)
Cellular network Sensor network
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19. Basics of Relaying
Why Use a Relay? RS-MS
link
 Save Tx energy: Cooperative
BS-RS MT #1
transmission
link
Coverage/ radio
- Reduced transmission distance. RS #1 range extension
RS #2
Traditional direct
 Performance improvement: MT #2
transmission
BS
- Enhance QoS, capacity, range. RS #3
MT #3
- Load balancing.
Capacity enhancement through
replacing low rate, unreliable links
with multiple high rate, reliable links
 CapEx benefit: Traditional service
boundary
- Temporary coverage, gradual rollout.
A. Chandra, C. Bose, and M. K. Bose, “Wireless relays for next generation broadband
networks,” IEEE Potentials, vol. 30, no. 2, pp. 39-43, Mar.-Apr. 2011.
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20. Basics of Relaying
Direct Path vs. Relayed Path
1st time slot
× ×
Relay Relay 2nd time slot
Source Destination Source
× Destination
Co-operative Strategies
1st time slot
Relay Relay 2nd time slot
Source Destination Source Destination
K. J. Ray Liu, A. K. Sadek, W. Su, and A. Kwasinski, Cooperative Communications and
Networking, Cambridge University Press, 2009.
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21. Basics of Relaying
Decoding at Relay
 Amplify and forward: Relay
- Relays act as analog repeaters.
Source Destination
 Decode and forward:
Relay
- Relays act as digital regenerative repeaters.
Source Destination
 Compress and forward:
Relay
- Relays quantize and compress.
Source Destination
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22. Basics of Relaying
Resource Allocation
 Persistent transmission:
- Relays always forward a processed version of their received signals.
 Selective relaying:
- Relays autonomously decide whether or not to forward.
 Incremental relaying:
- Relays provide redundancy only when explicitly requested by destination.
H. Katiyar, A. Rastogi, and R. Agarwal, “Cooperative communication: A review,” IETE
Tech. Review, vol. 28, no. 5, pp. 409-417, Sep.-Oct. 2011.
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23. Outline
Energy Efficiency – Why?
Energy Efficiency – How?
Basics of Relaying
Case Study
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24. Relay Placement
Collinear Model
Relayed
Path
Direct Path
(Reference level)
42.2 m (Optimum location)
Source Relay Destination
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25. Relay Placement
Non-linear Model
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26. Relay Placement
Non-linear Model
?
Source Destination
Relay
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27. Relay Placement
Energy Ratio
Source Destination
Relay
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28. Relay Placement
Open Problems
 Relay - To use or not to use:
- Always cooperate, or use relay only when the direct link fails?
 Relay selection:
- If there are many relay nodes, how many and which ones to select?
 Other issues:
- Multiple antennas at relay, distributed STC etc.
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29. Summary
 Value energy.
 Various means to reduce energy consumption.
 Use of wireless relays is one of them.
 A single collinear relay may save upto 35% energy.
 For non-linear setup, an energy efficient region may
be found to place the relay.
 Many open problems, we need you!
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30. Read More About It
Green Communication
1. G. Y. Li et al., “Energy efficient wireless communications: Tutorial, survey,
and open issues,” IEEE Wireless Commun. Magz., 18 (6), 28-35, 2011.
Modelling Energy Consumption
1. S. Cui et al., “Energy-efficiency of MIMO and cooperative MIMO
techniques in sensor networks,” IEEE JSAC, 22 (6), 1089-98, 2004.
2. G. G. de Oliveira Brante et al., “Energy efficiency analysis of some
cooperative and non-cooperative transmission schemes in wireless sensor
networks,” IEEE TCOM, 59 (10), 2671-77, 2011.
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31. Thank You!
Questions?
aniruddha.chandra@ieee.org
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