1. Chapter 14: Energy Efficient Next Generation Communications
HANDBOOK ON GREEN INFORMATION AND COMMUNICATION SYSTEMS
Chapter 14: Energy Efficient Next Generation Communications
1Jason B. Ernst
1University of Guelph, Canada

2. Outline Introduction Applications and Networks
Architectures, Techniques and Protocols
Trends, Limitations, Challenges and Open Problems
Conclusions & Future Directions

3. Introduction Motivation for Green Communications Energy as a Resource
Limited Spectrum available
Unlicensed spectrum saturated (802.11abgn Wi-Fi, Bluetooth, Zigbee, Microwave Ovens, Cordless Telephones …etc.)
Interference between competing technologies
Particularly troublesome in dense network deployments
“Cognitive Radio” techniques to use licensed spectrum on demand when not in use
Limits of information transmission – “Shannon Limit”

4. Introduction Motivation for Green Communications
Handheld & Battery Powered Devices
Batteries are heavy & expensive component
Reducing energy consumption allows smaller or cheaper power or increased lifetime
Many devices have multiple technologies (Wi-Fi, Bluetooth, Cellular), so choosing the best technology to reduce consumption in particular cases is important
Potential to extend overall lifetime of the network (in WSN or MANETs)

5. Introduction Motivation for Green Communications
Potential Health Effects
Recent studies showing evidence for and against the health effects on humans of wireless exposure
Are models for exposure accurate?
Are standards for exposure keeping people safe?
Reduced Interference & Contention
Interference and contention implemented poorly can be inefficient
Collisions require retransmissions, using more energy
As the number of hops increase, without caching, the retransmissions become cumulative and successful communication becomes more unlikely

6. Applications and Networks
Wireless Sensor (WSN) & Mobile Ad Hoc Networks (MANETs)
Sensor nodes are often dropped randomly
MANETs can also have random topologies and node placement
Both WSN and MANETs cannot rely on infrastructure
Geographically may be dense or sparse in some areas
Nodes are often battery powered, have low processing and memory capability
WSN focuses on transporting data to collection points
MANET traffic may travel to and from Internet or between peers

7. Applications and Networks
Heterogeneous Wireless Networks (HWNs)
“Convergence Networks”
Use multiple technologies, ex: Bluetooth, Wi-Fi, 3G, 4G, Satellite
Each individual technology is optimized without considering inter-operability
Traffic may have the ability to select a more energy efficient route by selecting which technology to transmit over when several are available

8. Applications and Networks
Delay Tolerant Networks
Useful when network partitioning is possible (WSNs or MANETS

9. Applications and Networks
Delay Tolerant Networks
Often employs caching at individual hops so retransmission is not required across all hops, only at those where the data was lost
Can also employ scheduled transmission if some nodes appear on a predictable timeline (ex: orbits, bus schedules etc.)

10. Applications and Networks
Interplanetary & Intergalactic Networks
Communication in space has several challenges
Line of sight is often interrupted (planets and orbits)
There are often not many links and routes to choose from
Energy is limited, often solar powered
Weight is limited (rules out large batteries, and generally limits how much communication equipment and processing may be done)
Some traffic must be reliable (firmware updates, orbit corrections)
Other traffic may not be reliable (streaming video)

11. Applications and Networks
Alternative Energy Networks
These networks are not powered by a typical electrical grid
Solar, wind, tidal, etc.
Power may come in bursts, may or may not be stored in batteries
Communication may be intermittent when power is unreliable

12. Applications and Networks
Military, Emergency and Disaster Scenarios
Communication networks for these applications may be integrated with aerial drones, uavs, rovers, trained dogs, soldiers, medics, police offers, heavy equipment (tanks, planes etc) which require light and compact devices for mobilty
Again cannot rely on electrical grid
Must be secure and reliable

13. Applications and Networks
Developing Countries and Rural Access Networks
Internet access in these areas is extremely expensive
Also necessary to provide education, emergency services and communication
Should provide a tradeoff between performance and cost
In developing countries, a van physically drives between villages collecting s which are sent opportunistically when the van returns to the city where infrastructure exists

14. Architectures, Techniques & Protocols
Topology Control
Many very different techniques to control the layout or arrangement of nodes in the network
Shut some nodes off completely periodically
Using a variety of cell sizes (smaller cells in dense areas so that fewer nodes are handled by one access point)
Adjust power levels dynamically to reduce neighbor connections (making routing choices easier)

15. Architectures, Techniques & Protocols
Power Control
Related to topology control, but not always done to control the layout of the network
May also be used to reduce destructive interference
Can be performed in conjunction with transmission schedules and compatibility matrices (used to determine which nodes may transmit at once without interfering – due to distance)

16. Architectures, Techniques & Protocols
Repeater and Relay Nodes
Can be used to reduce the distance required for long hops, reducing the transmission power
Especially useful in networks where battery life should be conserved
Similarly, special indoor antennas can be used within buildings so that mobile devices can use lower power when inside

17. Architectures, Techniques & Protocols
Caching, Clustering and Data Aggregation
Similar to repeater and relay nodes, cluster heads can be used to perform similar functionality
Nodes elect cluster heads within a nearby area and send all traffic through this node
The cluster head can then transmit several nodes’ traffic in burst

18. Architectures, Techniques & Protocols
Caching, Clustering and Data Aggregation

19. Architectures, Techniques & Protocols
Caching, Clustering and Data Aggregation
This reduces long hops to the destination, and avoids contention – reducing retransmissions
Techniques can also be used to switch cluster heads in the event the current cluster head is low on battery to keep the network “alive” longer
Particularly useful in WSN and MANETs

20. Architectures, Techniques & Protocols
Energy-Aware Handover Techniques
Traditional handover focuses on horizontal handovers (switching AP due to mobility)
With modern devices, vertical handover must also be considered
(switching access technology due to mobility or changing network conditions)
Choice becomes more complex since this is a cost and overhead associated with handover
Using the correct information such as capacity, transmission energy, congestion, battery life can lead to improved efficiency

21. Architectures, Techniques & Protocols
Access Point Selection
Typically based solely on signal strength when choosing between multiple Aps
More recent solutions propose considering congestion level, capacity etc.
It is also possible to avoid using the infrastructure and form small ad hoc networks that relay eventually back to an AP

22. Architectures, Techniques & Protocols
Energy-Efficient Routing Algorithms
Particularly effective in WSNs
Consider a “data-centric” approach rather than “destination-centric” approach
Query based routing, for example:
“Give me periodic reports about animal location in region A every t seconds”

23. Architectures, Techniques & Protocols
Energy-Efficient Transport Protocols
In some application it may be worthwhile to tradeoff reliability for energy-efficiency
One way to encourage green communication is to prioritize traffic from other “green networks” who make energy efficiency a priority
Another way is to encourage protocols which attempt to reduce re-transmissions, ie avoid protocols like UDP which may cause many retransmission by flooding into the network without any feedback

24. Architectures, Techniques & Protocols
Energy-Efficient Transport Protocols
Removing the end-to-end requirement of traditional protocols and use a store-and-forward approach

25. Architectures, Techniques & Protocols
Energy-Efficient Medium Access Protocols
When combined with certain routing approaches such as Directed Diffusion (data-centric) it may be beneficial to abandon using automatic repeat requests (ARQ) in MAC protocols and assume that reliability is handled at higher layers
When links are periodically poor, using ARQ will reduce higher layer retransmissions (ie retransmission across every hop)
Trade-off between each decision, dynamically switch between strategies?

26. Trends, Limitations, Challenges & Open Problems
Integration of specific solutions for targeted applications, networks and architectures into a complete green strategy / framework
Joint optimization across existing wireless technologies
Existing networks are optimized without interoperability in mind
Need to optimize with respect to interaction for converging network solutions
Standardization

27. Conclusions & Future Directions
Apply known green techniques from specific networks and applications to more general networks
Combine approaches where possible, for ex)
Data centric routing + link layer ARQ disabled
Extend horizontal handover techniques to vertical
When multiple access technologies are available, make choice based on:
Capacity, Congestion level, Energy requirements etc. so that retransmission and contention are reduced

28. Thanks for your attention!