Optimal Power Allocation and AP Deployment in
Green Wireless Cooperative Communications
Xiaoxia Zhang
x79zhang@bbcr.uwaterloo.ca
Department of Electrical and Computer Engineering
University of Waterloo
1
Outline
•
Introduction
•
System Model
•
Power Allocation for a Single-User Link
•
AP Deployment
•
Conclusions and Future Work
2
Introduction
•
Global Emission of CO2 in 2011
- Increased by 3%
- Reaching an all-time high of 34 billion tonnes
Figure 1. Global Emission of CO2 in 2011
3
Sustainable Energy
•
•
Eco-friendly renewable energy: solar, wind, tide, etc.
Occupied 16.7 % of global energy in 2011
Figure 2. Renewable Energy Share of Global Final Energy Consumption, 2011.
4
Green Technology in Wireless
Communications
•
In wireless communications
- Up to 90% of power consumption in BSs
•
- Energy cost is high and increasing
Green wireless networks: network devices powered
by sustainable energy
- Example: Huawei

Solar-powered base stations deployed over 1500
sites in over 30 countries and regions
o
Operation cost reduced over 60%
o
Carbon footprints reduced over 40%
Figure 3. A Green Base Station
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Motivation
•
Characteristics of Sustainable Energy
- Variable or intermittent in its capacity
- Highly dependent on the location and weather
•
Fulfillment of users’ QoS demand is challenging.
- Introduction of cooperative communication
- More efficient green wireless network
o
Device deployment
o
Resource allocation
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Objective
In a WLAN network where green APs are
deployed, we would like to maximize the overall
throughput by jointly allocating transmitting power
and deploying the green APs, subject to the
harvested energy constraint.
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Outline
•
Introduction
•
System Model
•
Power Allocation for a Single-User Link
•
AP Deployment
•
Conclusions and Future Work
8
System Model
•
A wireless local area network
(WLAN) where a green AP is
deployed.
•
Nodes could communicate with each
other in an ad hoc manner.
•
Transmission links are separated by
TDMA.
• AP can cooperate with the source
nodes to transmit data to the
destination.
•
Figure 4. A green wireless cooperative communication network.
n links in total.
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System Model
•
During each transmission period, only one source-destination
pair (si,di) exists.
AP
•
The AP functions as a relay node.
need extra
resources
- Two relaying protocols:
Amplify-and-Forward
Easy to implement
Noise cannot be eliminated
Decode-and-Forward
Coding cost
Noise free
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Achievable Rate for Single-User Relay
Channel
•
Information theoretic achievable rate
relay
decoding rate
•
destination
decoding rate
AWGN channel with path loss
path loss exponent
constant and identical for all links
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Achievable Rate for Single-User Relay
Channel
•
Noise variances received at the
relay and at the destination are
the same value.
•
Joint superposition
encoding/decoding to maximize
cooperation between source
and relay.
•
+
Generation of two codes.
relay
decoding rate
destination
decoding rate
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Problem Formulation
•
Objective: maximize overall throughput under instant available
power constraint.
Links are scheduled by TDMA
Power allocation on one link
does not affect other links
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Outline
•
Introduction
•
System Model
•
Power Allocation for a Single-User Link
•
AP Deployment
•
Conclusions and Future Work
14
Power Allocation for a Single-User
Link
•
•
Objective: jointly determine
the achievable rate.
,
and
to maximize
Note: optimum is achieved when
destination decoding rate = relay decoding rate
relay
decoding rate
destination
decoding rate
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Synchronous Case
• Destination decoding rate is the bottleneck.
• Increase
and reduce
to balance.
• Coherent transmission.
Optimal power allocation is:
Largest achievable rate is:
relay
decoding rate
destination
decoding rate
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Asynchronous Case
• Relay decoding rate is the bottleneck.
• Source will set
and
.
• Independent transmission.
Optimal power allocation is:
Largest achievable rate is:
relay
decoding rate
destination
decoding rate
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Outline
•
Introduction
•
System Model
•
Power Allocation for a Single-User Link
•
AP Deployment
•
Conclusions and Future Work
18
AP Deployment
• Optimal power allocation and maximum rate depends on
the location of AP.
Direct transmission without help of
relay can achieve highest rate.
Relay closer to the source
Synchronous case achieves higher rate
Relay closer to the destination
Asynchronous case achieves higher rate
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Optimal AP Deployment for a Single Link
• Two local maximum
Let
, the two possible relay positions to
maximize the throughput is the solutions to the following
two equations:
,
,
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Optimal AP Deployment
• Sustainable energy can only be exploited in some
specific locations due to the availability and neighboring
environment.
• Several candidate AP locations are considered.
• The optimal location can be decided based on the
overall throughput which is calculated by
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Simulation Results
Synchronous
Asynchronous
Figure 5. Rate comparison of three power allocation schemes when
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and
.
Simulation Results
Figure 6. Achievable rate of a single user link with different AP locations.
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Simulation Results
•100m×100m area
• 30 candidate locations
Figure 7. The overall throughput by our proposed AP deployment metric and random deployment method.
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Outline
•
Introduction
•
System Model
•
Power Allocation for a Single-User Link
•
AP Deployment
•
Conclusions and Future Work
25
Conclusions and Future Work

In this paper, a single-user channel achievable rate
maximization problem is formulated and the optimal
power allocation scheme is derived.
 A throughput upper bound of each single-user
channel is attained and the optimal AP deployment
is provided.
 In the future, we will consider the dynamic charging
and discharging buffer in the AP.
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Thanks!
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