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Optimal Power Allocation and AP Deployment in Green Wireless Cooperative Communications

Xiaoxia Zhangx79zhang@bbcr.uwaterloo.ca

Department of Electrical and Computer EngineeringUniversity of Waterloo

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Outline

• Introduction

• System Model

• Power Allocation for a Single-User Link

• AP Deployment

• Conclusions and Future Work

• 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

SAVE OUR

ENVIRONMENT!!!

Introduction

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• 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.

Sustainable Energy

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• 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 regionso Operation cost reduced over 60%

o Carbon footprints reduced over 40%

Figure 3. A Green Base Station

Green Technology in Wireless Communications

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• 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

Motivation

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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.

Objective

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Outline

• Introduction

• System Model

• Power Allocation for a Single-User Link

• AP Deployment

• Conclusions and Future Work

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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.

• n links in total.Figure 4. A green wireless cooperative communication network.

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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.

- Two relaying protocols:

Amplify-and-Forward

Decode-and-Forward

Easy to implementNoise cannot be eliminated

Coding cost Noise free

need extra resources

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• Information theoretic achievable rate

Achievable Rate for Single-User Relay Channel

relay decoding rate

destination decoding rate

• AWGN channel with path loss

path loss exponentconstant 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

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•Objective: jointly determine , and to maximize the achievable rate.

Power Allocation for a Single-User Link

• Note: optimum is achieved when

destination decoding rate = relay decoding rate

relay decoding rate

destination decoding rate

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•Destination decoding rate is the bottleneck.

•Increase and reduce to balance.

•Coherent transmission.

Synchronous Case

relay decoding rate

destination decoding rate

Optimal power allocation is:

Largest achievable rate is:

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•Relay decoding rate is the bottleneck.

•Source will set and .

•Independent transmission.

Asynchronous Case

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

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•Optimal power allocation and maximum rate depends on the location of AP.

AP Deployment

Direct transmission without help of relay can achieve highest rate.

Relay closer to the sourceSynchronous case achieves higher rate

Relay closer to the destinationAsynchronous case achieves higher rate

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•Two local maximum

Optimal AP Deployment for a Single Link

Let , the two possible relay positions to maximize the throughput is the solutions to the following two equations:

,

,

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•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

Optimal AP Deployment

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Simulation Results

Figure 5. Rate comparison of three power allocation schemes when and .

Synchronous

Asynchronous

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Simulation Results

Figure 6. Achievable rate of a single user link with different AP locations.

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Simulation Results

Figure 7. The overall throughput by our proposed AP deployment metric and random deployment method.

•100m×100m area• 30 candidate locations

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Outline

• Introduction

• System Model

• Power Allocation for a Single-User Link

• AP Deployment

• Conclusions and Future Work

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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!