Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Increased QoS through a Degraded Channel using a Cross-Layered HARQ Protocol

Elliot RangerBrad Gaynor

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Topics

• Problem Statement• Background• Previous Work• Our Approach• Work Breakdown• Expected Results

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Problem Scenario

• Disconnected Network– Noisy Channel– Mobility

• Nodes• Noise Sources

• Node D receives corrupted messages from Node C• Some WSN applications require increased (QoS)

A B C D

E

F

Noise

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Channel Characteristics

• Bandwidth– Capacity of Channel

• Fading– Frequency Selectivity

• Noise – Gaussian: Additive White Gaussian Noise (AWGN)– Non-Gaussian: Impulsive Noise

• Shannon (1948) – Maximum data rate that be reliably (error-free) transmitted over a certain

channel

http://www.aero.org/publications/crosslink/winter2002/04.html

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Previous QoS Methods

• Error Detection– Check for errors in the message

• Parity• Checksum

– Acknowledgement (ACK)• Indicates to sender that message was received correctly• Acknowledgement can also be corrupted

• Error Correction– Error Correction Code (ECC)

• Extra data used to correct bit errors• Added cost for high Signal to Noise Ratio (SNR) channels

– Automatic Repeat Request (ARQ)• Receiver node requests a retransmission from sender• Sender node sends a new copy of the message• Process repeats until

– Max # of attempts– Timeout– Successful transmission

http://www.aero.org/publications/crosslink/winter2002/04.html

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Forward Error Correction

• Forward Error Correction (FEC)– Linear Block Codes

• Repetition Codes• Hamming Codes and Simplex Codes• Walsh-Hadamard Codes

– Convolution Codes• Viterbi Codes

– Turbo Codes• Channel coding is more power efficient

– Compared to the un-encoded case, the same data rates are achieved using much less transmit power

http://www.aero.org/publications/crosslink/winter2002/04.html

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Hybrid Automatic Repeat Request (HARQ)

• HARQ Protocol– Message sent with

• Error detection• No error correction

– Incorrect messages cause ARQ– Replies to ARQ

• Not a retransmission of the message• Different encodings of the message• More codes received, better error correction at receiver

– Relies on Turbo Codes

http://www.aero.org/publications/crosslink/winter2002/04.html

A B C DL = Low

SNR = Low

E

F

Noise

L = HighSNR = Low

L = HighSNR = High

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Drawbacks

• HARQ– Retransmission requests are only

made to sender– Requires repeated transmission over a

single link• Expends the energy of a single node• Congests link during retransmissions

• Routing– Links are graded on probability of successful transmission over time– Noise characteristics of a link may have localized temporal differences

• Cannot grade links instantaneously because they change– Mobility of nodes– Variability of noise sources

A B C DL = Low

SNR = Low

E

F

Noise

L = HighSNR = Low

L = HighSNR = High

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Our Approach

• Take advantage of localized temporal uncertainty– A lower probability link may have

a better chance of success at any given time

• A lower probability link may be incorrectly labeled as such

• Varying network topology • Localized noise characteristics

• Send different FEC from multiple sources – Increases probability of receiving the correct message– Multiple transmit nodes relieve the strain on a single node

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Thesis

Thesis: HARQ using multiple, collaborative nodes results in increased probability of message reception and extends the overall lifetime of the network.

A B C D

Noise

E

F

A B C D

Noise

E

F

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Cross-Layer Implementation

• Cross-layer– Data must be shared between the network and data-link layers– Each layer must adapt to information from the other

Steps:1. Bad link is identified on the data-link layer2. Link information is passed to the network layer3. Network layer forms a cluster of nodes

• Cluster now responsible for maintaining link integrity• Link protocol adapts to take advantage of the cluster

4. If a reliable link is later found• Cluster dissolves• Routing table is updated

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Work Breakdown

• Implementation– Prior work

• HARQ• Turbo Codes

– Cross-layer support• Network Protocol• Network/Data-link layer interface

• Simulation– Model network

• Topology• Noise• Mobility

– Simulate implementation

Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005

Expected Results

• Expected Results– Increased QoS

• Reduced probability of network disconnects• Minimal impact on high SNR channels

• Comparison Points– Protocols (vs. our multi-node HARQ)

• No error control• ARQ• HARQ

– Metrics• Energy• Connectivity• Congestion/Throughput• Overhead (CPU, memory, etc.)