Tufts University. EE194-WIR Wireless Sensor Networks. February 17, 2005 Increased QoS through a Degraded Channel using a Cross-Layered HARQ Protocol Elliot Ranger Brad 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)

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

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

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

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

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

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.

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 layer 2.Link information is passed to the network layer 3.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 –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.)