Combining LT codes and XOR network coding for reliable and energy efcient transmissions in wireless sensor networks

Anya Apavatjrut , Katia Jaffres-Runser, Claire Goursaud and Jean-Marie Gorce

Combining LT codes and XOR network coding for reliable and energy efcient transmissions in wireless sensor networksSarnoff Symposium (SARNOFF), 2012 35th IEEEOutline Introduction Reaching reliability Gradient broadcast routingThrough coding: LT codesCombining LT codes and gradient broadcastingImproving energy with network coding XOR network coding heuristicXLT-GRABPerformance resultsIntroduction In large scale wireless sensor networkA node advertises its data to one or several sink nodes. The transmission is multi-hop between the data source node and the sink.High reliability is important.

To increase reliability Introducing redundancy through path diversityAdding a coding layer on top of the routing algorithmsensesensor3Multi-path routingSeveral copies of a same packet travel on multiple paths in parallelIncreasing transmission reliability Also increasing in energy expenditure for redundant transmission

Gradient broadcast algorithms Allow several nodes at a time to forward a same packet in broadcast based on pre-defined set of forwarding rulesA cost field is set in an initialization stageNodes are able to adjust locally to instantaneous changes in the network node failure or link failure. More flexibleIncreasing the number of copies traveling in the network. where all nodes of the network get assigned a cost proportional to their distance to the sink.

4Adding coding layer Each message m Encoded using a specific coding algorithm Adds redundancy to m to compensate for the losses Still retrieve m at the sinkHARQFountain codesA source S can potentially generate a limitless number of encoded packets until it receives an acknowledgement from D.D only acknowledges end to end a successful decoding to S This acknowledgement can be merged with the gradient cost field maintenance packets of the protocol.

is complex to implement a hop-by-hop acknowledgement for multi-hop broadcast transmissions as required by HARQ techniques.To further reduce the control overhead,5In this paperAdding fountain codes to a gradient broadcast algorithm Perfect reliability

To reduce the number of redundant packets travel in the network.Using network coding, relays re-combine the received packet along the multi-hop diffusion

We show in a simulation study how our implementation of a XOR network coding solution over an LT code [2] performs over a simple gradient broadcast algorithm: Reliability is maintained at a reduced energy and delay cost.Gradient broadcast routingBroadcast mode Any relay hearing a packet has to decide whether it can forward it or not. Only relays located closer to the sink than the previous hop relay are allowed to forward packets.

Cost field setupThe nodes distributively build the gradient cost fieldForwarding stage

[26] F. Ye, A. Chen, S. Liu, and L. Zhang. A scalable solution to minimumcost forwarding in large sensor networks. In International Conference onComputer Communications and Networks: Proceedings, pages 304309,2001.In order to push the packets towards the sink, 7Cost field setup

If Qp +L < QA then update QA=Qp+L L=the link costA new ADV packet is sent with a new packet cost Qp = QASink: ADV packet containing its own cost QQ=0floodingAll the other nodes have an initial cost Q = +

node AQA: ADV packet with packet cost Qp

Cost field setupThe node with the lowest value of QA sends its packet firstPrevents other nodes with higher costs from forwarding their ADV packet.With this algorithm, only one ADV packet per node is sent in the cost field setup stage. The link cost value can be expressed in various metrics (in hops, in meters, etc..). We consider a simple euclidian distance metric.Forwarding stageOnce a sensor S has a packet to send to the sink, it appends its own cost Qs to the packet and broadcasts it. All nodes receiving it decide to forward it if and only if their own cost Qi is lower than Qs.This algorithm is particularly reliable compared to single path routing Having very low control overhead but at the price of a very high packet redundancy. This probability is the same for all sensors in the network10Gradient broadcast routingFollowing works Creating additional forwarding rules to improve the tradeoff between reliability and energy consumption We control the amount of redundancy by introducing a forwarding probability pf . If the sensor is allowed to forward a packet based on its cost, it will do it with probability pf . [12], [13].

[12] K. Jaffr`es-Runser and C. Comaniciu. A probabilistic interference and energy aware gradient broadcasting algorithm for wireless sensornetworks. In Proceedings of IEEE ISWPC 2008, Santorini, Greece,2008.[13] K. Jaffr`es-Runser, C. Comaniciu, J.-M. Gorce, and R. Zhang. U-GRAB:A Utility-Based Gradient Broadcasting Algorithm for Wireless Sensor Networks. In IEEE Conference on Military Communications (MILCOM 2009), Boston, MA, USA, October 2009.Through coding: LT codesFountain codes provide both rate-less and universal property Transmission reliability can be assured without requiring channel state information

LT code Having lower decoding complexityBut at the price of a very high packet redundancy[17] M. Luby. LT Codes. In Foundations of Computer Science - FOCS 2002, pages 271, Vancouver, BC, Canada, November 2002. IEEE Computer Society.Combining LT codes and gradient broadcastingWe have considered a wireless sensor network 50 nodes spatially distributed following a Poisson distribution in a 2 dimensional space of 500m500m. Average node degree is of about three. The source first encodes the information with LT codes before broadcasting the encoded message. The message propagates in a relaying mesh from S to D following the gradient broadcast routing defined earlier.

The following simulation results are obtained using WSNet event-driven simulator [24].Combining LT codes and gradient broadcasting[24] WSNet. Worldsens simulator. http://wsnet.gforge.inria.fr/.

Combining LT codes and gradient broadcastingIn the simulations, a message is decomposed into K fragments.

The reliability is evaluated with the message success rate.For the LT-coding case, the message success rate is either one or zero, depending if the sink has been able to decoded the message or not. For the no LT-coding case, the source sends directly the K packets. In this case, the message success rate is measured by the ratio between the correctly received packets to the number of packets originally sent (i.e. K). Results are averaged over 1000 subsequent transmissions. The results in this figure are regressed with linear bezier approximation.15LT codes show a higher success rate on average for the same forwarding probabilities compared to the no coding case.Even if LT codes should ensure perfectly reliable transmissions, we do not always obtain a success rate equal to 1. Because of bad transmission conditions, nodes can keep trying (unsuccessfully) to relay the new packets of the source.Combining LT codes and gradient broadcastingwhat is not shown in these results is the additional coding overhead of LT codes and the fact that a substantialamount of copies arrive at the sink node.16XOR network coding heuristicTransmission with network coding More scalable and can lead to the optimization of complexity, throughput, transmission delay and security. In this paper ,we applying intra-flow network coding to fountain encoded packetsNetwork coding can play an efficient role to optimize the redundancyThe degree d of the packet to be created at the relay node is chosen with respect to the Robust Soliton distributionBuffered packets are then randomly selected and XOR-ed together until degree d is obtained or a MAXROUND value is reachedif d{1, 2} then a combination is only performed with probability p=0.2

Specific network codes have to be designed to maintain the degree distribution of LT coded packets at the sink .

17XOR network coding heuristic

[2] A. Apavatjrut, C. Goursaud, K. Jaffres-Runser, C. Comaniciu, and J.-M.Gorce. Toward increasing packet diversity for relaying lt fountain codes in wireless sensor networks. Communications Letters, IEEE, 15(1):52-54, January 201118XLT-GRABThe source sends each message m using an LT-codeK=100This ADV message serves two purposes Namely acknowledging m Updating the costs to account for topology changes in the network. The source keeps transmitting coded packets until an acknowledgement is received.

The sink acknowledges each message aftersuccessfully decoding it using an ADV message19A relay node forwards a packet based on a probability pf if it is located closer to the sink than the previous hop relay. A relay decides with a given XORing probability pxor to apply network coding using to forward a network coded packet instead of the received one.

XLT-GRAB

Performance resultsPerformance results are averaged over 50 consecutive message transmissions, each message encoded with an LT code.We have first evaluated the impact of pf and pxor on the number of duplicated packets .

pf < 0.4 : the network is not reliable at all for whichever XORing probabilitypf > 0.6 : the network is reliable for whichever XORing probability0.4 < pf < 0.6 : transitory area

the redundancy defined as the average number of identical (coded) packets received at the sink, the success ratio defined as the ratio between the number of correctly received messages to the number of transmitted messages,the end-to-end message transmission delay in seconds,the total energy consumed by all nodes of the network for emission and reception actions.pxor=0 corresponding to the non XORing case, and p=1 corresponding to the systematic XORing case. As expected,redundancy increases with the forwarding probability and decreases with the XORing probability as more diverse encoded packets are created through network coding21Performance results

the redundancy defined as the average number of identical (coded) packets received at the sink,

22Performance results

the success ratio defined as the ratio between the number of correctly received messages to the number of transmitted messages,

23The end-to-end message transmission delay in seconds

Performance results

The total energy consumed by all nodes of the network for emission and reception actions.

Performance results

the total energy consumed by all nodes of the network for emission and reception actions.

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