An Experimental Study of Fractional Cooperation in Wireless Mesh Networks

Transcription

1 An Experimental tudy of Frational Cooperation in Wireless Mesh Networks Anthony Cale, Nariman Farsad, and Andrew W. Ekford Dept. of Computer iene and Engineering, York University 47 Keele treet, Toronto, Ontario, Canada M3J 1P3 Astrat Frational ooperation is a deentralized, lowomplexity wireless networking protool in whih nodes have the aility to dynamially selet a fration of its resoures to ommit to forwarding, and where soures may use more than one relay to onvey information to the destination. In this paper, an implementation and a series of experiments are presented to demonstrate the pratial performane and effetiveness of frational ooperation. A low-omplexity MAC layer protool is used, whih employs frational ooperation using LT odes in the asene of entral oordination. Experimental results from realworld trials are given, whih show that this protool an maintain a reasonale throughput when nodes are aruptly entering and leaving, making it ideal for a dynamially hanging system, suh as an ad-ho network. The redundany of information seen in the network makes this sheme roust to unfavourale hannel onditions. I. INTODUCTION With the prevalene of wireless networks and ever inreasing appliations, the sale of wireless networks grows year y year. Conurrently, the need for high-speed data transmission also grows every year as the Internet eomes an essential part of soiety. However, as the size of the wireless networks grow, maintaining higher data rates eomes ever more hallenging. One solution to this prolem is spreading the load aross different nodes in the network. The nodes of wireless networks an e distriuted over wide areas, where multiple hops may e needed in order for information to reah a destination. Cooperation [1], [2] provides a possile solution to this prolem y allowing eah node to assist its neighours in transmitting information to a data sink. The asi ooperative system onsists of three nodes: a soure, a relay, and a destination. The relay an employ various ooperative shemes suh as deode-andforward [3] and demodulate-and-forward [4], [5], to assist the soure in transmitting its information its to the destination. Furthermore, where links are affeted y fading, ooperation an e used to inrease the spatial diversity of the system. In a large sale wireless network, there are multiple soures and multiple relays, where eah soure node is typially in radio range of more than one relay. Furthermore, in systems suh as wireless mesh networks (WMN) [6], eah node an e a soure and a relay. When a traditional ooperative sheme is employed, eah soure has a dediated numer of relay(s), where eah relay forwards all the soure s transmission its. However, relays might have their own information to transmit to the destination or might e unwilling to dediate all their resoures to relaying; furthermore, alloation of resoures is a diffiult prolem. To overome this issue, frational ooperation [7], [8] an e employed, where eah relay node an forward a fration of the soures transmission its. A related frational strategy in ellular networks was independently proposed in [1], where moiles are permitted to use multiple ase stations to inrease throughput to ell-edge users for multiple-input multiple-output (MIMO) systems. In this work, we develop a low-omplexity medium aess ontrol (MAC) layer protool, employing frational ooperation, and test it on pratial sensor networking hardware in real-world experiments. Our ontriutions in this work are as follows: We implement our MAC layer protool on Imote2 sensor nodes, reating a WMN. To the est of our knowledge, this is the first work where frational ooperation is implemented in a real wireless system. Compared to simulations, real-world experiments are unommon in the wireless literature. Through experiments on our implemented WMN, we show that the proposed protool an relialy transmit information over the network, even when eah node in the network relays a fration of the soure s transmission, and there is no entralized oordination of the frations. Using the proposed protool, nodes an relialy enter and exit the network at any time. We also show that when our protool is employed, the throughput dereases graefully as relay nodes exit the network. This is in ontrast to a traditional ooperative system, where an exodus of relay nodes an result in a sudden, preipitous drop in throughput. Based on the otained results we onlude that frational ooperation is suitale in large sale networks where nodes enter and exit the network onstantly. A. Frational Cooperation II. COMMUNICATION POTOCOL In frational ooperation, eah node selets a fration α [, 1], representing the fration of any neighour s transmission that it will relay; for example, if α =.5 and a neighour transmits a paket of 1 its, then the relay will hoose 5 of those 1 its for retransmission. The manner in whih these

2 elay 2 a ink oure, a elay 3 elay 1 Fig. 1: A multiple relay system oure Paket Coded Paket Forwarded Coded Paket its are seleted is explained elow. Both the hoie of α and the identities of the its to e relayed are made y the relay, and not oordinated with other nodes (however, it is assumed that the destination knows whih symols are seleted). ee also [7], [8] for more details. ine the relays are not oordinating the seletion of frations, it is impossile to ensure that every soure symol is ultimately seleted for relaying. As a result, it is essential to use an erasure-orreting ode to fill in the lanks and reover any symols whih were missed during the seletion proess. The LT ode [11] is used, as it has exellent deoding performane and low enoding omplexity (whih is essential for our hardware). B. elay Model The main goal of this protool is to ahieve a low omplexity ommuniation sheme whih inorporates frational oded ooperation. In order to fulfill this requirement, all nodes in the model ommuniate y roadasting. A few nodes are seleted as soures, ut all nodes (exept the sink) have the aility to ehave as relays. A soure annot e its own relay. Nodes in this system follow two rules: oure nodes transmit unoded information pakets. If a node reeives an unoded paket, it will selet a fration α of the its in the paket, use those its to reate a oded paket (using an LT ode), and roadast the oded paket to all its neighours. If a node reeives a oded paket, it will reroadast the paket in its entirety without deoding or re-enoding. A small example of this relay model is shown in Figure 1. A soure node roadasts paket a whih is seen y two relays. Coded pakets and are generated y these relays and are roadast. At this point, any relay whih reeives a oded paket will forward it without any proessing. ine this is roadast sheme, nodes may reeive dupliate pakets throughout this proess, for example, relay 1 forwarding paket ak to relay 2. These transmission links have een omitted from the figure for ease of viewing. ine the nodes are unaware of the suess of transmission, or the loation of the sink, we use a paket time to live (TTL) (measured as a maximum numer of hops) as a simple means of preventing flooding. Furthermore, we assume that the destination has knowledge of, ut no ontrol over, the identities of soure symols that the nodes are seleting for relay. As we will disuss elow, this is equivalent to knowledge of the LT ode employed y eah relay node; in pratie, this is implemented y making the destination aware of eah relay s random numer seed, whih an e expressed ompatly. Due to the nature of the protool, every soure paket sent will trigger the generation of a ertain numer of oded and forwarded pakets. A soure paket onsists of k symols, with l its per symol. Traffi an e expressed as a sum of the numer of soure, oded, and forwarded pakets generated per unit time. Eah one of the s soures will generate a paket with size kl. For every soure paket in the system, r oded pakets are generated with size αkl. For every oded paket in the system, r 1 forwarded pakets are generated with size αkl. We define the quantity Φ, expressed in its per seond (ps), to e the upper ound of traffi seen in the hannel provided all nodes are within range of eah other, and there is no interferene etween ommuniation links. Given s soures, r relays, and a soure paket rate λ expressed in pakets per seond, let n soure, n oded, and n forwarded represent the numer of soure its, oded its, and forwarded its generated per paket. Then Φ is given as follows: Φ=(n soure + n oded + n forwarded )sλ = (kl)+r(αkl)+r(αkl)(r 1) sλ = kl(1 + αr 2 )sλ When soure nodes are also allowed to relay, the aove equation eomes: (1) Φ=skl 1+α(r + s 1) 2 λ (2) To further redue unneessary forwarding, eah node is ale to ross-hek paket id numers against a uffer in order to determine dupliates. This guarantees us that Φ remains onstant for systems whih require a large TTL. C. Enoding and Deoding Proess This setion desries the enoding/deoding proess from the generation of a soure paket to its reonstrution at the sink; for ease of implementation, the proess is slightly different from the LT odes found in the literature, in that operations are performed symol-wise. The proess egins when a soure node roadasts an unoded information paket in the network. Eah relay node, upon reeiving an unoded paket, will egin its enoding step.

3 1 9 8 oure Transmissions Deoded Pakets at the ink Coded Pakets at the ink Total Traffi at the ink 7 Throughput (ps) elay atio α Fig. 3: Plot illustrating the deoding effiieny for varying α values Fig. 2: Experiment setup Given k information symols reeived, a relay will generate αk LT oded symols whih are then enapsulated into a oded paket and roadast. In [11], an LT oded symol is generated as follows: A degree 1 d k is hosen for the enoding symol with some proaility given y the roust solition distriution [11]. d symols are hosen uniformly at random from the set of k information symols. This suset of symols is referred to as the neighour set of the enoding symol. The value of the enoding symol is determined y performing the symol-wise exlusive-or (XO) operation etween the value of eah neighour symol. Given distriution onstants =.1, δ =.1, and k = 3, the roust solition distriution is approximated as: P r (d = 1, 2, 3, 4, 5, 6) = (.13,.26,.1,.5,.4,.42) The deoder needs to e aware of oth the degree of a oded symol and its set of neighour symols. As noted in the previous setion, this is aomplished y using a random seed in the enoding proess whih is utilized y a pseudorandom generator at the deoder to reprodue the exat distriutions used in the enoding proess. Upon reeiving a oded paket, the sink will rereate the degree and neighour set of eah oded symol (as seen in olumns 3 and 4 of Tale I). The sink handles the deoding of pakets individually suh that all oded symols are analyzed efore moving on to the next oded paket. A symol is onsidered deoded when its degree is equal to one (sine there is only one neighour symol x k, the value of the ode is equal to the x th information symol). One all degree one symols are handled, the algorithm tries to redue the degree of every other symol to one. This is done y omparing already deoded information symols to TABLE I: ink proessing of oded pakets generated in a one-soure, two-relay system Information ymol Coded ymol x id 1 seed Coded ymol y id 2 seed Code ymol Proessing ymol Degree Deoded Neighour Value Information et ymol id {1,2} 28 (,,,92) 1 1 {4} 92 (,,,92) 3 1 {2} 11 (,11,,92) (23,11,,92) id {1,2,3,4} 166 (23,11,23,92) 1 2 {2,4} 87 (23,11,,92) 3 3 {2,3,4} 177 unused the neighour set and oserving if that symol is an element of it. If so, a XO operation is performed etween the value of the oded symol and the deoded information symol, the element is removed from the neighour set, and the degree is redued y one. This proess is repeated until the soure message is fully reonstruted. A short example of this is seen in Tale I; this example assumes α =.75 and k =4. The soure analyzes symols id 1 (1, 2, 3) and determines that symols 1 and 3 an e automatially deoded. It then ross-referenes the seond deoded information symol with oded symol 2 to retrieve information symol 1. It then ontinues to proess symols id 2 (1, 2, 3) and following the same proedure, manages to deode the information paket. ine the paket was deoded after analyzing symol 2, all susequent symols and oded pakets reeived are disarded (this inludes pakets reeived y the forwarding proess).

4 1 9 8 oure Transmissions Deoded Pakets at the ink Coded Pakets at the ink Total Traffi at the ink 3 25 oure Transmissions Deoded Pakets at the ink Coded Pakets at the ink Total Traffi at the ink Throughput (ps) Throughput (ps) Time Elapsed (s) Time Elapsed (s) Fig. 4: One soure transmitting to four initial relays, one relay eing shutoff every 5s. D. Limitations The sink node experienes a performane penalty if the numer of information symols k is large. The main ontriuting fator to this prolem is that the deoding algorithm in this model was designed in a reursive fashion. It was shown in [11] that the deoding step runs in linearithmi time. Due to the simpliity of the ommuniation protool, large traffi is seen at the sink if there are too many nodes in lose proximity. Under these onditions, this model suffers from hannel saturation. III. EXPEIMENT This setion desries the implementation of our protool using Imote2 [12] sensor nodes whih are fatory onfigured to run Mirosoft.NET Miro Framework 2. and are programmed with the C# language. Figure 2 displays a setup used during experimentation. A. Enoding and Deoding imulation A simulation of the enoding/deoding proess was performed and it was shown that over 5 trials, given an information paket onsisting of k symols and onstants and δ, a reeption overhead of aout 6% was oserved. That is to say, the deoder would need to see 1.6k oded symols on average to suessfully omplete a single deode. Given α =.5, that would equate to 3.2 oded pakets. B. esults In this ontext, the primary features of frational ooperation are roustness, fairness, an inrease in transmission range, and the asene of entral oordination. Four experiments were designed to test these aspets of our protool. A sensitivity analysis was first done on α to determine what effet it would have to total throughput seen at the sink. Figure 3 desries these results. Eah data point was generated using a single soure transmitting to three relays for 2s. Inreasing α results in a trade-off etween higher deoding effiieny and extra traffi. Fig. 5: Three soures transmitting to six initial relays, one relay eing shutoff every 5s TABLE II: Parameters used in experiments Parameters Information symols, k 3 ymol length, l 8 its elay ratio, α.5 oure send rate, λ.5 pakets per seond The next experiment is set up using five nodes; one ating as the sink, one as the soure, and four as dediated relays. Nodes are positioned suh that they are all within range of eah other. During the ourse of the experiment, one relay node is shutoff every 5s. This experiment is designed to show that the protool is roust to relays leaving the system. For simpliity, we hose α to e the same for all relays. The results are divided into 2s time ukets, and are oserved over 2s in Figure 4. As relays are dropped from the system, the sink node experienes a derease in the amount of oded pakets it reeives, whih results in a redution of deoding effiieny. The traffi seen at the sink is onsistent with Equation 2. For every soure paket sent, there are r oded pakets generated (assuming all relays are within range of the soure). The simulation in etion III-A tells us that under the system parameters, an average of 3.2 oded pakets are required to suessfully deode. ine there are four relays present at the start of the experiment, we an expet to reah lose to 1% deoding effiieny during this period. The third experiment mimis the previous in all aspets, exept there are three soures roadasting instead of one. As previously mentioned in etion II-B, all nodes are ale to relay. As suh, for every s soures in the network we have an extra s 1 relays. This test is designed to show the roustness of the protool when there are a large numer of relays. As Figure 5 illustrates, the presene of two extra soure nodes greatly inreases sink traffi in our model; so muh so that it reahes hannel saturation and traffi no longer onforms to Equation 2. As we an see, the sink is deoding

5 D 15 oure Transmissions Deoded Pakets at the ink Coded Pakets at the ink Total Traffi at the ink Diret Unoded Pakets at the ink Throughput (ps) 1 5 Fig. 6: ix relays positioned over a field size of 18m 3m. A soure node is moving farther away from the sink destination Distane from the ink (m) Fig. 7: Plot illustrating the enefits of frational ooperation over diret non-routed paths at almost the same rate as the soure is transmitting, even as relays are eing dropped. This shows that provided there are enough relays in the system, an exodus of a few nodes will not ause signifiant performane degradation. The final experiment is set up as seen in Figure 6. ix relay nodes were positioned over an area of aout five square meters. A soure node is set up to roadast for 2s at fixed loations away from the sink. This test is intended to show to that our protool ahieves a signifiant oost in range over diret transmission. As depited in Figure 7, throughput of deoded pakets at the sink tends to graefully falloff with distane. Our protool ahieves over a three fold inrease in transmission range over a diret non-routed path. The extra redundany of dupliated pakets allows us to employ this protool in unfavourale hannel onditions and expet reasonale throughput. IV. CONCLUION This paper has presented a low-omplexity MAC layer protool whih employs frational ooperation using LT odes in the asene of entral oordination. A node has the aility to dynamially selet a relay ratio, whih ontrols how muh, if any, of its resoures it is willing to ommit to forwarding. As we have shown, a system with many relays is roust to noisy hannel onditions eause there is a large amount of dupliate information in the network. Due to the roadast nature of this protool, pakets are ale to avoid forwarding loops y ross-heking paket ids, thus making this sheme pratial for larger systems whih have a high paket TTL. Our approah is partiularly useful for hardware with omplexity onstraints, suh as the imote hardware often found in sensor networks. Further, our results prove that this protool an maintain deent throughput when nodes aruptly enter or exit the system, making this ideal for a dynamially hanging system suh as an ad-ho network. For future work, we plan to perform further experiments on a larger tested one the deoding algorithm is restrutured to run more effiiently. This would allow us to inrease the numer of information symols sent y a soure, resulting in greater system throughput. EFEENCE [1] A. endonaris, E. Erkip, and B. Aazhang, User ooperation diversitypart I: system desription, IEEE Trans. Commun., vol. 51, pp , Nov. 23. [2] A. endonaris, E. Erkip, and B. Aazhang, User ooperation diversity, Part II: Implementation aspets and performane analysis, IEEE Trans. Commun., vol. 51, pp , Nov. 23. [3] A. Nosratinia, T. Hunter, and A. Hedayat, Cooperative ommuniation in wireless networks, IEEE Commun. Mag., vol. 42, no. 1, pp , Otoer 24. [4] D. Chen and J. N. Laneman, Modulation and demodulation for ooperative diversity in wireless systems, IEEE Trans. on Wireless Commun., vol. 5, no. 7, pp , Jul. 26. [5] J. P. K. Chu and.. Adve, Implementation of o-operative diversity using message-passing in wireless sensor networks, in Pro. IEEE Gloeom, t. Louis, MO, pp , De. 25. [6] Y. Yan et al., Performane Analysis of IEEE82.11 Wireless Mesh Networks, IEEE Int. Conf. Commun., pp , May [7] A.W. Ekford et al., Low Complexity and Frational Coded Cooperation for Wireless Networks, IEEE Trans. Wireless Commun., vol. 7, pp , May 28. [8] A.W. Ekford et al., Low-omplexity strategies for ooperative ommuniations, in Cooperative Wireless Commun., ed. Y. Zhang, H.-H. Chen, and M. Guizani, CC Press: Boa aton, FL, pp , 29. [9] N. Farsad, A. W. Ekford, esoure Alloation via Linear Programming for Multi-oure, Multi-elay Wireless Networks, in Pro. IEEE International Conferene on Communiations (ICC), pp.1 5, May 21. [1] N. Kusashima et al., Frational Base tation Cooperation Cellular Network, 7th Int. Conf. on Inform., Commun. and ignal Proessing, pp. 1-5, 8-1, De. 29. [11] M. Luy, LT Codes, in Pro. 43rd ymp. on Found. of Comp. i., pp , Nov [12] Imote2: