Relayer Selection Strategies in Cellular Networks with Peer-to-Peer Relaying

Transcription

1 Relayer Seletion Strategie in Cellular Network with Peer-to-Peer Relaying V. Sreng, H. Yanikomeroglu, and D. D. Faloner Broadband Communiation and Wirele Sytem (BCWS) Centre Dept. of Sytem and Computer Engineering Carleton Univerity, Ottawa, Canada Abtrat We onider a TDMA ellular multihop network where relaying via wirele terminal that have a good ommuniation link to the bae tation i ued a a overage enhanement tehnique. Provided that the ubriber denity i not very low, relaying vi a wirele terminal an have a ignifiant impat on overage, apaity, and throughput. Thi i mainly due to the fat that the ignal only have to travel through horter ditane and/or improved path. In thi work, we invetigated the effet of relaying node eletion trategie (eentially a routing iue) and maximum relayer tranmit power level on overage. Our imulation reult how that with a very modet level of relaying node tranmit power and with ome moderate intelligene inorporated in the relaying node eletion heme, the (high data rate) overage an be improved ignifiantly through two-hop relaying without onuming any additional bandwidth. I. INTRODUCTION Peer-to-peer relaying tehnology originate in paket radio and ad-ho network where the traffi ignal have to be relayed through one or more intermediate node(), depending on the ditane between the oure and the detination node [1], [2]. Uing relaying via mobile terminal a a overage enhanement tehnique in a ellular network i a fairly novel onept, and i gaining a trong interet [3] and onideration in the reearh ommunity uh a the WWRF (World Wirele Reearh Forum) [4]. In [5] we preented a omparion of hannel eletion method for relaying purpoe, along with jut one pathlobaed relaying node eletion trategy. A major finding in that paper wa that the overage improvement obtained from relaying i rather inenitive to the relaying hannel eletion trategy provided that ome diligene i hown in relaying node eletion and power ontrol i applied. In thi paper, we invetigate the enitivity of relaying ytem to relaying node eletion (routing) and relaying node maximum power level, whih were not overed in [5]. Baially two form of relaying an be arried out - analog form or digital form. In the analog form of relaying, the ignal reeived are forwarded without any ignal proeing; hene, a the ignal i amplified, o i the noie. In the digital form, on the other hand, the reeived ignal are firt demodulated and deteted, and thu leaned from noie and interferene (a muh a poible), and then re-enoded and re-modulated before forwarding; thi proe i ometime referred to a deode-and-forward relaying [6],[7] or regenerative relaying [8]. Thu, when digital form of relaying i employed, there i no noie propagation from one hop to the next; however there i a danger of detetion error propagation [6],[7]. It hould be borne in mind that for delay enitive traffi, uh a voie, the digital form of relaying may not be uitable due to the inurred delay. Meanwhile, other traffi type, whih are le enitive to delay and whih require large bandwidth to aommodate high data rate, would require the digital form of relaying in order to prevent the noie propagation and thu to minimize the error rate. We onider only the digital form of relaying in thi paper. Routing i very ruial and hallenging in ad ho network. Routing protool mut be power-aware, robut, and reilient to hange in the network topology; moreover, thee protool hould ahieve all of thee goal without exeive overhead. Routing in ellular multihop network, on the other hand, i likely to be muh more manageable, mainly due to the preene of a entral node (whih i the ommon oure in the downlink and the ommon detination in the uplink) with muh more funtionality and intelligene, namely, the bae tation. Neverthele, routing (relaying node eletion) i till a non-trivial iue in ellular multihop network ine there will often be many andidate relaying node for a node 1 that require relaying aitane and not hooing the optimal relaying node an have potential impat on the overall performane improvement. Sine power i a very preiou entity at the wirele terminal, the power alloated for relaying other terminal ignal i an important parameter for the relaying terminal. In thi paper we only onider the downlink in two-hop ellular network; mobility i not taken into aount. II. RELAYING NODE AND RELAYING CHANNEL SELECTION SCHEMES In thi paper, we onider a relaying hannel eletion heme baed on the maximum SIR, whih we referred to in [5] a the Semi -Smart Channel Seletion. In our modeling, firt the relaying node eletion i performed for eah node 1 In thi paper, the term node, wirele terminal, and ubriber, are ued interhangeably.

2 that require relaying aitane, and the relaying hannel eletion proe then follow (i.e., the two eletion proee are equential). Chooing a relaying node olely baed on the phyial ditane will not be a effetive a hooing one baed on the pathlo (beaue of the hadowing effet); however, loation-baed relaying node eletion heme are attrative a they an be arried out quite imply with the aid of loation tehnique. In fat, loation-baed routing ha already been propoed for ad ho network in order to redue the routing overhead and to maintain a mall routing table at the mobile node [9]-[11]. Our reult for the loation-baed relaying node eletion heme are omewhat optimiti, ine we aume perfet knowledge of loation. In thi paper, the mall-ale multipath fading effet are exluded during the relaying node and hannel eletion proee ine it would be impratial to perform interrelaying node and inter-relaying hannel hand-off baed on multipath ondition. However, after the relaying node and hannel are eleted, the effet of multipath fading are inluded in the overage imulation. A. Relaying Node Seletion A.1 Seletion Baed on Phyial Ditane Let N denote the et of andidate relaying node defined a the node whih have an adequate link to the bae tation in a two-hop relaying network (ee (9) for the definition of adequate link). Let d n1 and d n2 be the ditane aoiated with the firt (between the bae tation to the andidate relaying node) and the eond (between the andidate relaying node to the relayed node) hop, repetively, along the n th route, where n N. Then, the eleted route, r, i determined a follow in eah eletion heme: Shortet Total Ditane (STD) Seletion: r = arg min ( d + d ) (1) n1 n2 Leat Longet Hop (LLH) Seletion: r = arg min (max( d n1, d n2 )) (2) alln N Shortet Relaying Hop Ditane (SRD) Seletion: r = min ( d ) (3) arg n2 alln N A.2 Seletion Baed on Pathlo A diued earlier, although it i expeted that a relaying node eletion baed on pathlo will be uperior to the one baed olely on ditane, the pathlo-baed eletion will inur higher ignaling overhead (mainly due the pathlo etimation tehnique whih are not onidered in thi paper); thi diadvantage mut be weighted againt the additional performane return. In the following, we invetigate three relaying node eletion heme in a two-hop relaying network: the firt i a heme that make the eletion baed on the leat pathlo in the two ombined hop; the eond i baed on a route that ha the lowet bottlenek (in term of pathlo); and the third i baed on the leat pathlo in only the eond hop (whih we all the relaying hop). Similar to the notation ued in the previou etion, let PL n1 and PL n2 denote the pathloe in db aoiated with the firt and the eond hop, repetively, along the n th route, where n N. Then, the eleted route, r, i determined a follow in eah eletion heme: Minimum Total Pathlo (MTP) Seletion: r = arg min ( PL + PL ) (4) n1 n 2 Leat Maximum Pathlo (LMP) Seletion (ued in [3]): r = arg min (max( PLn1, PLn 2 )) (5) all n N Minimum Relaying Hop Pathlo (MRP) Seletion: r = arg min{ PL } (6) n2 Fig. 1 illutrate a ell with a bae tation, a relayed node, and three andidate relaying node. Aoiated with eah andidate relaying node, there i a andidate path (route), marked a I, II, and III in Fig. 1. Path I orrepond to the minimum relaying hop pathlo eletion, path II orrepond to the minimum total pathlo eletion, and path III orrepond to the leat maximum pathlo eletion heme. There will be a many andidate path (route) a the number of node that have a good link with the bae tation (i.e., andidate relaying node). In pratie, however, eah node that require relaying aitane hould limit it andidate relaying node et to only a few in order to minimize the routing overhead. B. Channel Seletion One a relaying node ha been determined, an additional hannel mut be aigned for the relaying hop link to prevent feedbak loop. Baed on our previou paper [5], we propoe reuing hannel from the adjaent ell for relaying purpoe; the very appealing advantage of thi trategy i that no additional bandwidth i onumed in the relaying mode. One of the hannel eletion heme deribed in [5] i reintrodued here for onveniene. Let γ i be the ignal-tointerferene ratio (SIR) experiened at the relayed node i, on hannel, and B the et of all bae tation that ue hannel plu the relay node j whih will alo ue hannel for relaying purpoe (i.e., B i the augmented o-hannel et for hannel ). Then, G ji P j γ i =, (7) G P ki k k B, k j where G ji i the pathlo between the relayed node i and the relaying node j, and P j relaying node j. Similarly, G ki i the tranmitted power of the i the pathlo between the

3 relayed node i and a o-hannel bae tation k whoe hannel i being probed for reue, and Pk i the orreponding bae tation tranmit power. Alo, let L denote the et of all hannel in the adjaent ell (within the luter of the ubriber whih need relaying aitane). Then, a hannel, l, i eleted a follo w: Maximum SIR Seletion: l = arg max( γ ) (8) all L i III. SIMULATION MODEL We onider the downlink of a TDMA urban ellular network where two-hop relaying i ued whenever there doe not exit a uffiiently good diret link between a bae tation and a wirele terminal. Our propagation model onit of ditane-dependent attenuation with a propagation exponent of 4, lognormal hadowing (tandard deviation 10 db), and flat Rayleigh fading. The ame pathlo model i ued between a bae tation and a wirele terminal, and between one wirele terminal and another. The imulation area onit of 6x6 quare ell with wrap around edge. The luter ize i hoen to be 4. Only the 400x400 meter ell ize i onidered in thi paper, whih repreent an interferenelimited environment. Omnidiretional antenna are ued at both bae tation and wirele terminal. The arrier frequeny i taken to be 2.5 GHz and the tranmiion bandwidth 2 MHz. The thermal noie i alo onidered at the reeiver, with a noie figure of 8 db. Due to the large bandwidth and noie figure aumption, the digital form of relaying i ued throughout. We onidered only twohop relaying with ingle-la traffi where every wirele terminal ha the ame SINR requirement of 10 db (thi rather high value for SINR may orrepond to a partiular ontellation in a ytem employing adaptive modulation and oding). Simulation are run with the inorporation of tranmit power ontrol. Whenever power ontrol i employed, a naphot heme i ued for both relaying and non-relaying ae with a tep ize of 2 db. Power update are performed until the reeiver SINR fall between 10 and 12 db, or until a maximum of 10 update are reahed, whihever ome firt. The node within a ell are plaed randomly aording to a two-dimenional uniform ditribution. The number of hannel available in eah ell i aumed to be equal to the number of (ative) wirele terminal; that i, all the ell are aumed to be fully loaded before relaying, and therefore, a relaying hannel ha to be hoen among the already ued hannel in the adjaent ell. The imulation proe onit of the following tep: 1) Set the number of ubriber (node) per ell, S. 2) Plae the ubriber randomly aro the network while maintaining S ubriber per ell. Eah ubriber i given ervie through the loet bae tation. Aign an independent lognormal hadowing omponent between eah ubriber and it bae tation. 3) Determine whether eah ubriber ha a uffiiently good link with it bae tation or not. Toward that end, ollet 100 SINR ample for eah ubriber with eah ample undergoing independent Rayleigh fading (note that all of the 100 ample have the ame lognormal hadowing omponent). Exeute a naphot power ontrol before reording eah SINR ample. For a given ubriber, if 95% or more SINR ample turn out to be above the 10 db threhold, i.e., if Pr[ SINR 10 db] 95%, (9) then laify that ubriber a one having a uffiiently good link with it bae tation (uh a ubriber doe not require relaying; indeed, it ontitute a andidate relaying node for thoe ubriber whih need relaying aitane). Otherwie (i.e., if the omplement of (9) i true), laify it a a ubriber that ha a poor link (and therefore that ubriber will require relaying aitane). 4) For all ubriber that require relaying aitane, hooe a relaying node and a relaying hannel aording to the algorithm peified in Setion II (note that the multipath fading i not taken into aount in both relaying node and relaying hannel eletion heme). 5) With relaying inorporated, ollet one again 100 SINR ample for eah ubriber, inorporating independent Rayleigh fading in eah ample, alo inorporating naphot power ontrol for eah ample. For ubriber whih are erved by their bae tation through a ingle-hop, ue (11) to determine whether the link are uffiiently good; and for thoe ubriber whoe ignal are delivered in two-hop, ue the following riterion: Pr[{ SINR 1 & SINR2} 10 db] 95%. (10) In the above, ubript 1 and 2 orrepond to the firt and eond hop, repetively. In the SINR alulation, take into aount the interferene reated a a reult of relaying (an independent Rayleigh fading and a lognormal hadowing omponent are aoiated with all the interferene link). Reord the number of ubriber that have good link, aording to (9) or (10) a appropriate, only in the four ell that ontitute the innermot luter in the 9-luter wrap around network. 6) Repeat tep 2)-5) for a total of 1000 time. The fration of ubriber that have good link, out of the total number of 4 (number of ell in the innermot luter) x S (number of ubriber per ell) x 1000 (number of run) ubriber oberved, yield the deired overage value. 7) Go to tep 1) and repeat tep 1)-6) for variou S value.

4 IV. SIMULATION RESULTS For the reult hown below, the maximum bae tation tranmit power ued for any mobile node i 1 W (30 dbm). From Fig. 2, due to the hadowing effet we oberve that the relayer eletion heme baed on pathlo offer a onitently uperior performane in omparion to thoe baed on ditane alone, a expeted. It i worthy to note the performane differene among the three eletion heme. For the ditane-baed relayer eletion heme, the enhaned overage range from approximately 76% to 90%, at a ubriber denity of 64 and a maximum relaying node tranmit power of 20 mw, over the 70% ae with no relaying. Meanwhile, for the pathlo-baed eletion heme, we ee an even larger performane differene among the three heme. For intane, the highet overage improvement i approximately 97%, whih i baed on the Leat Maximum Pathlo (LMP) heme; meanwhile, the leat improvement, approximately 79%, i baed on the Minimum Total Pathlo (MTP) heme. Finally, Fig. 3 offer a perpetive on how the ubriber denity impat the range of maximum relaying node tranmit power level required to ahieve a maximum attainable overage improvement at that partiular ubriber denity. A an be oberved, at a low ubriber denity (16 and below) we ee that the ut off maximum relaying node tranmit power i approximately 30 dbm (1 W) before the performane improvement tart to aturate; while, at the high ubriber denity (32 and above), the ut off i approximately 24.8 dbm (300 mw). What thi reveal i that at a reaonably high ubriber denity, mot of the potential overage improvement an be obtained at fairly low relaying node maximum tranmit power level. V. DISCUSSIONS AND CONCLUSIONS In thi paper we invetigated the impat of variou relaying node eletion trategie on the ytem overage in a fully loaded ellular ytem. Our reult how that overage i enitive to the relaying node eletion trategy. Moreover, the degree of improvement depend on the denity and maximum tranmit power level of potential relaying node. In mall ell (interferene-limited ytem), the overage enhanement due to relaying an be inreaed ignifiantly (partiularly when uing pathlo-baed eletion heme) while uing only a low relaying node tranmit power. It hould be noted that thi dramati inreae i obtained with the ue of power ontrol. Moreover, thi performane improvement i obtained even without reerving extra hannel for relaying purpoe, ine the additional interferene due to relaying i onfined to within the viinity of the relaying/relayed node pair a a reult of power ontrol. Finally, baed on the reult in Fig. 3 it an be onluded that the higher the ubriber denity, the le tranmit power it i required of the relaying terminal ine a reaonably good relaying node eletion heme an be applied in order to aign the bet relaying/relayed node pair, whih will in turn have a poitive effet on the overage improvement. A imilar work i reported in [12] in whih ODMA (opportunity-driven multiple ae) type of relaying i onidered in a CDMA network to inreae the overage for voie ommuniation. Although a imilar finding to our i oberved there (i.e., overage i ignifiantly improved with relaying), it hould be noted that the power ontrol algorithm and the ignaling protool required are, in general, more tringent and omplex for CDMA-baed relaying network in omparion to TDMA-baed type; thi i mainly due to the inherent interferene-limited nature of CDMA network (we alo tried to make our imulation network a interferenelimited a poible by auming fully loaded ell). More information on ignaling and routing protool required to perform relaying in a ellular network an be found in [12]. Other iue that till remain open inlude relaying through fixed relayer veru wirele terminal (peer-to-peer relaying), two-hop veru multihop relaying, and mobility and handoff management in peer-to-peer relaying network. The advantage of uing fixed relayer inlude not onuming preiou reoure of the wirele terminal (whih would be the ae in peer-to-peer relaying), le handoff in a mobile environment, minimum ervie guarantee (finding a relayer will beome more diffiult in an unloaded peer-to-peer relaying network), and better eurity. The main diadvantage of fixed relayer, on the other hand, i the deployment ot. It i likely that future ellular network may upport relaying through both fixed relayer a well a wirele terminal. One final iue worth noting i that peer-to-peer relaying doe not work properly in the FDD (frequeny-diviion duplex) mode with the urrent wirele terminal tehnology. Thi i due to the fat in the FDD mode, the wirele terminal tranmit and reeived at two different frequeny band, although a relaying wirele terminal ha to be able to tranmit and reeive in both band. Therefore, TDD (timediviion duplex) i a more appropriate mode for peer-to-peer relaying. Sine fixed relayer may potentially be equipped with more advaned hardware and ignal proeing tehnique, their operation in the FDD mode may be poible. ACKNOWLEDGMENT Thi reearh wa upported by a grant from Nortel Network, Ottawa. In partiular, the author would like to thank to Dr. Baam Hahem (for pointing out the loationbaed tehnology a an aid in implifying routing) and Dr. Shalini Periyalwar of Nortel Network. REFERENCES [1] C. Chang and J. Chang, Optimal deign parameter in a multihop paket radio network uing random ae tehnique, IEEE Globeom, vol. 1, pp , [2] E. Royer, A review of urrent routing protool for ad ho mobile wirele network, IEEE Peronal Communiation, pp , April 1999.

5 [3] H. Yanikomeroglu, Fixed and mobile relaying tehnologie for ellular network, Seond Workhop on Appliation and Servie in Wirele Network (ASWN 02), pp , July 2002, Pari, Frane. [4] World Wirele Reearh Forum (WWRF) webite, [5] V. Sreng, H. Yanikomeroglu, D. Faloner, Coverage enhanement through two-hop relaying in ellular radio ytem, IEEE Wirele Commun. and Networking Conf. (WCNC 02), Marh 2002, Orlando, USA. [6] J. Laneman and G. Wornel, Energy effiient antenna haring and relaying for wirele network, IEEE Wirele Commun. and Networking Conf. (WCNC'00), pp. 7-12, Sept. 2000, Chiago, USA. [7] J. Boyer, D. D. Faloner, and H. Yanikomeroglu, A theoretial haraterization of the multihop wirele ommuniation hannel with diverity, IEEE Globeom, Nov. 2001, San Antonio, USA. [8] M. Hana and M. Alouini, End-to-end performane of tranmiion ytem with relay over Rayleigh fading hannel, ubmitted to IEEE Tranation on Wirele Communiation. [9] Y. Ko and N. Vaidya, Loation-aided routing (LAR) in mobile ad ho network, MobiCom, pp , [10] R. Jain, et. al, Geographial routing uing partial information for wirele ad ho network, IEEE Communiation Magazine, pp , Feb [11] I. Stojmenovi, Poition-baed routing in ad ho network, IEEE Communiation Magazine, pp , July [12] T. Roue, I. Band, and S. MLaughlin, Capaity and power invetigation of opportunity driven multiple ae (ODMA) network in TDD-CDMA baed ytem, IEEE Int. l Conf. on Commun. (ICC 02), pp , April 2002, New York, USA. Fig. 1. Illutration of three poible route baed on three different relaying node eletion heme (the number attahed to the edge indiate the pathloe in db). Fig. 2. Coverage v. number of ubriber per ell (with power ontrol, maximum relay node tx power: 20 mw (13 dbm)). Fig. 3. Coverage v. maximum relaying node tranmit power (with power ontrol, Leat Maximum Pathlo (LMP) eletion).