COLLABORATIVE BEAMFORMING FOR WIRELESS AD-HOC NETWORKS

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1 International Journal of Coputer Science and Counication Vol. 3, No. 1, January-June 2012, pp COLLABORATIVE BEAMFORMING FOR WIRELESS AD-HOC NETWORKS A.H. Karode 1, S.R. Suralkar 2, Manoj Bagde 3 1 Dept. of Electronics and Telecounication, SSBTCOET, Jalgaon, India, atul_karode@rediffail.co 2 Dept. of Electronics and Telecounication, SSBTCOET, Jalgaon, India, shekhar_srs@rediffail.co 3 Dept. of Electronics and Telecounication, SSBTCOET, Jalgaon, India, bagde.anoj7@gail.co ABSTRACT In this paper, we present collaborative/cooperative bea foring for wireless ad-hoc networks. Via collaborative beaforing, nodes in wireless network are able to transit a coon essage over a long distance in an energy efficient anner. To iprove the perforance, we have developed a cooperative beaforing algorith. The idea of cooperative beaforing is to take the help of neighboring nodes to for a bea of electroagnetic energy i.e. the idea of antenna array is used. Since, in this type of network the nodes cannot have antenna array to ipleent beaforing, we take the help of nearby node to participate in beaforing process. Source node in the network is allowed to transit. Collaborating nodes receives the transitted packets. Subsequently each collaborating node transits its received signal. One or ultiple beas are fored each focusing on one destination node and reinforcing the signal intended for a particular destination as copared to the other signals. The proposed work achieves higher throughput and iproved SNR in the ad-hoc network. Keywords: Beaforing, Collaborative, Throughput, Ad-hoc Networks. 1. INTRODUCTION In last few years, there has been treendous updation in wireless networks and a big interest has been developed in ad-hoc wireless networks. An Ad-hoc wireless network is a network, coprised of obile coputing devices that use wireless transission for counication, having no fixed infrastructure. The obile devices also serve as routers due to the liited range of wireless transission of these devices that is several devices ay need to route or relay a packet before it reaches its final destination. Ad-hoc wireless networks can be deployed quickly anywhere and anytie as they eliinate the coplexity of infrastructure setup. These networks find application in several areas; soe of the are ilitary, collaborative and distributed coputing wireless sensor networks and hybrid wireless networks. The special features of obile ad-hoc networks bring the technology great opportunities together with different challenges. A challenge in the area of wireless ad-hoc network is transission of signal over a long distance requires significant aount of energy in order to overcoe attenuation. Energy is usually a scarce coodity in wireless ad-hoc networks, as nodes typically operate on batteries, which in any cases are difficult to replace or recharge. Thus, energy-efficient schees for longdistance transission in wireless networks have recently been of uch interest. In soe such situations, ultihop ay be a preferred solution. However, there are several challenges in transitting real-tie services over ultiple hops. Also, ultihop networks require a high node density which akes routing difficult and affects the reliability of link [1]. Recently, a collaborative beaforing technique was proposed in [3], in which randoly distributed nodes in a network cluster for an antenna array and beafor data to a faraway destination without each node exceeding its power constraint. The destination receives data with high signal power. A challenge with ipleenting beaforing in ad-hoc networks is that the geoetry of the network ay change dynaically. In this paper we propose a schee that is based on the idea of collaborative beaforing, and reduces the tie required for inforation sharing. A preliinary version of the proposed schee appeared in [4]. The work in this paper contains error analysis that provides insight into the perforance of the proposed approach. The ain idea is as follows. Source node in the network is allowed to transit. Collaborating nodes receive the transitted packets. Subsequently, each collaborating node transits its received signal. One or ultiple beas are fored, each focusing on one destination node, and reinforcing the signal intended for a particular destination as copared to the other signals. The rest paper section II describes Beaforing Background. Section III describes the Proposed schee. Section IV presents Siulation. Finally section V Conclusions.

2 182 International Journal of Coputer Science and Counication (IJCSC) 2. BEAMFORMING BACKGROUND For siplicity, let us assue that source and destination are coplanar. We index source nodes using a subscript t, with t i denoting the i-th node. At slot n, one source node t needs to transit the signal s (n) to a faraway destination node q. Suppose that set of N nodes, designated as collaborating nodes c 1,..., c N, have access to s (n). The locations of these collaborating nodes follow a unifor distribution over a disk of radius R. We denote the location of c i in polar coordinates with respect to the origin of the disk by (r i, Ψ i ). Let d i (ϕ ), or siply d i, represent the distance between c i and the destination q, where ϕ is the aziuthal angle of q with respect to the origin of the disk. d 0 ( ϕ ) or d 0 denotes the distance between the origin of the disk and q, so the polar coordinates of q are (d 0, ϕ ). Moreover, let d i (ϕ) denote the distance between c i and soe receiving point with polar coordinate ( d 0, ϕ). The initial phases at the collaborating nodes are set to 2π Ψi()(), φ = 1,..., din φ i = N λ This requires knowledge of distances (relative to wavelength λ) between nodes and destination, and applies to the closed loop case [3]. Alternatively, the initial phase of node i can be 2π Ψi() φ = cos() ri φ Ψi λ which requires knowledge of the node's position relative to soe coon reference point, and corresponds to the open loop case [3]. The path losses between collaborating nodes and destination are assued to be identical for all nodes. The corresponding array factor given the collaborating nodes at radial coordinates r = [r 1,..., r N ] and aziuthal coordinates Ψ = [Ψ 1,..., Ψ N ] at location with polar coordinate (d 0, ϕ) is Finally, the average beapattern can be expressed as [3] J1(( α ;)) φ φ Pav () φ = E{ ( z F ) φ } z = 1+ 2 N N α( φ;) φ When plotted as a function of ϕ, P av (ϕ) exhibits a ain lobe around ϕ, and side lobes away fro ϕ. It equals one in the target direction, and the side lobe level approaches 1/N as the angle oves away fro the target direction. The statistical properties of the beapattern were analyzed in [3], where it was shown that under ideal channel and syste assuptions, directivity of order N can be achieved asyptotically with N sparsely distributed nodes. As we have noted, all of the collaborating nodes ust have the sae inforation to (1) (2) 2 (3) ipleent beaforing. Thus, the source nodes need to share their inforation sybols with all collaborating nodes in advance. 3. THE PROPOSED SCHEME Here we refine the odel of [3], focusing ore directly on the physical odel for the signal, fading channel and noise. In addition to the above assuptions, we will further assue the following: 1. The network is divided into clusters, so that nodes in a cluster can hear each other. 2. A slotted packet syste is considered, in which each packet requires one slot for its transission. Perfect synchronization is assued between nodes in the sae cluster. 3. Nodes transit packets consisting of phase-shift keying (PSK) sybols each having the sae power. Also, nodes operate under half-duplex ode, i.e., they cannot receive while they are transitting. 4. Counication takes place over flat fading channels. The channel gain during slot n between source t i and collaborating node c j is denoted by a ij (n). It does not change within one slot, but can change between slots. 5. The distances between collaborating nodes and destinations are uch greater than the axiu distance between source and collaborating nodes. Suppose that cluster C contains J nodes. During slot n, source nodes t 1... t K need to counicate with nodes q 1,..., q K that belong to clusters C 1,..., C K, respectively. The aziuthal angle of destination q i is denoted by ϕ i. The packet transitted by node t i consists of L sybols s j (n). Due to the broadcast nature of the wireless channel, non-source nodes in cluster C hear a collision, i.e., a linear cobination of the transitted sybols. More specifically, node c i hears the signal x i (n) = a ji ()()() n s n w n j + i (4) where w i (n) represents noise at the receiving node c i. Let q denote the destination of s (n). In slot n +, = 1... K, each collaborating node c i transits the signal x ()() n + = *()() x n µ a n e Ψ ϕ (5) i i i i where µ is a scalar used to adjust the transit power and is the sae for all collaborating nodes. µ is of the order of 1/N. Collaborating nodes need to know which are source nodes and then estiate the channel between all source nodes and theselves. Also, collaborating nodes require the knowledge of their initial phases. To obtain initial phases, collaborating nodes also require knowledge of the aziuths of the destinations so that the beas can be steered toward desired directions. Given the

3 Collaborative Beaforing for Wireless Ad-hoc Networks 183 collaborating nodes at radial coordinates r and aziuthal coordinates Ψ, the received signal at an arbitrary location with polar coordinates (d 0, ϕ), is j2 π /() λ di ϕ y( ϕ; r,)()() Ψ = bxi n + e + v n + (6) where v ( n + ) represents noise at the receiver during slot n SIMULATIONS AND RESULTS In this section we study the BER perfors of the proposed ethod via siulation. We assue a playground area of network as 8 8. This area is two diensional. Then 400 nodes with unifor distribution populate this area. Then this area is divided into four clusters. The siulation is perfored using 8, 16 and 32 nodes along with 2 and 4 nodes as a collaborating node. Figure 1 and figure 2 shows the BER versus SINR. It shows that BER for 32 nodes is better than other two i.e. 16 and 8 nodes. Figure 3 and figure 4 shows for different nubers of collaborating nodes. Also BER is better for 16 nodes as copared to 8 nodes. Hence we can say that BER depends on the nuber of collaborating nodes. Figure 3: BER vs. SNR; N= 2,4,8, 16, 32 Figure 4: BER vs. SNR; N= 8, 16, 32 Figure 1: BER vs. SNR; N = 8, 16,32 Figure 2: BER vs. SNR; N = 8, 16, 32; 2 8 Sybols It was assued that the network is randoly generated and during this case this generated network the position of network nodes is not changing and even the source and destination fixed. In this case the different collaborating nodes are considered and siulation was perfored for above case. Figure 5(a) to 5(e) shows the result for above case. Figure 5(a) shows for 2 nodes as a collaborating nodes. Siilarly figure fro 5 ( b) to 5 (e) shows for 6, 8, 16 and 32 as collaborating nodes. One can see that as collaborating nodes increase we get good bea pattern as in above case. If bea pattern obtained in the case of 2 nodes as collaborating with the case 6 nodes as collaborating it is observed that good bea are obtain in case of 6 nodes as collaborating nodes. Good bea pattern and ore directivity in desired direction are observed if nuber of collaborating nodes increases. The ain lobe is of interest which is in desired direction and other are the side lobe in any undesired direction. Side lobes are unwanted because they waste power.

4 184 International Journal of Coputer Science and Counication (IJCSC) Figure 5(d): Nodes=400,Collaborating Nodes=8 Figure 5(a): Nodes=400,Collaborating Nodes=2 Figure 5(e): Nodes=400,Collaborating Nodes=16 Figure 5(b): Nodes=400,Collaborating Nodes=4 Figure 5(c): Nodes=400,Collaborating Nodes=6 Figure 5(f): Nodes=400,Collaborating Nodes=32

5 Collaborative Beaforing for Wireless Ad-hoc Networks CONCLUSION We have proposed a schee for wireless ad-hoc networks that uses idea of collaborative beaforing. We have provided analysis of SEP(Sybol error probability), which shows how perforance depends on the nuber of collaborating nodes. Hence we conclude that the beaforing enhances the capacity and the power consuption reduction for retransission. On the other hand if a single source can already counicate with intend destination at the desired rate and range then beaforing can be eployed to reduce the transit power per source and to reduce the energy radiated in undesired directions, which can be exploited for energy efficient in any applications. REFERENCES [1] H. Gharavi and K. Ban, Multihop Sensor Network Design for Wide-band Counications, Proc. IEEE, 91(8), pp , Aug [2] R. Lin and A.P. Petropulu, New Wireless Mediu Access Protocol Based on Cooperation, IEEE Trans. Signal Process., 53(12), pp , Dec [3] H. Ochiai, P. Mitran, H.V. Poor and V. Tarokh, Collaborative Beaforing for Distributed Wireless ad-hoc Sensor Networks, IEEE Trans. Signal Process., 53(11), pp , Nov [4] A.P. Petropulu, L. Dong and H.V. Poor, A Highthroughput Cross-layer Schee for Distributed Wireless ad-hoc Networks, Proc. Conference on Inforation Sciences and Systes, Baltiore MD, Mar [5] Lun Dong Petropulu, A.P. Poor, H.V., Cooperative Bea Foring for Wireless ad-hoc Networks, IEEE Trans. 6 Aug [6] Andy Ganse, An Introduction to Bea Foring, Applied Physics Laboratory University of Washington Seattle. [7] Hsi-Lu Chao An Introduction to Cooperative Networks. Departent of Coputer Science National Chiao Tung University. [8] Tiago Cailo, Jorge Sá Silva, André Rodrigues, Fernando Boavida, A Topology Generator For Real Wireless Sensor Networks Deployent, Departent of Inforatics Engineering, University of Coibra. [9] R. Hekat, Fundaentals Properties of Wireless Mobile Ad-hoc Networks. Netherlands, [10] X. Zhou, Haley M. Jones, Salan Durrani and Adle Scott, Effects of Beaforing on the Connectivity of Ad Hoc Networks, IEEE Trans., 2007, pp [11] R. Mudubai, D.R. Brown III, U. Madhow and H.V. Poor Distributed Transit Beaforing Challenges and Recent Progress, Counications Magazine, IEEE Publication, February 2009, 47(2), on pp [12] Martin Haenggi, Analysis and Design of Diversity Schees for ad hoc Wireless Networks, Counications, IEEE Journal, on 23, Jan pp