Sensor Network Capacity Enhancement Through Spatial Concurrency Bharat B. Madan and Shashi Phoha Applied Research Lab, P

1. Sensor Network Capacity Enhancement Through Spatial Concurrency Bharat B. Madan and Shashi Phoha Applied Research Lab, Penn State University

2. Sensor Network Communication CH • Nodes need to send their data to the CH (Cluster Head, Fusion Center). • Single wireless channel (e.g., 802.11) – need to time multiplex a single channel to service multiple sensors. • Need for concurrency – so that CH can receive data concurrently from high data rate multiple sensors. • Multiple frequency channels – CH deploys multiple frequency channels. • Spatial concurrency – create multiple channels in space (via beam forming). q1 s1 s2 sN CH Array BF1 BF2 BFN y1 y2 yN

3. 2 3 1 4 q1 ti i 0 1 N-1 di Space Diversity Processing • Space diversity requires multiple element array antenna. • Duality between time sampling and space sampling. • Optimally control the link (or channel) gain Gij • Assumptions: • CH knows the location (direction) of each sensor node • Sources lie in the far field • Problem formulation: Steer null in the direction of nodes 2,3,4 while maintaining constant gain in the direction of node 1 • Possible Solutions • Constrained optimization • Synthesize array response CH • Mathematical Model

4. Constrained Optimization CH • Minimize total output power subject to C1*W=1 wN-1 w0 w1 S • In the process of minimizing total power, the array would null out all nodes (except for the contribution of node 1 due to the constraint) • Spatial parallelism: Have multiple beam-formers running in parallel • The ith beam-former will maintain contribution of the ith node, while nulling out all other nodes.

5. Beam Pattern Synthesis • BF1:Maintain fixed response towards the direction of θ1while simultaneously generating nulls in the directions θ2, θ3 ,.., θM-1 • Design BF2, BF3, ..,BFM-1in a similar manner to deal with (M-1) sensor nodes of a cluster • Computational Issues • In general, C may not be square and existence of unique solution can not be guaranteed • Pair wise multi-stage algorithm BF1 y1 BF2 y2 Antenna array CH BFM-1 yM-1

6. w1 w1 w1 w1 w2 w2 w2 w2 w1 w2 S S S S S Pair Wise Multi-stage Algorithm • Consider only 2-element array and two directional sources q1 q2 • 2-element array that constrains output due to source1 to a constant level, while simultaneously nulling out source2

7. Receive Side Concurrent Beam-forming CH 3 2 1 ti 4 i 0 1 1 N-1 CH BF2 BF3 BF4

8. + + + + + + + + + tw Transmit Side Concurrent Beam-forming • Wireless communication: effectively half-duplex, i.e., not possible to simultaneously receive and transmit (unlike in wired communication  collision detection is feasible). • Antenna array (a node) is not capable of transmitting and receiving at the same time (on the same channel). • How to exploit available concurrency? • Accumulate requests for a finite time, i.e., batch processing s1 BF1 s2 BF2 s3 BF3 2 1 3 Array Manifold : Channel gain offered by ith element to signal sj

9. Beam Pattern Synthesis • Let u = sin(θ) • Initializebeam pattern  • Synthesis problem: Control the pattern Ps(u), such that is minimized, subject to N=17, M=4

10. Applications • Wireless Ethernet Switch • Single hop sensor fusion P2,j2 2’ P1,f1 1 CH RTS 3’ P2, f2 P3, j3 1 2 2 4 P1,j1 1’ 3 P3,f3 3 • Receive/Transmit concurrent beam-former makes the wireless router behave like a wireless Ethernet switch. • Each nodes sends RTS (in some arbitrary order). • Mobile command and control. • Concurrent Jamming CH CTS CTS T2,j2 2’ 1 Jamming waveform synthesizer Concurrent Transmit Side Beam-former 2 4 3’ T3,j3 3 T1,j1 1’ • CH accumulates RTSs and sends CTS concurrently to all nodes. • Above scenario: node 3 is enemy; 1 and 2 are friends • Alternately, 1, 2 and 3: enemy nodes being suppressed concurrently • CH, Wireless router/switch, C2 center, Jammer, etc. not constrained to be stationary

11. Accumulate RTS/CTS Protocol • k=0,1,..,N number of requests (arrivals) in time window tw f(k) tw k’ N k • Associate a reward (penalty) function f(k) with event k • Linear f(k)

12. Simulation Results • New MAC protocol, Accumulated RTS/CTS (A-RTS/CTS) • N: # array elements • Reward versus arrival rate λ • Concave function behavior • For a given N, as λ increases, R first increases (due to N degrees of concurrency) • Beyond certain point, higher value of λ leads to reduced performance (due to the request accumulation window getting filled up quickly  hence higher idle time)

13. Conclusions • Concurrent beam-forming is a powerful concept for providing spatial concurrency. • Several possible applications: • Sensor network traffic capacity enhancement. • Wireless Ethernet switch for high speed routing. • Mobile Control & Command Center • Concurrent jamming. • Cellular phone network – cell capacity enhancement.