DOAALOHA Slotted ALOHA for Ad Hoc Networking Using

DOA-ALOHA: Slotted ALOHA for Ad Hoc Networking Using Smart Antennas Harkirat Singh & Suresh Singh Portland State University, OR, USA 1

Outline What is an ad hoc network Smart Antenna Overview Protocol description Implementation of the protocol within OPNET • Performance study of the protocol • Summary • • 2

Ad Hoc Networks A B C D § Formed by co-operating wireless nodes § No fixed network infrastructure § No centralized administration - Each node acts as a router 3

MAC in Wireless networks • Uses MAC protocol of IEEE 802. 11 based on Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) • Basic channel access method can not combat hidden and exposed terminal problems • RTS and CTS are used to reserve the channel for the entire duration of the transmission of data (including ACK) • Physical and virtual carrier sensing is used for Collision Avoidance 4

Antenna in Wireless networks • Uses Omni-Directional Mode • Limited spatial reuse of the channel A C B D If (C, D) are transmitting A & B cannot, with directional antenna simultaneous sessions are possible 5

Smart Antennas Schematic of a smart antenna (adaptive linear array) 6

Smart Antennas • Adaptive Antenna Arrays can direct the Radiation / receiving pattern (main lobe) towards the desired node • Signals received by multiple antennas are weighed and combined to maximize ‘SINR’ (Signal-to-Interference plus Noise ratio) • Weight Vectors obtained will give information about the desired node position • Weight Vectors can be computed to ‘Null’ undesired signals 7

Smart Antennas Received Power (Transmit power) *(Tx Gain) * (Rx Gain) Directional gain is higher, with Nulling Rx Gain can be negligible 8

Protocol Description § Direction-of-Arrival (DOA)-ALOHA is based on Slotted-ALOHA protocol DOA Minislot ACK Minislot DATA Transmission § During DOA Minislot Tx and Rx discovers each other § Tx sends pure tone towards intended Rx 9

Protocol Description § The largest minislot is for the data transmission § Receiver rejects the packet if not an intended destination § Receiver sends ACK if data correctly received § Sender performs back-off if no ACK received (similar to Slotted-ALOHA) § Do not do Collision Avoidance (CA) but exploit Nulling! 10

Protocol Description D B C A E F Node ‘A’ receives max power from node ‘B’, hence, places main lobe towards B and Nulls towards D & F 11

Implementation of the protocol within OPNET § Adaptive Antenna Array is implemented in Matlab and antenna module calls the Matlab routines § A node has no packet scheduled for transmission issues a remote interrupt to antenna to compute weights for omni-direction mode § Transmitter MAC calls antenna module with desired direction which invokes Matlab routines to determine weights 12

Implementation of the protocol within OPNET • • During the duration of the DOA-Minislot, dra-power pipeline stage computes the direction and the received power of all the signals Antenna module inserts (pw, dir) pair in a dynamic list • Max power direction is the desired direction and all the other received signals are interfereres • Antenna module invokes Matlab routine with input parameter (desired_DOA, interferers) and returns new weights • We use Minimum Mean Square Error ( MMSE) algorithm for Nulling 13

Implementation of the protocol within OPNET c a b d a c and b d, ‘d’ mistakenly Forms a beam towards ‘a’ § If a node beamforms incorrectly in a given timeslot, it remembers that direction in single-entry cache § During next slot node ignores maximum signal strength direction, if same, it selects second strongest signal § Cache is not updated if a node correctly receives the packet and cache is reset if no signal from that direction 14

Performance Study Simulation Parameters § Smart antenna implemented in Matlab and interfaced with Opnet § 2 Mbps channel and free space propagation § Grid Topology used § 4 simultaneous flows of CBR traffic considered § 512 Byte packet size used 15

Some Aligned Routes in Grid Aggregate Throughput (Kbps) Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) 16

Unaligned Routes in Grid Aggregate Throughput (Kbps) Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) 17

“Random” Topology Aggregate Throughput (Kbps) Sending rate (Tx) vs Aggregate Throughput Sending Rate (Kbps) 18

Conclusion and Future Work • Power control • Impact on Routing • Extend study to multipath environments 19

Thank You 20

References • J. C. Liberti and T. S. Rappaport. Smart Antennas for Wireless Communications. Prentice Hall, 1999. • Nitin H. Vaidya Romit Roy Choudhury, Xue Yang, and Ramanathan. Using directional antennas for medium access control in ad hoc networks. In ACM/SIGMOBILE Mobi. Com 2002, 23 – 28 Sep 2002. • www. eas. asu. edu/~trccomm/nsf/presentations/ Mar_21_Ravi_Govindarajula. pdf • http: //www. crhc. uiuc. edu/~croy/presentation. html 21