2Outline Introduction Multiple Access Protocols Mode Selection Criteria Mobile Ad hoc Networks (MANETS)AntennasMultiple Access ProtocolsMode Selection CriteriaMotivationsAssumptionsNode ModelAntenna PatternSimulation ParametersPerformance EvaluationApplicability of Mode Selection Criteria to Multiple Beam AntennasConclusionsFuture Work
3Mobile Ad Hoc Networks (MANETs) Peer-to-peer connectivityLack of fixed infrastructure relaysAbsence of centralized authorityMulti-hop forwarding to ensure network connectivityApplicationsMilitary.. Combat Systems, reconnaissanceRescue, medical emergency, telemedicine
4Antenna Types Omni-directional antenna Directional antenna Transmits power equally in all directionsDirectional antennaConcentrates power in a directed zoneSmart AntennaHas the in-built intelligence to change direction according to requirement (steer the beam)Multiple-Beam Smart AntennaSimultaneous transmission/reception in more than one directionsMultiple Input Multiple Output (MIMO)Multiple streams of data in same channel.
6Antennas and MANETsOmni-directional communication suffers from poor spatial reuseDirectional communication leads to better spatial reuse, reduces co-channel interference and provides range extension
7Multiple Access Protocols MAC Proposals differ based onHow RTS/CTS transmitted (omni, directional)Transmission range of directional antennasChannel access schemesOmni or directional NAVsAntenna ModelTwo Operation modesOmni & DirectionalOmni Mode:Omni Gain = GoIdle node stays in Omni modeDirectional Mode:Capable of beamforming in specified directionDirectional Gain = Gd (Gd > Go) --> Range ExtensionDirectional Gain = Gd (Gd = Go) --> Spatial ReuseRange ExtensionSpatial Reuse
8Directional vs. Omni-directional The Problem of utilizing directional antennas to improve the performance of ad hoc networks is non-trivialProsHigher gain (Reduced interference)Spatial ReuseConsPotential possibility to interfere with communications taking place far awayHidden TerminalDeafness
9Motivations Which mode? Omni-directional or directional Analyze various topologies involving neighboring transmissions or receptionsFormulate mode selecting criteria for medium access control (MAC) for MANETs with heterogeneous technologies
10Assumptions Two modes of operation: omni and directional Directional transmission of RTS/CTS/DATA/ACK in directional modeTransmission range of directional antennas is same as that of omni-directional ==> Spatial Reuse4-Way CSMA for medium access controlThe channel is symmetric
11Node ModelThe node model of advance MANET available in OPNET is modified to facilitate directional mode of communicationIn directional mode, the antenna (tx_rx_ant) points in the desired direction with the help of antenna pointing processor (tx_rx_point)
12Antenna PatternConical directional antenna pattern of main lobe having beam-width of 45 degrees and a gain of 0 dBi. The gain in remaining spherical side-lobe is confined to -20dBi
14Performance Evaluation – Deaf Deaf communicating pair scenarioReceivers in same beam of the transmitterTransmitters in same beam of the receiverBoth the transmitters are deaf to each other communicationOmni-directional mode performs better
15Performance Evaluation – Deaf Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode
16Performance Evaluation – Deaf Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver. However, average delay is nearly same in both cases
17Performance Evaluation – Common Receiver Common receiver scenarioTwo or more transmitters with common receiverUsually both the transmitters are deaf to each other communicationOmni-directional mode performs better
18Performance Evaluation – Common Receiver Degradation of throughput (~15%) in directional mode of communication as compared to omni-directional mode
19Performance Evaluation – Common Receiver Retransmission attempts are higher (~12 times) in directional communication due increased collisions at the receiver.
20Performance Evaluation – Linear_Pair_SameBeam Another communicating pair in the same beam of the transmitterThroughput of C-D pair suffers due to interference from A-B ongoing communication in directional modeFor optimal performance C switches to omni mode while other remains in directional mode
21Performance Evaluation – Linear_Pair_SameBeam Switching C to omni-directional mode while remaining nodes in directional mode gives optimal throughput
22Performance Evaluation – Linear_Pair_SameBeam Delay is less in directional mode as all newly generated packets are transmitted while packets in queue are dropped after maximum retransmission attempts
23Performance Evaluation – Linear_Pair_SameBeam Retransmission attempts by node C are much higher in directional mode owing to higher BER (i.e. collisions) at node D
24Performance Evaluation – Tx_0 Another node transmitting in same directionAgain switching the mode of intermediate transmitting node to omni-directional mode while remaining with directional mode yields optimal performance
25Performance Evaluation – Tx_0 Average throughput in directional mode is about 15% lower than in omni-directional mode
26Performance Evaluation – Tx_0 BER is much higher in directional mode due to interference from transmitters as they are deaf to each other
27Performance Evaluation – Tx_90 and Rx_90 Another non-interfering transmitter or receiver in the communicating beamsOmni-mode restricts simultaneous transmissions, hence directional mode is recommendedTx_90Rx_90
28Performance Evaluation – Tx_90 and Rx_90 Directional communication achieves maximum possible throughput in all cases owing to better spatial reuse
29Performance Evaluation – Tx_90 and Rx_90 Delay is more in omni-directional communication due to increased media access delay at the transmitters
30Performance Evaluation – Tx_90 and Rx_90 Due to increased channel contention at the transmitters packet retransmission attempts are more in omni-directional mode
31Performance Evaluation – Linear, Parallel and X topologies Only the intended receiver or transmitter in the communicating beamsBoth the transmitters are deaf to each other communicationNo other communicating node in those beamsDirectional mode outperforms omni-directional mode of communicationLinearXParallel
32Performance Evaluation – Linear, Parallel and X topologies Traffic Received (packets/sec) vs. time
33Mode Selection Criteria All nodes in omni-directional mode in the following cases:Deaf communicating pair scenarioReceivers in same beam of the transmitterTransmitters in same beam of the receiverBoth the transmitters are deaf to each other communicationCommon receiver scenarioTwo or more transmitters with common receiverIntermediate transmitting node in omni-directional mode while other nodes in directional mode for the following cases:Another communicating pair in the same beam of the transmitterAnother node transmitting in same directionAll nodes in directional mode, in the remaining cases including:Another non-interfering transmitter or receiver in the communicating beamsOnly the intended receiver or transmitter in the communicating beams
34Applicability of Mode Selection Criteria to Multiple Beam Antennas Can either transmit or receive multiple packets simultaneously. This requires:Packet receptions in different beams at the node to commence at the same timePacket transmissions by a node in multiple beams to begin simultaneouslyA node cannot both send and receive data at the same timeCan simulate omni-directional mode by transmitting in all possible beams simultaneouslyCan multiple beam antennas achieve optimal performance by transmitting control packets in beams having transmitters and receivers only ???
35Conclusions Directional mode Better spatial reuseEnhances system capacityDeafness and hidden terminal problemsHowever there are some cases where omni-directional mode performs betterDeaf communicating pair scenarioInterference from side-lobes cannot be ruled outCommon receiver scenarioMode Selection Criteria forms the basis of developing MAC protocols for MANETs using heterogeneous antenna technologiesDynamically switching a node from directional to omni-directional or vice versa depending on the neighboring nodes
36Future Work Work needs to be extended for multi-hop topologies Extensive study needs to be done with more communicating pairs within the vicinity so that performance varies with the node densityGame Theoretic approach for mode selection criteria in such scenariosPerformance of multiple beam antennas transmitting control packets in beams having transmitters and receivers only, need to be evaluated
41Antenna Pattern of 7-element uniform equally spaced circular array. Beam FormationBeam FormingTechnique in which the gain pattern of an adaptive array is steered to a desired direction through either beam steering or null steering signal processing algorithmsAdaptive beam forming algorithms can provide substantial gains (of the order of 10log(M) dB, where M is number of array elements) as compared to omni directional antenna systemAntenna Pattern of 7-element uniform equally spaced circular array.
42Smart Antenna System Switched Beam Consists of a set of predefined beams.Allows selection of signal from desired user.Beams have narrow main lobe and small side-lobes.Signals received from side-lobes can be significantly attenuated.Uses a linear RF network, called a Fixed Beam-forming Network (FBN) that combines M antenna elements to form up to M directional beams.
43General Smart Antenna Architecture Source: Chris Loadman, Zhizhang Chen and Dylan Jorgensen, “An Overview of Adaptive Antenna Technologies For Wireless Communications,” In Proc. o Communication Networks and Services Research Conference (CNSR), pp 15-19, 2003.
44Features and Benefits of Smart Antenna Systems Source:
45The global market for smart antennas growth Source: US analyst firm Visant Strategies
46A terminal with 16 antennas mounted on a laptop Source: Alexiou, A.and Haardt, M., “Smart antenna technologies for future wireless systems: trends and challenges,” IEEE Communications Magazine, Vol. 42, pp , Sept. 2004