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Cross Layer Architectures for Tactical Ad Hoc Networks equipped with Space-Time Code Processing Capabilities

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Neighbor Discovery ... its neighbors, and the channel conditions on the link to ... neighbors to refresh channel weights and to possibly schedule data transfers ... – PowerPoint PPT presentation

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Title: Cross Layer Architectures for Tactical Ad Hoc Networks equipped with Space-Time Code Processing Capabilities


1
Cross Layer Architectures for Tactical Ad Hoc
Networks equipped with Space-Time Code Processing
Capabilities
Presented by Prof. J.J. Garcia-Luna-Aceves UC
Santa Cruz PI Srikanth V. Krishnamurthy, UC
Riverside Student Gentian Jakllari UC Riverside
2
Premise and Motivation
  • Ad Hoc Networks with omni-directional antenna
    capabilities are limited in capacity
  • Sophisticated antenna equipment -- space-time
    encoding and decoding capabilities becoming the
    reality.
  • Antennas are becoming smaller.
  • New challenge is in developing suitable network
    architectures to exploit the presence of these
    capabilities.
  • Layered approach fails to effectively exploit
    these capabilities Need for layers to work
    together -- a cross layered approach.

3
Architectural Outlook
Discover and Maintain Appropriate Routes
Feedback To Lower Layers on Topology Construction
Network Layer
Support Scheduling based On Interference Zones
and Generated Traffic
MAC Layer
Physical Layer
Adaptive Antennas Provide Feedback To Higher
Layers On Interference Zones/ Tune In Response to
Needs
4
Outline
  • Objectives
  • Discussion of Approach
  • MAC Layer Design
  • Preliminary Simulations and Results
  • Routing Interactions and Higher Layer
    Dependencies
  • Future Plans.

5
Objectives
  • Design and development of a cross-layer
    architecture towards exploiting the physical
    capabilities
  • Adapt to changes in traffic conditions
  • Adapt to mobility
  • Adapt to changes in network topology

6
Approach
  • MAC Layer to support searching for neighbors,
    maintaining connectivity in scenarios of mobility
    and flexibility to smart scheduling approaches.
  • Multi-path routing strategy to provide
    reliability via redundancy -- tightly
    intercoupled with the MAC layer.
  • Interactions with transport layer (or
    applications) to provide adaptability to changes
    in traffic patterns/requirements.

7
The MAC Layer
  • Full exploitation of directed (weighted)
    transmissions
  • Many of the solutions at the MAC layer for use
    with directional antennas still rely on
    omni-directional reception of RTS/CTS messages.
  • Need to direct antennas correctly
  • The communicating pair must be aware of where and
    when and with what weighting co-efficients to
    point their antennas correctly in order that they
    can successfully exchange information.
  • Neighbor Discovery
  • Each node should be aware of the location of its
    neighbors, and the channel conditions on the link
    to each neighbor -- a difficult challenge in
    conditions of mobility.

8
Design Approach at MAC layer
  • Divide Time into three phases
  • Searching -- Allow for the routing protocol to
    search for new neighbors
  • Polling -- Periodically re-establish contact with
    known neighbors to refresh channel weights and to
    possibly schedule data transfers
  • Data Transfer --Exchange actual data with
    neighbors

9
The Basic Timing Diagram
Search Segment
Data
Poll
1 2
A
B
1
2
Pilot tone
Sub-slots to facilitate handshakes during search
Includes PSON / RPSON control messages to
indicate use/non-use of the particular polling
slot.
10
Advantages of our Scheme
  • Obviates the need for omni-directional
    transmissions.
  • Proactive maintenance of neighborhood
    information (polling) makes it robust to
    mobility.
  • Provision of pilot tones prior to data access
    allows refinement of channel weights.
  • Scheduled access considerably reduces the
    possibility of collisions.
  • Provides an interface with the routing layer
    (searching for and maintaining neighbors).

11
Preliminary Simulations and Results
  • Assume directional antennas
  • Use OPNET simulator
  • For now We call our MAC PMAC for polling
    based MAC.
  • Compare with prior scheme using Circular RTS
    messages Korakis et al Mobihoc 2003 and IEEE
    802.11 standard.
  • With the Circular RTS, if a nodes position is
    unknown, the RTS message transmitted
    directionally and sequentially in all directions
    to try and reach the neighbor.

12
Simulation Parameters
Search Segment Length 20 slots
Poll Segment Length 4 slots
Data Transfer Length 800 slots
PSON 20 bytes
RPSON 14 bytes
Data Packet 512 bytes
Frame size 1.64 s
Data Rate 2 Mbps
Simulation Time 500 s
Number of Antenna Elements 8
13
Simulation Results Grid Topology
  • 16 nodes and No mobility
  • Under heavy load CRTS suffers from collisions due
    to asymmetry in gain due to omni-directional
    transmission of CTS messages.
  • In PMAC all the transmission are directional.
    Thus, the gain is always symmetric and this
    leads to a significant increase in network
    throughput -- collisions are especially reduced
    at high loads.

14
Simulation Results Random Topology
  • 15 nodes Nodes move with a speed of 10 m/s.
  • Both protocols offer significant improvement over
    802.11
  • CRTS requires a high control overhead per data
    packet sent (circular transmissions) while it is
    much smaller for PMAC.
  • CRTS requires a overhead proportional to the
    number of elements of the antenna array while for
    PMAC it is constant.
  • Thus, one would expect that the benefits with
    PMAC would increase with narrower beamwidths --
    also applicable with adaptive antenna arrays.

15
Topology Control
  • Polling each and every neighbor is expensive --
    too many polling slots.
  • Whom to poll ? -- Traffic pattern dependent.
  • Input from higher layers.
  • Connectivity also depends on routing -- if a
    route is via a neighbor, link to the neighbor to
    be maintained.

16
Routing Enforcing Parallelism
  • Our objective is to integrate the MAC protocol
    with a multipath routing protocol.
  • Compute maximally disjoint paths to provide for
    parallel streaming if possible.
  • Can reduce delays (improve quality of service)
    during congested periods -- allows for bypassing
    congested areas.

17
Interactions with MAC layer
  • Tuning the topology based on routing decisions.
  • In times of partitions include an aggressive
    search phase to look for new neighbors or paths
    to excluded partitions.
  • Choose paths (and therefore links that ought to
    be maintained) based on the optimization of
    desired metrics of interest -- energy efficiency,
    sustained data rate etc.

18
Future Plans
  • Inclusion of appropriate physical layer models
    into simulation framework.
  • Include adaptive antennas (space-time processors
    into framework).
  • Provide a unified framework with
    MAC/routing/transport layers.
  • Investigate the interactions between layers and
    tune design choices.
  • And of course, work with other team members
    towards ensuring the success of this project !
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