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Opportunistic Spectrum Access in Cognitive Radio Networks. Project Team: Z. Ding and X. Liu (co-PIs) S. Huang and E. Jung (GSR) University of California, Davis. (Well known) Motivations for Cognitive Radio Networks . Spectrum scarcity. More wireless services.

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opportunistic spectrum access in cognitive radio networks

Opportunistic Spectrum Access in Cognitive Radio Networks

Project Team:

Z. Ding and X. Liu (co-PIs)

S. Huang and E. Jung (GSR)

University of California, Davis

well known motivations for cognitive radio networks
(Well known) Motivations for Cognitive Radio Networks
  • Spectrum scarcity.
  • More wireless services.
  • Inefficient static spectrum allocation.
  • Existence of a large amount of under-utilized spectrum.
  • Advantage of flexible and cognitive spectrum access scheme needed: cognitive radio.
opportunistic spectrum access
Opportunistic Spectrum Access
  • Design Objectives:
  • Non-intrusiveness
  • Spectral efficiency
  • Cost efficiency
  • Decentralized
three basic access schemes
Three basic access schemes

PU -- primary user (licensee of the channel)

SU -- secondary user (cognitive ratio)

problem formulation
Problem Formulation
  • Assumptions:
  • Exponentially distributed idle period
  • General primary busy period distribution
  • Perfect sensing
  • Knowledge of average idle time/busy time
  • Constraint Metrics:
  • Bounded collision probability
  • Bounded overlapping time
  • Optimization problem:
fundamental limits of opportunistic spectrum access
Fundamental limits of opportunistic spectrum access
  • Primary channel with exponentially distributed idle period
  • Bounded collision probability constraints
  • Maximum achievable throughput of a secondary user

 --- collision probability bound

 --- percentage of idle time (by primary users)

observations
Observations
  • VX, VAC and KS schemes have indistinguishable throughput performance, under collision probability constraint;
  • The smaller the packet length, the larger the throughput.
  • The result can be extended to systems with multiple primary users and multiple secondary users (treat all secondary users as a “super” secondary user)
fixed length packet wins
Fixed length packet wins
  • Under the collision probability constraint, the secondary user achieves the maximum throughput when it transmits fixed length packets
overhead consideration
Overhead Consideration
  • Optimal packet length achieves trade-off between overhead and collision probability
multi band multiple secondary systems
Multi-band multiple secondary systems
  • No synchronization between secondary users and primary users
  • No control channel for secondary users
  • Collision probability constraint
  • Perfect sensing
smart antenna technique applied in cognitive radio networks
Smart Antenna Technique Applied in Cognitive Radio Networks
  • Design Objective:
  • Maximize the QoS of SUs while protecting PUs
  • Design MAC Protocols to take advantages of smart antenna technologies
  • System Setup:
  • One primary Tx (PT), one primary Rx (PR)
  • One cognitive Tx (CT) , one cognitive Rx (CR)
  • PT and CT transmit simultaneously to PR and CR, respectively
  • Performance metric:
  • talk-able zone of CR
optimal beamforming problem with constraints
Optimal Beamforming Problem with Constraints
  • Can be solved efficiently by convex optimization method
simulation results 1
Simulation Results (1)
  • PT uses omni-directional antenna
  • PRs are evenly distributed over the area centered at PT
  • Interference to PR is less than 0.1 of the received signal power
  • Spectrum efficiency increased at least by:
simulation results 2
Simulation Results (2)
  • PT uses Transmit beamforming
  • PRs are evenly distributed over the area centered at PT
  • Interference to PR is less than 0.1 of the received signal power
  • Spectrum efficiency increased at least:
integration of mac phy design in cognitive radio networks
Integration of MAC/PHY design in Cognitive Radio Networks
  • Design Objective:
  • Under the collision probability constraint, increase the capacity of secondary users
  • A cross-layer approach
  • Channel models
  • Rich scattering environment: Rayleigh fading MISO channel from CT to CR and PR
  • Rayleigh SISO fading channel from PT to PR and CR
received signal model
Received signal model
  • Idea:
    • when overlapping happens, primary user can decode its signal as long as the interference power from secondary user is very small.
    • Transmit beamforming helps in this scenario, since it can mitigate the interference to primary users;
  • Collision probability:
conclusions
Conclusions
  • Opportunistic spectrum access of secondary users can increase the spectrum efficiency of system
  • Smart antenna technique enables concurrent transmission of primary users and secondary users, and reduces interference to primary user
  • Integration of PHY/MAC layer can improve system’s spectrum efficiency
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