WPMC2013_SBhattarai

1
Optimum Transmit Beamforming Scheme for Underlay
Cognitive Radio Networks
Sudeep Bhattarai, Liang Hong, Sachin Shetty
Department of Electrical and Computer Engineering
Tennessee State University
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364
GWS 2013 Atlantic City, NJ, June 26, 2013

2
Outline
• Introduction
• Problem Statement and Research Objective
• System Model
• Problem Formulation
• Optimum Beamforming for Stationary and Mobile SUs
• Simulation Results
• Conclusions
Optimum Transmit Beamforming Scheme for Underlay Cognitive Radio Networks
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

Introduction
• The deployment of numerous broadband wireless applications
with different service requirements leads to a huge demand on
the expensive radio spectrum.
• Spectrum shortage becomes a significant challenge towards the
implementation of next generation communication networks.
• Cognitive radio is a promising paradigm in wireless
communication that enables efficient use of frequency resources
− Coexistence of licensed primary users (PUs) and unlicensed secondary
users (SUs, or cognitive users) in the same frequency band
− Cognitive capabilities
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

• PUs have priority access to the available radio frequency bands.
• SUs have restricted access, subject to a constrained degradation
on the PUs’ performance
• Basic spectrum sharing approaches:
− Spectrum overlay: SUs use the frequency bands that are not currently
occupied by PUs
− Spectrum underlay: SUs co-exist with PUs in the same frequency bands,
under the strict constraint that the interference introduced by the SUs to
the PUs is under certain threshold
Our focus: Underlay Cognitive Radio System
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

• Transmit beamforming
− Transmitter equipped with multiple antennas adjusts the weights assigned
to each antenna component to obtain desired radiation pattern in different
directions.
 Secondary transmitter with antenna
array can control the level of
interference to the PUs by placing nulls
at the direction of primary receiver
 Maximize the SU’s received power for
a given transmit power constraint by
focusing the radiation pattern along the
direction of the secondary receiver.
Primary
Receiver
Secondary
Receiver
Secondary
Transmitter
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

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• Existing beamforming schemes mainly considered the cases
where the secondary receiver is stationary with perfect channel
information to the secondary transmitter
• Existing beamforming schemes cannot guarantee the uniform
transmission power for moving SU within its mobile range
Problem Statement and Research Objectives
This Research
Develop an optimum transmit beamforming scheme that
supports stationary as well as mobile secondary users in a
cognitive radio network.
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

7
Direction estimates
of all users is
known at SU-Tx
Channel information
for all stationary
users is known at
SU-Tx
Assumptions
Other existing users
are located spatially
apart from the
desired SU-Rx
System Model
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

8
SU-Rx
PU
SU-Tx
(antenna array)
w1
w2
wN
hp1
hp2
hpN
hs1
hs2
hsN
If x denotes the message signal, then the
signal received by SU-Rx is,
where
= AWGN
Similarly, the interference received by
PU from SU-Tx is,
where
= AWGN
hs: Channel coefficients vector for SU-
RX
hp: Channel coefficients vector for PU-
RX
w: Beamforming weight vector
α(θs): steering vector for SU
α(θp): steering vector for PU
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

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Non-Convex
Difficult to solve
Very strict constraints
Difficult to get solution
Problem Formulation
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

10
Optimum Beamforming for Stationary and Mobile SU
Non-convex objective function can be converted to
convex form by simple mathematical manipulation.
Non-Convex,
Difficult to solve
Convex,
Easy to solve
Maximize:
The constraints can be relaxed by putting small positive constant
Є instead of 0
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

11
Case I: Stationary SU-Rx:
• Narrow beam towards secondary
receiver
• M = 1
• Channel information Hp and Hs is
known
(1)
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

12
Case II: Mobile SU-Rx:
• Uniform transmission power in the
moving range of secondary receiver
• Channel information hs is unknown,
set to 1 for simplicity in solving
optimization problem
(2)
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

13
• The optimization problems (1) and (2) can be solved by using
any available second-order cone programming (SOCP)
solvers.
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

14
Simulation Results
 SU-Tx is equipped with 9 omnidirectional antennas
 Tolerable interference level for PU and other existing SUs, ε,
is set to -100dB
 All channels are flat Rayleigh fading channels.
 Total number of bits transmitted = 1 million
 Modulation Scheme: BPSK
 Maximum likelihood demodulator is used at the receiver side
to decode the received signal
 Matlab Optimization toolbox is used for SOCP solver
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

15
Radiation pattern for Stationary SU-Rx at 30°, PU at 80°:
Signal steered only in the direction of the SU-Rx without
interference in PU’s direction
Polar plot of beam pattern Log plot of beam pattern
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

16
BER performance with and without beamforming:
Stationary SU-Rx PU
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364
BER
BER

17
Mobile SU-Rx between (30°,40°), PU at 80°:
Signal steered equally in the moving range of the mobile SU-
Rx without interference in PU’s direction
Polar plot of beam pattern Log plot of beam pattern
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

18
BER performance with and without beamforming:
Mobile SU-Rx PU
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364
BER
BER

19
Comparison of beam patterns with different angles for SU-Rx
moving between 30 and 40
Radiation pattern is almost independent of angle selection of SU-Rx
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

20
Conclusions
• Proposed a scheme that allows stationary as well as mobile
SU to co-exist with PU.
• Transmission power towards secondary receiver is maximized
while the interference to primary receiver is constrained
• The transmission power is constant in an area of angles when
SU is not stationary
• Probability of successful transmission for SUs is significantly
boosted with the proposed beamforming technique.
• Performance of primary users is not affected with the addition
of secondary user in the same frequency band.
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

21
Thank you!
Questions?
Dr. Liang Hong
LHONG@TNstate.edu
(615) 963-5364

WPMC2013_SBhattarai

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