Cognitive radio is a promising solution to the spectrum shortage problem that allows opportunistic use of spectrum. This document discusses interference reduction techniques for OFDM-based cognitive radios with single-input single-output (SISO) and multiple-input single-output (MISO) antenna configurations. For SISO systems, active interference cancellation and adaptive symbol transition methods are analyzed. For MISO systems, a joint antenna optimization method is proposed to minimize interference at the primary receiver by designing the transmitted sequences from each secondary transmitter antenna. Simulation results show the tradeoffs between different interference reduction techniques and demonstrate improved performance of the joint antenna optimization method over separate antenna optimization.

1. COGNITIVE RADIO
(Cross-Band Interference reduction Trade-Offs in SISO and MISO
OFDM-Based Cognitive Radios)
Presented By
Ms. Samikshya S. Ghosalkar
M.E.(EXTC)
Under the Guidance of
Dr.( Mrs.) Saylee Gharge

2. Introduction To Cognitive Radio
• Spectrum Utilization
The extensive growth of wireless applications over the past decade has caused an
increasing demand for radio spectrum resources.
Within the current spectrum regulatory framework, almost all of the available
bands have been allocated to existing applications, which has resulted in shortage
of spectrum.
• Solution
Cognitive radio, introduced by J. Mitola, is a promising solution to the spectrum
shortage problem that suggests using spectrum in an opportunistic manner.
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3. Objective Of Cognitive Radio
• Cognitive Radio is a software radio whose control processes leverage situational
knowledge and intelligent processing to work towards achieving some goal related to
the needs of the user application and network.
• Cognitive radio is a technology in which wireless equipment recognizes the spectrum
environment and uses frequencies effectively by properly selecting the frequency band
and the communication method.
• When several wireless communication systems simultaneously use cognitive radio
technology, in a given area, the priority level of each system for frequency utilization is
set and the frequency resource is shared between these systems according to the priority
level.
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4. Cognitive Antenna Transmitter
There are two types of cognitive antenna transmitters:
• Single-Antenna Cognitive Transmitter
• Multiple-antenna Cognitive Transmitter
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5. Single-Antenna Cognitive Transmitter
• In this case, the high sidelobes of data subcarriers of a single-antenna secondary
transmitter causes interference to the primary users.
• There are two methods :
Active Interference Cancellation (AIC)
Adaptive Symbol Transition (AST)
• Active Interference Cancellation (AIC) method, is performed in the frequency
domain. A few subcarriers are inserted at the border of the primary bandwidth.
• These subcarriers are referred to as cancellation carriers. They donot carry any data,
but are modulated by data dependent complex values.
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6. Fig.2.1Using cancellation carriers to reduce the interference power in the
primary band.
Single-Antenna Cognitive Transmitter
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7. Single-Antenna Cognitive Transmitter
• In the AST method, each OFDM symbol is extended in the time domain with a
complex valued data dependent extension.
• The idea relies on the fact that smoother the transition between successive OFDM
symbols the lower the sidelobe levels.
• To find the extension vector, the total interference of the two OFDM symbols and the
spectrum of extension in the primary band cancel each other.
• The Least Square (LS) optimization is used to find the extension vector. The AST
method reduces interference at the cost of throughput degradation as portion of time is
not used to send useful data.
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8. Multiple-antenna Cognitive Transmitter
•In multiple-antenna cognitive systems, the total interference to the primary user
results from the interference powers caused by each antenna separately.
• In an improved AIC,the cancellation carriers are inserted in the transmission
symbols of each antenna and the values of cancellation carriers are optimized
jointly over all antenna.
• In this method, cancellation carriers are transmitted through only one antenna
and are designed to cancel the interference resulting from other subcarriers of the
same antenna and all subcarriers of other antennas.
• In multiple antenna systems to involve the effect of the channel because the
received signal spectrum is the superposition of transmitted signals from each
antenna passed through different fading channels.
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9. Cognitive OFDM System Model
Block Diagram Of OFDM Transmitter
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10. • The resulting vector X = [X0,X1, ...,XN−1]T then passes through the inverse fast
Fourier transform (IFFT) module and produces the time domain vector
x = [x0, x1, ..., xN−1]T where
• The above equation in matrix form as
• The modified DFT matrix as
where A is the submatrix of WN,N consisting of the last G columns of WN,N.
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11. • Hence, the time domain OFDM symbol including the cyclic prefix is expressed as
• We use an upsampled DFT defined by NL × N matrix,
• Hence the upsampled spectrum of X is calculated as
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12. Single- Antenna Cognitive Transmitter
• The method is based on jointly minimizing the interference over time and frequency at
the location of primary receiver,using knowledge of the channel.
• The weights of the cancellation carriers and the values of the extension are jointly
optimized such that the interference to the primary user is minimized.
• To find the optimum weights for the cancellation carriers of each OFDM symbol, only
the spectrum of that symbol is considered during the calculations.
• The objective is to compute the complex values of the cancellation carriers (denoted by
the vector μ) and extension (denoted by the vector η) of the second symbol.
Cognitive Radio Antenna Transmitter Joint Time/Frequency
Optimization
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13. • There is a single primary user whose bandwidth is spread over B consecutive
subcarriers [Xt+1,Xt+2, . . . , Xt+B], where B < N.
• Let Xd
(k) denote the kth OFDM symbol in which tones within the primary band and the
cancellation carriers are set to zero,i.e.,
Xd(k) = [X0
(k) , . . .,X t−g
(k), 0, . . . , 0,X t+B+g+1
(k), . . . , X N−1
(k)]T
where g is the number of subcarriers used as cancellation carriers on each side of the
primary band and Xopt(k) denotes the kth OFDM symbol in which the optimum
cancellation carries are inserted from the previous round.
• We denote the upsampled frequency response of the channel between the secondary
transmitter and the primary receiver by h = [h0, h1, . . . , h NL−1]T.
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14. H =
in which a is the length of the extension and 0a is the zero vector of length a. η(k−1)
denotes the optimal extension vector of the (k − 1)th symbol calculated in the previous
iteration.Hence, the interference vector is
= S(t+1)L,(t+B)L
which is a subvector of S containing indexed elements (t +1)L through (t + B)L of S.
||S||2 represents the amount of interference power to the primary user and is to be
minimized.
S
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15. Multiple- Antenna Cognitive Transmitter
• In this case,a good improvement in interference reduction can be achieved.
• The set of the secondary transmitter antennas and primary receiver antenna forms a
multiple-input single-output (MISO) system.
• Let hi = [hi,0, hi,1, . . . , hi,NL−1]T denote the upsampled frequency response of the
channels between the ith secondary transmitter antenna and the primary receiver antenna.
Therefore the upsampled spectrum of the received signal at the primary receiver is
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16. Multiple- Antenna cognitive system.
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17. Simulation Results and Discussions
• Single Wideband Interference
Single sideband interference ,power spectrum of the OFDM signal:N=256, no. of primary
bands=1,B=32.
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18. Effect of adding extension samples on the amount of interference reduction in single
wideband interference case.
Trade- Off Study
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19. • Multiple Narrowband Interference
Multiple narrowband interference, power spectrum of the output OFDM
signal:N=256,no. of primary bands=6, B=4.
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20. Effect of adding extension samples on the amount of interference reduction in multiple
wideband interference case.
Trade- Off Study
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21. Comparison of the spectra of MISO-OFDM signal at the primary receiver in
the frequency selective fading channel;4 cancellation carriers on each side of
the primary band and a time extension of length 4 are used.
Simulation Results And Discussions
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22. Conclusion
• In the single-antenna case, a new joint time/frequency scheme to investigate the trade-
off between active interference cancellation and adaptive symbol transition techniques.
The method optimizes jointly over the symbol extension and cancellation subcarriers to
minimize the interference to the primary user.
• In view of symbol extension, it is shown that for a single wideband primary, most of
the gain in interference cancellation is achieved by adding the first extension sample.
• In the multiple-antenna case, a new method, called the joint antenna method, in which
the transmitted sequences from the secondary transmitter antennas are designed such
that the interference at the primary receiver antenna is minimized.
• Simulation results also demonstrate significant improvement in jointly optimizing over
two antennas compared to two separate antenna interference minimization.
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23. References
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24. [5] H. Mahmoud and H. Arslan, “Sidelobe suppression in OFDM-based spectrum sharing
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[6] S. Brandes, I. Cosovic, and M. Schnell, “Reduction of out-of-band radiation in OFDM
based overlay systems,” in Proc. 2005 IEEE International Symp. New Frontiers Dynamic
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25. Any Questions ??

26. THANK YOU !!