COGNITIVE RADIO(Cross-Band Interference reduction Trade-Offs in SISO and MISO

OFDM-Based Cognitive Radios)

Presented ByMs. Samikshya S. Ghosalkar

M.E.(EXTC)

Under the Guidance of Dr.( Mrs.) Saylee Gharge

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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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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Cognitive Antenna Transmitter

There are two types of cognitive antenna transmitters:

• Single-Antenna Cognitive Transmitter

• Multiple-antenna Cognitive Transmitter

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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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Fig.2.1Using cancellation carriers to reduce the interference power in the primary band.

Single-Antenna Cognitive Transmitter

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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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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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Cognitive OFDM System Model

Block Diagram Of OFDM Transmitter

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• 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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• 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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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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• 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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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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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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Multiple- Antenna cognitive system.

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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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Effect of adding extension samples on the amount of interference reduction in single wideband interference case.

Trade- Off Study

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• 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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Effect of adding extension samples on the amount of interference reduction in multiple wideband interference case.

Trade- Off Study

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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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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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References

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[5] H. Mahmoud and H. Arslan, “Sidelobe suppression in OFDM-based spectrum sharing

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Any Questions ??

THANK YOU !!