| 1. |
- Haghighi, Kasra, 1978, et al.
(författare)
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An LLR-based Cognitive Transmission Strategy for Higher Spectrum Reutilization
- 2011
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Ingår i: IEEE GLOBECOM 2011 - Cognitive Radio and Networks Symposium. - 9781424492688
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Konferensbidrag (refereegranskat)abstract
- Reutilization of the spectrum licensed to services with low occupancy is of great interest for cognitive radios (CRs). To achieve this goal, we introduce a simple hidden Markov model which captures the primary users activity, signal uncertainties, and noise. For evaluating the performance of any CR, two new criteria are presented entitled spectrum utilization ratio (UR) and interference ratio (IR). Based on this model and new measures, a new a-posterior log-likelihood-ratio based CR is designed and implemented. Its performance is compared with standard energy-detection based spectrum-sensing CR. We demonstrate more than 300% increase in UR for up to 1% allowed interference at the SNR of -5dB.
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| 2. |
- Haghighi, Kasra, 1978
(författare)
-
Cognitive Sensing and Transmission Strategies
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many frequency bands for wireless services are severely underutilized by the primary users (PU) to which these bands are assigned. This motivates cognitive radios (CR), which identifies vacant spectrum and transmit accordingly. Many algorithms, some as simple as energy detection, are used to sense the spectrum. However, in the wideband sensing or, generally, low-SNR regimes, the detection performance of such algorithms is limited. To address that, a new class of statistical tests known as sequential tests have been introduced, which accumulates a test statistic until it reaches one of two thresholds. In paper A, we introduce new time-varying thresholds for sequential spectrum sensing.These new thresholds, for an SNR of -10 dB, in comparison with standard sequential detection with parallel (fixed) thresholds with similar probabilities of mis-detection andfalse-alarm, performs 54% faster in terms of maximum detection time (90 percentile). In papers B and C, we study CR transmission strategies that are based on noisy observations of the PU activities, which is modeled as a two-state discrete-output hidden Markov model (HMM). We introduce a transmission strategy which is based on comparing an a-posterior probability (APP) log-likelihood ratio (LLR) with a threshold. The objective is to maximize the utilization ratio (UR), i.e., the relative number of the PU-idle slots that are used by the CR, subject to that the interference ratio (IR), i.e., the relative number of PU-active slots that are used by the CR, is below a certain level. We demonstrate a more than 300% increase in UR over standard energy detection, for the same IR value, at the SNR of -5 dB. Finally, in paper C, we use a continuous-output HMM to model the received signal, and calculate an APP LLR based on that. We show that this strategy is the optimum in the sense of maximizing the UR, given a certain maximum allowed IR, among all CRs. Moreover, two practical schemes for calculating the transmission threshold are introduced. Numerical results show that the first method yields a threshold which is close to optimum when the PU use a large fraction of the available spectrum (i.e., when the PU activity level is high). However, the method fails to give a valid threshold for (i.e., a threshold that respect the maximum allowed IR) low SNRs when the PU activity level is low. The second method is analytically proven to always give a valid threshold, regardless of SNR and PU activity level. The resulting UR is reasonably high when the PU activity level is high, but quite low when the PU activity level is low. In addition, an upper-bound for the UR of any CR strategy is presented. Simulation results have showed over 116% improvement in UR at SNR of -3dB and IR level of 10% with PU state estimation over energy detection.
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| 3. |
- Haghighi, Kasra, 1978, et al.
(författare)
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IP Prefix Lookup at OC-768 Rate on Reconfigurable Hardware
- 2002
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Ingår i: 9th Intl. Conf. on Telecommunication, ICT2002.
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Konferensbidrag (refereegranskat)abstract
- This paper reports on the design of an IPv4 prefix lookupengine that supports 166 million lookups per second, andan estimated 850k prefix entries. It also supports dynamictable update operations that can be issued from anexternal controller. The design employs a prefix searchalgorithm that is fine-tuned for hardware implementation.It takes advantage of pipelining and utilizes one FPGAchip and seven ZBT® SRAM chips. We have implementedthis engine as an HDL core in Verilog.
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| 4. |
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| 5. |
- Haghighi, Kasra, 1978
(författare)
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Model-Based Cognitive Radio Strategies
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Many frequency bands for wireless services are severely underutilized by the primary users (PU) to which these bands are assigned. This motivates a new class of wireless communication devices known as cognitive radios (CR), which identify vacant spectrum and transmit accordingly.In this thesis, the PU traffic model knowledge as well as all the observations available to the CR are included in the CR transmission decisions. A transmission strategy is introduced that is based on comparing an a-posterior probability (APP) log-likelihood ratio (LLR) with a threshold. The objective is to maximize the utilization ratio (UR) subject to that the interference ratio (IR) is below a certain level. In papers A and B, we study CR transmission strategies that are based on all noisy observations of the PU activities, even when the CR itself is transmitting. Paper A demonstrates a more than 300% increase in UR over standard energy detection, for the same IR value, at the PU signal to CR noise power ratio (SNR) of -5dB. Then, in paper B, weuse a continuous-output hidden Markov model for the received signal and calculate an APP LLR based on this model. This paper shows that this strategy is the optimum in the sense of maximizing the UR, given a certain maximum allowed IR, among all CRs.Moreover, two practical schemes for calculating the transmission threshold are introduced.Numerical results show that the first method yields a threshold that is close to optimum when the PU use a large fraction of the available spectrum (i.e., when the PU activity level is high).The second method is analytically proven to always give a valid threshold.Simulation results show a 116% improvement in UR with PU state estimation over energy detection, at an SNR of -3dB and IR level of 10%.In paper C, we extend paper B to consider that PU activities cannot be observed when CR is transmitting, in other words they are censored.This new strategy, entitled CLAPP, calculates a new LLR, which is compared with a threshold. This threshold is computed with a bisection search method. Simulation results show that CLAPP has a 52% gain in UR over the best censored energy detection scheme for a maximum IR level of 10% and an SNR of -2dB. In paper D, we introduce new time-varying thresholds for sequential spectrum sensing. These new thresholds, for an SNR of -10dB, in comparison with standard sequential detection with parallel (fixed) thresholds with similar probabilities of misdetection and false alarm, performs 54% faster in terms of maximum detection time (90 percentile).
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| 6. |
- Haghighi, Kasra, 1978
(författare)
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MRI Brain Abnormality Detection Using Fuzzy Neural Networks
- 1996
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Ingår i: 11th Intl. Conf. of Applications of Artificial Intelligence in Engineering. - 1853124109
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Konferensbidrag (refereegranskat)abstract
- In this paper an expert system for detection of brain abnormalities is proposed. First preceding methods for segmentation of MR images are reviewed and their limitations are discussed. In the proposed method, MR images (three images from one slice: T1, T2 and Proton Density) are acquired from a scanner or directly from MRI system. For noise deletation two filters (median and bandreject lowpass) are used (This stage is optional). They make a clean view of MR images. It is necessary to have precise detections. So by implementing a gray-scale to color transformation algorithm (it is a radially symmetric butterworth band-reject filter), system can recognize the differences between tissues accurately. Now we have three colored images (T1, T2 and Proton Density) from the last section that better represent tissues and it is possible to say that those tissues with the same color in each of these three images may be same tissues. The combination of fuzzy systems and neural Networks make a powerful tool for pattern recognition problems. So a fuzzified neural network with outputs to a back-propagation network for tissues recognition must be used. Therefore a fuzzy neuron and a fuzzified network are introduced. The output of the back-propagation network is the type of tissue under process. The results of the last section are fed into another network that uses a knowledge-base to make a suggestion for treatment (This level is also optional). Because of the time limitation MATLAB 4.0 for Windows is used for expert system simulation. It has several abilities for matrix calculations and graphing that make the work easier (some of the base modules are written in C++).
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| 7. |
- Haghighi, Kasra, 1978, et al.
(författare)
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On Optimum Causal Cognitive Spectrum Reutilization Strategy
- 2012
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Ingår i: IEEE Journal on Selected Areas in Communications. - 0733-8716 .- 1558-0008. ; 30:10, s. 1911-1921
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we study opportunistic transmission strategies for cognitive radios (CR) in which causal noisy observation from a primary user(s) (PU) state is available. PU is assumed to be operating in a slotted manner, according to a two-state Markov model. The objective is to maximize utilization ratio (UR), i.e., relative number of the PU-idle slots that are used by CR, subject to interference ratio (IR), i.e., relative number of the PU-active slots that are used by CR, below a certain level. We introduce an a-posteriori LLR-based cognitive transmission strategy and show that this strategy is optimum in the sense of maximizing UR given a certain maximum allowed IR. Two methods for calculating threshold for this strategy in practical situations are presented. One of them performs well in higher SNRs but might have too large IR at low SNRs and low PU activity levels, and the other is proven to never violate the allowed IR at the price of a reduced UR. In addition, an upper-bound for the UR of any CR strategy operating in the presence of Markovian PU is presented. Simulation results have shown a more than 116% improvement in UR at SNR of -3dB and IR level of 10% with PU state estimation. Thus, this opportunistic CR mechanism possesses a high potential in practical scenarios in which there exists no information about true states of PU.
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| 8. |
- Haghighi, Kasra, 1978
(författare)
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Sensing or transmission: Causal cognitive radio strategies with censorship
- 2013
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Ingår i: IEEE Transactions on Wireless Communications. - 1558-2248 .- 1536-1276. ; Submitted
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces a novel opportunistic transmission strategy for cognitive radios (CRs). The primary user (PU) is assumed to transmit in a time-slotted manner according to a two-state Markov model, and the CR is either sensing, that is, obtaining a causal, noisy observation of a primary user (PU) state, or transmitting, but not both at the same time. In other words, the CR observations of the PU are censored whenever the CR is transmitting. The objective of the CR transmission strategy is to maximize the utilization ratio (UR), i.e., the relative number of the PU-idle slots that are used by the CR, subject to that the interference ratio (IR), i.e., the relative number of the PU-active slots that are used by the CR, is below a certain level. We introduce an a-posteriori LLR-based CR transmission strategy, called CLAPP, and evaluate this strategy in terms of the achievable UR for different PU model parameters and received SNRs. The performance of CLAPP is compared with a simple censored energy detection scheme. Simulation results show that CLAPP has 52% gain in UR over the best censored energy detection scheme for a maximum IR level of 10% and an SNR of -2dB.
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| 9. |
- Haghighi, Kasra, 1978, et al.
(författare)
-
Sensing or Transmission: Causal Cognitive Radio Strategies with Censorship
- 2014
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Ingår i: IEEE Transactions on Wireless Communications. - 1558-2248 .- 1536-1276. ; 13:6, s. 3031-3041
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Tidskriftsartikel (refereegranskat)abstract
- This paper introduces a novel opportunistic transmission strategy for cognitive radios (CRs). The primary user (PU) is assumed to transmit in a time-slotted manner according to a two-state Markov model, and the CR is either sensing, that is, obtaining a causal, noisy observation of a primary user (PU) state, or transmitting, but not both at the same time. In other words, the CR observations of the PU are censored whenever the CR is transmitting. The objective of the CR transmission strategy is to maximize the utilization ratio (UR), i.e., the relative number of the PU-idle slots that are used by the CR, subject to that the interference ratio (IR), i.e., the relative number of the PU-active slots that are used by the CR, is below a certain level. We introduce an a-posteriori LLR-based CR transmission strategy, called CLAPP, and evaluate this strategy in terms of the achievable UR for different PU model parameters and received signal-to-noise ratios (SNRs). The performance of CLAPP is compared with a simple censored energy detection scheme. Simulation results show that CLAPP has 52% gain in UR over the best censored energy detection scheme for a maximum IR level of 10% and an SNR of -2dB. © 2002-2012 IEEE.
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| 10. |
- Haghighi, Kasra, 1978, et al.
(författare)
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Wideband Sequential Spectrum Sensing with Varying Thresholds
- 2010
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Ingår i: Proceedings IEEE Global Communications Conference, Wireless Communications Symposium, Miami, Florida, USA. - 9781424456383
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Konferensbidrag (refereegranskat)abstract
- In this contribution, time varying threshold sequentialdetectors are employed for energy detection-based spectrumsensing in low-SNR regimes. Sequential detection is proven to befaster (on average) than any other multi-sample detector for a setof given probabilities of detection and false-alarm. In this report,exact performance of a sequential detector for spectrum sensingis analyzed using the direct method. The theoretical resultspresented herein are verified with Monte-Carlo simulations. Itis shown that for a SNR of −10 dB, among tests with Wald andtriangular thresholds with similar probabilities of mis-detectionand false-alarm, triangular performs 54% faster in terms ofmaximum detection time (90 percentile).
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