In our thesis, we investigate the detection of malicious behavior in cooperative networks. Our model contain one source, one destination and two relays where relays adopt Orthogonal Space Time Block Code (OSTBC) to achieve spatial diversity. Cooperative communication takes two phases to forward signal to the destination. During first phase, source broadcasts data symbols to relay and destination, and a few tracing symbols are inserted randomly in data symbols. The values and positions of tracing symbols are known at source and destination. The random placement of tracing symbols is to prevent relays evade the detection of malicious behavior. In second phase, relays adopt orthogonal space time block code to forward the received signals after decoding source message successfully. We consider two scenarios based on the decoding capability at relays. The first scenario assumes perfect source¡Vrelay links. So, relays can always decode symbols correctly. Second scenario considers decoding failures at relays. In both scenarios, relays have a certain probability to perform maliciously. After receiving symbols at destination, the destination extracts and detect the tracing symbols. The malicious behavior of relay is detected depending on the value of the correlation between detected and exact tracing symbols. Moreover, depending on the average received energy, we can distinguish whether relays behaves as in outage. Through computer simulation, we can verify that our proposed tracing algorithm and decoding strategy reduce bit error rate.

Cooperative systems exploit spatial diversity to improve communication quality. But system performances could be severely degraded in the existance malicious relay nodes. In this thesis, we consider a two-relay decode-and-forward (DF) cooperative network. Relay nodes adopt Orthogonal Space Time Block Code (OSTBC) to achieve spatial diversity. Assume that relay nodes may misbehave with a certain probability. If a relay is malicious, it will garble transmission signals, resulting in severe damage to system performance. In the literature, detecting malicious relays requires perfect channel state information. However, misbehavior of the relay will first lead to inaccurate channel estimation. Therefore, we propose a novel detecting misbehavior scheme to deal with the dilemma. Since misbehavior of relays influences statistical properties of the estimated channel coefficients, destination can detect misbehaving by comparing the distribution of channel estimates. The most important of all is that we don¡¦t need channel state information to enhance detecting performance. Through simulation results, we verify proposed scheme can detect misbehavior effectively without channel state information. Compared with signal-to-noise ratio, the number of received tracing symbols has more significant impact on detecting misbehavior of the relay.

To achieve cooperative diversity in cognitive radio network, Decode and Forward (DF) protocol is implemented at Cognitive Radios (CRs) using Orthogonal Space Time Block Coding (OSTBC). The 2-hop communication between source and destination is completed with the help of Cognitive Relays (CRs) using Multiple Input Multiple Output (MIMO) technology within the network. To achieve spatial diversity and good code rate Alamouti 2×2 STBC is used for transmission. CR is using the decoding (Decode and Forward (DF)) strategy and without amplifying ability before forwarding data towards destination provide better performance. The main objective of this thesis is to detect Primary User (PU) spectrum availability or non-availability for the use of Secondary Users (SU). The Alamouti STBC encoded data is broadcasted to wireless Rayleigh faded channel through transmitter having two transmitting antennas. The CRs are preferred to place close with PU to detect transmitted signal and because of having decoding capability CRs decode the collected data using Maximum Likelihood (ML) decoding technique then re-encode the decoded data for further transmission towards receiver. The energy of PU signal received at relays is calculated using energy detector used at cognitive controller having authority to make final decision about presence or absence of PU signal within the spectrum by comparing calculated energy of PU received signal with a predefined value. If the calculated signal energy is less than threshold value it is pretended as the absence of PU and in the other case spectrum is assumed as occupied by PU. Decoding PU signal at relays before forwarding towards destination provide better performance in terms of detection probability and decreasing probability of false alarming as the Signal to Noise (SNR) increases. The proposed cooperative spectrum sensing using DF protocol at cognitive relays with Alamouti STBC is implemented and results are validated by MATLAB simulation. / +46 455 38 50 00

In this report, we introduce cooperative spectrum sensing using orthogonal space time block coding (OSTBC) in order to achieve cooperative diversity in the cognitive radios (CRs) network. Transmit diversity or gain is achieved by introducing more than one antenna on the transmitter and receiver side, but in small electronic mobile devices it looks impractical. The signals received from the primary users (PUs) are amplified by the cognitive relays and further forwarded to the cognitive controller where decisions are made on the basis of the information collected from each cognitive relay. The cooperative relaying protocol used here in cognitive relays is based on an amplifying-forward (AF) scheme. Alamouti scheme in OSTBC has been proposed to achieve better detection performance in CR network. The energy detector performance is analyzed over an independent Rayleigh fading channel. In CR network the secondary user (SU) shares PU's frequency band if it fi nds PU is not in its vicinity. The SU starts using the licensed band and leaves the band as soon as it finds the PU is present or going to use the same band. The detection of the spectrum holes by CRs has to be more agile and intelligent. The main objective of the CRs network is to use the free holes without causing any interference to the PUs. The energy detection technique is simple and outperforms other sensing techniques in cooperative cognitive radio networks. The energy detector collects information from different users, compares it with a certain prede fined threshold () value and then makes a fi nal decision. Detection and false alarm probabilities are derived and manipulated using OSTBC on PU and SU through AF protocol in cooperative communication. The performance of the system is analyzed with single and multiple relays and with and without direct path between the PUs and SUs. Maximum ratio (MRC) and selection combining (SC) schemes are used in energy detector and the results are compared with and without direct link between PU and SU. The analysis is performed by placing the relay close to the PUs. Our results are processed and validated by computer simulation.

ITC/USA 2008 Conference Proceedings / The Forty-Fourth Annual International Telemetering Conference and Technical Exhibition / October 27-30, 2008 / Town and Country Resort & Convention Center, San Diego, California / Although multiple-input multiple-output (MIMO) systems have become increasingly popular, the existence of real time results to compare with those predicted by theory is still surprisingly limited. In this work the hardware description of a MIMO wireless communication system using orthogonal space time block codes (OSTBC) is discussed for two antennas at both the transmitter and receiver. A numerical example for a frequency flat time correlated channel is given to show the impact of channel estimation.

Spectrum sensing is an important aspect of cognitive radio systems. In order to efficiently utilize the spectrum, the role of spectrum sensing is essential in cognitive radio networks. The transmitter detection based techniques: energy detection, cyclostationary feature detection, and matched filter detection, is most commonly used for the spectrum sensing. The Energy detection technique is implemented in the 2-hop cooperative cognitive radio network in which Orthogonal Space Time Block Coding (OSTBC) is applied with the Decode and Forward (DF) protocol at the cognitive relays. The Energy detection technique is simplest and gives good results at the higher Signal to Noise Ratio (SNR) values. However, at the low SNR values its performance degrades. Moreover, each transmitter detection technique has a SNR threshold, below which it fails to work robustly. This thesis aims to find the most reliable and accurate spectrum sensing technique in the 2-hop cooperative cognitive radio network. Using Matlab simulations, a comparative analysis of three transmitter detection techniques has been made in terms of higher probability of detection. In order to remove the shortcomings faced by all the three techniques, the Fuzzy-combined logic sensing approach is also implemented and compared with transmitter detection techniques. / Atti Ur Rehman (atti.rehmman@gmail.com) ph: +358-440458080

Au cours de la dernière décennie, les communications optiques en espace libre (FSO) ont pris de l’ampleur dans les deux domaines académiques et industriels. L’importance de FSO s’appuie sur la possibilité de faire un système de transmission économique et écologique avec un débit élevé et sans licence à l’opposition des systèmes de transmission radiofréquences (RF). Dans la plupart des travaux antécédents sur les systèmes multi-émetteurs, seulement les canaux décorrélés ont été considérés. Un canal décorrélé nécessite un espace suffisant entre les émetteurs. Cette condition devient difficile et non-réalisable dans certaines applications. Pour cette raison, nos études se focalisent sur les performances des codes à répétition RC (Repitition Codes) et les codes OSTBC (Orthogonal Space-Time Block Codes) dans des canaux log-normaux corrélés en utilisant une modulation d’intensité et une détection directe (IM/DD). En addition, les effets des différentes conditions météorologiques sur le taux d’erreur moyen (ABER) sont étudiés. Les systèmes FSO à multi-entrées/ multi-sorties MIMO (Multiple-Input Multiple-Output) avec une modulation SSK (Space Shift Keying) ont été abordés. Les résultats obtenus montrent que la SSK est supérieure aux RC avec une modulation d’impulsion (Multiple Pulse Amplitude Modulation) pour toute efficacité spectrale égale ou supérieure à 4 bit/s/Hz. Nous avons aussi analysé les performances d’un système à sauts multiples (Multi-Hop) et des relais à transmission directe (forward relays). Nos simulations montrent que le système ainsi considéré est efficace pour atténuer les effets météorologiques et les pertes géométriques dans les systèmes de communication FSO. Nous avons montré qu’un tel système avec plusieurs entrées et une sortie (MISO, i.e. multiple-input single-output) à sauts multiples est supérieur à un système MISO avec un lien direct (direct link) avec une forte atténuation. Pour satisfaire la demande croissante des réseaux de communication à débits élevés, la communauté scientifique s'intéresse de plus en plus aux systèmes FSO avec des relais full-duplex (FD). Pour ces derniers systèmes, nous avons étudié la probabilité d'erreur moyenne (ABER) et nous avons analysé leurs performances. En considérant des différentes conditions de transmission, les performances de relais FD ont été comparées à celles d'un système avec un lien direct ou des relais half-duplex. Les résultats obtenus montrent que les relais FD ont le minimum ABER. En conséquence, les résultats obtenus dans cette thèse sont très prometteurs pour la prochaine génération de FSO. / Free-space optical (FSO) communication has been the subject of ongoing research activities and commercial attention in the past few years. Such attention is driven by the promise of high data rate, license-free operation, and cheap and ecological friendly means of communications alternative to congested radio frequency communications. In most previous work considering multiple transmitters, uncorrelated channel conditions have been considered. An uncorrelated channel requires sufficient spacing between transmitters. However, this can be difficult and may not be always feasible in some applications. Thereby, this thesis studies repetition codes (RCs) and orthogonal space-time block codes performance in correlated log-normal FSO channels using intensity modulation and direct detection (IM/DD). Furthermore, the effect of different weather conditions on the average bit error rate (ABER) performance of the FSO links is studied. Multiple-input multiple-output (MIMO) FSO communication systems using space shift keying (SSK) modulation have been also analyzed. Obtained results show that SSK is a potential technique for spectral efficiencies equal or greater than 4 bits/s/Hz as compared to RCs with multiple pulse amplitude modulations. The performance analysis of a multi-hop decode and forward relays for FSO communication system using IM/DD is also considered in this thesis. It is shown that multi-hop is an efficient technique to mitigate atmospheric turbulence and different weather attenuation effects and geometric losses in FSO communication systems. Our simulation results show that multiple-input single-output (MISO) multi-hop FSO systems are superior to direct link and MISO systems over links exhibiting high attenuation. Meeting the growing demand for higher data rates communication networks, a system with full-duplex (FD) relays is considered. For such a system, the outage probability and the ABER performance are analyzed under different turbulence conditions, misalignment error and path loss effects. FD relays are compared with the direct link and half-duplex relays. Obtained results show that FD relays have the lowest ABER and the outage probability as compared to the two other systems. Finally, the obtained results in this thesis are very promising towards the next generation of FSO systems.