Mutual Interference of Frequency Hopping with Collision Avoidance Systems

The aim of this article is to quantify and analyze mutual interference of Frequency Hopping with Collision Avoidance (FH/CA) systems. The FH/CA system is a frequency hopping system where stations select the least jammed channel from several possible before the next jump. The article describes a mathematical model that allows determining the upper limit of the probability of collision of multiple FH/CA systems operated in a common band. The dependence obtained for mutual interference of FH/CA systems is compared with the dependence for mutual interference of conventional FH systems. The result of the comparison is a conclusion that, in terms of mutual interference, it is more advantageous to operate the FH/CA systems than the conventional FH systems

John Jeffries Martin: Myths of renaissance individualism. Basingstoke, New York, Palgrave Macmillan, 2004, 187 str

John Jeffries Martin: Myths of renaissance individualism. Basingstoke, New York, Palgrave Macmillan, 2004, 187 str

Domain-wall coercivity in ferromagnetic systems with nonuniform local magnetic field

Domain-wall (DW) coercive field, H-CW, which characterizes pinning of DW's in soft magnetic materials, decreases strongly with increasing value of gradient, G, of the effective local DW-position-restoring magnetic field. Particular shapes of the dependence, H-CW(G), can be calculated from the mean energy dissipation of the DW moving over the particular profile of the DW pinning field, H-p. In this paper, H-CW(G) is calculated from a wall-pinning field, H-p, which is expressed as a stochastic function of the DW coordinate, x(DW). The wall-pinning field, H-p, is described as a Wiener-Levy stochastic process modified by two correlation lengths in such a way that H-p is stationary for large DW displacements and dH(p) /dx(DW) is well defined for small DW displacements. The computed H-CW(G) is close to a hyperbolic decrease, but it approaches finite values if G-->O and it decreases in a much steeper way than alpha 1/G for high values of G, which agrees with the experimental observations. Experimentally, the dependence H-CW(G) was measured on close-packed arrays of cylindrical bubble domains in two thin films of magnetic garnets, where the local field gradient, G, was controlled within the range 10(9)-10(10) A/m(2) by changing distances between neighboring DW's. The DW coercive field, H-CW, extrapolated from the measured values for G-->O was close to 80 A/m for both samples, while H-CW(G approximate to 10(10) A/m(2)) was several times smaller. Fitting the calculated H-CW(G) dependence to the experimental data, we obtained values of the Wiener-Levy correlation lengths well comparable to the DW width parameters

Possibilities of Error Controls in Frequency hopping Stations

Doktorská práce se zabývá problematikou návrhu kódového zabezpečení pro stanice s kmitočtovým skákáním v pásmu s intenzivním rušením. U digitálních modulací dochází vlivem rušení k chybnému určení modulačního stavu na straně přijímače. Důsledkem jsou chybně přenesené symboly zprávy. Vzniklé chyby během přenosu je možné eliminovat za použití zabezpečovacích kódů. Zároveň je také možné těmto chybám předcházet pomocí algoritmů (technik) frekvenčního skákání, které provádí výběr vhodného komunikačního kanálu. Vhodným komunikačním kanálem se rozumí kanál s nižší pravděpodobností vzniku chybných symbolů ve zprávě. Hlavní přínos této práce spočívá v návrhu nové techniky frekvenčního skákání s předcházením kolizí (FH/CA). Stanice s technikou FH/CA provádí před každým skokem měření úrovně signálu v několika uvažovaných kanálech. Na základě provedených měření je pak vybrán nejvhodnější kanál, tj. kanál s nejnižší naměřenou hodnotou úrovně signálu. S vyšší pravděpodobností tak dochází ke skoku na kanál neobsazený nějakým přenosem. Pomocí matematického modelu byla porovnána výkonnost nově navržené techniky FH/CA se současně užívanými technikami FH a AFH. Porovnávacím kritériem byla pravděpodobnost kolize mezi komunikačním systémem FH/CA a statickými rušiči (zařízení vysílající nepřetržitě na stálém kmitočtu) nebo dynamickými rušiči (tj. jinými FH nebo AFH systémy). Porovnáním hodnot pravděpodobnosti rušeného přenosu byly zjištěny nesporné teoretické výhody nové techniky FH/CA oproti současně užívaným technikám FH a AFH. Technika FH/CA vždy dosahuje lepších nebo stejných výsledků v porovnání s technikou FH v případě rušení jak statickými tak i dynamickými rušiči. Technika FH/CA je většinově výhodnější než technika AFH v pásmu se statickými a dynamickými rušiči. Významný přínos techniky FH/CA je vidět zejména v oblasti dynamických rušičů. Naopak v oblasti statických rušičů je technika FH/CA oproti technice AFH v určitých situacích horší. Správnost matematických modelů byla ověřena simulačním modelem, který byl vytvořen v rámci řešení této doktorské práce v prostředí MATLAB. Na základě získaných dat z modelu byl následně proveden návrh kódového zabezpečení pro stanice s kmitočtovým skákáním s novou technikou frekvenčního skákání FH/CA, které jsou určeny pro přenos dat o malém objemu v pásmu s intenzivním rušením.The doctoral thesis deals with design of coding for frequency hopping stations in band with intensive jamming. In digital modulations erroneous determination of the modulation state occurs due to jam at the receiver side. The result is erroneously transferred symbols of the message. Errors created during the transmission can be eliminated by using error control systems. It is also possible to prevent these errors by using algorithms (techniques) of frequency hopping which select the appropriate channel. Appropriate communication channel is a channel with a lower probability of erroneous symbol in the message. The main contribution of this thesis is to design a new frequency hopping technique with collision avoidance (FH/CA). The station with FH/CA technique measures signal levels in the considered several channels before every jump. Based on the measurements the most appropriate channel with the lowest value of measured signal level is selected. Therefore, it is more probable that a jump to an unoccupied channel with a transmission will occur. Using a mathematical model, the performance of the newly proposed FH/CA technique is compared with the currently used techniques FH and AFH. Comparison criteria are the probability of a collision between an FH/CA communication system and a static (device transmitting continuously at a fixed frequency) or dynamic jammer (i.e. other FH or AFH systems). By comparing the values of the probability of jammed transmission, indisputable theoretical advantages of the new FH/CA technique were found, compared to the currently used FH and AFH techniques. The FH/CA technique always has better or equal results compared with the FH technique in the case of interference by static and dynamic jammers. The FH/CA technique in a band with static and dynamic jammers usually has better results than the AFH technique. A significant contribution of the FH/CA technique can be seen in the case of dynamic jammers. On the other hand, in the case of static jammers the FH/CA technique is in certain situations worse than the AFH technique. The accuracy of the mathematical models were successfully verified on a simulation model that was created as a part of this thesis in the MATLAB environment. Based on the obtained data from the model there was designed coding for frequency hopping stations with the new technique of frequency hopping FH/CA which is designed for small-volume data transfer in a band with intensive jamming.

"Le genti della citta, delle isole e del contado, le quale al tutto volevano partirsi”. Migrations from the Venetian to the Ottoman Territory and Conversions of Venetian Subjects to Islam in the Eastern Adriatic in the Sixteenth Century

Migrations from the Venetian to the Ottoman territory and conversions of Venetian subjects to Islam were commonplace in the eastern Adriatic in the sixteenth century and were thus one of the most important factors within the frame of Ottoman- Venetian relations. Main internal reasons for migrations and religious conversions were of economic, social, existential, material and personal nature, while external reasons were above all influenced by Ottoman-Venetian wars and Ottoman raids to the Venetian territory

Cartesian Knowledge and Confirmation

Bayesian conceptions of evidence have been invoked in recent arguments regarding the existence of God, the hypothesis of multiple physical universes, and the Doomsday Argument. Philosophers writing on these topics often claim that, given a Bayesian account of evidence, our existence or something entailed by our existence (perhaps in conjunction with some background knowledge or assumption) may serve as evidence for each of us. In this paper, I argue that this widespread view is mistaken. The mere fact of one's existence qua conscious creature cannot serve as evidence on the standard Bayesian conception of evidence because knowledge of one's existence is a necessary part of the background knowledge relative to which all epistemic probabilities are defined. It follows that some formulations of the fine-tuning argument (for theism or a multiverse), the argument from consciousness (for theism) and a rejoinder to the Doomsday argument are mistaken

A Conflict between Indexical Credal Transparency and Relevance Confirmation

According to the probabilistic relevance account of confirmation, E confirms H relative to background knowledge K just in case P(H/K&E) \u3e P(H/K). This requires an inequality between the rational degree of belief in H determined relative to two bodies of total knowledge which are such that one (K&E) includes the other (K) as a proper part. In this paper, I argue that it is quite plausible that there are no two possible bodies of total knowledge for ideally rational agents meeting this requirement. Hence, the positive relevance account may have to be rejected