An ever increasing number of wirelessly-enabled applications places a very high demand on stringent spectral resources. Cognitive radios have the potential of enhancing spectral efficiency by improving the usage of channels that are already licensed for a specific purpose. Research on cognitive radios involves answering questions such as: how can a cognitive radio transmit at a high data rate while maintaining the same quality of service for the licensed user? There are multiple forms of cognition studied in literature, and each of these models must be studied in detail to understand its impact on the overall system performance. Specifically, the information-theoretic capacity of such systems is of great interest. Also, the design of cognitive radio is necessary to achieve those capacities in real applications. In this dissertation, we formulate different problems that relate to the performance of such systems and methods to increase their efficiency. This dissertation discusses, firstly, the means of "sensing" in cognitive systems, secondly, the optimal resource allocation algorithms for interweave cognitive radio, and finally, the fundamental limits of partially and overly cognitive overlay systems. / text

Darbe apžvelgiamas, išanalizuojamas IEEE 802.11 standartas, jo architektūra, fizinis lygmuo, kanalinis lygmuo. Aprašoma tyrime naudojama aparatinė ir programinė įranga, tyrimo vykdymo eiga, metodika. Eksperimentiškai ištirtos bevielių lokalių tinklų IEEE 802.11 standarto kokybinės charakteristikos. Pateikiamas kokybinių charakteristikų teorinis įvertinimas. Naudojantis tyrimo rezultatais ir surinkta informacija, sukuriama programinė įranga, skirta bevielių lokalių tinklų laidumo, vėlinimo laiko, aprėpties zonos nustatymui, radimui, skaičiavimui, įvertinimui. Aptariami, pateikiami programinės įrangos kūrimo etapai: reikalavimų specifikacija, architektūros projektavimas, programavimas, testavimas; apibendrinami gauti rezultatai, pateikiamos išvados. / The aim of project work is to research of wireless local network qualitative characteristics: throughput, maximum range, delay. The standart IEEE 802.11, its architecture, physical layer, MAC layer is analysed. Hardware and software, used in research, the methods of research are discribed. Research of qualitative characteristics of wireless local network is done. On the basis of research results the software intended for calculation, determination of value of qualitative characteristics of wireless local network, software is created. The main stages of software creation: specification, designing of architecture, programming, testing are analysed. Obtained results are discussed. Practical, experimental researches are carried out according to recommendations of IEEE 802.11, IEEE 802.3 standarts.

When using a wireless repeater in a network, an overlap zone between the repeater and the primary access point that holds the internet connection is created. The theoretical throughput for connections through the repeater is reduced by half since the repeater can’t send and receive data at the same time when using only one channel which was the case in this thesis. To solve the problem with the overlap zone, an algorithm that runs in the form of a bash script on two openWRT-based access points where one acts as a repeater and the other one is a gateway with an internet connection was developed. The script uses the signal strength value (RSSI) to decide whether a wireless device is located within the overlap zone by comparing the measured value against a set threshold where the gateway still delivers higher throughput than the repeater. Wireless devices in the overlap zone are blocked on the repeater and will then connect to the gateway instead. Wireless devices that are located outside the overlap zone will connect to the access point that yields the strongest signal strength. The throughput to the gateway was measured at several locations with different signal strengths to both the gateway and the repeater. Wireless devices within the overlap zone that were connected to the repeater before the script was run gained a higher throughput after the script was run and made it connect directly to the gateway instead. / Vid användning av en trådlös repeater i ett nätverk skapas en överlappningszon mellan repeatern och den primära anslutningspunkten som har uppkopplingen mot internet. Den möjliga datagenomströmningshastigheten för uppkopplingar via repeatern halveras eftersom repeatern inte kan sända och ta emot samtidigt på samma kanal. För detta examensarbete fanns inte möjligheten att använda fler kanaler. För att lösa problemet med överlappningszonen utvecklades en algoritm som körs i form av ett bash-skript på två openWRT-baserade anslutningspunkter där den ena agerar som repeater och den andra är en gateway med internetuppkoppling. Skriptet använder värdet på signalstyrka (RSSI) för att avgöra om en trådlös enhet befinner sig i överlappningszonen genom att kontrollera uppmätt värde mot ett tröskelvärde där gatewayen fortfarande ger högre datagenomströmningshastighet än repeatern. Trådlösa enheter i överlappningszonen blockeras på repeatern och kopplar då upp till gatewayen istället. Utanför överlappningszonen kopplas enheter upp till den anslutningspunkt som har starkast signalstyrka. Datagenomströmningen till gatewayen mättes på flera platser med olika signalstyrkor till både gatewayen och repeatern. Trådlösa enheter i överlappningszonen som var uppkopplade till repeatern innan skriptet kördes fick en högre datagenomströmningshastighet efter att skriptet kört och styrt om anslutningen direkt till gatewayen istället.