Coexistence of spectrum sensing of IEEE 802.11 systems for cognitive radio applications

Abstract

Most of today's short range wireless communication is based on the IEEE 802.11 standard, also referred to as Wi-Fi, which has been developed to exchange data using a certain portion of radio spectrum. However, many of today's new technologies, such as Wi-Fi, have been allocated to unlicensed spectrum bands while spectrum is scarcely available and therefore needs to be used efficiently. Nowadays, a vast majority of the Wi-Fi systems operates in the license-exempt 2.4 GHz ISM band that becomes more and more crowded due to the growing usage of Wi-Fi. On top of that, a growing number of non Wi-Fi technologies become active in this band as well. In this thesis the coexistence of Wi-Fi systems with other technologies is addressed to deal with congestion and interference issues. The focus is on spectrum sensing and detection techniques, that can be used by cognitive radio applications, to enhance the coexistence of Wi-Fi systems in the ISM band. The improvement in coexistence can be achieved through the detection of both interference sources and congestion mechanisms which in turn can assist in a more efficient usage of the radio spectrum. In addition, improvements can be made on the monitoring side to assess the spectrum usage better and thus more precisely by minimizing the influences of the measurement equipment on the observation data.First, we investigate spectrum sensing for monitoring short-range wireless technologies. We show the value of using mobile spectrum monitoring equipment and results are presented concordantly. Additionally the impact of Automatic Gain Control (AGC), an essential building block in spectrum monitoring receivers, is investigated in order to enhance spectrum sensing performance. By doing so, techniques are developed and applied to remove the AGC influences from the monitoring spectrum data. The results show that interference due to the AGC could lead to an overestimation of the actual spectrum usage by 60% in the 2.4 GHz ISM band.Secondly, the influence of interference on Wi-Fi systems has been investigated. This entails respectively interference due to non Wi-Fi technologies and co-channel interference due to overlapping Wi-Fi systems.A measurement tool has been developed to assess spectrum utilization and to measure the congestion simultaneously. The obtained results show the inefficient use of the wireless medium due to a large amount of transmission overhead which may lead to only 21% of actual data content transmitted in highly congested areas.Thirdly, the alternative use of Wi-Fi communication in the licensed UHF/VHF bands has been investigated to alleviate the congestion and interference issues in the ISM band and to access the large amount of additional available bandwidth. The operation of Wi-Fi systems in these bands is considered to be allowed under iv the condition of secondary use, i.e. avoiding interference to users with the primary rights (e.g. TV-broadcasting stations). Recently the IEEE 802.11af standard has been set up, based on cognitive radio, to provide Wi-Fi access by...