Key Technologies In NC-OFDM Based Dynamic Spectrum Sharing Systems

The"cognitive radio"concept, proposed by Dr. Mitola, is aiming to improve thespectrum utilization. In cognitive radio networks, wireless devices share a length of spec-trum in a dynamic way. As a physical layer enabling mechanism, non-contiguous or-thogonal frequency division multiplexing (NC-OFDM) is promising because of its high?exibility and e?ciency as well as being easy to implement. However, the dynamic andcomplexed spectrum environment makes the design of dynamic spectrum sharing systemchallenging. The thesis studies the NC-OFDM based dynamic spectrum sharing system,including spectrum sensing, spectrum analysis, spectrum allocation and spectrum hand-o?.Firstly, the thesis proposes a hybrid spectrum sensing approach in order to tradeo? the spectrum sensing overhead and sensing accuracy for wideband OFDM systems.This hybrid approach incorporates two signal detection techniques—-energy detectionand wavelet transform based detection. With energy detection used to quickly detectthe existing users and the wavelet transform based detection used in spectrum boundarieslocalization, the hybrid spectrum sensing approach is e?ective to shorter the sensing time.The idea of integration of multiple techniques into one method can also be utilized inother systems, which should not be neglected.Secondly, the thesis formulates an analytical model to determine the multi-user in-terference, i.e., cross-band interference, in distributed OFDM/NC-OFDM based dynamicspectrum sharing systems. Network asynchronism and user heterogeneity are used tocharacterize the cross-band interference. Based on the interference analysis, the thesiscompares three mitigation methods and points out that inserting frequency guardband be-tween adjacent spectrum blocks is the most e?ective one. Furthermore, the thesis quan-titatively analyses the required frequency guardband. Through the study on cross-bandinterference, we find that the multi-user interference is considerable in dynamic spectrumsharing systems.Next, the thesis makes a study on the dynamic spectrum allocation strategies inNC-OFDM based dynamic spectrum sharing systems. Based on two known spectrumallocation strategies, the thesis proposes two improved ones taking into consideration thefrequency guardband and channel fading property. The two improved strategies show di?erent performance under di?erence settings, indicating that fully taking advantageof the physical layer characteristics is an e?ective way to improve the performance ofdynamic spectrum allocation.Last, the thesis discusses link synchronization and link maintenance to improve therobustness of spectrum hando?. In order to combat the interference in link synchroniza-tion, the thesis presents a protective synchronization scheme—-ProSync. ProSync has ashrinked preamble at the transmitter and a double-filter structure at the receiver, so thesynchronization can be improved with the data retrieval una?ected. Besides, the thesisdesigns a temporary dual-monitor aided link maintenance protocol to raise the robustnessand lower the system overhead.This thesis is a guideline to the implementation of NC-OFDM based dynamic spec-trum sharing systems, and related researches can be extended to other cognitive radiosystems.