Faced with imminent spectrum scarcity largely due to inflexible licensed band arrangements, cognitive radio (CR) has been proposed to facilitate higher spectrum utilization by allowing cognitive users (CUs) to access the licensed bands without causing harmful interference to primary users (PUs). To achieve this without the aid of PUs, the CUs have to perform spectrum sensing reliably detecting the presence or absence of PU signals. Without reliable spectrum sensing, the discovery of spectrum opportunities will be inefficient, resulting in limited utilization enhancement. This dissertation examines three major techniques for spectrum sensing, which are matched filter, energy detection, and cyclostationary feature detection. After evaluating the advantages and disadvantages of these techniques, we narrow down our research to a focus on cyclostationary feature detection (CFD). Our first contribution is to boost performance of an existing and prevailing CFD method. This boost is achieved by our proposed optimal and sub-optimal schemes for identifying best hypothesis test points. The optimal scheme incorporates prior knowledge of the PU signals into test point selection, while the sub-optimal scheme circumvents the need for this knowledge. The results show that our proposed can significantly outperform other existing schemes. Secondly, in view of multi-antenna deployment in CR networks, we generalize the CFD method to include the multi-antenna case. This requires effort to justify the joint asymptotic normality of vector-valued statistics and show the consistency of covariance estimates. Meanwhile, to effectively integrate the received multi-antenna signals, a novel cyclostationary feature based channel estimation is devised to obtain channel side information. The simulation results demonstrate that the errors of channel estimates can diminish sharply by increasing the sample size or the average signal-to-noise ratio. In addition, no research has been found that analytically assessed CFD performance over fading channels. We make a contribution to such analysis by providing tight bounds on the average detection probability over Nakagami fading channels and tight approximations of diversity reception performance subject to independent and identically distributed Rayleigh fading. For successful coexistence with the primary system, interference management in cognitive radio networks plays a prominent part. Normally certain average or peak transmission power constraints have to be placed on the CR system. Depending on available channel side information and fading types (fast or slow fading) experienced by the PU receiver, we derive the corresponding constraints that should be imposed. These constraints indicate that the second moment of interference channel gain is an important parameter for CUs allocating transmission power. Hence, we develop a cooperative estimation procedure which provides robust estimate of this parameter based on geolocation information. With less aid from the primary system, the success of this procedure relies on statistically correlated channel measurements from cooperative CUs. The robustness of our proposed procedure to the uncertainty of geolocation information is analytically presented. Simulation results show that this procedure can lead to better mean-square error performance than other existing estimates, and the effects of using inaccurate geolocation information diminish steadily with the increasing number of cooperative cognitive users.

One of the challenges of distributed cooperative orthogonal frequency division multiplexing systems is that the multiple carrier frequency offsets (CFOs) simultaneously present at the receiver. According to our knowledge up to now, even the CFOs are known at the receiver, the way to perfectly eliminate the effect of CFOs is still an open problem. This thesis proposes a scheme to mitigate the effect due to multiple CFOs by using the concept of intercarrier interference self-cancellation in transitional OFDM systems, a scheme where the data are simultaneously modulated on symmetric subcarriers between two transmit antennas. Before processing FFT, two values related to CFOs are used to adjust the time-domain signal resulting in better signal-to-interference ratio in even and odd subcarriers respectively. After that, the data are combined by applying maximum ratio combining and then decoded. Simulation results are given to demonstrate the effectiveness of the proposed scheme as compared to previous scheme.

This thesis explores the practical application of Physical Layer Security (PLS) inMultiple-Input Multiple-Output (MIMO) systems, particularly focusing on ShortPacket Communication (SPC). The aim is to enhance the security of wireless com-munications against eavesdropping threats. By employing advanced techniques suchas Maximum Ratio Transmission (MRT) and Maximum Ratio Combining (MRC),along with Beamforming, the study demonstrates how these methods can signifi-cantly strengthen the signal integrity in MIMO systems.In developing a comprehensive system model that integrates PLS into MIMO,the research provides a dual approach of evaluation. Rigorous theoretical analy-sis coupled with MATLAB simulations are utilized to validate the effectiveness ofthe proposed model. These methods not only underscore the feasibility of PLSin real-world applications but also highlight the potential improvements in wirelesscommunication security, offering a valuable contribution to the field.