Design And Key Techniques Of Cognitive OFDM Transmission Systems

Cognitive radio has changed the way of using the electromagnetic spectrum which is licensed by governments. It aims for efficient spectrum utilization, and allows unlicensed users to opportunistically access spectrum holes of licensed users. Because of its adaptive characteristic on spectrum resource control, OFDM is a good candidate for cognitive radio, and combining them results in a frequency-agile transmission system.With theoretical analysis and algorithm simulation, this work focuses on design of a systematic scheme and research on the key techniques including spectrum sensing and mask exchange, channel estimation, and interference suppression for cognitive OFDM transmission systems.In Chapter 1, the background of cognitive radio is introduced, together with the contributions of spectrum regulation departments, standardization organizations, and scholars. The research advances on spectrum sensing and dynamic spectrum management are also presented.In Chapter 2, a framework of the cognitive OFDM transmission system is built. The feasibility of using OFDM in cognitive radios is analyzed, and the network environment of cognitive radio systems is explained. The cycle of the cognitive OFDM transmission system is described then, which is composed by three phases-spectrum sensing, spectrum mask exchange, and data transmission. The flow of the transceiver is showed while the use of rateless codes in dynamic spectrum environment is pointed out. Later, the design method of system parameters like symbol duration, guard interval, and sub-carrier spacing are introduced. The way to calculate the bandwidth efficiency and evaluate the system performance is also presented.In Chapter 3, this work puts an emphasis on spectrum sensing and mask exchange which constitute the first two phases of the system cycle. In regard to spectrum sensing, local detectors are studied, and energy detector is proposed to be used in the system. At the same time, the optimal sensing time is deduced. A physical layer method is then proposed to exchange the spectrum mask information, with simulation results implying that nearly identical masks for both sides can be achieved.In the rest of the thesis, the key techniques of the third phase of the system cycle-data transmission is studied.In Chapter 4, the channel estimation algorithm for cognitive OFDM receiver is studied. After Least Square (LS) criterion is proposed to estimate the channel responses at the pilots, it is suggested that time-domain interpolation can only be used after the channel responses of the unevenly distributed pilots are well processed first. However, piecewise linear interpolation should be used if the number of the processed data is less than the channel taps. Simulation results demonstrate the sound performance of such a method.In Chapter 5, the interference suppression methods for cognitive OFDM transmitter are studied. Windowing and deactivating subcarriers are proposed to suppress the interference on primary bands from the side lobes of OFDM signal. An active method is proposed to use interference avoidance subcarriers to suppress the interference on primary subcarriers, and simulation results prove the effectiveness of the method.Though considering effectiveness and complexity at the same time, the thesis leaves much to do in the future, which includes distributed cooperative spectrum sensing, semi-blind channel estimation, power/rate/frequency joint control, and so on.