Date of Award


Degree Type


Degree Name

Master of Engineering Science


Electrical and Computer Engineering


Dr. Xianbin Wang


This thesis defines a new Orthogonal Frequency Division Multiplexing (OFDM) system with Precoded In-band Pilots (PIP) tailored for cognitive radio (CR) communications. The motivation, principle, system design, implementation architecture, and CR application specific considerations of proposed PIP-OFDM system are investigated in this thesis. Principles and limitations of existing spectrum sensing techniques for cognitive radio communications are first analyzed and compared, with a focus on implementation challenges of pilot-based spectrum sensing for OFDM signals due to its robust performance in low signal-to-noise ratio (SNR) conditions. Several technical difficulties which haven’t been well addressed in previous pilot-based OFDM spectrum sensing studies, including impact of cyclic prefix, frequency offset between transmitter and spectrum sensing device, and noise uncertainty in the sensing threshold design, are taken into consideration in the analysis. Considering the poor performance of existing spectrum sensing techniques on user identification in cognitive radio network, where multiple secondary users may coexist, a precoded in-band pilots design is proposed in this thesis to enhance the user identification capabilities at low SNRs. The pilots in proposed PIP-OFDM system consist of uniform pilots and identification pilots. Each secondary user is associated with a unique identification pilot signal for identification purpose. Encoding of identification pilots is investigated, which will be used at the spectrum sensing device to identify the active user on the frequency band of interest. 111 Abstract To demodulate/decode identification pilots for user identification purpose, synchronization between transmitter and spectrum sensing device needs to be established. The synchronization in PIP-OFDM system, which is different from that in traditional OFDM systems, is subsequently investigated. Coarse time and frequency synchronization are achieved by correlation respectively in time and frequency domain. Through phase shift estimation in time domain, fine frequency synchronization is reached using a modified maximum likelihood estimation algorithm exploiting the redundancy in cyclic prefix. Based on this observation, a fine time synchronization algorithm is proposed in this thesis using redundant information on specifically designed uniform pilots. A multiple OFDM symbols processing strategy is used to improve the synchronization performance of PIP-OFDM system considering the poor performance of synchronization at low SNR. With the developed synchronization strategies, channel estimation in PIP-OFDM system is achieved using well developed estimation techniques in frequency domain. User identification is subsequently realized through demodulating the identification pilots. Theoretical performance and simulation results of user identification in PIP-OFDM system are provided to further confirm the effectiveness of the proposed design.



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