No description available.

Cognitive radio has recently been proposed as a promising approach for efficient utilization of radio spectrum. However, there are several challenges to be addressed across all layers of a cognitive radio system design, from application to hardware implementation. From the physical layer point-of-view, two key challenges are spectrum sensing and an appropriate signaling scheme for data transmission. The modulation techniques used in cognitive radio not only should be efficient and flexible but also must not cause (harmful) interference to the primary (licensed) users. Among all the proposed signaling schemes for cognitive radio, orthogonal frequency division multiplexing (OFDM) has emerged as a promising one due to its robustness against multipath fading, high spectral efficiency, and capacity for dynamic spectrum use. However, OFDM suffers from high out-of-band radiation which is due to high sidelobes of subcarriers. In this thesis, we consider spectral shaping in OFDM-based cognitive radio systems with focus on reducing interference to primary users created by by out-of-band radiation of secondary users' OFDM signal. In the first part of this research, we first study the trade-o between time-based and frequency-based methods proposed for sidelobe suppression in OFDM. To this end, two recently proposed techniques, active interference cancellation (AIC) and adaptive symbol transition (AST), are considered and a new joint time-frequency scheme is developed for both single-antenna and multi-antenna systems. Furthermore, knowledge of wireless channel is used in the setting of the proposed joint scheme to better minimize interference to the primary user. This scheme enables us to evaluate the trade-o between the degrees of freedom provided by each of the two aforementioned methods. In the second part of this research, a novel low-complexity technique for reducing out-of-band radiation power of OFDM subcarriers for both single-antenna and multi-antenna systems is proposed. In the new technique, referred to as a phase adjustment technique, each OFDM symbol is rotated in the complex plane by an optimal phase such that the interference to primary users is minimized. It is shown that the phase adjustment technique neither reduces the system throughput, nor does increase the bit-error-rate of the system. Moreover, the performance of the technique in interference reduction is evaluated analytically in some special cases and is verified using numerical simulations. Due to high sensitivity of OFDM systems to time and frequency synchronization errors, performance of spectral shaping techniques in OFDM is significantly affected by timing jitter in practical systems. In the last part of this research, we investigate the impact of timing jitter on sidelobe suppression techniques. Considering AIC as the base method of sidelobe suppression, we first propose a mathematical model for OFDM spectrum in presence of timing jitter and evaluate the performance degradation to AIC due to timing jitter. Then, a precautionary scheme based on a minimax approach is proposed to make the technique robust against random timing jitter.

Zugl.: Erlangen, Nürnberg, University, Diss., 2008.

Zugl.: Hamburg, Techn. Univ., Diss., 2008

Zugl.: Karlsruhe, Univ., Diss., 2009 / Hergestellt on demand

Zugl.: Kaiserslautern, Techn. Univ., Diss., 2009

Techn. University, Diss., 2003--Berlin.

Techn. University, Diss., 2005--Kaiserslautern.

No description available.

Typically, in the semiconductor industry pn heterojunctions have been used as either light-emitting diodes (LED) or photodiodes by applying forward current bias or reverse voltage bias, respectively. However, since both devices use the same structure, the light emitting and detecting properties could be combine in one single device, namely LED-based photodetector. Therefore, by integrating LED-based photodetectors as either transmitter or receiver, optical wireless communication could be easily implemented for bidirectional visible light communication networks at low-cost. Therefore, this dissertation focus on the investigation of the photodetection characteristics of InGaN LED-based photodetectors for visible light communication in the blue region. In this regard, we obtain external quantum efficiency of 10 % and photoresponse rise time of 71 μs at 405-nm illumination, revealing high-performance photodetection characteristics. Furthermore, we use orthogonal frequency division multiplexing quadrature amplitude modulation codification scheme to enlarge the operational bandwidth. Consequently, the transmission rate of the communication is efficiently enhanced up to 420 Mbit/s in visible light communication.