PIP-OFDM System Design and Application for Cognitive Radio Communications

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

Synchronization in wireless communications

The last decade has witnessed an immense increase of wireless communications services in order to keep pace with the ever increasing demand for higher data rates combined with higher mobility. To satisfy this demand for higher data rates, the throughput over the existing transmission media had to be increased. Several techniques were proposed to boost up the data rate: multicarrier systems to combat selective fading, ultra wide band (UWB) communications systems to share the spectrum with other users, MIMO transmissions to increase the capacity of wireless links, iteratively decodable codes (e.g., turbo codes and LDPC codes) to improve the quality of the link, cognitive radios, and so forth

An Efficient Data-aided Synchronization in L-DACS1 for Aeronautical Communications

L-band Digital Aeronautical Communication System type-1 (L-DACS1) is an emerging standard that aims at enhancing air traffic management (ATM) by transitioning the traditional analog aeronautical communication systems to the superior and highly efficient digital domain. L-DACS1 employs modern and efficient orthogonal frequency division multiplexing (OFDM) modulation technique to achieve more efficient and higher data rate in comparison to the existing aeronautical communication systems. However, the performance of OFDM systems is very sensitive to synchronization errors. L-DACS1 transmission is in the L-band aeronautical channels that suffer from large interference and large Doppler shifts, which makes the synchronization for L-DACS more challenging. This paper proposes a novel computationally efficient synchronization method for L-DACS1 systems that offers robust performance. Through simulation, the proposed method is shown to provide accurate symbol timing offset (STO) estimation as well as fractional carrier frequency offset (CFO) estimation in a range of aeronautical channels. In particular, it can yield excellent synchronization performance in the face of a large carrier frequency offset.Comment: In the proceeding of International Conference on Data Mining, Communications and Information Technology (DMCIT

Spectrum Sensing of DVB-T2 Signals in Multipath Channels for Cognitive Radio Networks

© 2018 VDE VERLAG GMBHIn this paper, spectrum sensing of digital video broadcasting-second generation terrestrial (DVB-T2) signals in different fading environments with energy detection (ED) is considered. ED is known to achieve an increased performance among low computational complexity detectors, but it is susceptible to noise uncertainty. By taking into consideration the edge pilot and scattered pilot periodicity in DVB-T2 signals, a low computational complex noise power estimator is proposed. It is shown analytically that the choice of detector depends on the environment, the detector requirements, the available prior knowledge and with the noise power estimator. Simulation confirm that with the noise power estimator, ED significantly outperforms the pilot correlation-based detectors. Simulation also show that the proposed scheme enables ED to obtain increased detection performance in fading channels

Cognitive Radio for Emergency Networks

In the scope of the Adaptive Ad-hoc Freeband (AAF) project, an emergency network built on top of Cognitive Radio is proposed to alleviate the spectrum shortage problem which is the major limitation for emergency networks. Cognitive Radio has been proposed as a promising technology to solve todayâ?~B??~D?s spectrum scarcity problem by allowing a secondary user in the non-used parts of the spectrum that aactully are assigned to primary services. Cognitive Radio has to work in different frequency bands and various wireless channels and supports multimedia services. A heterogenous reconfigurable System-on-Chip (SoC) architecture is proposed to enable the evolution from the traditional software defined radio to Cognitive Radio