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Research Areas

Below is a list of current or recent research topics we work on with brief summaries. For details please contact to any of the team members listed under each project.

Advanced Transceiver Design for Continuous Phase Modulation

Team: Baris Ozgul, Mutlu Koca, Hakan Delic

This research aims to propose efficient transmitter and receiver designs for continuous phase modulation (CPM), which is a non-linear modulation scheme with excellent power and bandwidth properties. The research topics include the application of iterative receiver algorithms, precoding, and space-time processing for CPM as mentioned below. The optimal joint equalization/demodulation of CPM signals over frequency-selective multipath fading channels encounter computational problems because of the intensive search performed over a single super trellis. In the thesis, soft-input soft-output linear time-domain/frequency-domain equalizer algorithms are devised for coded CPM, which are followed by CPM demodulator and the channel decoder. Near optimum performance is attained with low-complexity by employing iterative processing. The receiver performance is analyzed via EXIT charts. As another solution to reduce the receiver complexity, a space-time pre-equalizer is proposed for CPM. The proposed method preserves the spectral efficiency by allowing small envelope variations. Since the channel equalization functions are moved to the transmitter, the receiver performs CPM demodulation only. For coded CPM, further performance gain can be obtained by exploiting demodulation/decoding iterations at the receiver.  

Robust Multiuser Detection in Ultra-Wideband Systems

Team: Nazli Guney, Hakan Delic

UWB transmission, where information is sent over impulse-like radio (IR) waveforms, has emerged as an exciting technology for commercial wireless communication systems and local area networking. Despite their advantages especially in short range, such as low power consumption, carrier-free transmission, utilization of low-complexity transceivers, capability to co-exist with existing broadband systems, ultra wideband systems can only be realized after major design issues have been solved. Our research in this area involves the design low-power UWB multiple access schemes with low complexity multi-user receivers. Our investigation involves code optimization for multiple access protocols, transceiver design for UWB systems employing time-hopping and multi-band orthogonal pulses, design of low complexity demodulation and decoding structures and interference cancellation algorithms, space-time signal processing to enable the use of multiple transmit/receive antennas in UWB systems.  

Robust Signal Acquisition and Detection in Ultra-Wideband Systems

Team: Ersen Ekrem, Mutlu Koca, Hakan Delic

The research conducted for this thesis aims to find robust solutions to timing acquisition and channel estimation for ultra wideband systems subject to non-Gaussian noise. Low power constraints on the UWB systems, using very short pulses to obtain higher rates and the multipath channel with extreme delay spread makes these problems more challenging with respect to other spread spectrum systems. For timing acquisition, noncoherent methods, i.e. the methods that do not employ a priori channel knowledge, are investigated and for channel estimation, frequency-domain techniques inspired by single-carrier frequency domain equalization are explored. For robust detection and estimation, minimax robust techniques such as M-estimates, least favorable pair, are employed. 

Ultra-Wideband User Cooperative Networks

Team: Ozgur Dalkilic, Cumhur Oazan Yalcin, Mutlu Koca

Ultra wideband communication devices, which are restricted in size and power, can achieve some benefits of the MIMO systems by means of user cooperation. Cooperation between single-antenna users involves transmission of the same data by partners hence creation of a virtual antenna array. By introducing additional diversity, user cooperation improves capacity and decreases outage probability. In this project we investigate different ways of implementing cooperative communications in UWB systems. Major challenge is the frequency selective nature of the UWB channel, which introduces ISI and breaks down the orthogonality of the signals transmitted from different users. 

Space-Time-Polarization Codes

Team: Onur Ozyesil, Mutlu Koca

MIMO communications systems employing space-time codes are known to have huge performance gains over single antenna systems. The capacity of the MIMO systems can be further increased -theoretically doubled- if the systems use dual polarized antennas. For this reason, in this project we investigate the use of dual polarized antennas in space-time block coded systems and evaluate the theoretical limits of the performance and design iterative receiver architectures which overcome the cross-polar interference, channel fading and spatial and polarization correlation effects. 

Multitone Interference in MIMO-OFDM Systems

Team: Celal Esli, Hakan Delic

We consider MIMO OFDM systems subject to mult-tone interference and derive the analytical bounds for the anti-jamming performance of space-frequency coding in these systems. We also design iterative receivers for joint interference cancellation and space-frequency detection for OSTBC coded OFDM systems. 

Turbo Space-Time Equalization for WiMAX System

Team: Mutlu Koca

We consider space-time turbo equalization methods for complex coded modulation signals transmitted over broadband wireless channels, which is characterized by severe multipath fading as envisioned by IEEE 802.16 standard. The multipath fading channel, protected by an encoder/decoder pair, is described as a convolutional encoder and the received signal is viewed as the output of a serially concatenated coding system. It is well known that in the narrowband transmission case, the equivalent encoder model for the channel can be represented by reasonable complexity and optimum symbol detection is achieved by iterative equalization/decoding schemes where soft-information is shared between the constituent decoders. However, for fixed wireless systems operating at high data rates, the multipath delay spread becomes large which makes direct application of trellis based equalization methods impossible.

Turbo Processing for Video over IP

Team: Ceyhan Kasap, Mutlu Koca

We investigate the joint use of multiple description coding and channel coding for systems transmitting data over heterogeneous networks so as to achieve resilience against packet losses and protection against channel errors. Observing that the two coding blocks form a serial concatenated code, we consider a turbo decoding scheme passing soft messages between a SISO channel decoder and a source reconstructor. We also investigate the problem of optimal redundancy allocation between the blocks of such heterogeneous coding structures. Another objective of this research is to apply iterative processing on advanced quantization methods such as trellis coded quantization.

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