Abgeschlossene Forschungspraktika


 Demonstration of Novel Applications of Convex Optimization in Communications and Signal Processing
Andreas Feder, 01.09.2017-09.04.2018

Over the last decade, convex optimization methods have received significant interest in communications and signal processing.Numerous schemes based on convex optimization algorithms have been designed for signal detection, beamforming filter design,channel equalization, resource allocation, classification in machine learning, regression and smoothing, etc. One goal of this research internship is to identify interesting recent novel contributions in the literature on applications of convex optimization to communications and signal processing and provide a summary of some promising schemes. Afterwards, a software based demo of some schemes will be established, highlighting the benefits of convex optimization for practical applications. For example, some wireless information and power transmission scheme might be designed via convex optimization, and the corresponding performance might be simulated and compared  to some benchmarks schemes.

Drone-Based Communication Using FSO Backbone
Hedieh Ajam, 01.07.2016 - 15.11.2017
The proposal can be found here.

 Compressed Channel Sensing in THz communications 
Viktoria Schram, 07.2017

 THz communications is one of the candidates for the future ultrafast wireless transmission. However, the peculiarities of propagation in the THz band result in a very high frequency-selective channel and a much longer channel impulse response (CIR) compared to that of a wireless microwave channel. Moreover it has been observed that the indoor THz channel is sparse, since most of the channel coefficients are either zero or nearly zero. Conventional linear channel estimation strategies, such as the least squares, are ill-suited to fully exploit the characteristics of the THz channel. In contrast, compressed channel sensing seems to be a good candidate.
The basic principle of this concept is to reconstruct the sparse channel h from a number of measurements: y=Ah, taken by the help of a sensing matrix A. To achieve this goal, the following two main tasks have to be solved.
1. How to find a suitable sensing matrix?
2. What reconstruction algorithm is appropriate and exploits the sparse channel characteristics?
In this research-internship  we investigate this by studying how different parameters, like channel length, number of measurements and additional noise influence the channel estimation results. We do also analyze achievable performance bounds. As first approaches the “Dantzig-Selector” according to the work of U. Bajwa et. al (http://ieeexplore.ieee.org/document/5454399/) and the GPSR algorithm according to the work of A. T. Figueiredo et. al (http://ieeexplore.ieee.org/document/4407762/) are studied.

Filter design for Filter-and-Forward Relaying in Magnetic Induction based WUSNs (finished)
Zhao Mengyu, 01.2016-05.2016

Wireless Underground Sensor Networks (WUSNs) present a wide area of research possibilities. The challenge is to establish a reliable fast, and robust communication in the underground medium. Magnetic Induction (MI) based WUSNs make use of induction coils for signal transmission which provides a great benefit in conductive medium (soil) compared to the traditional EM waves and therefore dramatically improves the performance. However, high path loss is still inevitable due to harsh propagation conditions. In original works, MI waveguides (passive multihop relaying) have been proposed, in order to reduce the path loss. Later, this idea has been shown to not improve the performance due to a very limited bandwidth of the MI channel for this technique. Recently, an active MI relay has been introduced and its theoretical performance with different relaying schemes has been shown. In opposite to the MI waveguides, active relaying seems very promising and provides high rate gains. Also, it has been shown, that Amplify-and-Forward (AF) relaying is not suitable for MI based WUSNs. Hence, Filter-and-Forward (FF) relaying has been recommended, since it enables high data rates close to the performance of Decode-and-Forward (DF) relaying and does not require symbol decision at the relay. However, only the theoretical bounds of FF relaying using infinite impulse response (IIR) filters have been computed. In this project, the design of a practical filter with a finite impulse response (FIR) was further investigated.

Buffer-Aided Relaying with Discrete Transmission Rates (finished)
Wayan Wicke, 10.2014-10.2015

In today's cooperative communications networks, the relays receive information in one time slot and transmit it in the following time slot independent of the quality of the receiving and transmitting channels. However, in wireless environments, the qualities of the receiving and transmitting channels vary with time and using predefined reception and transmission slots usually leads to a large performance loss.  By using a buffer at the relay, the relay can receive and store information when the transmit channel is weak and can transmit the stored information when the transmit channel is strong.  Buffer-aided relaying is a relatively new concept that has the potential to significantly improve the performance of wireless relay networks. The objective of this research project is to investigate the optimal buffer-aided relaying protocol which maximizes the throughput of the three-node relay network consisting of a source, a relay node, and a destination. Thereby,  as a practical constraint, source and relay can only transmit with  rates selected from a predefined discrete set of available rates.  Since this protocol is expected to introduce an unbounded delay, a modified buffer-aided protocol which limits the delay without severely limiting the throughput is also to be investigated. The performance of these buffer-aided protocols shall then be compared to the performance of current non-buffer-aided relaying protocols where the transmit nodes can transmit with any desired rate.

Energy Transfer in Access Limited Environments (Finished)
Xiaoyang Chen, 04.2015 - 08.2015

Wireless Underground Sensor Networks (WUSNs) present a variety of new research challenges. Recently a magneto-inductive (MI) waveguide technique has been proposed to overcome the very harsh propagation conditions in WUSNs. One of the major problems of the WUSNs is the energy consumption in case of access limited deployment. Assuming, that only one sensor node can be charged from outside, the received energy needs to be distributed over the whole network. In order to simplify and improve the charging, the possibility of energy transfer in magnetic induction based WUSNs needs to be investigated. In addition, a simultaneous information and energy transfer technique may dramatically improve the battery lifetime and the system performance.

Interference Alignment for SC-FDMA Systems with Widely Linear Filtering 
Mohamed Soliman

In expectation of a growing interference level in cellular systems in the near future, elaborate interference management techniques are necessary in addition to the traditional interference avoidance approaches employed so far. A first step has been done when Coordinated Multi-Point (CoMP) transmission was introduced in the 4G LTE-A standard, allowing an exchange of information between base stations from neighbouring cells. For the network of the next generation, CoMP will additionally embrace most likely centralized processing and distributed cooperation. This new architecture gives way to more advanced cooperation techniques such as interference alignment. Interference alignment is a technique that recently has attracted attention due to its capability to theoretically increase the sum rate of a network without any bound. This is realized by jointly designing precoding filters for the transmitters such that the interference falls into a reserved subspace at the receiver while leaving the remaining subspace interference-free for the desired user. Although theory shows that a complete and perfect alignment is possible, under real conditions residual interference still will be present due to a low signal-to-noise ratio and a limited number of signaling dimensions. Task of the research internship is to investigate widely linear (WL) filtering at the transmitter and the receiver side to overcome the issue associated with limited number of signaling dimensions. For WL filtering the imaginary and the real part of the filter input are processed seperately and subsequently combined linearly. When additionally applying real-valued transmit symbols, a WL system model results wich has doubled dimensions. This translates into more degrees of freedom for the filter design. As the joint optimization over precoding, receive matrices of the users implies an exhaustive search, a suboptimal approach will be examined. The transceiver design is conducted for the 4G formats single-carrier frequencydivision multiple access (SC-FDMA).

Information Theoretic Performance Bounds for Practical Full-Duplex Communication (Finished)
Erik Sippel, 10.2014-04.2015

Abstract: In today's wireless communications networks, the nodes are usually assumed to operate  in either a full-duplex (FD) or half-duplex (HD) mode. FD nodes can transmit and receive at the same time and in the same frequency  band. However, due to the self-interference caused by concurrent transmission and reception, the design of ideal FD nodes is very challenging in practice and demands precise and expensive components. As a result, HD communication has attracted significant attention for practical wireless networks. HD nodes are not allowed to transmit and  receive  at  the  same  time  and  in  the  same  frequency  band  simultaneously.  However, HD communication suffers from a loss  in spectral efficiency compared to FD communication, e.g., ideal FD communication can enhance the spectral efficiency by up to 100% percent compared to HD communication. The objective of this research project is to study practical FD communication. In fact, in contrast to conventional ideal FD communication which neglects the self-interference and conventional HD communication which avoids self-interference by orthogonal transmission and reception, the effect of self-interference has to be taken into account for the study of the practical FD communication. The focus of this research work was on a simple three-node relay network consisting of a source, a practical FD relay node, and a destination. We determined the fundamental performance limits of practical FD communication and proposed effective coding schemes to achieve these bounds.