## Fachbereich Elektrotechnik und Informationstechnik

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- Contributions to increase the power efficiency of OFDM communications (2011)
- This thesis has the goal to propose measures which allow an increase of the power efficiency of OFDM transmission systems. As compared to OFDM transmission over AWGN channels, OFDM transmission over frequency selective radio channels requires a significantly larger transmit power in order to achieve a certain transmission quality. It is well known that this detrimental impact of frequency selectivity can be combated by frequency diversity. We revisit and further investigate an approach to frequency diversity based on the spreading of subsets of the data elements over corresponding subsets of the OFDM subcarriers and term this approach Partial Data Spreading (PDS). The size of said subsets, which we designate as spreading factor, is a design parameter of PDS, and by properly choosing , depending on the system designer's requirements, an adequate compromise between a good system performance and a low complexity can be found. We show how PDS can be combined with ML, MMSE and ZF data detection, and it is recognized that MMSE data detection offers a good compromise between performance and complexity. After having presented the utilization of PDS in OFDM transmission without FEC encoding, we also show that PDS readily lends itself for FEC encoded OFDM transmission. We display that in this case the system performance can be significantly enhanced by specific schemes of interleaving and utilization of reliabiliy information developed in the thesis. A severe problem of OFDM transmission is the large Peak-to-Average-Power Ratio (PAPR) of the OFDM symbols, which hampers the application of power efficient transmit amplifiers. Our investigations reveal that PDS inherently reduces the PAPR. Another approch to PAPR reduction is the well known scheme Selective Data Mapping (SDM). In the thesis it is shown that PDS can be beneficially combined with SDM to the scheme PDS-SDM with a view to jointly exploit the PAPR reduction potentials of both schemes. However, even when such a PAPR reduction is achieved, the amplitude maximum of the resulting OFDM symbols is not constant, but depends on the data content. This entails the disadvantage that the power amplifier cannot be designed, with a view to achieve a high power efficiency, for a fixed amplitude maximum, what would be desirable. In order to overcome this problem, we propose the scheme Optimum Clipping (OC), in which we obtain the desired fixed amplitude maximum by a specific combination of the measures clipping, filtering and rescaling. In OFDM transmission a certain number of OFDM subcarriers have to be sacrificed for pilot transmission in order to enable channel estimation in the receiver. For a given energy of the OFDM symbols, the question arises in which way this energy should be subdivided among the pilots and the data carrying OFDM subcarriers. If a large portion of the available transmit energy goes to the pilots, then the quality of channel estimation is good, however, the data detection performs poor. Data detection also performs poor if the energy provided for the pilots is too small, because then the channel estimate indispensable for data detection is not accurate enough. We present a scheme how to assign the energy to pilot and data OFDM subcarriers in an optimum way which minimizes the symbol error probability as the ultimate quality measure of the transmission. The major part of the thesis is dedicated to point-to-point OFDM transmission systems. Towards the end of the thesis we show that the PDS can be also applied to multipoint-to-point OFDM transmission systems encountered for instance in the uplinks of mobile radio systems.

- A Gravitational Task Model for Target Sensitive Real-Time Applications (2011)
- For many years real-time task models have focused the timing constraints on execution windows defined by earliest start times and deadlines for feasibility. However, the utility of some application may vary among scenarios which yield correct behavior, and maximizing this utility improves the resource utilization. For example, target sensitive applications have a target point where execution results in maximized utility, and an execution window for feasibility. Execution around this point and within the execution window is allowed, albeit at lower utility. The intensity of the utility decay accounts for the importance of the application. Examples of such applications include multimedia and control; multimedia application are very popular nowadays and control applications are present in every automated system. In this thesis, we present a novel real-time task model which provides for easy abstractions to express the timing constraints of target sensitive RT applications: the gravitational task model. This model uses a simple gravity pendulum (or bob pendulum) system as a visualization model for trade-offs among target sensitive RT applications. We consider jobs as objects in a pendulum system, and the target points as the central point. Then, the equilibrium state of the physical problem is equivalent to the best compromise among jobs with conflicting targets. Analogies with well-known systems are helpful to fill in the gap between application requirements and theoretical abstractions used in task models. For instance, the so-called nature algorithms use key elements of physical processes to form the basis of an optimization algorithm. Examples include the knapsack problem, traveling salesman problem, ant colony optimization, and simulated annealing. We also present a few scheduling algorithms designed for the gravitational task model which fulfill the requirements for on-line adaptivity. The scheduling of target sensitive RT applications must account for timing constraints, and the trade-off among tasks with conflicting targets. Our proposed scheduling algorithms use the equilibrium state concept to order the execution sequence of jobs, and compute the deviation of jobs from their target points for increased system utility. The execution sequence of jobs in the schedule has a significant impact on the equilibrium of jobs, and dominates the complexity of the problem --- the optimum solution is NP-hard. We show the efficacy of our approach through simulations results and 3 target sensitive RT applications enhanced with the gravitational task model.