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- Doctoral Thesis (3) (remove)

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- Fachbereich Elektrotechnik und Informationstechnik (3) (remove)

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.

The present thesis deals with multi-user mobile radio systems, and more specifically, the downlinks (DL) of such systems. As a key demand on future mobile radio systems, they should enable highest possible spectrum and energy efficiency. It is well known that, in principle, the utilization of multi-antennas in the form of MIMO systems, offers considerable potential to meet this demand. Concerning the energy issue, the DL is more critical than the uplink. This is due to the growing importance of wireless Internet applications, in which the DL data rates and, consequently, the radiated DL energies tend to be substantially higher than the corresponding uplink quantities. In this thesis, precoding schemes for MIMO multi-user mobile radio DLs are considered, where, in order to keep the complexity of the mobile terminals as low as possible, the rationale receiver orientation (RO) is adopted, with the main focus to further reduce the required transmit energy in such systems. Unfortunately, besides the mentioned low receiver complexity, conventional RO schemes, such as Transmit Zero Forcing (TxZF), do not offer any transmit energy reductions as compared to conventional transmitter oriented schemes. Therefore, the main goal of this thesis is the design and analysis of precoding schemes in which such transmit energy reductions become feasible - under virtually maintaining the low receiver complexity - by means of replacing the conventional unique mappings by the selectable representations of the data. Concerning the channel access scheme, Orthogonal Frequency Division Multiplex (OFDM) is presently being favored as the most promising candidate in the standardization process of the enhanced 3G and forthcoming 4G systems, because it allows a very flexible resource allocation and low receiver complexity. Receiver oriented MIMO OFDM multi-user downlink transmission, in which channel equalization is already performed in the transmitter of the access point, further contributes to low receiver complexity in the mobile terminals. For these reasons, OFDM is adopted in the target system of the considered receiver oriented precoding schemes. In the precoding schemes considered the knowledge of channel state information (CSI) in the access point in the form of the channel matrix is essential. Independently of the applied duplexing schemes FDD or TDD, the provision of this information to the access point is always erroneous. However, it is shown that the impact of such deviations not only scales with the variance of the channel estimation errors, but also with the required transmit energies. Accordingly, the reduced transmit energies of the precoding schemes with selectable data representation also have the advantage of a reduced sensitivity to imperfect knowledge of CSI. In fact, these two advantages are coupled with each other.

The present thesis deals with a novel approach to increase the resource usage in digital communications. In digital communication systems, each information bearing data symbol is associated to a waveform which is transmitted over a physical medium. The time or frequency separations among the waveforms associated to the information data have always been chosen to avoid or limit the interference among them. By doing so, n the presence of a distortionless ideal channel, a single receive waveform is affected as little as possible by the presence of the other waveforms. The conditions necessary to meet the absence of any interference among the waveforms are well known and consist of a relationship between the minimum time separation among the waveforms and their bandwidth occupation or, equivalently, the minimum frequency separation and their time occupation. These conditions are referred to as Nyquist assumptions. The key idea of this work is to relax the Nyquist assumptions and to transmit with a time and/or frequency separation between the waveforms smaller than the minimum required to avoid interference. The reduction of the time and/or frequency separation generates not only an increment of the resource usage, but also a degradation in the quality of the received data. Therefore, to maintain a certain quality in the received signal, we have to increase the amount of transmitted power. We investigate the trade-off between the increment of the resource usage and the correspondent performance degradation in three different cases. The first case is the single carrier case in which all waveforms have the same spectrum, but have different temporal locations. The second one is the multi carrier case in which each waveform has its distinct spectrum and occupies all the available time. Finally, the hybrid case when each waveform has its unique time and frequency location. These different cases are framed within the general system modelling developed in the thesis so that they can be easily compared. We evaluate the potential of the key idea of the thesis by choosing a set of four possible waveforms with different characteristics. By doing so, we study the influence of the waveform characteristics in the three system configurations. We propose an interpretation of the results by modifying the well-known Shannon capacity formula and by explicitly expressing its dependency on the increment of resource usage and on the performance degradation. The results are very promising. We show that both in the case of a single carrier system with a time limited waveform and in the case of a multi-carrier system with a frequency limited waveform, the reduction of the time or frequency separation, respectively, has a positive effect on the channel capacity. The latter, depending on the actual SNR, can double or increase even more significantly.