- 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.