Doctor's Theses (authored and supervised):
"Wireless OFDM Systems: Channel Prediction and System Capacity";
Supervisor, Reviewer: F. Hlawatsch, H. Bölcskei;
Institut für Nachrichtentechnik und Hochfrequenztechnik,
The general theme of this thesis is orthogonal frequency division multiplexing (OFDM)
communications over time and frequency selective fading channels. We propose and study
linear prediction techniques for acquiring channel state information (CSI) in OFDM receivers,
and we perform an information-theoretic analysis of the performance of OFDM
After a review of the generic discrete-time pulse-shaping OFDM system (which comprises
conventional cyclic-pre x OFDM systems as a special case), we consider the transmission
over a time and frequency selective fading channel. We arrive at an approximate
multiplicative system input-output relation in which intersymbol and interchannel interference
Based on this approximate input-output relation, we propose decision-directed channel
predictors that are capable of yielding up-to-date CSI without regular transmission of pilot
symbols. We derive the minimum mean-square error (MMSE) predictor and a reducedcomplexity
version that allows for an e cient DFT implementation. We also develop
adaptive predictors that do not need statistical prior knowledge and can track nonstationary
channels. Several applications of channel prediction are discussed, and the excellent
performance of the proposed techniques is demonstrated by computer simulations.
The second major contribution of this thesis is an information-theoretic analysis of the
performance of OFDM systems transmitting over time and frequency selective channels.
We study the system capacity of wideband OFDM communications in the absence of CSI
at the transmitter and the receiver. Using a codebook that is \peaky" in time and frequency,
we show that OFDM can approach the in nite-bandwidth channel capacity. On
the other hand, using a \nonpeaky" constant-modulus signaling scheme, we show that the
information rate is reduced by a penalty term that is related to the predictability of the
fading channel. We quantify the impact of the spread and shape of the scattering function
on this penalty term. Finally, we formulate an upper and a lower bound on system capacity
and demonstrate by simulations that both bounds are close to the AWGN channel capacity
for large ranges of bandwidth and for practically relevant system parameters.
Electronic version of the publication:
Created from the Publication Database of the Vienna University of Technology.