9:30, SPCOM-L3.1
OPTIMAL OFDM SYSTEM DESIGN THROUGH OPTIMAL SPHERE COVERINGS
T. STROHMER, S. BEAVER
Standard OFDM systems are associated with a rectangular grid in the time-frequency plane. However such a setup is in general not optimum for pulse shaping OFDM systems for doubly dispersive channels. We introduce lattice-OFDM systems (LOFDM), which are OFDM systems constructed with respect to general lattices in the time-frequency plane. We show how to design optimum pulse shapes for LOFDM system. Furthermore we demonstrate by theoretical considerations and numerical simulations that LOFDM systems using hexagonal-type lattices outperform ordinary OFDM systems with regard to robustness against ISI/ICI.

9:50, SPCOM-L3.2
CHIRPED-OFDM FOR TRANSMISSIONS OVER TIME-VARYING CHANNELS WITH LINEAR DELAY/DOPPLER SPREADING
S. BARBAROSSA, R. TORTI
In this work we show that the optimal digital communication strategy for transmissions over time-varying channels with spread function maximally concentrated along a line of the delay-Doppler domain consists in multiplexing the input symbol block with an IFFT, as in OFDM, and modulating the IFFT output with a chirp signal whose sweep rate is matched to the channel. We show how to allocate the transmit power optimally across the chirped subcarriers, derive the limits of applicability of the proposed chirped-OFDM scheme and compute the resulting BER curves.

10:10, SPCOM-L3.3
IMPULSE NOISE CANCELLATION IN MULTICARRIER TRANSMISSION
F. ABDELKEFI, P. DUHAMEL, A. GABAY
A parallel between Reed Solomon codes in the complex field and multicarrier transmission using OFDM is first presented. This shows that when the signal is sent over some channel composed of Gaussian plus impulse noise, the impulse noise can be removed by a procedure similar to channel decoding, using information carried by the "syndrome". These results are first derived in a simple situation (oversampled DMT, additive channel), which is merely of theoretical interest. Several extensions are then provided in order to increase the practical usefulness of the method. Simulations combining classical convolutive codes with the above mentioned approach are provided.

10:30, SPCOM-L3.4
SPREAD OFDM PERFORMANCE WITH MMSE EQUALIZATION
M. DEBBAH, P. LOUBATON, M. DE COURVILLE
In this paper, upper bounds on the bit error probability of an OFDM/CDMA system in a single user context are derived for a linear minimum mean-square-error (MMSE) receiver structure. All the calculus are performed in presence of a frequency-selective Rayleigh fading channel both in a coded (convolutive Forward Error Correcting scheme) and uncoded scenario. Surprisingly, we show that coded SOFDM outperforms COFDM if the signal to noise ratio is greater than a given threshold which only depends on code rate and not on the code structure. These theoretical results provide tools to optimally combine coding and spreading for increasing the spectral efficiency of OFDM/CDMA schemes.

10:50, SPCOM-L3.5
OPTIMAL TRAINING AND REDUNDANT PRECODING FOR BLOCK TRANSMISSIONS WITH APPLICATION TO WIRELESS OFDM
S. OHNO, G. GIANNAKIS
The adoption of orthogonal frequency-division multiplexing (OFDM) by wireless local area networks and audio/video broadcasting standards testifies to the importance of recovering block precoded transmissions propagating through frequency-selective FIR channels. Existing block transmission standards invoke bandwidth-consuming error control codes to mitigate channel fades and training sequences to identify the FIR channels. To enable low-complexity block-by-block receiver processing, we design redundant precoders with cyclic prefix (CP) and superimposed training sequences for optimal channel estimation and guaranteed symbol recovery regardless of the underlying FIR frequency-selective channels. Numerical results are presented to access the performance of the designed training and precoding schemes.

11:10, SPCOM-L3.6
OPTIMAL EMBEDDING OF KNOWN SYMBOLS FOR OFDM
S. ADIREDDY, L. TONG, H. VISWANATHAN
The problem of placing known symbols optimally for OFDM is considered. The channel is assumed to be quasi-static with a finite impulse response. Under the assumption that neither the transmitter and receiver know the channel, we optimize the training by maximizing a lower bound of the mutual information. It is shown that the lower bound is maximized by placing the known symbols periodically. Optimum energy trade-off between the training and the data is also obtained and illustrated through simulation.