16 research outputs found
Analysis of a Multicarrier Communication System Based on Overcomplete Gabor Frames
A multicarrier signal can be seen as a Gabor family whose coefficients are the symbols to be transmitted and whose generators are the time-frequency shifted pulse shapes to be used. In this article, we consider the case where the signaling density is increased such that inter-pulse interference is unavoidable. Such an interference is minimized when the Gabor family used is a tight frame. We show that, in this case, interference can be approximated as an additive Gaussian noise. This allows us to compute theoretical and simulated bit-error-probability for a non-coded system using a quadrature phase-shift keying constellation. Such a characterization is then used in order to predict the convergence of a coded system using low-density parity check codes. We also study the robustness of such a system to errors on the received bits in an interference cancellation context
FTN multicarrier transmission based on tight Gabor frames
A multicarrier signal can be synthesized thanks to a symbol sequence and a Gabor family (i.e., a regularly time-frequency shifted version of a generator pulse). In this article, we consider the case where the signaling density is increased such that inter-pulse interference is unavoidable.Over an additive white Gaussian noise channel, we show that the signal-to-interference-plus-noise ratio is maximized when the transmitter and the receiver use the same tight Gabor frame. What is more, we give practical efficient realization schemes and show how to build tight frames based on usual generators. Theoretical and simulated bit-error-probability are given for a non-coded system using quadrature amplitude modulations. Such a characterization is then used to predict the convergence of a coded system using low-density parity-check codes. We also study the robustness of such a system to errors on the received bits in an interference cancellation context
Analysis of a FTN Multicarrier System: Interference Mitigation Based on Tight Gabor Frames
Cognitive radio applications require flexible waveforms to overcome several challenges such as opportunistic spectrum allocation and white spaces utilization. In this context, multicarrier modulations generalizing traditional cyclic-prefix orthogonal frequency-division multiplexing are particularly justified to fit time-frequency characteristics of the channel while improving spectral efficiency.In our theoretical framework, a multicarrier signal is described as a Gabor family the coefficients of which are the symbols to be transmitted and the generators are the time-frequency shifted pulse shapes to be used. In this article, we consider the case where non-rectangular pulse shapes are used with a signaling density increased such that inter-pulse interference is unavoidable. Such an interference is minimized when the Gabor family used is a tight frame. We show that, in this case, interference can be approximated as an additive Gaussian noise. This allows us to compute theoretical and simulated bit-error-probability for a non-coded system using a quadrature phase-shift keying constellation. Such a characterization is then used in order to predict the convergence of a coded system using low-density parity check codes. We also study the robustness of such a system to errors on the received bits in an interference cancellation context
On Time-Variant Distortions in Multicarrier Transmission with Application to Frequency Offsets and Phase Noise
Phase noise and frequency offsets are due to their time-variant behavior one
of the most limiting disturbances in practical OFDM designs and therefore
intensively studied by many authors. In this paper we present a generalized
framework for the prediction of uncoded system performance in the presence of
time-variant distortions including the transmitter and receiver pulse shapes as
well as the channel. Therefore, unlike existing studies, our approach can be
employed for more general multicarrier schemes. To show the usefulness of our
approach, we apply the results to OFDM in the context of frequency offset and
Wiener phase noise, yielding improved bounds on the uncoded performance. In
particular, we obtain exact formulas for the averaged performance in AWGN and
time-invariant multipath channels.Comment: 10 pages (twocolumn), 5 figure
Eigenvalue Estimates and Mutual Information for the Linear Time-Varying Channel
We consider linear time-varying channels with additive white Gaussian noise.
For a large class of such channels we derive rigorous estimates of the
eigenvalues of the correlation matrix of the effective channel in terms of the
sampled time-varying transfer function and, thus, provide a theoretical
justification for a relationship that has been frequently observed in the
literature. We then use this eigenvalue estimate to derive an estimate of the
mutual information of the channel. Our approach is constructive and is based on
a careful balance of the trade-off between approximate operator
diagonalization, signal dimension loss, and accuracy of eigenvalue estimates.Comment: Submitted to IEEE Transactions on Information Theory This version is
a substantial revision of the earlier versio
Joint bit allocation and precoding for filterbank transceivers in NOFDM systems
Recently, the non orthogonal frequency division multiplexing (NOFDM) systems have
attracted increased interest. They have several advantages over traditional OFDM
systems: higher bandwidth efficiency; reduced sensitivity to carrier frequency offsets,
oscillator phase noise and narrowband interference; and reduced intersymbol/intercarrier
interference (ISI/ICI). In particular, low ISI/ICI will be important for future systems
where Doppler frequencies will be larger (equivalently, channel variations will be
faster) due to higher carrier frequencies and higher mobile velocities.
In the first part of this thesis the duality of multicarrier systems and Gabor frames
is discussed and applied to the design of a generalized multicarrier system based on
a filterbank structure. The efficient polyphase implementation is also discussed.
In this thesis the channel capacity of a GMC systems is evaluated through the
diagonalization of an equivalent matrix model where intersymbol and intercarrier
interferences have been included. Exploiting the majorization theory, the mutual information
can be represented as a Schur-concave function and it is maximized through
a joint transceiver design adding a linear precoder at the transmitter and a LMMSE
equalizer at the receiver. The capacity is derived by the eigenvalue decomposition
of the global system matrix including the noise colored by the receiver filtering and
employing a power allocation of the transmitted power according to the well-known
water-filling solution.
This thesis investigates also the behaviour of the NOFDM systems when a power
and bit allocation algorithm (like the Campello one) is employed in order to satisfy
a certain QoS constrain. A comparison of the performances with OFDM systems is
included.
Finally a simple application of the cognitive radio paradigm employing filterbankbased
multicarrier systems is developed and some interesting results are showed