1,965 research outputs found
Improving the Spectral Efficiency of Nonlinear Satellite Systems through Time-Frequency Packing and Advanced Processing
We consider realistic satellite communications systems for broadband and
broadcasting applications, based on frequency-division-multiplexed linear
modulations, where spectral efficiency is one of the main figures of merit. For
these systems, we investigate their ultimate performance limits by using a
framework to compute the spectral efficiency when suboptimal receivers are
adopted and evaluating the performance improvements that can be obtained
through the adoption of the time-frequency packing technique. Our analysis
reveals that introducing controlled interference can significantly increase the
efficiency of these systems. Moreover, if a receiver which is able to account
for the interference and the nonlinear impairments is adopted, rather than a
classical predistorter at the transmitter coupled with a simpler receiver, the
benefits in terms of spectral efficiency can be even larger. Finally, we
consider practical coded schemes and show the potential advantages of the
optimized signaling formats when combined with iterative detection/decoding.Comment: 8 pages, 8 figure
Iterative pre-distortion of the non-linear satellite channel
Digital Video Broadcasting - Satellite - Second Generation (DVB-S2) is the
current European standard for satellite broadcast and broadband communications.
It relies on high order modulations up to 32-amplitude/phase-shift-keying
(APSK) in order to increase the system spectral efficiency. Unfortunately, as
the modulation order increases, the receiver becomes more sensitive to physical
layer impairments, and notably to the distortions induced by the power
amplifier and the channelizing filters aboard the satellite. Pre-distortion of
the non-linear satellite channel has been studied for many years. However, the
performance of existing pre-distortion algorithms generally becomes poor when
high-order modulations are used on a non-linear channel with a long memory. In
this paper, we investigate a new iterative method that pre-distorts blocks of
transmitted symbols so as to minimize the Euclidian distance between the
transmitted and received symbols. We also propose approximations to relax the
pre-distorter complexity while keeping its performance acceptable
Advanced transceivers for spectrally-efficient communications
In this thesis, we will consider techniques to improve the spectral
efficiency of digital communication systems, operating on the whole transceiver
scheme. First, we will focus on receiver schemes having detection algorithms
with a complexity constraint. We will optimize the parameters of the reduced
detector with the aim of maximizing the achievable information rate. Namely, we
will adopt the channel shortening technique. Then, we will focus on a technique
that is getting very popular in the last years (although presented for the
first time in 1975): faster-than-Nyquist signaling, and its extension which is
time packing. Time packing is a very simple technique that consists in
introducing intersymbol interference on purpose with the aim of increasing the
spectral efficiency of finite order constellations. Finally, in the last
chapters we will combine all the presented techniques, and we will consider
their application to satellite channels.Comment: PhD Thesi
On Low Complexity Detection for QAM Isomorphic Constellations
Despite of the known gap from the Shannon's capacity, several standards are
still employing QAM or star shape constellations, mainly due to the existing
low complexity detectors. In this paper, we investigate the low complexity
detection for a family of QAM isomorphic constellations. These constellations
are known to perform very close to the peak-power limited capacity,
outperforming the DVB-S2X standard constellations. The proposed strategy is to
first remap the received signals to the QAM constellation using the existing
isomorphism and then break the log likelihood ratio computations to two one
dimensional PAM constellations. Gains larger than 0.6 dB with respect to QAM
can be obtained over the peak power limited channels without any increase in
detection complexity. Our scheme also provides a systematic way to design
constellations with low complexity one dimensional detectors. Several open
problems are discussed at the end of the paper.Comment: Submitted to IEEE GLOBECOM 201
Advanced techniques for spectrally efficient DVB-S2X systems
We investigate different techniques to improve the spectral efficiency of systems based on the DVB-S2 standard, when the transmitted signal bandwidth cannot be increased because it has already been optimized to the maximum value allowed by transponder filters. We will investigate and compare several techniques to involve different sections of the transceiver scheme. The techniques that will be considered include the use of advanced detection algorithms, the adoption of time packing, and the optimization of the constellation and shaping pulses. The LDPC codes recently proposed for the evolution of the DVB-S2 standard will be considered, as well as the adoption of iterative detection and decoding. Information theoretical analysis will be followed by the study of practical modulation and coding schemes
On the use of multiple satellites to improve the spectral efficiency of broadcast transmissions
We consider the use of multiple co-located satellites to improve the spectral efficiency of broadcast transmissions. In particular, we assume that two satellites transmit on overlapping
geographical coverage areas, with overlapping frequencies. We first describe the theoretical framework based on network information theory and, in particular, on the theory for multiple access channels. The application to different scenarios will be then considered, including the bandlimited additive white Gaussian noise channel with average power constraint and different models for the nonlinear satellite channel. The comparison with the adoption of frequency division multiplexing (FDM) is also provided. The main conclusion is that a strategy based on overlapped signals is convenient with respect to FDM, although it requires the adoption of a multiuser detection strategy at the receiver
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