6,849 research outputs found
Link Optimization in Future Generation Satellite Systems
No full textIn recent years, communication networks have seen a huge growth in the amount of
requested throughput, pushed from the combination of two main drivers: the
introduction of new services and the improvement of existing ones, requiring
increased amount of traffic (e.g. higher quality of video content). These
effects mandate the constant evolution of current systems in order to cope with
the growing user demand and should be tackled from multiple angles. On the one
hand, better utilization of available resources might help in the short term to
keep up with the market and has always been an important priority for operators
of terrestrial and satellite networks alike. On the other hand, acquisition and
exploitation of currently unused resources might fuel the growth for a
significantly longer period of time, ensuring longevity and thus enabling
future-proofing of current systems. Both these topics are addressed in this
thesis with specific applications relevant to satellite communication networks.
In the first part, this thesis focuses on maximization of the user capacity by
better exploiting the available radio resources. Motivated by the substantial
capacity gains enabled by a higher bandwidth allocation, we
investigate the optimization of satellite systems employing full-frequency
reuse on the user downlink. Unlike most of the literature on the subject, usually
resorting to precoding techniques to mitigate the interference, we propose a
combination of predistortion and precoding to jointly counteract on-board non-linear
distortions and multi-user interference. First, a flexible framework for
the optimization of transmit processing schemes in communication chains is
presented. This framework expands on the application of the well known gradient
descent technique by applying it to the maximization of the received Signal
to Noise plus Interference ratio in complex communication systems. To do so, it
identifies a suitable mathematical representation of various key blocks of the
system and exploits the chain rule of the derivative to compute the overall
gradient as a cascade of the single components. Afterwards, this framework is
validated by optimizating the coefficients of the proposed predistortion
architecture for the satellite system in analysis. The obtained results
highlight the flexibility of the developed optimization framework and the
benefits of the suggested predistortion strategy compared to existing state of
the art solutions.
In the second part of the thesis, the focus is shifted towards investigating the
exploitation of novel resources by looking at the use of optical frequencies for
ground-to-space feeder links. The topic is introduced by a survey of existing benefits
and limitations of free space optical communications. Subsequently, the
implications of employing optical frequencies in long distance ground-to-space feeder
links with transparent satellites are addressed. Furthermore, a powerful and flexible
simulation tool was developed and exploited during the course of this thesis to
model and assess the Physical (PHY) layer performance of hybrid optical/Radio
Frequencies (RF) satellite networks. This tool is presented together with the scenarios and results obtained as part of the project ONSET
(Optical Feeder Links Study for Satellite Networks - ESA Contract No. 40000113462/15/NL/NDe).
Finally, the thesis investigates a scenario that combines the transmit processing
techniques analyzed in the first part and the context of optical feeder links
evaluated in the second part. A hybrid optical/RF system is considered with an
electrical predistorter in place to counteract the impairments induced by the
combined effects of electrical and optical non-linearities encountered along the
end-to-end chain. The developed mathematical framework is exploited to jointly
optimize the predistortion coefficients and the working point for the
electro-optical modulator. The performance results obtained after the
optimization procedure demonstrate the efficacy of the proposed approach for
hybrid optical/RF systems with analog modulations
Code-multiplexed UWB Transmitted-Reference Radio
No full textIn traditional transmitted reference (TR) ultrawideband
systems the reference component is time-shifted and
orthogonal relative to the data-bearing signal. This paves the
way to a correlation receiver in which the local template is
derived from the incoming waveform using a delay line. As analog
delay lines are difficult to implement with current technology,
an alternative TR system has recently been proposed in which
reference and data components are made orthogonal by a
frequency shift rather than a time shift. The resulting receiver
has no delay lines and has better performance compared to
the traditional scheme. In the present paper we discuss a third
way to achieve orthogonality, i.e., by modulating reference and
data components with two distinct code sequences. Even in this
case the receiver has no delay lines. However, it is simpler to
implement and has better performance than the frequency-shift
based receiver
Frame Frequency Estimation in Ultra-Wideband Communications
No full textThis paper investigates the estimation of the frame
frequency in ultrawideband (UWB) communication systems. An
estimation method is proposed that exploits the transmission of a
periodic pulse sequence at the frame frequency. The samples of
the received waveform are used to compute a cost function that
depends on a trial value of the incoming pulse frequency. The
location of the maximum provides an estimate of the transmitted
frequency. The performance of the estimator is assessed theoretically
and is compared to the Cramer–Rao lower bound. It is shown
that in certain conditions the estimator achieves the bound at high
SNR values. Simulations validate the theory and show the degradations
in the estimation performance caused by multiple-access
interference. They also give an idea of the estimation accuracy
needed in a correlation receiver
Code-Multiplexed Transmitted-Reference UWB Systems in a Multi-User Environment
No full textRecent studies have investigated alternative methods
to the realization of transmitted-reference ultra-wideband
communication systems. In addition to the conventional scheme,
in which data and reference are separated in time, two other
methods have been proposed in which the separation is achieved
through a frequency shift or via code multiplexing. In a singleuser
scenario the alternative methods have advantages in terms
of error rate and receiver implementation. In this paper the
three schemes are compared in a multi-user environment. It is
shown that even in the presence of multiple access interference
the unconventional solutions are preferable and, in particular,
the code multiplexing method has the best performance and is
the simplest to implement
Channel Estimation for the Uplink of a DS-CDMA System
No full textThis letter deals with channel estimation in the uplink of a direct-sequence code-division multiple-access system operating in a multipath environment. The parameters of interest are the delays and the complex attenuations incurred by the signal echoes along the propagation paths. We propose an iterative approach for estimating the channel parameters of a new user entering the system. The method is based on the space-alternating generalized expectation-maximization algorithm and exploits a training sequence. In comparison to other estimation algorithms, it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional problems. In addition, it can be effectively used in applications over fast-fading channels. Computer simulations are employed to assess the performance of the proposed scheme. It is found that it is resistant to multiuser interference and has accuracy close to the Cramer-Rao lower bound even with very short training sequences
Synchronization for Differential Transmitted Reference UWB Receivers
No full textThis paper investigates timing recovery for differential
transmitted reference (DTR) ultra-wideband receivers. In
DTR systems the symbol energy is distributed over several time
slots (frames) and a single pulse is transmitted per frame. The
information is conveyed by differentially encoding the pulse
polarities. Equal polarities correspond to a bit +1 while alternate
polarities correspond to a bit −1. The detector computes the
decision statistic by correlating pulses from adjacent frames and
summing the contributions from all the frames of a symbol.
Synchronization consists in providing accurate timing for the
correlation computations. A timing algorithm is proposed requiring
a limited amount of circuitry in addition to that needed
for detection purposes. No training sequences are needed as the
synchronizer operates in a blind mode. Two possible receiver
architectures are envisioned, analog or digital. The effects of the
timing errors on the receiver performance are investigated by
simulation. In general, they are found to be negligible, except
when the implementation is digital and the analog-to-digital
converter resolution is limited to 1 bit
DOA and Channel Parameter Estimation for Wideband CDMA Systems
No full textWe consider the uplink of a direct-sequence code-division multiple-access system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters (path gains and delays) and the directions of arrival of the signal echoes from a user entering the network. We propose an estimator that operates in an iterative fashion and exploits knowledge of the transmitted symbols (training sequence). Compared to other existing schemes, it is simpler to implement as it reduces a complicated multidimensional optimization problem to a sequence of one-dimensional searches. Computer simulations indicate that the proposed scheme is useful even in applications over rapidly varying channels where the training sequence must be short compared with the channel decorrelation time
Joint DOA and Channel Parameter Estimation for Code-Division Multiple-Access Systems
No full textWe consider the uplink of a DS-CDMA system and we assume that the base station is endowed with a linear antenna array. Transmission takes place over a multipath channel and the goal is to estimate the channel parameters and the directions of arrival of the signals from a user entering the network. Maximum likelihood estimation of all these parameters is not feasible as it involves a search over a multidimensional domain. Through suitable approximations we replace the above search by a sequence of mono-dimensional searches. This results in an estimation algorithm of reasonable complexity for third-generation cellular applications. The performance of the algorithm is assessed by simulation in a scenario inspired by the specifications of the FDD component of the UMTS standard. It is found that the channel parameters and the directions of arrival can be estimated with accuracy close to the Cramer-Rao bound
Impact of MAI and Channel Estimation Errors on the Performance of Rake Receivers in UWB Communications
No full textThe performance of Rake receivers for ultrawidebandwidth
communications is discussed, taking into account the
effects of multiple access interference (MAI) and channel estimation
errors. Two alternative signaling formats are considered:
time-hopping pulse-position modulation (TH-PPM) and TH pulseamplitude
modulation (TH-PAM). The channel exhibits multipath
propagation and its impulse response is either assumed known or
is estimated with least squares methods. Computer simulations
show that, even with perfect channel knowledge (PCK), TH-PAM
is superior to TH-PPM. The superiority increases with the number
of users and becomes substantial in the presence of channel
estimation errors. An intuitive explanation of this fact is provided
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