1,721,102 research outputs found
GLRT-Based Combining Schemes for PPM IR-UWB Systems with Multiple Receive Antennas
No full textThis paper deals with impulse-radio ultra wideband
systems with multiple receive antennas. For binary PPM
modulations, we propose various combining/detection schemes derived by the generalized likelihood ratio criterion. These schemes first combine delayed versions of the waveforms received at the different antennas and then compute the energies of the resulting
signal over intervals smaller than half the symbol period.
The decision statistic is finally obtained by a linear combination of these energies. The proposed algorithms can be implemented
using currently available technology. They outperform a detection scheme that simply combines the decision statistics obtained (at
each antenna) by means of a conventional energy-based detector. Their advantage increases with the number of antennas
A 'reciprocity' property of the unbiased Cramér-Rao bound for vector parameter estimation
No full textIn this letter, we consider Cramér-Rao bounds (CRBs) on the variance of unbiased vector parameter estimators. It is well known that the CRB for the estimation of a parameter \alpha, assuming that a second parameter \beta is unknown, is not smaller than the CRB computed assuming \beta known. The performance loss can be measured by the ratio between these two bounds. In this work, we derive a 'reciprocity' property of the CRB for vector parameter estimation, which indicates that the performance loss in estimating \alpha when \beta is unknown equals the performance loss in estimating \beta with \alpha unknown. Though this property is of mainly theoretical interest, some examples are given which show that it can be useful also in practical applications. © 1994-2012 IEEE
Code-Multiplexing-Based One-Way Detect-and-Forward Relaying Schemes for Multiuser UWB MIMO Systems
Full text linkIn this paper, we consider decode-and-forward (DF) one-way relaying schemes for multiuser impulse-radio ultrawideband (UWB) communications. We assume low-complexity terminals with limited processing capabilities and a central transceiver unit (i.e., the relay) with a higher computational capacity. All nodes have a single antenna differently from the relay in which multiple antennas may be installed. In order to keep the complexity as low as possible, we concentrate on noncoherent transceiver architectures based on multiuser code-multiplexing transmitted-reference schemes. We propose various relaying systems with different computational complexity and different levels of required channel knowledge. The proposed schemes largely outperform systems without relay in terms of both bit error rate (BER) performance and coverage
Tomlinson-Harashima Precoding in MIMO Systems: A Unified Approach to Transceiver Optimization Based on Multiplicative Schur-Convexity
No full textThis paper deals with the design of a multicarrier
multiple-input multiple-output system which performs nonlinear
Tomlinson-Harashima precoding at the transmitter and linear
equalization at the receiver. The goal is the joint optimization of
the processing matrices at both ends of the wireless link, assuming
a constraint on the overall transmit power. We consider different
design criteria which require the maximization/minimization of
objective functions that depend on the mean square errors over
the individual data streams. A unified framework for the solution
of a great number of optimization problems is developed, based
on the concept of multiplicative Schur-convexity. We show that
many widely used design criteria can be easily accommodated in
our framework. Our analysis is centered on two theorems which,
in the case of cooperative processing among the different subcarriers,
provide closed-form solutions to a number of optimization
problems. On the other hand, when an independent processing is
assumed at each subcarrier closed-form solutions may not exist.
Nonetheless, exploiting the results of the two theorems, we show
that for a variety of cost functions the optimization problem can
be efficiently solved. Theoretical analysis and computer simulations
are used to assess the performance of the proposed schemes
and make comparisons with linear transceiver architectures.
It turns out that nonlinear precoding provides better results
than linear prefiltering when the cost function is multiplicatively
Schur-convex. In contrast, when the cost function is multiplicatively
Schur-concave the nonlinear scheme is equivalent to the
linear one
Efficient Maximum-Likelihood Based Clock and Phase Estimators for OQPSK Signals
Full text linkIn this paper we propose an algorithm for joint carrier phase and timing estimation with OQPSK modulations. The derivation is based on the maximum-likelihood criterion, and exploits a very efficient algorithm for the detection of differentially encoded -PSK symbols already described in literature. Though we are mainly interested in measuring the phase and clock parameters, estimates of the transmitted symbols are also obtained as by-products. The resulting scheme has a feedforward structure and provides phase and timing information in a fixed time, differently from closed-loop architectures. It can be implemented in digital form and is particularly suitable for burst mode transmissions. Its performance is investigated by simulation and the results are compared with Cramér-Rao bounds. It turns out that the estimation accuracy is very close to the theoretical limits, even with short observation intervals and small values of the excess bandwidth. In such conditions, the proposed estimators largely outperform other schemes already known in literature. Their superiority becomes less significant as the signal bandwidth increases
A Differential Receiver for UWB Systems
No full textWe consider a UWB communication system operating
in a typical indoor environment and we propose a novel differential
receiver in which encoding is performed symbol-by-symbol
rather than frame-by-frame, as is normally done. The detector
structure is derived in a single-user scenario using a generalized
likelihood ratio test approach. Its performance is assessed by
simulation in the presence of multiple-access interference. The
proposed receiver has a simple structure and outperforms other
differential schemes of comparable complexity
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
Frequency Estimation and Timing Acquisition in the Uplink of a DS-CDMA System
No full textWe consider the uplink of a direct-sequence code-division multiple-access system operating over a multipath fading channel, and we aim at estimating the propagation delay and the frequency offset of a new user entering the network. The joint estimation of these parameters is investigated using a weighted-least-squares approach. The exact solution turns out to be too complex for practical purposes, as it involves a numerical search over a bidimensional domain. As an alternative, we propose a suboptimal procedure, in which the propagation delay is first computed with a monodimensional search and then is exploited to derive the frequency offset in closed form. Both data-aided and nondata-aided estimation methods are considered. The proposed synchronizers have a reasonable complexity and are suited for third-generation cellular systems. Their performance is assessed by simulation in a scenario inspired by the specifications of the frequency-division duplexing component of the Universal Mobile Telecommunications Systems standard. It is found that they provide accurate estimates and are useful even in applications over rapidly varying channels
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
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