33 research outputs found
Pilot-aided ML schemes for joint beamforming and channel estimation in WCDMA systems
This paper proposes semi-blind channel estimation and interference cancellation schemes for the reception of pilot-aided signals in WCDMA systems. It is shown that the performance of classical training-based schemes is severely degraded due to the code-multiplexing (as opposed to time-multiplexing) of traffic and pilot signals. The time-dispersivity of the channel destroys the orthogonality between the traffic signal and the pilot, and consequently the traffic signal appears as a new interfering signal for the channel estimation/beamforming algorithm. In order to avoid this effect, we propose techniques that exploit both the presence of the training sequence and the structure of the traffic signal in a semi-blind fashion.Peer ReviewedPostprint (published version
Joint beamforming and channel estimation for pilot-aided WCDMA systems
The problem of joint beamforming and channel estimation for multi-rate multi-code systems is addressed. Usual schemes perform this filtering/estimation operation making use of a training sequence time-multiplexed with the transmitted data. However if pilot and traffic signals are transmitted simultaneously using distinct code allocation-as it is the case in recent standards such as cdma2000 or WCDMA-these schemes tend to fail. This paper proposes semi-blind techniques to overcome the uplink auto-interfering effects of such systems. It is shown that the semi-blind approach yields substantially better performance results thanks to the implicit modeling of the unknown traffic data.Peer ReviewedPostprint (published version
Uplink beamforming for the FDD mode of UTRA
This paper presents some link level simulation results for the evaluation of adaptive antennas in the uplink of the FDD mode of UTRA (UMTS terrestrial radio access). Two families of algorithms were initially considered, the basic difference between them being their ability/disability to suppress the contribution from W-CDMA directional interfering sources. Two distinct schemes were established as representatives for each family and their performance was evaluated in presence of some illustrative interfering scenarios. In the light of the results it is shown that time-reference beamforming algorithms suffer from severe beam pattern distortion effects when applied as such. This in turn causes harsh performance degradation in terms of raw BER, especially at high SINR levels. It is shown that these shortcomings are essentially caused by the uplink multiplexing of the traffic channel, which is seen by the base station as a powerful interfering source coming from the direction of arrival of the desired user.Peer ReviewedPostprint (published version
Spatial filtering for pilot-aided WCDMA systems: a semi-blind subspace approach
This paper proposes a spatial filtering technique for
the reception of pilot-aided multirate multicode direct-sequence
code division multiple access (DS/CDMA) systems such as wideband
CDMA (WCDMA). These systems introduce a code-multiplexed
pilot sequence that can be used for the estimation of the
filter weights, but the presence of the traffic signal (transmitted
at the same time as the pilot sequence) corrupts that estimation
and degrades the performance of the filter significantly. This is
caused by the fact that although the traffic and pilot signals are
usually designed to be orthogonal, the frequency selectivity of the
channel degrades this orthogonality at hte receiving end. Here,
we propose a semi-blind technique that eliminates the self-noise
caused by the code-multiplexing of the pilot. We derive analytically
the asymptotic performance of both the training-only and
the semi-blind techniques and compare them with the actual simulated
performance. It is shown, both analytically and via simulation,
that high gains can be achieved with respect to training-onlybased
techniques.Peer Reviewe
ML approaches to channel estimation for pilot-aided multi-rate DS/CDMA systems
This paper analyzes the asymptotic performance of maximum likelihood (ML) channel estimation algorithms in wideband code division multiple access (WCDMA) scenarios. We concentrate on systems with periodic spreading sequences (period larger than or equal to the symbol span) where the transmitted signal contains a code division multiplexed pilot for channel estimation purposes. First, the asymptotic covariances of the training-only, semi-blind conditional maximum likelihood (CML) and semi-blind Gaussian maximum likelihood (GML) channel
estimators are derived. Then, these formulas are further simplified assuming randomized spreading and training sequences under the approximation of high spreading factors and high number of codes. The results provide a useful tool to describe the performance of the channel estimators as a function of basic
system parameters such as number of codes, spreading factors, or traffic to training power ratio.Peer Reviewe
Adaptive Beamforming for High Bit Rate Services in the FDD Mode of UTRA
Most time-reference beamforming algorithms suffer from severe beampattern distortion effects when applied to high bit rate services in WCDMA, causing serious performance degradation in terms of output BER, especially at high input SINR levels. These shortcomings are essentially caused by the uplink multiplexing of the traffic channel, which is seen by the base station as a powerful interfering source coming from the direction of arrival of the desired user. In this paper, a semi-blind beamforming technique is proposed as a valid solution to overcome this effect. The suggested scheme resorts to a conditional maximum likelihood approach to the underlying estimation problem and is designed to operate in an iterative fashion.Peer ReviewedPostprint (published version
Two-stage code reference beamformer in mobile communications
This paper addresses a new architecture for blind adaptive beamforming when dealing with frequency hopping (FH) modulation in cellular mobile communications systems. The proposed code reference beamformer (CRB) takes advantage of the inherent frequency diversity to estimate beforehand the noise plus interference correlation matrix, which is employed as the first part of the framework. Then, a second stage is adaptively obtained without any a priori knowledge of either the direction of arrival or the array manifold. Using this information, the first stage is in turn readjusted and, as a result, the scheme is able to track non-stationary scenarios following the channel variations with no previous referencesPeer ReviewedPostprint (published version
Capacity of MIMO Channels: asymptotic evaluation under correlated fading
This paper investigates the asymptotic uniform power allocation capacity of frequency nonselective multiple-input
multiple-output channels with fading correlation at either the
transmitter or the receiver. We consider the asymptotic situation,
where the number of inputs and outputs increase without bound
at the same rate. A simple uniparametric model for the fading
correlation function is proposed and the asymptotic capacity per
antenna is derived in closed form. Although the proposed correlation
model is introduced only for mathematical convenience, it
is shown that its shape is very close to an exponentially decaying
correlation function. The asymptotic expression obtained provides
a simple and yet useful way of relating the actual fading
correlation to the asymptotic capacity per antenna from a purely
analytical point of view. For example, the asymptotic expressions
indicate that fading correlation is more harmful when arising at
the side with less antennas. Moreover, fading correlation does not
influence the rate of growth of the asymptotic capacity per receive
antenna with high Eb /N0.Peer Reviewe
Smart antennas in software radio base stations
The application of adaptive antenna techniques to fixed-architecture base stations has been shown to offer wide-ranging benefits, including interference rejection capabilities or increased coverage and spectral efficiency.
Unfortunately, the actual implementation of
these techniques to mobile communication scenarios has traditionally been set back by two fundamental reasons. On one hand, the lack of flexibility of current transceiver architectures does not allow for the introduction of advanced add-on functionalities. On the other hand, the
often oversimplified models for the spatiotemporal characteristics of the radio communications channel generally give rise to
performance predictions that are, in practice, too optimistic. The advent of software radio architectures represents a big step toward the
introduction of advanced receive/transmit
capabilities. Thanks to their inherent flexibility
and robustness, software radio architectures
are the appropriate enabling technology for the
implementation of array processing techniques.
Moreover, given the exponential progression of
communication standards in coexistence and
their constant evolution, software reconfigurability
will probably soon become the only costefficient
alternative for the transceiver
upgrade. This article analyzes the requirements
for the introduction of software radio techniques
and array processing architectures in
multistandard scenarios. It basically summarizes
the conclusions and results obtained within
the ACTS project SUNBEAM,1 proposing
algorithms and analyzing the feasibility of
implementation of innovative and softwarereconfigurable
array processing architectures in
multistandard settings.Peer Reviewe
Nonlinear system inversion applied to random variable generation
A method to design random variable (RV) generators with the same probability density function (PDF) as a given RV record is presented. The resulting RV generator is a nonlinear system that when driven by a uniformly distributed RV, provides an output RV with the desired PDF distribution. The analytical description of the desired PDF is not needed; in fact, only a data record of the desired RV is used. Inversion of nonlinear systems and nonlinear system adaptive design are used.Peer ReviewedPostprint (published version