9 research outputs found
Blind Estimation of Multiple Carrier Frequency Offsets
Multiple carrier-frequency offsets (CFO) arise in a distributed antenna
system, where data are transmitted simultaneously from multiple antennas. In
such systems the received signal contains multiple CFOs due to mismatch between
the local oscillators of transmitters and receiver. This results in a
time-varying rotation of the data constellation, which needs to be compensated
for at the receiver before symbol recovery. This paper proposes a new approach
for blind CFO estimation and symbol recovery. The received base-band signal is
over-sampled, and its polyphase components are used to formulate a virtual
Multiple-Input Multiple-Output (MIMO) problem. By applying blind MIMO system
estimation techniques, the system response is estimated and used to
subsequently transform the multiple CFOs estimation problem into many
independent single CFO estimation problems. Furthermore, an initial estimate of
the CFO is obtained from the phase of the MIMO system response. The Cramer-Rao
Lower bound is also derived, and the large sample performance of the proposed
estimator is compared to the bound.Comment: To appear in the Proceedings of the 18th Annual IEEE International
Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC),
Athens, Greece, September 3-7, 200
Parameter estimation and equalization techniques for communication channels with multipath and multiple frequency offsets
We consider estimation of frequency offset (FO) and equalization of a wireless communication channel, within a general framework which allows for different frequency offsets for various multipaths. Such a scenario may arise due to different Doppler shifts associated with various multipaths, or in situations where multiple basestations are used to transmit identical information. For this general framework, we propose an approximative maximum-likelihood estimator exploiting the correlation property of the transmitted pilot signal. We further show that the conventional minimum mean-square error equalizer is computationally cumbersome, as the effective channel-convolution matrix changes deterministically between symbols, due to the multiple FOs. Exploiting the structural property of these variations, we propose a computationally efficient recursive algorithm for the equalizer design. Simulation results show that the proposed estimator is statistically efficient, as the mean-square estimation error attains the Crame´r-Rao lower bound. Further, we show via extensive simulations that our proposed scheme significantly outperforms equalizers not employing FO estimation
Advanced classification of OFDM and MIMO signals with enhanced second order cyclostationarity detection
With the emergence of cognitive radio and the introduction of new modulation techniques such as OFDM and MIMO, the problem of Modulation Classification (MC) becomes more challenging and complicated. In the first part of the thesis, we explore the automatic modulation classification to blindly distinguish OFDM from single carrier signals. We use the fourth order cumulants; an approach which in the past has been also applied to classify single carrier signals. A blind OFDM parameter estimation scheme was then followed, which includes the estimation of number of subcarriers, CP length, timing and frequency offset and the oversampling factor for the OFDM signal. For the second part of the thesis, we improve the statistical signal processing techniques that were used in the first part. Particularly, the second order cyclostationarity based methods have been examined and improved. Based on the fact that most of the cyclostationary communication signals has a real cyclostationary part and a complex non-cyclostaionary part, we suggest an approach that enhance the second order cyclostationarity and hence increase its probability of detection. Using such improved second-order cyclostationarity, we present an improved synchronization method based on second order cyclostationarity. With the proposed approach, it is shown that the timing estimator, is independent of the frequency offset estimator, and therefore performs better than the previously proposed class of blind synchronization methods. To negate the dependence of the blind synchronization scheme on the prior knowledge of the raised cosine pulse shaping filters, we proposed a blind roll-off factor estimator based on the second order cyclostationarity. Last, we address the MIMO classification problem, wherein we estimate the number of transmitting antennas. Here the second order cyclostationarity test has been applied in distinguishing STC from BLAST modulation
Investigation into synchronization for partial response signals and the development of a clock recovery scheme for 49QPRS signals
ThesisData communication is used increasingly in modern society. It is against this background
that research is conducted worldwide toward the improvement of existing, as well as the
development of new, improved communication techniques.
Correlative encoding of data before transmission IS a very frequency-effective
communication technique. The extent to which any communication technique is used,
however, is dependent on a wide variety of factors. This study regarding the
synchronisation of 49QPRS signals was undertaken with this in mind.
Since digital signal processing (DSP) is used increasingly in modern communication
systems, both a data transmitter and receiver were implemented by making use of this
technique. Not only would this result in a system with all the desirable characteristics
inherent to DSP, but, by making limited changes to the supporting software, the
evaluation of a wide variety of alternatives became feasible.
During the study a system making use of a pilot tone at one third the frequency of the
carrier frequency was developed. The receiver recovers this signal by means of DSP
techniques and its frequency is tripled. The phase of this recovered signal is crosscorrelated
every 650 ~s in time with a locally generated signal of the correct frequency -
and the phase of the locally generated signal is adjusted accordingly. It was found that the
accuracy and stability of the locally generated signal were such that sufficient
synchronisation was obtained in this manner. The quality of synchronisation is a function
of the level of the pilot tone and if this tone should decrease to below a certain value,
unacceptably large phase adjustments have to be made. This results in a senous
degradation of the spectral purity of the recovered signal. However, the system as
described exhibits extremely good noise immunity. During the development of the clock frequency recovery system, a baseband filter with a
unique frequency response was defined. Making use of this, in conjunction with a limited
amount of pre-processing, and an absolute value rectifier, recovery of the clock frequency
becomes possible. In order to limit the amount of processing by the receiver, the baseband
filter was implemented in its entirety in the transmitter. The recovered signal showed a
moderate amount of amplitude variation, but an extremely stable synchronising signal
could be derived from this.
During the study both levels of synchronisation required by a hypothetical 49QPRS data
communication system were therefore investigated fully and solutions found
High-performance signal acquisition algorithms for wireless communications receivers
Due to the uncertainties introduced by the propagation channel, and RF and
mixed signal circuits imperfections, digital communication receivers require efficient
and robust signal acquisition algorithms for timing and carrier recovery, and interfer-
ence rejection.
The main theme of this work is the development of efficient and robust signal
synchronization and interference rejection schemes for narrowband, wideband and
ultra wideband communications systems. A series of novel signal acquisition schemes
together with their performance analysis and comparisons with existing state-of-the-
art results are introduced. The design effort is first focused on narrowband systems,
and then on wideband and ultra wideband systems.
For single carrier modulated narrowband systems, it is found that conventional
timing recovery schemes present low efficiency, e.g., certain feedback timing recov-
ery schemes exhibit the so-called hang-up phenomenon, while another class of blind
feedforward timing recovery schemes presents large self-noise. Based on a general re-
search framework, we propose new anti-hangup algorithms and prefiltering techniques
to speed up the feedback timing recovery and reduce the self-noise of feedforward tim-
ing estimators, respectively.
Orthogonal frequency division multiplexing (OFDM) technique is well suited for
wideband wireless systems. However, OFDM receivers require high performance car-rier and timing synchronization. A new coarse synchronization scheme is proposed for
efficient carrier frequency offset and timing acquisition. Also, a novel highly accurate
decision-directed algorithm is proposed to track and compensate the residual phase
and timing errors after the coarse synchronization step. Both theoretical analysis
and computer simulations indicate that the proposed algorithms greatly improve the
performance of OFDM receivers.
The results of an in-depth study show that a narrowband interference (NBI) could
cause serious performance loss in multiband OFDMbased ultra-wideband (UWB) sys-
tems. A novel NBI mitigation scheme, based on a digital NBI detector and adaptive
analog notch filter bank, is proposed to reduce the effects of NBI in UWB systems.
Simulation results show that the proposed NBI mitigation scheme improves signifi-
cantly the performance of a standard UWB receiver (this improvement manifests as
a signal-to-noise ratio (SNR) gain of 9 dB)
New advances in symbol timing synchronization of single-carrier, multi-carrier and space-time multiple-antenna systems
In this dissertation, the problem of symbol timing synchronization for the following three different communication systems is studied: 1) conventional single-carrier
transmissions with single antenna in both transmitter and receiver; 2) single-carrier
transmissions with multiple antennas at both transmitter and receiver; and 3) orthogonal frequency division multiplexing (OFDM) based IEEE 802.11a wireless local
area networks (WLANs).
For conventional single-carrier, single-antenna systems, a general feedforward
symbol-timing estimation framework is developed based on the conditional maximum
likelihood principle. The proposed algorithm is applied to linear modulations and two
commonly used continuous phase modulations: MSK and GMSK. The performance
of the proposed estimator is analyzed analytically and via simulations.
Moreover, using the newly developed general estimation framework, all the previously proposed digital blind feedforward symbol timing estimators employing second-order statistics are cast into a unified framework. The finite sample mean-square
error expression for this class of estimators is established and the best estimators are
determined. Simulation results are presented to corroborate the analytical results.
Moving on to single-carrier, multiple-antenna systems, we present two algorithms. The first algorithm is based on a heuristic argument and it improves the
optimum sample selection algorithm by Naguib et al. so that accurate timing estimates can be obtained even if the oversampling ratio is small. The performance of
the proposed algorithm is analyzed both analytically and via simulations.
The second algorithm is based on the maximum likelihood principle. The data
aided (DA) and non-data aided (NDA) ML symbol timing estimators and their cor-
responding CCRB and MCRB in MIMO correlated ??at-fading channels are derived.
It is shown that the improved algorithm developed based on the heuristic argument
is just a special case of the DA ML estimator. Simulation results under different
operating conditions are given to assess and compare the performances of the DA
and NDA ML estimators with respect to their corresponding CCRBs and MCRBs.
In the last part of this dissertation, the ML timing synchronizer for IEEE 802.11a
WLANs on frequency-selective fading channels is developed. The proposed algorithm
is compared with four of the most representative timing synchronization algorithms,
one specically designed for IEEE 802.11a WLANs and three other algorithms designed for general OFDM frame synchronization
New advances in synchronization of digital communication receivers
Synchronization is a challenging but very
important task in communications. In digital communication systems, a hierarchy of synchronization problems has to be considered: carrier synchronization, symbol timing synchronization and frame synchronization. For bandwidth efficiency and burst transmission reasons, the former two synchronization steps tend to favor non-data aided (NDA or blind) techniques, while in general, the last one is usually solved by inserting repetitively known
bits or words into the data sequence, and is referred to as a data-aided (DA) approach.
Over the last two decades, extensive research work has been carried out to design nondata-aided timing recovery and carrier synchronization algorithms. Despite their importance and spread
use, most of the existing blind synchronization algorithms are derived in an ad-hoc manner without exploiting optimally the entire available statistical information. In most cases their
performance is evaluated by computer simulations, rigorous and complete performance analysis has not been performed yet. It turns out that a theoretical oriented approach is indispensable for
studying the limit or bound of algorithms and comparing different methods.
The main goal of this dissertation is to develop several novel signal processing frameworks that enable to analyze and improve
the performance of the existing timing recovery and carrier synchronization algorithms. As byproducts of this analysis, unified methods for designing new computationally and statistically efficient (i.e., minimum variance estimators)
blind feedforward synchronizers are developed.
Our work consists of three tightly coupled research directions. First, a general and unified framework is proposed to develop optimal nonlinear least-squares (NLS) carrier recovery scheme for burst transmissions. A family of
blind constellation-dependent optimal "matched" NLS carrier estimators is proposed for synchronization of burst transmissions fully modulated by PSK and QAM-constellations in additive white Gaussian noise
channels. Second, a cyclostationary statistics
based framework is proposed for designing computationally and statistically efficient robust blind symbol timing recovery for time-selective flat-fading channels. Lastly, dealing with the problem of frame synchronization, a simple and efficient data-aided approach is
proposed for jointly estimating the frame boundary, the frequency-selective channel and the carrier frequency offset