29 research outputs found
Multiuser Detection and Channel Estimation for Multibeam Satellite Communications
In this paper, iterative multi-user detection techniques for multi-beam
communications are presented. The solutions are based on a successive
interference cancellation architecture and a channel decoding to treat the
co-channel interference. Beams forming and channels coefficients are estimated
and updated iteratively. A developed technique of signals combining allows
power improvement of the useful received signal; and then reduction of the bit
error rates with low signal to noise ratios. The approach is applied to a
synchronous multi-beam satellite link under an additive white Gaussian channel.
Evaluation of the techniques is done with computer simulations, where a noised
and multi-access environment is considered. The simulations results show the
good performance of the proposed solutions.Comment: 12 page
Multiuser Detection with Decision-Feedback Detectors and PIC in MC-CDMA System
In this paper we propose an iterative parallel decision feedback (P-DF) receivers associated with parallel interference cancellation (PIC) for multicarrier code division multiple access (MC-CDMA) systems in a Rayleigh fading channel (cost 207). First the most widely detection techniques, minimum mean-squared error MMSE, Maximum Likelihood ML and PIC were investigated in order to compare their performances in terms of Bit Error Rate (BER) with parallel feedback detection P-DFD. A MMSE DF detector that employs parallel decision-feedback (MMSE-P-DFD) is considered and shows almost the same BER performance with MMSE and ML, which present a better result than the other techniques. In a second time, an iterative proposed method based on the multi-stage techniques P-DFD (parallel DFD with two stages) and PIC was exploited to improve the performance of the system
Asynchronous CDMA Systems with Random Spreading-Part II: Design Criteria
Totally asynchronous code-division multiple-access (CDMA) systems are
addressed. In Part I, the fundamental limits of asynchronous CDMA systems are
analyzed in terms of spectral efficiency and SINR at the output of the optimum
linear detector. The focus of Part II is the design of low-complexity
implementations of linear multiuser detectors in systems with many users that
admit a multistage representation, e.g. reduced rank multistage Wiener filters,
polynomial expansion detectors, weighted linear parallel interference
cancellers. The effects of excess bandwidth, chip-pulse shaping, and time delay
distribution on CDMA with suboptimum linear receiver structures are
investigated. Recursive expressions for universal weight design are given. The
performance in terms of SINR is derived in the large-system limit and the
performance improvement over synchronous systems is quantified. The
considerations distinguish between two ways of forming discrete-time
statistics: chip-matched filtering and oversampling
Channel Estimation And Multiuser Detection In Asynchronous Satellite Communications
In this paper, we propose a new method of channel estimation for asynchronous
additive white Gaussian noise channels in satellite communications. This method
is based on signals correlation and multiuser interference cancellation which
adopts a successive structure. Propagation delays and signals amplitudes are
jointly estimated in order to be used for data detection at the receiver. As, a
multiuser detector, a single stage successive interference cancellation (SIC)
architecture is analyzed and integrated to the channel estimation technique and
the whole system is evaluated. The satellite access method adopted is the
direct sequence code division multiple access (DS CDMA) one. To evaluate the
channel estimation and the detection technique, we have simulated a satellite
uplink with an asynchronous multiuser access.Comment: 14 pages, 9 figure
Combined turbo coding and interference rejection for DS-CDMA.
Thesis (Ph.D.)-University of KwaZulu-Natal, Durban, 2004.This dissertation presents interference cancellation techniques for both the Forward Error
Correction (FEC) coded and the uncoded Direct Sequence Code Division Multiple
Access (DS-CDMA) systems. Analytical models are also developed for the adaptive and
the non-adaptive Parallel Interference Cancellation (PlC) receivers. Results that are
obtained from the computer simulations of the PlC receiver types confirm the accuracy of
the analytical models that are developed. Results show that the Least Mean Square
(LMS) algorithm based adaptive PlC receivers have bit error rate performances that are
better than those of the non-adaptive PlC receivers.
In the second part of this dissertation, a novel iterative multiuser detector for the Turbo
coded DS-CDMA system is developed. The performance of the proposed receiver in the
multirate CDMA system is also investigated. The developed receiver is found to have an
error rate performance that is very close to the single user limit after a few numbers of
iterations. The receiver is also resilient against the near-far effect. A methodology is also
presented on the use of the Gaussian approximation method in the convergence analysis
of iterative interference cancellation receivers for turbo coded DS-CDMA systems
Improved successive interference cancellation schemes using selective pre-cancellation and cross- correlation
Master'sMASTER OF ENGINEERIN
Multiuser Detection in Asynchronous Multibeam Communications
This paper deals with multi-user detection techniques in asynchronous
multibeam satellite communications. The proposed solutions are based on
successive interference cancellation architecture (SIC) and channel decoding
algorithms. The aim of these detection methods is to reduce the effect of
cochannel interference due to co-frequency access, and consequently, improves
the capacity of the mulitbeam communications systems, by improving frequency
reuse. Channel estimation allows the determination of interference
coefficients, which helps their effects compensation. The developed multiuser
detections techniques are iterative. Therefore, detection quality is improved
from a stage to another. Moreover, a signals combining method, which is
integrated into these detection solutions, enhances their capability. The
proposed solutions are evaluated through computer simulations, where an
asynchronous multibeam satellite link is considered over an AWGN channel. The
obtained simulation results showed the robustness of these multi-user detection
techniques.Comment: 14 pages, 6 figure
Interference characterization and suppression for multiuser direct-sequence spread-spectrum system
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2002.Includes bibliographical references (p. 175-184).In this thesis we investigate efficient modulation and detection techniques for the uplink (i.e. transmission from mobile to base station) of a DS-CDMA network. Specifically, the thesis contains three parts. In the first part, we focus on the mobile transmitter. In particular, we evaluate and compare the spectral efficiency of two promising variable rate DS-CDMA transmission techniques, multicode (MCD) and variable-spreading-gain (VSG), under the presence of multiple-access (user-to-user) interferences (MAI) and multipath interferences. The uniqueness of our study is that in bit-error-rate evaluation, instead of approximating the interference as Gaussian noise (which has been done in most of the previous studies), we incorporate both power and distribution of interferences into consideration. We show where the Gaussian assumption may give misleading answers and how our results in these cases are different from those obtained in the past. In part two and three of the thesis, we focus on the base station receiver. Specifically, we present effective joint detection techniques that have good performance-complexity tradeoff. Part two of the thesis introduces a class of novel multistage parallel interference cancellation algorithms based on stage-by-stage minimum mean-squared error (MMSE) optimization. We show that this scheme is capable of achieving significantly better performance than other algorithms with similar complexity. Part three of the thesis presents a low-complexity dual-mode multiuser detector that dynamically switches its detection mode between the matched-filter receiver and the decorrelator. We show that this detector is capable of achieving the performance of a decorrelator but with significant savings in processing power and complexity.by Mingxi Fan.Ph.D
Adaptive DS-CDMA multiuser detection for time variant frequency selective Rayleigh fading channel
The current digital wireless mobile system such as IS-95, which is based on direct sequence Code Division Multiple Access (DS-CDMA) technology, will not be able to meet the growing demands for multimedia service due to low information exchanging rate. Its capacity is also limited by multiple accessed interference (MAI) signals.
This work focuses on the development of adaptive algorithms for multiuser detection (MUD) and interference suppression for wideband direct sequence code division multiple access (DS-CDMA) systems over time-variant frequency selective fading channels. In addition, channel acquisition and delay estimation techniques are developed to combat the uncertainty introduced by the wireless propagation channel. This work emphasizes fast and simple techniques that can meet practical needs for high data rate signal detection.
Most existing literature is not suitable for the large delay spread in wideband systems due to high computational/ hardware complexity. A de-biasing decorrelator is developed whose computational complexity is greatly reduced without sacrificing performance. An adaptive bootstrap symbolbased signal separator is also proposed for a time-variant channel. These detectors achieve MUD for asynchronous, large delay spread, fading channels without training sequences.
To achieve high data rate communication, a finite impulse response (FIR) filter based detector is presented for M-ary QAM modulated signals in a multipath Rayleigh fading channel. It is shown that the proposed detector provides a stable performance for QAM signal detection with unknown fading and phase shift. It is also shown that this detector can be easily extended to the reception of any M-ary quadrature modulated signal.
A minimum variance decorrelating (MVD) receiver with adaptive channel estimator is presented in this dissertation. It provides comparable performance to a linear MMSE receiver even in a deep fading environment and can be implemented blindly. Using the MVD receiver as a building-block, an adaptive multistage parallel interference cancellation (PIC) scheme and a successive interference cancellation (SIC) scheme were developed. The total number of stages is kept at a minimum as a result of the accurate estimating of the interfering users at the earliest stages, which reduces the implementation complexity, as well as the processing delay. Jointly with the MVD receiver, a new transmit diversity (TD) scheme, called TD-MVD, is proposed. This scheme improves the performance without increasing the bandwidth. Unlike other TD techniques, this TDMVD scheme has the inherent advantage to overcome asynchronous multipath transmission. It brings flexibility in the design of TD antenna systems without restrict signal coordination among those multiple transmissions, and applicable for both existing and next generation of CDMA systems.
A maximum likelihood based delay and channel estimation algorithm with reduced computational complexity is proposed. This algorithm uses a diagonal simplicity technique as well as the asymptotically uncorrelated property of the received signal in the frequency domain. In combination with oversampling, this scheme does not suffer from a singularity problem and the performance quickly approaches the Cramer-Rao lower bound (CRLB) while maintaining a computational complexity that is as low as the order of the signal dimension