761 research outputs found
Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols
In this paper, we describe direct-sequence code-division multiple-access
(DS-CDMA) systems with quadriphase-shift keying in which channel estimation,
coherent demodulation, and decoding are iteratively performed without the use
of any training or pilot symbols. An expectation-maximization
channel-estimation algorithm for the fading amplitude, phase, and the
interference power spectral density (PSD) due to the combined interference and
thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate
codes. After initial estimates of the fading amplitude, phase, and interference
PSD are obtained from the received symbols, subsequent values of these
parameters are iteratively updated by using the soft feedback from the channel
decoder. The updated estimates are combined with the received symbols and
iteratively passed to the decoder. The elimination of pilot symbols simplifies
the system design and allows either an enhanced information throughput, an
improved bit error rate, or greater spectral efficiency. The interference-PSD
estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication
Multiuser MIMO-OFDM for Next-Generation Wireless Systems
This overview portrays the 40-year evolution of orthogonal frequency division multiplexing (OFDM) research. The amelioration of powerful multicarrier OFDM arrangements with multiple-input multiple-output (MIMO) systems has numerous benefits, which are detailed in this treatise. We continue by highlighting the limitations of conventional detection and channel estimation techniques designed for multiuser MIMO OFDM systems in the so-called rank-deficient scenarios, where the number of users supported or the number of transmit antennas employed exceeds the number of receiver antennas. This is often encountered in practice, unless we limit the number of users granted access in the base station’s or radio port’s coverage area. Following a historical perspective on the associated design problems and their state-of-the-art solutions, the second half of this treatise details a range of classic multiuser detectors (MUDs) designed for MIMO-OFDM systems and characterizes their achievable performance. A further section aims for identifying novel cutting-edge genetic algorithm (GA)-aided detector solutions, which have found numerous applications in wireless communications in recent years. In an effort to stimulate the cross pollination of ideas across the machine learning, optimization, signal processing, and wireless communications research communities, we will review the broadly applicable principles of various GA-assisted optimization techniques, which were recently proposed also for employment inmultiuser MIMO OFDM. In order to stimulate new research, we demonstrate that the family of GA-aided MUDs is capable of achieving a near-optimum performance at the cost of a significantly lower computational complexity than that imposed by their optimum maximum-likelihood (ML) MUD aided counterparts. The paper is concluded by outlining a range of future research options that may find their way into next-generation wireless systems
Near-Instantaneously Adaptive HSDPA-Style OFDM Versus MC-CDMA Transceivers for WIFI, WIMAX, and Next-Generation Cellular Systems
Burts-by-burst (BbB) adaptive high-speed downlink packet access (HSDPA) style multicarrier systems are reviewed, identifying their most critical design aspects. These systems exhibit numerous attractive features, rendering them eminently eligible for employment in next-generation wireless systems. It is argued that BbB-adaptive or symbol-by-symbol adaptive orthogonal frequency division multiplex (OFDM) modems counteract the near instantaneous channel quality variations and hence attain an increased throughput or robustness in comparison to their fixed-mode counterparts. Although they act quite differently, various diversity techniques, such as Rake receivers and space-time block coding (STBC) are also capable of mitigating the channel quality variations in their effort to reduce the bit error ratio (BER), provided that the individual antenna elements experience independent fading. By contrast, in the presence of correlated fading imposed by shadowing or time-variant multiuser interference, the benefits of space-time coding erode and it is unrealistic to expect that a fixed-mode space-time coded system remains capable of maintaining a near-constant BER
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
Approaching universal frequency reuse through base station cooperation
Base Station (BS) architectures are a promising cellular wireless solution to mitigate
the interference issues and to avoid the high frequency reuse factors implemented
in conventional systems. Combined with block transmission techniques, such as Orthogonal
Frequency-Division Multiplexing (OFDM) for the downlink and Single-Carrier with
Frequency-Domain Equalization (SC-FDE) for the uplink, these systems provide a significant
performance improvement to the overall system. Block transmission techniques are
suitable for broadband wireless communication systems, which have to deal with strongly
frequency-selective fading channels and are able to provide high bit rates despite the channel
adversities. In BS cooperation schemes users in adjacent cells share the same physical
channel and the signals received by each BS are sent to a Central Processing Unit (CPU)
that combines the different signals and performs the user detections and/or separation,
which can be regarded as a Multi-User Detection (MUD) technique. The work presented
in this thesis is focused on the study of uplink transmissions in BS cooperations systems,
considering single carrier block transmission schemes and iterative receivers based on the
Iterative-Block Decision Feedback Equalization (IB-DFE) concept, which combined with
the employment of Cyclic Prefix (CP)-assisted block transmission techniques are appropriate
to scenarios with strongly time-dispersive channels. Furthermore, the impact of the
sampling and quantization applied to the received signals from each Mobile Terminal (MT)
to the corresponding BS is studied, with the achievement of the spectral characterization
of the quantization noise. This thesis also provides a conventional analytical model for the
BER (Bit Error Rate) performance complemented with an approach to improve its results.
Finally, this thesis addresses the contextualization of BS cooperation schemes in clustered
C-RAN (Centralized-Radio Access Network)-type solutions.As arquitecturas BS cooperation são uma solução promissora de redes celulares sem
fios para atenuar o problema da interferência e evitar os factores de reuso elevados, que
se encontram implementados nos sistemas convencionais. Combinadas com técnicas de
transmissão por blocos, como o OFDM para o downlink e o SC-FDE no uplink, estes
sistemas fornecem uma melhoria significativa no desempenho geral do sistema. Técnicas
de transmissão por blocos são adequadas para sistemas de comunicações de banda larga
sem fios, que têm que lidar com canais que possuem um forte desvanescimento selectivo
na frequência e são capazes de fornecer ligações com taxas de transmissão altas apesar
das adversidades do canal. Em esquemas BS cooperation os terminais móveis situados em
células adjacentes partilham o mesmo canal fÃsico e os sinais recebidos em cada estação
de base são enviados para uma Unidade Central de Processamento (CPU) que combina
os diferentes sinais recebidos associados a um dado utilizador e realiza a detecção e/ou
separação do mesmo, sendo esta considerada uma técnica de Detecção Multi-Utilizador
(MUD). O trabalho apresentado nesta tese concentra o seu estudo no uplink de transmissões
em sistemas BS cooperation, considerando transmissões em bloco de esquemas monoportadoras
e receptores iterativos baseados no conceito B-DFE, em que quando combinados
com a implementação de técnicas de transmissao por blocos assistidas por prefixos cÃclicos
(CP) são apropriados a cenários com canais fortemente dispersivos no tempo. Além disso, é
estudado o impacto do processo de amostragem e quantização aplicados aos sinais recebidos
de cada terminal móvel para a estação de base, com a obtenção da caracterização espectral
do ruÃdo de quantização. Esta tese também fornece um modelo analÃtico convencional para
a computação do desempenho da taxa de erros de bit (BER), com um método melhorado
para o mesmo. Por último, esta tese visa a contextualização dos sistemas BS cooperation
em soluções do tipo C-RAN
A probabilistic method for blind multiuser detection using array observations
In this paper, a blind algorithm for detecting active users in a DS-CDMA system is presented. This probabilistic algorithm relies on the theory of hidden Markov models (HMM) and is completely blind in the sense that no knowledge of the signature sequences, channel state information or training sequences is required for any user. Additionally, observation through an array of sensors is also considered. Performance is verified via computer simulations, showing the near-far resistance of the analyzed procedure.Peer ReviewedPostprint (published version
Linearisation, error correction coding and equalisation for multi-level modulation schemes
University of Technology, Sydney. Faculty of Engineering.Orthogonal frequency division multiplexing (OFDM) has been standardised for digital audio broadcasting (DAB), digital video broadcasting (DVB) and wireless local area networks (WLAN). OFDM systems are capable of effectively coping with frequency- selective fading without using complex equalisation structures. The modulation and demodulation processes using fast fourier transform (FFT) and its inverse (IFFT) can be implemented very efficiently. More recently, multicarrier code division multiple access (MC-CDMA) based on the combination of OFDM and conventional CDMA has received growing attention in the field of wireless personal communication and digital multimedia broadcasting. It can cope with channel frequency selectivity due to its own capabilities of overcoming the asynchronous nature of multimedia data traffic and higher capacity over conventional multiple access techniques.
On the other hand, multicarrier modulation schemes are based on the transmission of a given set of signals on large numbers of orthogonal subcarriers. Due to the fact that the multicarrier modulated (MCM) signal is a superposition of many amplitude modulated sinusoids, its probability density function is nearly Gaussian. Therefore, the MCM signal is characterised by a very high peak-to-average power ratio (PAPR). As a result of the high PAPR, the MCM signal is severely distorted when a nonlinear high power amplifier (HPA) is employed to obtain sufficient transmitting power. This is very common in most communication systems, and decreases the performance significantly. The simplest way to avoid the nonlinear distortion is substantial output backoff (OBO) operating in the linear region of the HPA. However, because of the high OBO, the peak transmit power has to be decreased. For this reason, many linearisation techniques have been proposed to compensate for the nonlinearity without applying high OBO. The predistortion techniques have been known and studied as one of the most promising means to solve the problem. In this thesis, an improved memory mapping predistortion technique devised to reduce the large computational complexity of a fixed point iterative (FPI) predistorter is proposed, suitable especially for multicarrier modulation schemes. The proposed memory mapping predistortion technique is further extended to compensate for nonlinear distortion with memory caused by a shaping linear filter. The case of varying HPA characteristics is also considered by using an adaptive memory mapping predistorter which updates the lookup table (LUT) and counteracts these variations. Finally, an amplitude memory mapping predistorter is presented to reduce the LUT size.
Channel coding techniques have been widely used as an effective solution against channel fading in wireless environments. Amongst these, particular attention has been paid to turbo codes due to their performance being close to the Shannon limit. In-depth study and evaluation of turbo coding has been carried out for constant envelope signaling systems such as BPSK, QPSK and M-ary PSK. In this thesis, the performance of TTCM-OFDM systems with high-order modulation schemes, e.g. 16-QAM and 64-QAM, is investigated and compared with conventional channel coding schemes such as Reed-Solomon and convolutional coding. The analysis is performed in terms of spectral efficiency over a multipath fading channel and in presence of an HPA. Maximum a-priori probability (MAP), soft output Viterbi algorithm (SOVA) and pragmatic algorithms are compared for non-binary turbo decoding with these systems. For this setup, iterative multiuser detection in TTCM/MC-CDMA systems with M-QAM is introduced and investigated, adopting a set of random codes to decrease the PAPR. As another application of TTCM, the performance of multicode CDMA systems with TTCM for outer coding over multipath fading channels is investigated
Adaptive step-size blind multi-user detectors for DS-CDMA systems under deterministically and Markovian time-varying multipath environments
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