543 research outputs found
A combined channel-modified adaptive array MMSE canceller and viterbi equalizer
In this thesis, a very simple scheme is proposed which couples a maximum-likelihood sequence estimator (MLSE) with a X-element canceller. The method makes use of the MLSE\u27s channel estimator to modify the locally generated training sequence used to calculate the antenna array weights. This method will increase the array\u27s degree of freedom for interference cancellation by allowing the dispersive, desired signal to pass through the array undisturbed. Temporal equalization of the desired signal is then accomplished using maximum-likelihood sequence estimation. The T-spaced channel estimator coefficients and the array weights are obtained simultaneously using the minimum mean square error criteria. The result is a X-element receiver structure capable of canceling X- 1 in-band interferences without compromising temporal equalization
Near far resistant detection for CDMA personal communication systems.
The growth of Personal Communications, the keyword of the 90s, has already the signs of a technological revolution. The foundations of this revolution are currently set through the standardization of the Universal Mobile Telecommunication System (UMTS), a communication system with synergistic terrestrial and satellite segments. The main characteristic of the UMTS radio interface, is the provision of ISDN services. Services with higher than voice data rates require more spectrum, thus techniques that utilize spectrum as efficiently as possible are currently at the forefront of the research community interests. Two of the most spectrally efficient multiple access technologies, namely. Code Division Multiple Access (CDMA) and Time Division Multiple Access (TDMA) concentrate the efforts of the European telecommunity.This thesis addresses problems and. proposes solutions for CDMA systems that must comply with the UMTS requirements. Prompted by Viterbi's call for further extending the potential of CDMA through signal processing at the receiving end, we propose new Minimum Mean Square Error receiver architectures. MMSE detection schemes offer significant advantages compared to the conventional correlation based receivers as they are NEar FAr Resistant (NEFAR) over a wide range of interfering power levels. The NEFAR characteristic of these detectors reduces considerably the requirements of the power control loops currently found in commercial CDMA systems. MMSE detectors are also found, to have significant performance gains over other well established interference cancellation techniques like the decorrelating detector, especially in heavily loaded system conditions. The implementation architecture of MMSE receivers can be either Multiple-Input Multiple Output (MIMO) or Single-Input Single-Output. The later offers not only complexity that is comparable to the conventional detector, but also has the inherent advantage of employing adaptive algorithms which can be used to provide both the dispreading and the interference cancellation function, without the knowledge of the codes of interfering users. Furthermore, in multipath fading channels, adaptive MMSE detectors can exploit the multipath diversity acting as RAKE combiners. The later ability is distinctive to MMSE based receivers, and it is achieved in an autonomous fashion, without the knowledge of the multipath intensity profile. The communicator achieves its performance objectives by the synergy of the signal processor and the channel decoder. According to the propositions of this thesis, the form of the signal processor needs to be changed, in order to exploit the horizons of spread spectrum signaling. However, maximum likelihood channel decoding algorithms need not change. It is the way that these algorithms are utilized that needs to be revis ed. In this respect, we identify three major utilization scenarios and an attempt is made to quantify which of the three best matches the requirements of a UMTS oriented CDMA radio interface. Based on our findings, channel coding can be used as a mapping technique from the information bit to a more ''intelligent" chip, matching the ''intelligence" of the signal processor
Limiting Performance of Conventional and Widely Linear DFT-precoded-OFDM Receivers in Wideband Frequency Selective Channels
This paper describes the limiting behavior of linear and decision feedback
equalizers (DFEs) in single/multiple antenna systems employing
real/complex-valued modulation alphabets. The wideband frequency selective
channel is modeled using a Rayleigh fading channel model with infinite number
of time domain channel taps. Using this model, we show that the considered
equalizers offer a fixed post signal-to-noise-ratio (post-SNR) at the equalizer
output that is close to the matched filter bound (MFB). General expressions for
the post-SNR are obtained for zero-forcing (ZF) based conventional receivers as
well as for the case of receivers employing widely linear (WL) processing.
Simulation is used to study the bit error rate (BER) performance of both MMSE
and ZF based receivers. Results show that the considered receivers
advantageously exploit the rich frequency selective channel to mitigate both
fading and inter-symbol-interference (ISI) while offering a performance
comparable to the MFB
Physical layer network coding based communication systems in frequency selective channels
PhD ThesisThe demand for wireless communications is growing every day which requiresmore
speed and bandwidth. In two way relay networks (TWRN), physical
layer network coding (PLNC) was proposed to double the bandwidth. A
TWRN is a system where two end users exchange data through a middle node
called the relay. The two signals are allowed to be physically added before being
broadcasted back to the end users. This system can work smoothly in flat
fading channels, but can not be applied straightforward in frequency selective
channels. In a multipath multi-tap FIR channel, the inter-symbol interference
(ISI) spreads through several symbols. In this case, the symbols at the relay
are not just an addition of the sent symbols but also some of the previous
symbols from both sides. This not only causes a traditional PLNC to fail but
also a simple one equalizer system will not solve the problem. Three main
methods have been proposed by other researchers. The OFDM based PLNC
is the simplest in terms of implementation and complexity but suffers from
the disadvantages of the OFDMlike cyclic prefix overhead and frequency offset.
The main disadvantage, however is the relatively low BER performance
because it is restricted to linear equalizers in the PLNC system. Another
approach is pre-filtering or pre-equalization. This method also has some disadvantages
like complexity, sensitivity to channel variation and the need of
a feedback channel for both end nodes. Finally, the maximum likelihood
sequence detector was also proposed but is restricted to BPSK modulation
and exponentially rising complexity are major drawbacks. The philosophy in
this work is to avoid these disadvantages by using a time domain based system.
The DFE is the equalizer of choice here because it provides a non-trivial
BER performance improvement with very little increase in complexity. In
this thesis, the problem of frequency selective channels in PLNC systems can
be solved by properly adjusting the design of the system including the DFE.
The other option is to redesign the equalizer to meet that goal. An AF DFE
system is proposed in this work that provides very low complexity especially
at the relay with little sensitivity to channel changes. A multi-antenna DNF
DFE system is also proposed here with an improved performance. Finally, a
new equalizer is designed for very low complexity and cost DNF approach
with little sacrifice of BER performance. Matlab was used for the simulations
with Monte Carlo method to verify the findings of this work through finding
the BER performance of each system. This thesis opens the door for future
improvement on the PLNC system. More research needs to be done like testing
the proposed systems in real practical implementation and also the effect
of adding channel coding to these systems.Iraqi Government, Ministry of
Higher Educatio
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 físico 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 cíclicos
(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 ruído de quantização. Esta tese também fornece um modelo analítico 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
Wireless Channel Equalization in Digital Communication Systems
Our modern society has transformed to an information-demanding system, seeking voice, video, and data in quantities that could not be imagined even a decade ago. The mobility of communicators has added more challenges. One of the new challenges is to conceive highly reliable and fast communication system unaffected by the problems caused in the multipath fading wireless channels. Our quest is to remove one of the obstacles in the way of achieving ultimately fast and reliable wireless digital communication, namely Inter-Symbol Interference (ISI), the intensity of which makes the channel noise inconsequential.
The theoretical background for wireless channels modeling and adaptive signal processing are covered in first two chapters of dissertation.
The approach of this thesis is not based on one methodology but several algorithms and configurations that are proposed and examined to fight the ISI problem. There are two main categories of channel equalization techniques, supervised (training) and blind unsupervised (blind) modes. We have studied the application of a new and specially modified neural network requiring very short training period for the proper channel equalization in supervised mode. The promising performance in the graphs for this network is presented in chapter 4.
For blind modes two distinctive methodologies are presented and studied. Chapter 3 covers the concept of multiple cooperative algorithms for the cases of two and three cooperative algorithms. The select absolutely larger equalized signal and majority vote methods have been used in 2-and 3-algoirithm systems respectively. Many of the demonstrated results are encouraging for further research.
Chapter 5 involves the application of general concept of simulated annealing in blind mode equalization. A limited strategy of constant annealing noise is experimented for testing the simple algorithms used in multiple systems. Convergence to local stationary points of the cost function in parameter space is clearly demonstrated and that justifies the use of additional noise. The capability of the adding the random noise to release the algorithm from the local traps is established in several cases
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