44 research outputs found
Spread spectrum techniques for indoor wireless IR communications
Multipath dispersion and fluorescent light
interference are two major problems in indoor
wireless infrared communications systems. Multipath
dispersion introduces intersymhol interference
at data rates above 10 Mb/s, while
fluorescent light induces severe narrowband
interference to baseband modulation schemes
commonly used such as OOK and PPM. This
article reviews the research into the application
of direct sequence spread spectrum techniques
to ameliorate these key channel impairments
without having to resort to complex signal processing
techniques. The inherent properties of a
spreading sequence are exploited in order to
combat the ISI and narrowband interference. In
addition, to reduce the impact of these impairments,
the DSSS modulation schemes have
strived to be bandwidth-efficient and simple to
implement. Three main DSSS waveform techniques
have been developed and investigated.
These are sequence inverse keying, complementary
sequence inverse keying, and M-ary biorthogonal
keying (MBOK). The operations of
the three systems are explained; their performances
were evaluated through simulations and
experiments for a number of system parameters,
including spreading sequence type and length.
By comparison with OOK, our results show that
SIK, CSIK, and MBOK are effective against
multipath dispersion and fluorescent light interference
becausc the penalties incurred on the
DSSS schemes are between 0-7 dB, while the
penalty on OOK in the same environment is
more than 17 dB. The DSSS solution for IR
wireless transmission demonstrates that a transmission
waveform can he designed to remove
the key channel impairments in a wireless IR
system
Applications of perfect difference codes in fiber-optics and wireless optical code-division multiplexing/multiple-access systems
After establishing itself in the radio domain, Spread spectrum code-division
multiplexing/multiple-access (CDMA) has seen a recent upsurge in optical
domain as well. Due to its fairness, flexibility, service differentiation and
increased inherent security, CDMA is proved to be more suitable for the bursty
nature of local area networks than synchronous multiplexing techniques like
Frequency/Wavelength Division Multiplexing (F/WDM) and Time Division
Multiplexing (TDM). In optical domain, CDMA techniques are commonly known
as Optical-CDMA (O-CDMA). All optical CDMA systems are plagued with the
problem of multiple-access interference (MAI). Spectral amplitude coding (SAC)
is one of the techniques used in the literature to deal with the problem of MAI.
The choice of spreading code in any CDMA system is another way to ensure the
successful recovery of data at the receiving end by minimizing the effect of MAI
and it also dictates the hardware design of the encoder and decoder.
This thesis focuses on the efficient design of encoding and decoding hardware.
Perfect difference codes (PDC) are chosen as spreading sequences due to their
good correlation properties. In most of the literature, evaluation of error
probability is based on the assumptions of ideal conditions. Such assumptions
ignore major physical impairments such as power splitting losses at the
multiplexers of transmitters and receivers, and gain losses at the receivers, which
may in practice be an overestimate or underestimate of the actual probability of
error.
This thesis aims to investigate thoroughly with the consideration of practical
impairments the applications of PDCs and other spreading sequences in optical
communications systems based on spectral-amplitude coding and utilizing codedivision
as multiplexing/multiple-access technique. This work begins with a
xix
general review of optical CDMA systems. An open-ended practical approach has
been used to evaluate the actual error probabilities of OCDM/A systems under
study. It has been concluded from results that mismatches in the gains of
photodetectors, namely avalanche photodiode (APDs), used at the receiver side
and uniformity loss in the optical splitters results in the inaccurate calculation of
threshold level used to detect the data and can seriously degrade the system bit
error rate (BER) performance. This variation in the threshold level can be
compensated by employing techniques which maintain a constant interference
level so that the decoding architecture does not have to estimate MAI every time
to make a data bit decision or by the use of balanced sequences.
In this thesis, as a solution to the above problem, a novel encoding and decoding
architecture is presented for perfect difference codes based on common zero code
technique which maintains a constant interference level at all instants in CDM
system and thus relieves the need of estimating interference. The proposed
architecture only uses single multiplexer at the transmitters for all users in the
system and a simple correlation based receiver for each user. The proposed
configuration not only preserves the ability of MAI in Spectral-Amplitude Coding
SAC-OCDM system, but also results in a low cost system with reduced
complexity. The results show that by using PDCs in such system, the influence of
MAI caused by other users can be reduced, and the number of active users can be
increased significantly.
Also a family of novel spreading sequences are constructed called Manchestercoded
Modified Legendre codes (MCMLCs) suitable for SAC based OCDM
systems. MCMLCs are designed to be used for both single-rate and Multirate
systems. First the construction of MCMLCs is presented and then the bit error rate
performance is analyzed.
Finally the proposed encoding/decoding architecture utilizing perfect difference
codes is applied in wireless infrared environment and the performance is found to
be superior to other codes
On architecture and scalability of optical multi-protocol label switching networks using optical-orthogonal-code label.
Wen Yonggang.Thesis (M.Phil.)--Chinese University of Hong Kong, 2001.Includes bibliographical references.Abstracts in English and Chinese.Chapter 1 --- IntroductionChapter 1.1 --- Multi-Protocol Label Switching (MPLS) Technology --- p.1Chapter 1.2 --- Objective of this Thesis --- p.4Chapter 1.3 --- Reference --- p.5Chapter 2 --- Optical MPLS Network and Optical Label SchemesChapter 2.1 --- Optical MPLS Network --- p.7Chapter 2.2 --- Optical Label Schemes --- p.10Chapter 2.2.1 --- Time-division OMPLS scheme --- p.12Chapter 2.2.2 --- Wavelength-division OMPLS scheme --- p.16Chapter 2.2.3 --- Frequency-division OMPLS scheme --- p.22Chapter 2.2.3.1 --- UCSB Testbed --- p.23Chapter 2.2.3.2 --- UC-Davis Testbed --- p.26Chapter 2.2.3.3 --- NCTU-Telecordia Testbed --- p.28Chapter 2.2.4 --- Code-division OMPLS scheme --- p.30Chapter 2.2.4.1 --- Coherent Code-Division Label Scheme --- p.30Chapter 2.2.4.2 --- Noncoherent Code-Division Label Scheme --- p.32Chapter 2.3 --- Reference --- p.35Chapter 3 --- Architecture of OOC-based OMPLS networkChapter 3.1 --- Infrastructure of OOC-label switch router (code converter) --- p.37Chapter 3.1.1 --- Architecture of the Proposed Code Converter --- p.38Chapter 3.1.2 --- Enhancement of the Code Converter --- p.41Chapter 3.2 --- Implementation of the OOC code converter --- p.43Chapter 3.2.1 --- Encoders/Decoders --- p.43Chapter 3.2.1.1 --- All-parallel encoders/decoders --- p.43Chapter 3.2.1.2 --- All-serial encoders/decoders --- p.45Chapter 3.2.1.3 --- Serial-to-parallel encoder/decoders --- p.47Chapter 3.2.1.4 --- Comparison of the three kinds of encoders/decoders --- p.49Chapter 3.2.2 --- Time-Gate-Intensity-Threshold (TGIT) Device --- p.50Chapter 3.2.3 --- Optical Space Switch Array --- p.54Chapter 3.2.3.1 --- All-optical Space Switch --- p.54Chapter 3.2.3.2 --- Optical switching technologies --- p.56Chapter 3.2.3.2.1 --- Scalability --- p.56Chapter 3.2.3.2.2 --- Switching Speed --- p.57Chapter 3.2.3.2.3 --- Reliability --- p.57Chapter 3.2.3.2.4 --- Losses --- p.58Chapter 3.2.3.2.5 --- Port-to-Port repeatability --- p.58Chapter 3.2.3.2.6 --- Cost --- p.59Chapter 3.2.3.2.7 --- Power Consumption --- p.60Chapter 3.3 --- Reference --- p.61Chapter 4 --- Scalability of OOC-based MPLS networkChapter 4.1 --- Limitation on Label Switching Capacity --- p.63Chapter 4.1.1 --- Upper Bound --- p.65Chapter 4.1.2 --- Lower Bound --- p.66Chapter 4.2 --- Limitation on Switching Cascadability --- p.70Chapter 4.2.1. --- Limit Induced by the Inter-channel Crosstalk --- p.70Chapter 4.2.2 --- Limits Induced by the Residue Intensity of Sidelobes --- p.74Chapter 4.3 --- Appendix --- p.78Chapter 4.3.1 --- Derivation of Chip Intensity --- p.78Chapter 4.3.2 --- The 5% residue power criterion --- p.81Chapter 4.4 --- Reference --- p.83Chapter 5 --- ConclusionChapter 5.1 --- Summary of the Thesis --- p.85Chapter 5.2 --- Future work --- p.8
Dark signalling and code division multiple access in an optical fibre LAN with a bus topology
This thesis describes an optical fibre network that uses a bus topology and Code Division Multiple Access (CDMA). Various potential configurations are analysed and compared and it is shown that a serious limitation of optical CDMA schemes using incoherent correlators is the effect of optical beating due to the presence of multiple incoherent optical signals at the receiver photodiode. The network proposed and analysed in this thesis avoids beating between multiple optical fields because it only uses a single, shared, optical source. It does this through the SLIM (Single Light-source with In-line Modulation) configuration in which there is a continuously-operating light source at the head-end of a folded bus, and modulators at the nodes to impose signals on the optical field in the form of pulses of darkness which propagate along the otherwise continuously bright bus. Optical CDMA can use optical-fibre delay-line correlators as matched filters, and these may be operated either coherently or incoherently.Coherent operation is significantly more complex than incoherent operation, but incoherent correlators introduce further beating even in a SLIM network. A new design of optical delay-line correlator, the hybrid correlator, is therefore proposed, analysed and demonstrated. It is shown to eliminate beating. A model of a complete network predicts that a SLIMbus using optical CDMA with hybrid correlators can be operated at TeraBaud rates with the number of simultaneous users limited by multiple access interference (MAI), determined only by the combinatorics of the code set
Optimization of multidimensional equalizers based on MMSE criteria for multiuser detection
PhD ThesisThis thesis is about designing a multidimensional equalizer for uplink
interleaved division multiple access (IDMA) transmission. Multidimensional
equalizer can be classified into centralized and decentralized
multidimensional equalizer. Centralized multidimensional equalizer
(MDE) have been used to remove both inter-symbol interference
(ISI) and multiaccess interference (MAI) effects from the received signal.
In order to suppress MAI effects, code division multiple access
(CDMA) has been used with MDE to minimize the correlation between
users' signals. The MDE structure can be designed using linear
equalizer (MLE) or decision feedback equalizer (MDFE). Previous
studies on MDE employed adaptive algorithms to estimate filter co-effi cients during the training mode, i.e. the symbol equalization was
not optimal, for two users. In our work, we applied MDE on IDMA
receiver for multipath selective fading channels and also derived new
equations to obtain the optimal filter taps for both types of MDE
equalizers, i.e. MDFE and MLE, based on the minimum mean square
error (MMSE) criterion. The optimal filter taps are calculated for
more than two users. Moreover, we investigated the performance of
the optimal MDFE using both IDMA (MDFE-IDMA) and CDMA
(MDFE-CDMA) detectors.
Generally, the MDE equalizer suffers from residual MAI interference
effects at low signal-to-noise-ratios (SNR) due to the delay inherent
in the convergence of the crossover filter taps. Therefore, a new decentralized
multidimensional equalizer has been proposed to IDMA
detector. Within design of decentralized equalizer, the convergence
problem has been resolved by replacing the crossover filters with parallel
interference canceler (PIC) for removing MAI dispersion. The
proposed decentralized multidimensional equalizer shows a higher efficiency in removing MAI interference when compared with existing
receivers in the literature. However, this is achieved at the expense
of higher computational complexity compared to centralized multidimensional
equalization
Direct sequence spread spectrum techniques in local area networks
This thesis describes the application of a direct sequence spread spectrum modulation scheme to the physical layer of a local area networks subsequently named the SS-LAN. Most present day LANs employ erne form or another of time division multiplexing which performs well in many systems but which is limited by its very nature in real time, time critical and time demanding applications. The use of spread spectrum multiplexing removes these limitations by providing a simultaneous multiple user access capability to the channel which permits each and all nodes to utilise the channel independent of the activity being currently supported by that channel. The theory of spectral spreading is a consequence of the Shannon channel capacity in which the channel capacity may be maintained by the trading of signal to noise ratio for bandwidth. The increased bandwidth provides an increased signal dimensionality which can be utilised in providing noise immunity and/or a simultaneous multiple user environment: the effects of the simultaneous users can be considered as noise from the point of view of any particular constituent signal. The use of code sequences at the physical layer of a LAN permits a wide range of mapping alternatives which can be selected according to the particular application. Each of the mapping techniques possess the general spread spectrum properties but certain properties can be emphasised at the expense of others. The work has Involved the description of the properties of the SS-LAN coupled with the development of the mapping techniques for use In the distribution of the code sequences. This has been followed by an appraisal of a set of code sequences which has resulted in the definition of the ideal code properties and the selection of code families for particular types of applications. The top level design specification for the hardware required in the construction of the SS-LAN has also been presented and this has provided the basis for a simplified and idealised theoretical analysis of the performance parameters of the SS-LAN. A positive set of conclusions for the range of these parameters has been obtained and these have been further analysed by the use of a SS-LAN computer simulation program. This program can simulate any configuration of the SS-LAN and the results it has produced have been compared with those of the analysis and have been found to be in agreement. A tool for the further analysis of complex SS-LAN configurations has therefore been developed and this will form the basis for further work
Sistemas de comunicação por luz visível na segurança rodoviária
Doutoramento em MAP-TeleEsta tese apresenta um estudo exploratório sobre sistemas de comunicação por luz visível e as suas aplicações em sistemas de transporte inteligentes como forma a melhorar a segurança nas estradas. Foram desenvolvidos neste trabalho, modelos conceptuais e analíticos adequados à caracterização deste tipo de sistemas. Foi desenvolvido um protótipo de baixo custo, capaz de suportar a disseminação de informação utilizando semáforos. A sua realização carece de um estudo detalhado, nomeadamente: i) foi necessário obter modelos capazes de descrever os padrões de radiação numa área de serviço pré-definida; ii) foi necessário caracterizar o meio de comunicações; iii) foi necessário estudar o comportamento de vários esquemas de modulação de forma a optar pelo mais robusto; finalmente, iv) obter a implementação do sistema baseado em FPGA e componentes discretos.
O protótipo implementado foi testado em condições reais. Os resultados alcançados mostram os méritos desta solução, chegando mesmo a encorajar a utilização desta tecnologia em outros cenários de aplicação.This thesis presents a study carried out on the exploration of visible light communication (VLC) for road safety applications in intelligent transportation systems (ITS). We developed conceptual and analytical models for the usage of VLC technologies for human safety. A low cost VLC prototype traffic broadcast system was hardware designed and implemented. In order to realize this prototype a number of exhaustive steps have been designed and implemented.
An optimized illumination distribution was achieved in a defined service area from LED-based traffic lights associated with a VLC emitter. A traffic light system set-up was modeled and designed for optimum performance. The optical wireless channel was characterized and examined. Depending on the characteristics of the channel and specific applications, a robust modulation technique based on direct sequence spread spectrum using sequence inverse keying (DSSS SIK) was analyzed, developed, and implemented. The complete prototype VLC transceiver system was then implemented with field programmable gate arrays (FPGA) and discrete components.
Simulation and experimental validation of system was performed in different scenarios and environments. The obtained results have shown the merits of our approach. A number of findings was experienced which are illustrated at the end. These observations would enhance and encourage potential research in the area and optimize performance of VLC systems for a number of interesting applications in future. A summary of future research challenges is presented at the end
Hierarchy Based Construction of Signature Matrices for Simplified Decoding in Overloaded CDMA
The overloaded CDMA system, as the solution to the capacity limit of its conventional counterpart, has drawn frequent interest of the researchers in the past. While there exists numerous proposals on the construction of uniquely decodable (UD) signature matrices for overloaded CDMA system with very high value of overloading factor, most of them lag the efficient multiuser detector (MUD) for noisy transmission. Here, by efficient, we imply the MUD to have acceptable BER performance and simplified in design. Whereas the lack of efficiency of several MUDs is primarily due to the impact of excess level of multiple access interference (MAI) because of the rise in the number of active users, its random nature prohibits its accurate estimation and elimination. Under such constraints, if the signature matrices can be intelligently constructed so as to generate a defined and controlled pattern (hierarchy) of MAI so that the designed MUD will exploit the knowledge of this hierarchy to remove the MAI completely and attain better error performance at much lower cost of complexity. We consider this as the motivation for research in this thesis. First, we propose the ternary signature matrix with orthogonal subsets (TSMOS), where the matrix with index-k comprises of k orthogonal subsets with each having different number signatures, and all subsets besides the first (largest) one are of ternary type. The correlation (interference) pattern among the signatures is mapped into a twin tree hierarchy, which is further leveraged to design a simplified MUD using the linear decoding blocks like matched filter (MF) to provide errorfree and better error performance for noiseless and noisy transmission respectively. Next, we generalize the construction of TSMOS to multiple structures i.e.; Type I, Type II, Type III and mixed versions and reveal the complementary feature of 50% signatures of the largest (binary) subset that further results in their optimality. Further, we propose the non-ternary version of SMOS (called as 2k-SMOS), where the binary alphabets in each of the k subsets are different from each other. With vii no complementary feature, 50% signatures of its largest subset are also found to be optimal. The superiority of 2k-SMOS over TSMOS is also verified for an overloading capacity of 150%. Next, we propose and discuss the hybrid SMOS (HSMOS), where the subsets from TSMOS and 2k-SMOS are used as the constituents to produce multiple SMOS structures, of which TSMOS and 2k-SMOS are treated as the special cases. For better understanding of the features of the whole family of SMOS (with an overloading capacity of 200%), the gradual change in the twin tree hierarchy and BER performance of the left and right child of the individual subsets are studied. Similar to SMOS, we also introduce the hierarchy based low density signature (HLDS) matrix, where any UD matrix satisfying particular criterion can be considered as the basis set. For hadamard matrix as the basis set, we design a MUD that uses the MF to implement the decision vector search (DVS) algorithm, which is meant to exploit the advantageous hierarchy of constellation of the transmitted vector to offer errorfree decoding. For noisy channel, the marginal degradation in the level of BER of the MUD (DVS) as compared to the optimum joint maximum likelihood decoder (MLD) is worthy to be overlooked when compared with the significant gain achieved in terms of complexity. For the smallest dimension of the hadamard matrix as the basis, the MUD is further simplified to offer recovery using a comparison driven decision making algorithm, also known as comparison aided decoding (CAD). Despite simplicity, the error performance of the MUD (CAD) is observed to be very close to that of MUD (DVS)
From Theory to Practice: Sub-Nyquist Sampling of Sparse Wideband Analog Signals
Conventional sub-Nyquist sampling methods for analog signals exploit prior
information about the spectral support. In this paper, we consider the
challenging problem of blind sub-Nyquist sampling of multiband signals, whose
unknown frequency support occupies only a small portion of a wide spectrum. Our
primary design goals are efficient hardware implementation and low
computational load on the supporting digital processing. We propose a system,
named the modulated wideband converter, which first multiplies the analog
signal by a bank of periodic waveforms. The product is then lowpass filtered
and sampled uniformly at a low rate, which is orders of magnitude smaller than
Nyquist. Perfect recovery from the proposed samples is achieved under certain
necessary and sufficient conditions. We also develop a digital architecture,
which allows either reconstruction of the analog input, or processing of any
band of interest at a low rate, that is, without interpolating to the high
Nyquist rate. Numerical simulations demonstrate many engineering aspects:
robustness to noise and mismodeling, potential hardware simplifications,
realtime performance for signals with time-varying support and stability to
quantization effects. We compare our system with two previous approaches:
periodic nonuniform sampling, which is bandwidth limited by existing hardware
devices, and the random demodulator, which is restricted to discrete multitone
signals and has a high computational load. In the broader context of Nyquist
sampling, our scheme has the potential to break through the bandwidth barrier
of state-of-the-art analog conversion technologies such as interleaved
converters.Comment: 17 pages, 12 figures, to appear in IEEE Journal of Selected Topics in
Signal Processing, the special issue on Compressed Sensin
Proceedings of the Second International Mobile Satellite Conference (IMSC 1990)
Presented here are the proceedings of the Second International Mobile Satellite Conference (IMSC), held June 17-20, 1990 in Ottawa, Canada. Topics covered include future mobile satellite communications concepts, aeronautical applications, modulation and coding, propagation and experimental systems, mobile terminal equipment, network architecture and control, regulatory and policy considerations, vehicle antennas, and speech compression