797 research outputs found
Near-capacity MIMOs using iterative detection
In this thesis, Multiple-Input Multiple-Output (MIMO) techniques designed for transmission over narrowband Rayleigh fading channels are investigated. Specifically, in order to providea diversity gain while eliminating the complexity of MIMO channel estimation, a Differential Space-Time Spreading (DSTS) scheme is designed that employs non-coherent detection. Additionally, in order to maximise the coding advantage of DSTS, it is combined with Sphere Packing (SP) modulation. The related capacity analysis shows that the DSTS-SP scheme exhibits a higher capacity than its counterpart dispensing with SP. Furthermore, in order to attain additional performance gains, the DSTS system invokes iterative detection, where the outer code is constituted by a Recursive Systematic Convolutional (RSC) code, while the inner code is a SP demapper in one of the prototype systems investigated, while the other scheme employs a Unity Rate Code (URC) as its inner code in order to eliminate the error floor exhibited by the system dispensing with URC. EXIT charts are used to analyse the convergence behaviour of the iteratively detected schemes and a novel technique is proposed for computing the maximum achievable rate of the system based on EXIT charts. Explicitly, the four-antenna-aided DSTSSP system employing no URC precoding attains a coding gain of 12 dB at a BER of 10-5 and performs within 1.82 dB from the maximum achievable rate limit. By contrast, the URC aidedprecoded system operates within 0.92 dB from the same limit.On the other hand, in order to maximise the DSTS systemâs throughput, an adaptive DSTSSP scheme is proposed that exploits the advantages of differential encoding, iterative decoding as well as SP modulation. The achievable integrity and bit rate enhancements of the system are determined by the following factors: the specific MIMO configuration used for transmitting data from the four antennas, the spreading factor used and the RSC encoderâs code rate.Additionally, multi-functional MIMO techniques are designed to provide diversity gains, multiplexing gains and beamforming gains by combining the benefits of space-time codes, VBLASTand beamforming. First, a system employing Nt=4 transmit Antenna Arrays (AA) with LAA number of elements per AA and Nr=4 receive antennas is proposed, which is referred to as a Layered Steered Space-Time Code (LSSTC). Three iteratively detected near-capacity LSSTC-SP receiver structures are proposed, which differ in the number of inner iterations employed between the inner decoder and the SP demapper as well as in the choice of the outer code, which is either an RSC code or an Irregular Convolutional Code (IrCC). The three systems are capable of operating within 0.9, 0.4 and 0.6 dB from the maximum achievable rate limit of the system. A comparison between the three iteratively-detected schemes reveals that a carefully designed two-stage iterative detection scheme is capable of operating sufficiently close to capacity at a lower complexity, when compared to a three-stage system employing a RSC or a two-stage system using an IrCC as an outer code. On the other hand, in order to allow the LSSTC scheme to employ less receive antennas than transmit antennas, while still accommodating multiple users, a Layered Steered Space-Time Spreading (LSSTS) scheme is proposed that combines the benefits of space-time spreading, V-BLAST, beamforming and generalised MC DS-CDMA. Furthermore, iteratively detected LSSTS schemes are presented and an LLR post-processing technique is proposed in order to improve the attainable performance of the iteratively detected LSSTS system.Finally, a distributed turbo coding scheme is proposed that combines the benefits of turbo coding and cooperative communication, where iterative detection is employed by exchanging extrinsic information between the decoders of different single-antenna-aided users. Specifically, the effect of the errors induced in the first phase of cooperation, where the two users exchange their data, on the performance of the uplink in studied, while considering different fading channel characteristics
Optical code-division multiple access system and optical signal processing
This thesis presents our recent researches on the development of coding devices, the
investigation of security and the design of systems in the optical cod-division multiple
access (OCDMA) systems. Besides, the techniques of nonlinear signal processing used
in the OCDMA systems fire our imagination, thus some researches on all-optical signal
processing are carried out and also summarized in this thesis.
Two fiber Bragg grating (FBG) based coding devices are proposed. The first coding
device is a superstructured FBG (SSFBG) using ±Ï/2-phase shifts instead of
conventional 0/Ï-phase shifts. The ±Ï/2-phase-shifted SSFBG en/decoders can not only
conceal optical codes well in the encoded signals but also realize the reutilization of
available codes by hybrid use with conventional 0/Ï-phase-shifted SSFBG en/decoders.
The second FBG based coding device is synthesized by layer-peeling method, which
can be used for simultaneous optical code recognition and chromatic dispersion
compensation.
Then, two eavesdropping schemes, one-bit delay interference detection and
differential detection, are demonstrated to reveal the security vulnerability of differential
phase-shift keying (DPSK) and code-shift keying (CSK) OCDMA systems.
To address the security issue as well as increase the transmission capacity, an
orthogonal modulation format based on DPSK and CSK is introduced into the OCDMA
systems. A 2 bit/symbol 10 Gsymbol/s transmission system using the orthogonal
modulation format is achieved. The security of the system can be partially guaranteed.
Furthermore, a fully-asynchronous gigabit-symmetric OCDMA passive optical
network (PON) is proposed, in which a self-clocked time gate is employed for signal
regeneration. A remodulation scheme is used in the PON, which let downstream and
upstream share the same optical carrier, allowing optical network units source-free. An
error-free 4-user 10 Gbit/s/user duplex transmission over 50 km distance is reazlied.
A versatile waveform generation scheme is then studied. A theoretical model is
established and a waveform prediction algorithm is summarized. In the demonstration,
various waveforms are generated including short pulse, trapezoidal, triangular and
sawtooth waveforms and doublet pulse.
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In addition, an all-optical simultaneous half-addition and half-subtraction scheme is
achieved at an operating rate of 10 GHz by using only two semiconductor optical
amplifiers (SOA) without any assist light.
Lastly, two modulation format conversion schemes are demonstrated. The first
conversion is from NRZ-OOK to PSK-Manchester coding format using a SOA based
Mach-Zehnder interferometer. The second conversion is from RZ-DQPSK to RZ-OOK
by employing a supercontinuum based optical thresholder
Parameters of Communication Systems Based on OFDM-CDMA
CĂl disertaÄnĂ prĂĄce leĆŸĂ v oblasti modelovĂĄnĂ a vyhodnocenĂ bezdrĂĄtovĂœch komunikaÄnĂch systĂ©mĆŻ s dvojrozmÄrnĂœm rozprostĂrĂĄnĂm signĂĄlu a jejich klĂÄovĂœch parametrĆŻ v zĂĄvislosti na vybranĂœch vlastnostech modernĂho bezdrĂĄtovĂ©ho komunikaÄnĂho ĆetÄzce. VĂœzkumnĂ© metody pouĆŸitĂ© v tĂ©to prĂĄci spoÄĂvajĂ pĆedevĆĄĂm ve vĂœvoji softwarovĂ©ho simulĂĄtoru pro prostĆedĂ Matlab, s jehoĆŸ pomocĂ, a s vyuĆŸitĂm statistickĂ©ho pĆĂstupu, jsou navrĆŸenĂ© algoritmy ovÄĆeny. DĂĄle je pouĆŸit simulĂĄtor fyzickĂ© vrstvy dle 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), vyvinutĂœ na TechnickĂ© univerzitÄ ve VĂdni. Tento pĆedstavuje ideĂĄlnĂ platformu pro implementaci metody dvojrozmÄrnĂ©ho (2D) rozprostĂrĂĄnĂ a jejĂ vyhodnocenĂ s pĆihlĂ©dnutĂm k souÄasnĂœm bezdrĂĄtovĂœm komunikaÄnĂm systĂ©mĆŻm. ZjiĆĄtÄnĂ prezentovanĂĄ v tĂ©to prĂĄci pĆedstavujĂ pĆedevĆĄĂm ovÄĆenĂ ĂșÄinnosti systĂ©mu nazvanĂ©ho jako Variable Spreading Factor - Orthogonal Code Frequency Division Multiplex (VSF-OFCDM), kterĂœ vyuĆŸĂvĂĄ principu 2D rozprostĂrĂĄnĂ signĂĄlu a zjiĆĄtÄnĂ, ĆŸe VSF-OFCDM systĂ©m pĆekonĂĄvĂĄ systĂ©my vyuĆŸĂvajĂcĂ Orthogonal Division Frequency Multiplex (OFDM), nebo Code Division Multiple Access (CDMA). DĂĄle byla navrĆŸena metoda 2D rozprostĂrĂĄnĂ signĂĄlu v systĂ©mu LTE, kde se tĂ©ĆŸ potvrdila jejĂ ĂșÄinnost. DĂky ĂșÄinnÄjĆĄĂmu potlaÄenĂ vlivu rychlĂ© variace pĆenosovĂ©ho kanĂĄlu v zĂĄvislosti na frekvenci a Äase, dosahuje systĂ©m VSF-OFCDM znatelnÄ vyĆĄĆĄĂ datovĂ© prostupnosti.The focus of this research is in the area of modeling and evaluating of the wireless systems with two dimensional signal spreading, itâs key parameters and dependencies on other features in modern wireless communication chain. The research method adopted in this dissertation includes a development of Matlab based simulators which exploits a statistical approach to show a contribution of proposed algorithms. Furthermore, a model of physical layer of the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), developed by the Vienna University of Technology, was utilized as a simulation environment suitable for implementation of a two dimensional (2D) signal spreading method and its evaluation as well as comparison of achieved results with the state-of-the-art systems. The findings from this research provide evidence that the Variable Spreading Factor - Orthogonal Code Frequency Division Multiplex (hereafter VSF-OFCDM) employing a 2D spreading is a promising wireless access scheme superior to Orthogonal Division Frequency Multiplex (OFDM) or Code Division Multiple Access (CDMA) and is capable to significantly increase the data rates in wireless transmission due to the capability of such system to effectively cope with fast time and frequency fluctuations in the wireless transmission channel.
Investigation of the impact of fibre impairments and SOA-based devices on 2D-WH/TS OCDMA codes
In seeking efficient last-mile solutions for high-capacity, optical code division multiple access (OCDMA) emerges as a promising alternative high-speed optical network that can securely support a multitude of simultaneous users without requiring extensive equipment. This multiplexing technique has recently been the subject of comprehensive research, highlighting its potential for facilitating high-bandwidth multi-access networking. When contrasted with techniques such as wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM), OCDMA offers a more effective and equitable split of available fibre bandwidth among the users. This thesis presents my research focused on the incoherent OCDMA under the influence of optical fibre impairments that uses picosecond multiwavelength pulses to form two-dimensional wavelength hopping time-spreading (2D-WH/TS) incoherent OCDMA codes.
In particular, self-phase modulation, temperature induced fibre dispersion, chromatic dispersion, as well as the impact of semiconductor optical amplifier SOA devices deployment on 2D-WH/TS OCDMA code integrity were investigated. These aspects were investigated using a 17-km long bidirectional fibre link between Strathclyde and Glasgow University. In particular, I investigated the impact of temporal skewing among OCDMA code carriers and the importance of selecting small range of wavelengths as code carriers where wide range manifest high dependency on wavelength. This wavelength dependency is exploited furthermore to measure the induced temperature dispersion coefficient accurately and economically. I have conducted experiments to characterise the impact of SOA-device on 2D OCDMA code carries which is evaluated under different bias conditions. This evaluation addressed the potential challenges and ramifications of the gain recovery time of SOA and its wavelength dependency with respect to gain ratio and self-phase modulation (SPM). The OCDMA code was built using multiplexers and delay lines to create a 2D OCDMA code to allow studying the impact of deploying a SOA under different conditions on each wavelength.
The concept described above is then extended to the investigation of the SOAâs impact on a 2D-WH/TS OCDMA prime code under high bias current/gain conditions. The overall performance of two different 2D-WH/TS OCDMA systems deploying the SOA was also calculated. I have also investigated the possibility of manipulating chirp in 2D-WH/TS incoherent OCDMA to counteract the self-phase modulation-induced red shift by using single mode fibre and lithium crystals. I have investigated the performance of the picosecond code based optical signal when subjected to temperature variations similar to that experience by most buried fibre systems. I have proposed and demonstrated a novel technique, which I examined analytically and experimentally, that utilises a SOA at the transmitter to create a new code with a new wavelength hopping and spreading time sequences to achieve a unique physical improved secure incoherent OCDMA communication method. A novel fully automated tuneable compensation testbed is also proposed of an autonomous dispersion management in a WH/TS incoherent OCDMA system. The system proposed manipulates the chirp of OCDMA code carriers to limit chromatic dispersion detrimental effect on transmission systems.In seeking efficient last-mile solutions for high-capacity, optical code division multiple access (OCDMA) emerges as a promising alternative high-speed optical network that can securely support a multitude of simultaneous users without requiring extensive equipment. This multiplexing technique has recently been the subject of comprehensive research, highlighting its potential for facilitating high-bandwidth multi-access networking. When contrasted with techniques such as wavelength division multiplexing (WDM) and optical time division multiplexing (OTDM), OCDMA offers a more effective and equitable split of available fibre bandwidth among the users. This thesis presents my research focused on the incoherent OCDMA under the influence of optical fibre impairments that uses picosecond multiwavelength pulses to form two-dimensional wavelength hopping time-spreading (2D-WH/TS) incoherent OCDMA codes.
In particular, self-phase modulation, temperature induced fibre dispersion, chromatic dispersion, as well as the impact of semiconductor optical amplifier SOA devices deployment on 2D-WH/TS OCDMA code integrity were investigated. These aspects were investigated using a 17-km long bidirectional fibre link between Strathclyde and Glasgow University. In particular, I investigated the impact of temporal skewing among OCDMA code carriers and the importance of selecting small range of wavelengths as code carriers where wide range manifest high dependency on wavelength. This wavelength dependency is exploited furthermore to measure the induced temperature dispersion coefficient accurately and economically. I have conducted experiments to characterise the impact of SOA-device on 2D OCDMA code carries which is evaluated under different bias conditions. This evaluation addressed the potential challenges and ramifications of the gain recovery time of SOA and its wavelength dependency with respect to gain ratio and self-phase modulation (SPM). The OCDMA code was built using multiplexers and delay lines to create a 2D OCDMA code to allow studying the impact of deploying a SOA under different conditions on each wavelength.
The concept described above is then extended to the investigation of the SOAâs impact on a 2D-WH/TS OCDMA prime code under high bias current/gain conditions. The overall performance of two different 2D-WH/TS OCDMA systems deploying the SOA was also calculated. I have also investigated the possibility of manipulating chirp in 2D-WH/TS incoherent OCDMA to counteract the self-phase modulation-induced red shift by using single mode fibre and lithium crystals. I have investigated the performance of the picosecond code based optical signal when subjected to temperature variations similar to that experience by most buried fibre systems. I have proposed and demonstrated a novel technique, which I examined analytically and experimentally, that utilises a SOA at the transmitter to create a new code with a new wavelength hopping and spreading time sequences to achieve a unique physical improved secure incoherent OCDMA communication method. A novel fully automated tuneable compensation testbed is also proposed of an autonomous dispersion management in a WH/TS incoherent OCDMA system. The system proposed manipulates the chirp of OCDMA code carriers to limit chromatic dispersion detrimental effect on transmission systems
High Capacity CDMA and Collaborative Techniques
The thesis investigates new approaches to increase the user capacity and improve the error
performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation
and collaborative spreading and space diversity techniques. Collaborative Coding Multiple
Access (CCMA) is also investigated as a separate technique and combined with CDMA. The
advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the
uplink and downlink are proposed and evaluated.
Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The
practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive
approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM)
property of digital signals to blindly suppress interference during the despreading process and obtain
amplitude estimation with minimum mean squared error for use in cancellation stages. Two
new blind adaptive receiver designs employing successive and parallel interference cancellation
architectures using the CM algorithm (CMA) referred to as âCMA-SICâ and âBA-PICâ, respectively,
are presented. These techniques have shown to offer near single user performance for large
number of users. It is shown to increase the user capacity by approximately two fold compared
with conventional IC receivers. The spectral efficiency analysis of the techniques based on output
signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore,
an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a
simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI
without any knowledge of channel amplitudes and allows large number of users compared with
equal gain and maximum ratio combining techniques normally used in practice.
New user collaborative schemes are proposed and analysed theoretically and by simulations
in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First,
a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA
are designed and analysed. Next, a new user collaborative scheme with successive interference
cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce
MAI and achieve improved diversity. To further improve the performance of C-SIC under high
system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed.
It is shown to minimize the residual MAI, leading to improved user capacity and a more robust
system. It is known that collaborative diversity schemes incur loss in throughput due to the need of
orthogonal time/frequency slots for relaying sourceâs data. To address this problem, finally a novel
near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time
encoding methods. At the receiver collaborative joint detection is performed to separate each
paired usersâ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth
as inter-user channel SNR becomes high.
A novel approach of âUser Collaborationâ is introduced to increase the user capacity of CDMA
for both the downlink and uplink. First, collaborative group spreading technique for the downlink
of overloaded CDMA system is introduced. It allows the sharing of the same single spreading
sequence for more than one user belonging to the same group. This technique is referred to as
Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative
coding is used for each group to form a composite codeword signal of the users and then a
single orthogonal sequence is used for the group. At each userâs receiver, decoding of composite
codeword is carried out to extract the userâs own information while maintaining a high SINR performance.
To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions,
Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative
coding multiple access and space-time coding principles. A new scheme for uplink of CDMA
using the âUser Collaborationâ approach, referred to as CS-CDMA-UL is presented next. When
usersâ channels are independent (uncorrelated), significantly higher user capacity can be achieved
by grouping multiple users to share the same spreading sequence and performing MUD on per
group basis followed by a low complexity ML decoding at the receiver. This approach has shown
to support much higher number of users than the available sequences while also maintaining the
low receiver complexity. For improved performance under highly correlated channel conditions,
T-user collaborative coding is also investigated within the CS-CDMA-UL system
Recommended from our members
Advanced Techniques for High-Throughput Cellular Communications
The next generation wireless communication systems require ubiquitous high-throughput mobile connectivity under a range of challenging network settings (urban versus rural, high device density, mobility, etc). To improve the performance of the system, the physical layer design is of great importance. The previous research on improving the physical layer properties includes: a) highly directional transmissions that can enhance the throughput and spatial reuse; b) enhanced MIMO that can eliminate
contention, enabling linear increase of capacity with number of antennas; c) mmWave technologies which operate on GHz bandwidth to over substantially higher throughput; d) better cooperative spectrum sharing with cognitive radios; e) better multiple access method which can mitigate multiuser interference and allow more multi-users.
This dissertation addresses several techniques in the physical layer design of the next generation wireless communication systems. In chapter two, an orthogonal frequency division with code division multiple access (OFDM-CDMA) systems is proposed and a polyphase code is used to improve multiple access performance and make the OFDM signal satisfy the peak to average ratio (PAPR) constraint. Chapter three studies the I/Q imbalance for direct down converter. For wideband transmitter and receiver that use direct conversion for I/Q sampling, the I/Q imbalance becomes a critical issue. With higher I/Q imbalance, there will be higher degradation in quadrature amplitude modulation, which degrades the throughput tremendously. Chapter four investigate a problem of spectrum sharing for cognitive wideband communication. An energy-efficient sub-Nyquist sampling algorithm is developed for optimal sampling and spectrum sensing. In chapter ve, we study the channel estimation of millimeter wave full-dimensional MIMO communication. The problem is formulated as an atomic-norm minimization problem and algorithms are derived for the channel estimation in different situations.
In this thesis, mathematical optimization is applied as the main approach to analyze and solve the problems in the physical layer of wireless communication so that the high-throughput is achieved. The algorithms are derived along with the theoretical analysis, which are validated with numerical results
Algorithms for 5G physical layer
There is a great activity in the research community towards the investigations of the various aspects of 5G at different protocol layers and parts of the network. Among all, physical
layer design plays a very important role to satisfy high demands in terms of data rates, latency, reliability and number of connected devices for 5G deployment. This thesis addresses
he latest developments in the physical layer algorithms regarding the channel coding, signal detection, frame synchronization and multiple access technique in the light of 5G use
cases. These developments are governed by the requirements of the different use case scenarios that are envisioned to be the driving force in 5G.
All chapters from chapter 2 to 5 are developed around the need of physical layer algorithms dedicated to 5G use cases. In brief, this thesis focuses on design, analysis, simulation and
he advancement of physical layer aspects such as 1. Reliability based decoding of short length Linear Block Codes (LBCs) with very good properties in terms of minimum hamming
istance for very small latency requiring applications. In this context, we enlarge the grid of possible candidates by considering, in particular, short length LBCs (especially extended CH codes) with soft-decision decoding; 2. Efficient synchronization of preamble/postamble in a short bursty frame using modified Massey correlator; 3. Detection of Primary User
activity using semiblind spectrum sensing algorithms and analysis of such algorithms under practical imperfections; 4. Design of optimal spreading matrix for a Low Density Spreading (LDS) technique in the context of non-orthogonal multiple access. In such spreading matrix, small number of elements in a spreading sequences are non zero allowing each user to
spread its data over small number of chips (tones), thus simplifying the decoding procedure using Message Passing Algorithm (MPA)
Multi-carrier CDMA using convolutional coding and interference cancellation
SIGLEAvailable from British Library Document Supply Centre-DSC:DXN016251 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
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