790 research outputs found
Design and analysis approaches to compact directional antennas for cognitive radio
Cognitive radio (CR) ist eine neuartige Technologie, die es erlaubt die
spektralen Funkressourcen intelligent und effektiv zu nutzen. Jüngste
Messkampagnen beweisen, dass die zugewiesenen Frequenzbänder der
lizenzierenden Benutzer nicht effizient genutzt werden. Außerdem benötigen
moderne Funktechnologien mehr Spektrum, um wachsenden
DatenÃijbertragungsrate- und Quality-of- Service-Anforderungen gerecht zu
werden. Cognitive radio erlaubt die Sekundärnutzung von nicht vollständig
genutzten Frequenzbereichen, wobei die Primärnutzung durch Lizenzinhaber
nicht gestört werden darf.Seit der ersten Erwähnung von Cognitive radio im
Jahr 1999 lag der Fokus auf Frequenz- bzw. spektralen Ressourcen.
Allerdings ist dies für die Anforderungen von zukunftsweisenden
Funktechnologien nicht ausreichend. Eine Kombination aus der Betrachtung
von Frequenz, Raum/Richtung und Zeit ermöglicht eine noch effizientere
Nutzung des Funkspektrums. Dabei kommen Technologien wie beispielsweise die
Schätzung der Empfangsrichtung und die Interferenzunterdrückung zum
Einsatz. In dieser Arbeit werden Methoden des Entwurfs und der Analyse von
direktiven Multibandantennen zur Bereitstellung richtungs- und
frequenzabhängiger Funktionalitäten vorgestellt. Dies geschieht mit Hilfe
orthogonal angeordneter Multibandantennen und mit kompakten
Multibandantennenarrays.Die entworfenen Antennen wurden mit Hilfe von
Simulationen, Messungen und durch die Emulation von
channel-sounder-Messungen analysiert. Als Referenzantennensystem dient eine
konzentrische Anordnung aus Monopolantennenarrays und Absorberplatten
zwischen den Antennenelementen. Dieses Referenzantennensystem wurde für die
Durchführung von Machbarkeitsstudien in Messkampagnen eingesetzt. Mit einem
aus neun Elementen bestehenden Array können entsprechend neun
Freiheitsgrade erzielt werden. Diese setzen sich aus drei wählbaren
Frequenzbändern (GSM 900 MHz, GSM 1800 MHz, und IEEE 802.11b/g) und drei
Richtungen pro Frequenzband zusammen. Das Referenzantennensystem ist in der
Lage, Frequenzbänder und Signaleinfallsrichtungen mit einem
Signal-zu-Interferenz-Verhältnis von 20 dB unter den reflexionsarmen
Bedingungen in einer Absorberkammer aufzulösen. Für das Band des GSM 1800
wurde eine Feldmessung in der Umgebung von vier Basisstationen
durchgeführt.Das spektrale Sensing erfolgte nach dem Prinzip der
Leistungsdetektion. Möglichkeiten zur richtungsselektiven Kommunikation
konnten in einer Vielzahl von GSM-Kanälen für ca. 50 % der Beobachtungszeit
detektiert werden. Durch die Reduzierung der Zwischenelementabstände konnte
eine kompakte Antenne des konzentrischen Antennenarrays konstruiert werden.
Dies führt zu einer gegenseitigen Verkopplung der Antennenelemente und
damit zu einer Beeinflussung der Stromverteilung und schließlich der
Antennenrichtdiagramme. Um diese Effekte zu minimieren, wurde ein
multibandfähiges Anpassungs- und Entkopplungsnetzwerk entworfen, welches
die Entkopplung und Anpassung der Antennenelemente mit Modenspezifischen
Lasten ermöglicht. Die Rekonfigurierbarkeit in jedem Frequenzband wird
durch kapazitive Justierung mit Hilfe von Varaktordioden erreicht. Das
multibandfähige Anpassungs- und Entkopplungsnetzwerk und das
rekonfigurierbare Netzwerk für GSM 900 wurden auf einer Leiterplatte
realisiert und im Hinblick auf Entkopplung, Anpassung, und
Strahlungsdiagramme der Ports getestet. Die 10 dB-Bandbreite für Anpassung
und Entkopplung der statischen Netzwerke ist ca. 30 MHz. Das
rekonfigurierbare Netzwerk stellt eine Bandbreite von mehr als 100 MHz
bereit, die mit insgesamt 5 Stufen erreicht wird.Die Richtdiagramme waren
in verschiedenen Richtungen mit einem Korrelationskoeffizient kleiner als
30 % orthogonal, und in verschiedenen Frequenzbereichen mit einer
Korrelation besser als 70 % selbstähnlich. Schließlich wurde das Verhalten
von Richtantennen in heterogenen Ausbreitungsszenarien durch Simulation und
Emulation untersucht. Dies beinhalteten Kanalmodelle für Simulation von
statischen Szenarien und vorhandenen channel-sounder-Messungen zur
Emulation der Mobilitätsszenarien. Verschiedene gemessene und analytisch
bestimmte Richtdiagramme wurden verwendet, um die Verfügbarkeit von
richtungsabhängigen Kommunikationsressourcen für Cognitive radio zu
untersuchen.Simulationen mit analytischen Richtdiagrammen von uniform
zirkularer Arrays zeigten, dass die Empfangssignalstärke über die
Einfallsrichtungen proportional zum Nebenkeulenpegel der direktiven
Richtdiagramme ist. Ein Nebenkeulenpegelvon 20 dB eines Antennenarrays mit
6 Elementen wurde als Optimum gefunden. Die richtungsabhängigen
Sendemöglichkeiten von ca. 50 % wurden mit einem Sensing-Schwellwert
kleiner -120 dB für mobile Szenarien ermittelt. Die Verfügbarkeit
richtungsabhängiger Ressourcen ist abhängig von dem Schwellwert des
gewählten Algorithmus für das spektrale Sensing.Zusammenfassend lässt sich
sagen, dass sorgfältig konstruierte direktive Antennen die Existenz
richtungsabhängiger Ressourcen für Cognitive radio aufspüren können.
Anpassungs- und Entkopplungsnetzwerke für kompakte Antennenarrays können
mittels kommerziell verfügbaren konzentrierten Bauelementen mit engen
Toleranzen hergestellt werden. Die Rekonfigurierbarkeit solcher Netzwerke
kann mittels Varaktordioden erreicht werden. Richtungsabhängige
Kommunikation ist mit den vorgeschlagenen Antennen sowohl in statischen als
auch mobilen Szenarien möglich.Cognitive radio is an emerging radio technology, promising
intelligent and effective use of spectrum resources. State-of-the-art
measurement campaigns show that the allocated spectrum is not efficiently
used by the licensed users. On the other hand, future radio technologies
require more spectrum to meet high capacity and quality of service
requirements. Cognitive radio proposes secondary usage of the
under-utilised spectrum resources while preserving the access-rights of the
licensed (primary) users.Since the introduction of cognitive radio, in
1999, the focus of cognitive radio communications has been on frequency
resources. However, frequency resourcealone may not be sufficient to fulfil
the needs of future communication systems. A combination of frequency,
space/direction, and time can ensure a more efficient use of the spectrum,
by employing techniques like direction-of-arrival estimation, interference
mitigation, etcetera. Approaches to design and analyse compact multi-band
directional antennas, required to support directional as well as frequency
resources, are proposed in this thesis. Design of such antennas was
accomplished with orthogonal arrangement of multi-band antennas, and with
compact multi-band antenna arrays. Analysis of directional antennas was
carried out with simulations, measurement campaigns, and emulation of
channel sounder measurements. A concentric arrangement of monopole antenna
arrays was used as a reference antenna system, where directional patterns
were obtained using metallic/absorber walls between antenna elements. This
reference antenna system was used to perform proof-of-principle measurement
campaigns. With an antenna array of nine elements, nine degrees-of-freedom
(frequency-directional resources) were obtained at the antenna ports. These
consist of three selectable frequency bands, namely GSM 900 MHz, GSM 1800
MHz, and IEEE 802.11b/g, and three directions per frequency band. The
reference antenna system was capable of separating frequency and directions
with a signal-to-interference-ratio of 20 dB, inside an anechoic chamber.
An outdoor measurement of such an antenna system was carried out for GSM
1800 MHz, at a location surrounded by four base-stations. Power detection
was used as the spectrum sensing algorithm. The opportunity to communicate
in a certain direction using the occupied frequency channels was observed
for about 50 % of the sensing time for various GSM channels.This concentric
arrangement was made compact by reducing the inter-element spacing. The
reduction of inter-element spacing results in mutual coupling between the
antenna elements, which disturbs the current distribution and hence the
beam patterns of the antenna arrays. To reduce this negative effect, a
multiband decoupling and matching network was designed to mitigate the
element coupling and to match the elements with mode-specific loads.
Reconfigurable networks were designed with the help of the capacitive
tuning of varactor diodes. The multi-band decoupling and matching network,
and the reconfigurable network for GSM 900 MHz were manufactured on a
printed circuit board, and tested in terms of decoupling, matching, and
resulting port beam patterns. The 10 dB bandwidth for matching and
decoupling by the fixed network, for compact antenna arrangement with an
inter-element spacing of lambda/6, was about 30 MHz. Reconfigurable network
provided a bandwidth above 100 MHz, achievable with five reconfigurable
states. The patterns were orthogonal in different directions with
correlation coefficients less than 30 % and self-similar at different
frequency bands with a correlation better than 70 %.Finally, the behaviour
of directional antennas under heterogeneous propagation scenarios was
studied using simulation and emulation. This involved channel models for
statistical simulation of static scenarios, and existing channel sounder
measurements for emulation of mobility scenarios. Various measured and
analytical beam patterns were used to study the availability of directional
communications resources for cognitive radio. Simulations with analytical
patterns of uniform circular arrays indicated that the received signal
strength is directly proportional to the side-lobe level of the directional
patterns. A side-lobe level of 20 dB, achievable with an array of 6
elements, was found to be optimum. The opportunity to communicate in
certain directions using the occupied frequency channels (directional
opportunity) was obtained for 50% of the total snapshots for a threshold
level lower than -120 dB, in mobility scenarios. The availability of
directional resources was dependent on the threshold level chosen for the
spectrum sensing algorithm.It is concluded that well-designed directional
antennas can identify the existence of directional resources for cognitive
radio communications. Exploitation of unexplored antenna strategies for
cognitive radio empowers a cognitive node with significant additional
degrees-of-freedom. However, angular distribution of multipath, mobility of
primary or secondary user, and speed of detection influence the usability
of directional resources for cognitive radio. Decoupling and matching
networks for compact arrays can be fabricated with off-the-shelf lumped
elements with tight tolerances. Such networks can be made reconfigurable
using varactor diodes. The work presented in the thesis is expected to
facilitate the design of future directional antennas for cognitive radios
resulting in more efficient utilisation of the spectrum
Mobile to mobile channel modelling for wireless communications
Wireless communication has been experiencing many recent advances in mobile to mobile (M2M) applications. M2M communication systems differ from conventional fixed to mobile systems by having both transmitter and receiver in low elevation and in motion. This raises the need to come up with new channel models and perform statistical analysis on M2M communication channels looking from a different perspective. This need motivated us to perform the research outlined in this thesis. In reviewing the literature we found that though in general the M2M channel models are sparse, a major gap exists in the non geometrical stochastic based mathematical channel models. In filling this gap, we develop a novel mathematical non geometrical stochastic multiple input multiple output (MIMO) M2M channel model for two dimensional (2D) and three dimensional (3D) scattering environments. This model is based on the underlying physics of free space wave propagation and can be used as a framework for any environment by selecting suitable complex scattering gain functions. In addition, we extend this novel model to multicarrier M2M which is the first multicarrier channel model in the non geometrical stochastic M2M category. Based on our novel M2M channel model, we carry out an extensive analysis in space-time correlation, space-frequency correlation and second order channel statistics. With the choice of suitable parameters, this analysis and channel model can be used for any wireless environment. Thus, we claim that our novel channel model together with the analysis performed in this thesis can be taken as a generalized framework. A significant contribution of our analysis is the consideration of the impact of transmitter and receiver speed to space-time and space-frequency correlation, which is not available in the literature. Using a von Mises-Fisher distribution as the angular power distribution, the usefulness of the derived temporal correlation function is discussed. The simulation results corroborate the fact that both space-time and space-frequency correlations are reduced when transmitter or receiver speed increases. The rate of reduction of space-time correlation in von Mises-Fisher distribution scattering environment is more than in the isotropic environment. Under second order channel statistics, we consider Rice, Rayleigh and Nakagami fading channels in four different non-isotropic scattering environments with angle of departure (AoD) and angle of arrival (AoA) distributions given by (i) separable Truncated Gaussian, (ii) separable von-Mises, (iii) truncated Gaussian bivariate and (iv) truncated Laplacian bivariate distributions. We show that the major second order statistics, namely, the level crossing rate (LCR) and the average fade duration (AFD), in different fading channels can be expressed in terms of known scattering coefficients of the AoD and AoA distributions. As the channel models and their respective measurements provide reliable knowledge of the channel for the design and analysis of M2M systems, the proposed channel model and the corresponding analysis will be useful for the design, testing and performance evaluation of future M2M communication systems
Massive MIMO Extensions to the COST 2100 Channel Model: Modeling and Validation
To enable realistic studies of massive multiple-input multiple-output
systems, the COST 2100 channel model is extended based on measurements. First,
the concept of a base station-side visibility region (BS-VR) is proposed to
model the appearance and disappearance of clusters when using a
physically-large array. We find that BS-VR lifetimes are exponentially
distributed, and that the number of BS-VRs is Poisson distributed with
intensity proportional to the sum of the array length and the mean lifetime.
Simulations suggest that under certain conditions longer lifetimes can help
decorrelating closely-located users. Second, the concept of a multipath
component visibility region (MPC-VR) is proposed to model birth-death processes
of individual MPCs at the mobile station side. We find that both MPC lifetimes
and MPC-VR radii are lognormally distributed. Simulations suggest that unless
MPC-VRs are applied the channel condition number is overestimated. Key
statistical properties of the proposed extensions, e.g., autocorrelation
functions, maximum likelihood estimators, and Cramer-Rao bounds, are derived
and analyzed.Comment: Submitted to IEEE Transactions of Wireless Communication
On the Performance Gain of NOMA over OMA in Uplink Communication Systems
In this paper, we investigate and reveal the ergodic sum-rate gain (ESG) of
non-orthogonal multiple access (NOMA) over orthogonal multiple access (OMA) in
uplink cellular communication systems. A base station equipped with a
single-antenna, with multiple antennas, and with massive antenna arrays is
considered both in single-cell and multi-cell deployments. In particular, in
single-antenna systems, we identify two types of gains brought about by NOMA:
1) a large-scale near-far gain arising from the distance discrepancy between
the base station and users; 2) a small-scale fading gain originating from the
multipath channel fading. Furthermore, we reveal that the large-scale near-far
gain increases with the normalized cell size, while the small-scale fading gain
is a constant, given by = 0.57721 nat/s/Hz, in Rayleigh fading
channels. When extending single-antenna NOMA to -antenna NOMA, we prove that
both the large-scale near-far gain and small-scale fading gain achieved by
single-antenna NOMA can be increased by a factor of for a large number of
users. Moreover, given a massive antenna array at the base station and
considering a fixed ratio between the number of antennas, , and the number
of users, , the ESG of NOMA over OMA increases linearly with both and
. We then further extend the analysis to a multi-cell scenario. Compared to
the single-cell case, the ESG in multi-cell systems degrades as NOMA faces more
severe inter-cell interference due to the non-orthogonal transmissions.
Besides, we unveil that a large cell size is always beneficial to the ergodic
sum-rate performance of NOMA in both single-cell and multi-cell systems.
Numerical results verify the accuracy of the analytical results derived and
confirm the insights revealed about the ESG of NOMA over OMA in different
scenarios.Comment: 51 pages, 7 figures, invited paper, submitted to IEEE Transactions on
Communication
Massive MIMO channel modelling for 5G wireless communication systems
Massive Multiple-Input Multiple-Output (MIMO) wireless communication systems,
equipped with tens or even hundreds of antennas, emerge as a promising technology
for the Fifth Generation (5G) wireless communication networks. To design and evaluate
the performance of massive MIMO wireless communication systems, it is essential
to develop accurate, flexible, and efficient channel models which fully reflect the characteristics
of massive MIMO channels. In this thesis, four massive MIMO channel
models have been proposed.
First, a novel non-stationary wideband multi-confocal ellipse Two-Dimensional (2-D)
Geometry Based Stochastic Model (GBSM) for massive MIMO channels is proposed.
Spherical wavefront is assumed in the proposed channel model, instead of the plane
wavefront assumption used in conventional MIMO channel models. In addition, the
Birth-Death (BD) process is incorporated into the proposed model to capture the
dynamic properties of clusters on both the array and time axes.
Second, we propose a novel theoretical non-stationary Three-Dimensional (3-D) wideband
twin-cluster channel model for massive MIMO communication systems with
carrier frequencies in the order of gigahertz (GHz). As the dimension of antenna arrays
cannot be ignored for massive MIMO, nearfield effects instead of farfield effects
are considered in the proposed model. These include the spherical wavefront assumption
and a BD process to model non-stationary properties of clusters such as cluster
appearance and disappearance on both the array and time axes.
Third, a novel Kronecker Based Stochastic Model (KBSM) for massive MIMO channels
is proposed. The proposed KBSM can not only capture antenna correlations but
also the evolution of scatterer sets on the array axis. In addition, upper and lower
bounds of KBSM channel capacities in both the high and low Signal-to-Noise Ratio
(SNR) regimes are derived when the numbers of transmit and receive antennas are
increasing unboundedly with a constant ratio.
Finally, a novel unified framework of GBSMs for 5G wireless channels is proposed.
The proposed 5G channel model framework aims at capturing key channel characteristics
of certain 5G communication scenarios, such as massive MIMO systems, High
Speed Train (HST) communications, Machine-to-Machine (M2M) communications,
and Milli-meter Wave (mmWave) communications
Spatio-Temporal processing for Optimum Uplink-Downlink WCDMA Systems
The capacity of a cellular system is limited by two different phenomena, namely
multipath fading and multiple access interference (MAl). A Two Dimensional (2-D)
receiver combats both of these by processing the signal both in the spatial and temporal
domain. An ideal 2-D receiver would perform joint space-time processing, but at the
price of high computational complexity. In this research we investigate computationally
simpler technique termed as a Beamfom1er-Rake. In a Beamformer-Rake, the output of a
beamfom1er is fed into a succeeding temporal processor to take advantage of both the
beamformer and Rake receiver. Wireless service providers throughout the world are
working to introduce the third generation (3G) and beyond (3G) cellular service that will
provide higher data rates and better spectral efficiency. Wideband COMA (WCDMA)
has been widely accepted as one of the air interfaces for 3G. A Beamformer-Rake
receiver can be an effective solution to provide the receivers enhanced capabilities
needed to achieve the required performance of a WCDMA system.
We consider three different Pilot Symbol Assisted (PSA) beamforming techniques,
Direct Matrix Inversion (DMI), Least-Mean Square (LMS) and Recursive Least Square
(RLS) adaptive algorithms. Geometrically Based Single Bounce (GBSB) statistical
Circular channel model is considered, which is more suitable for array processing, and
conductive to RAKE combining. The performances of the Beam former-Rake receiver are
evaluated in this channel model as a function of the number of antenna elements and
RAKE fingers, in which are evaluated for the uplink WCDMA system. It is shown that,
the Beamformer-Rake receiver outperforms the conventional RAKE receiver and the
conventional beamformer by a significant margin. Also, we optimize and develop a
mathematical formulation for the output Signal to Interference plus Noise Ratio (SINR)
of a Beam former-Rake receiver.
In this research, also, we develop, simulate and evaluate the SINR and Signal to Noise
Ratio (Et!Nol performances of an adaptive beamforming technique in the WCDMA
system for downlink. The performance is then compared with an omnidirectional antenna
system. Simulation shows that the best perfom1ance can be achieved when all the mobiles
with same Angle-of-Arrival (AOA) and different distance from base station are formed in
one beam
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