10 research outputs found
Capacity of a Massive MIMO-OFDM system using Millimetric waves
In modern day society, wirelesses communications are fundamental to the communications worldwide. But with the saturation of the usable spectrum for communications, the need for capacity is one of the most important challenges faced nowadays.
With the development of technology, there is a much great need for quality of the service and bigger data streams. Increasing the capacity of a telecommuni-cation system will allow to cope with that need.
The solution for the need for capacity, can be solved with the use of massive multiple input multiple output (MIMO) techniques, since MIMO system ap-proach have already shown to increase the capacity of a wireless system.
The implications of this approach will make systems more complex and more energy consuming, in order to sustain the massive MIMO system. But such implementation combined with millimeter waves will allow the increase of the capacity of a system
Frequency-domain receiver design for doubly-selective channels
This work is devoted to the broadband wireless transmission techniques, which are serious candidates to be implemented in future broadband wireless and cellular systems, aiming at providing high and reliable data transmission and concomitantly high mobility.
In order to cope with doubly-selective channels, receiver structures based on OFDM
and SC-FDE block transmission techniques, are proposed, which allow cost-effective implementations, using FFT-based signal processing.
The first subject to be addressed is the impact of the number of multipath components,
and the diversity order, on the asymptotic performance of OFDM and SC-FDE, in
uncoded and for different channel coding schemes. The obtained results show that the
number of relevant separable multipath components is a key element that influences the performance of OFDM and SC-FDE schemes.
Then, the improved estimation and detection performance of OFDM-based broadcasting systems, is introduced employing SFN (Single Frequency Network) operation.
An initial coarse channel is obtained with resort to low-power training sequences estimation, and an iterative receiver with joint detection and channel estimation is presented.
The achieved results have shown very good performance, close to that with perfect channel estimation.
The next topic is related to SFN systems, devoting special attention to time-distortion
effects inherent to these networks. Typically, the SFN broadcast wireless systems employ OFDM schemes to cope with severely time-dispersive channels. However, frequency errors, due to CFO, compromises the orthogonality between subcarriers. As an alternative approach, the possibility of using SC-FDE schemes (characterized by reduced envelope fluctuations and higher robustness to carrier frequency errors) is evaluated, and a technique, employing joint CFO estimation and compensation over the severe time-distortion effects, is proposed.
Finally, broadband mobile wireless systems, in which the relative motion between
the transmitter and receiver induces Doppler shift which is different or each propagation path, is considered, depending on the angle of incidence of that path in relation to the direction of travel. This represents a severe impairment in wireless digital communications systems, since that multipath propagation combined with the Doppler effects, lead to drastic and unpredictable fluctuations of the envelope of the received signal, severely affecting the detection performance. The channel variations due this effect are very difficult to estimate and compensate. In this work we propose a set of SC-FDE iterative receivers implementing efficient estimation and tracking techniques. The performance results show that the proposed receivers have very good performance, even in the presence of significant Doppler spread between the different groups of multipath components
Time diversity solutions to cope with lost packets
A dissertation submitted to Departamento de Engenharia Electrotécnica of Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Engenharia Electrotécnica e de ComputadoresModern broadband wireless systems require high throughputs and can also have very high
Quality-of-Service (QoS) requirements, namely small error rates and short delays. A high spectral efficiency is needed to meet these requirements. Lost packets, either due to errors or collisions, are usually discarded and need to be retransmitted, leading to performance degradation.
An alternative to simple retransmission that can improve both power and spectral
efficiency is to combine the signals associated to different transmission attempts.
This thesis analyses two time diversity approaches to cope with lost packets that are
relatively similar at physical layer but handle different packet loss causes. The first is a lowcomplexity Diversity-Combining (DC) Automatic Repeat reQuest (ARQ) scheme employed in a Time Division Multiple Access (TDMA) architecture, adapted for channels dedicated to a single user. The second is a Network-assisted Diversity Multiple Access (NDMA) scheme, which is a multi-packet detection approach able to separate multiple mobile terminals transmitting simultaneously in one slot using temporal diversity. This thesis combines these techniques with Single Carrier with Frequency Division Equalizer (SC-FDE) systems, which are widely recognized as the best candidates for the uplink of future broadband wireless systems.
It proposes a new NDMA scheme capable of handling more Mobile Terminals (MTs)
than the user separation capacity of the receiver. This thesis also proposes a set of analytical tools that can be used to analyse and optimize the use of these two systems. These tools are then employed to compare both approaches in terms of error rate, throughput and delay performances, and taking the implementation complexity into consideration.
Finally, it is shown that both approaches represent viable solutions for future broadband wireless communications complementing each other.Fundação para a Ciência e Tecnologia - PhD grant(SFRH/BD/41515/2007); CTS multi-annual funding project PEst-OE/EEI/UI0066/2011, IT
pluri-annual funding project PEst-OE/EEI/LA0008/2011, U-BOAT project PTDC/EEATEL/
67066/2006, MPSat project PTDC/EEA-TEL/099074/2008 and OPPORTUNISTICCR
project PTDC/EEA-TEL/115981/200
An Optimization Theoretical Framework for Resource Allocation over Wireless Networks
With the advancement of wireless technologies, wireless networking has become ubiquitous owing to the great demand of pervasive mobile applications. Some fundamental challenges exist for the next generation wireless network design such as time varying nature of wireless channels, co-channel interferences, provisioning of heterogeneous type of services, etc. So how to overcome these difficulties and improve the system performance have become an important research topic.
Dynamic resource allocation is a general strategy to control the interferences and enhance the performance of wireless networks. The basic idea behind dynamic resource allocation is to utilize the channel more efficiently by sharing the spectrum and reducing
interference through optimizing parameters such as the
transmitting power, symbol transmission rate, modulation scheme, coding scheme, bandwidth, etc. Moreover, the network performance can be further improved by introducing diversity, such as
multiuser, time, frequency, and space diversity. In addition, cross layer approach for resource allocation can provide advantages such as low overhead, more efficiency, and direct end-to-end QoS provision.
The designers for next generation wireless networks face the common problem of how to optimize the system objective under the user Quality of Service (QoS) constraint. There is a need of unified but general optimization framework for resource allocation
to allow taking into account a diverse set of objective functions with various QoS requirements, while considering all kinds of diversity and cross layer approach. We propose an optimization
theoretical framework for resource allocation and apply these ideas to different network situations such as:
1.Centralized resource allocation with fairness constraint
2.Distributed resource allocation using game theory
3.OFDMA resource allocation
4.Cross layer approach
On the whole, we develop a universal view of the whole wireless networks from multiple dimensions: time, frequency, space, user, and layers. We develop some schemes to fully utilize the resources. The success of the proposed research will significantly
improve the way how to design and analyze resource allocation over wireless networks. In addition, the cross-layer optimization nature of the problem provides an innovative insight into vertical
integration of wireless networks
Multiple-access interference rejecting receivers in DS-CDMA communication system
SIGLEAvailable from British Library Document Supply Centre-DSC:DXN037068 / BLDSC - British Library Document Supply CentreGBUnited Kingdo
Modelling, Dimensioning and Optimization of 5G Communication Networks, Resources and Services
This reprint aims to collect state-of-the-art research contributions that address challenges in the emerging 5G networks design, dimensioning and optimization. Designing, dimensioning and optimization of communication networks resources and services have been an inseparable part of telecom network development. The latter must convey a large volume of traffic, providing service to traffic streams with highly differentiated requirements in terms of bit-rate and service time, required quality of service and quality of experience parameters. Such a communication infrastructure presents many important challenges, such as the study of necessary multi-layer cooperation, new protocols, performance evaluation of different network parts, low layer network design, network management and security issues, and new technologies in general, which will be discussed in this book
Exploitation of signal information for mobile speed estimation and anomaly detection
Although the primary purpose of the signal received by amobile handset or smartphone is to
enable wireless communication, the information extracted can be reused to provide a number
of additional services. Two such services discussed in this thesis are: mobile speed estimation
and signal anomaly detection. The proposed algorithms exploit the propagation environment
specific information that is already imprinted on the received signal and therefore do not
incur any additional signalling overhead. Speed estimation is useful for providing navigation
and location based services in areas where global navigation satellite systems (GNSS) based
devices are unusable while the proposed anomaly detection algorithms can be used to locate
signal faults and aid spectrum sensing in cognitive radio systems.
The speed estimation algorithms described within this thesis require a receiver with at least
two antenna elements and a wideband radio frequency (RF) signal source. The channel transfer
function observed at the antenna elements are compared to yield an estimate of the device
speed. The basic algorithm is a one-dimensional and unidirectional two-antenna solution.
The speed of the mobile receiver is estimated from a knowledge of the fixed inter-antenna
distance and the time it takes for the trailing antenna to sense similar channel conditions previously
observed at the leading antenna. A by-product of the algorithm is an environment
specific spatial correlation function which may be combined with theoretical models of spatial
correlation to extend and improve the accuracy of the algorithm. Results obtained via
computer simulations are provided.
The anomaly detection algorithms proposed in this thesis highlight unusual signal features
while ignoring events that are nominal. When the test signal possesses a periodic frame
structure, Kullback-Leibler divergence (KLD) analysis is employed to statistically compare
successive signal frames. A method of automatically extracting the required frame period
information from the signal is also provided. When the signal under test lacks a periodic
frame structure, information content analysis of signal events can be used instead. Clean
training data is required by this algorithm to initialise the reference event probabilities. In
addition to the results obtained from extensive computer simulations, an architecture for
field-programmable gate array (FPGA) based hardware implementations of the KLD based
algorithm is provided. Results showing the performance of the algorithms against real test
signals captured over the air are also presented.
Both sets of algorithms are simple, effective and have low computational complexity – implying
that real-time implementations on platforms with limited processing power and energy
are feasible. This is an important quality since location based services are expected to be an
integral part of next generation cognitive radio handsets